Future Directions for Sustainable Poultry Feeding and Product Quality: Alternatives from Insects, Algae and Agro-Industrial Fermented By-Products

Abstract

Due to global increases in poultry meat and egg production, consumers request sustainable agricultural practices, requiring alternative solutions for future feeding. Global egg production increased by over 41% between 2000 and 2020, from 51 to 87 million tonnes, at an average increasing rate of 3%. Similarly, the production of poultry meat reached 145 million tonnes in 2023 and continues to increase, which amplifies the pressure on sustainable alternative feed solutions. Commercial poultry diets are typically based on a cereal (corn or wheat) as an energy source and a quality protein source, especially soybean meal (SBM), to provide essential amino acids. Soybean production is associated with deforesting and land use in several countries, sensitiveness to supply chains and price volatility. As a response to these challenges over the last decade, research and commercial innovation have intensively focused on alternative and novel feed resources that can be integrated into both broiler and layer diets. Some future candidate ingredients are insect meal, algae, agro-industrial by-products such as distiller’s dried grains with solubles (DDGS), brewery spent grains (BSG) and fermented feedstuffs (oilseed cakes/meals). Literature data showed that moderate inclusion of these alternative ingredients can be partly integrated in poultry diets, without compromising egg or meat quality. In some cases, studies showed improvements of productive performances and specific quality traits (yolk color, fatty acids and antioxidant compounds), offering potential to valorize waste streams, improve local circularity and provide functional ingredients for animals and humans. However, challenges still remain, especially in terms of nutrient variability, digestibility limitations, higher processing costs and still-evolving regulations which constrain mainstream adoption of some of these potential future alternatives.

1. Introduction

In the last two decades, global poultry production, which includes both eggs and meat sectors, has been one of the most dynamic and rapidly growing segments of animal agriculture, and it continues to expand. According to the Food and Agriculture Organization, poultry meat production increased from a few million tonnes in the early 1960s to more than 142 million tonnes in 2023 [1]. The largest growth was noted in Asia, Latin America and Africa [1]. Similarly, the egg sector has shown a similar trajectory, with global hen production surpassing 20 million tonnes in the 1960s and increasing to about 87 million tonnes in 2023, representing one of the fastest growth rates among all livestock-derived foods [2]. Taken together, both poultry meat and eggs now provide the largest single share of animal protein sources consumed worldwide and are critical to both food security and human nutrition [3].

Urbanization, rising incomes and shifting dietary preferences are the primary demand-side drivers for both poultry meat and eggs. More than half of the world’s population now lives in urban areas, where animal-source proteins are part of the daily food sources. This trend favors poultry because of its affordability, short production cycles and adaptability into diverse processed and ready-to-cook products and meals. For eggs, demand growth is also linked to their role as a low-cost, nutrient-dense food product rich in protein, essential fatty acids and micronutrients, often consumed daily across the world in different cultures [4]. Recent projections from United Nations, World Bank and Organization for Economic Cooperation and Development (OECD-FAO) [5] analyses indicate continued increases in global consumption per capita of both meat and eggs through at least 2030–2050, with the most rapid growth occurring in low- and middle-income countries [6,7]. This suggests that animal feed will be a critical component of the integrated food chain in the future. In this line global compound feed production has surpassed 1 billion tonnes (1.2 to 1.3 billion tonnes) annually, of which the proportion destined for poultry sectors represents 40 to 43% (525.8 million tonnes), according to an Agri-Food Outlook report [6]. This significant increase could explain why changes in raw feed materials (replacing or reducing soy intake) have large effects on agri-food systems.

Poultry are the most efficient converters of feed into animal products compared to ruminants. Broiler chickens have a feed conversion ratio between 1.5 kg of feed and 2.0 kg of feed necessary for 1 kg of growth. These values depend on the growing stage (starter, grower, finisher), hybrid and genetics, while laying hens have nutritional requirements designed for egg production. Commercial poultry diets are typically based on a cereal (corn or wheat) as an energy source and a quality protein source, especially soybean meal (SBM), to provide essential amino acids. The inclusion levels of SBM in the compound feeds (15 to 40%) vary by type of poultry (layers or broilers) and growing stage, which makes SBM the nutritional reference ingredient.

On the supply side, poultry feed is the dominant cost and environmental driver across both broiler and layer systems. Feed commonly represents 60–70% of total production costs in broiler meat systems and up to 75% in egg production, making producers highly vulnerable to volatility in commodity markets [6,7,8]. For eggs, reliance on conventional feed not only influences productivity but also important egg quality parameters (yolk color, shell strength and nutrient composition), which are sensitive to diet formulation [8,9,10]. These constraints and quality concerns intensify the need to identify alternative ingredients to provide higher-quality poultry products for consumers.

According to the same report presented by the OECD-FAO [5] the global demand for protein is increasing across both animal and plant sources as the worldwide population continuously grows. It was stated that, by 2030, global demand for plant proteins is projected to increase up to 43% compared with current levels. At the same time, animal protein demand is expected to increase by approximately 33% by 2030, primarily due to increased meat consumption in emerging economies. These aspects threaten to intensify the environmental footprint of food systems. Currently, agri-food systems generate about 16.5 billion tonnes of carbon dioxide equivalent (Gt CO₂-eq) per year, representing roughly 32% of global emissions, with farm to gate crop and livestock activities alone contributing approximately 8.1 Gt CO₂-eq [1,2,5].

In these circumstances, the FAO is stressing the importance of finding alternatives to conventional animal feed, because of its limited amount and the competition between humans and animals, as viable mitigation strategies to enhance sustainability in poultry production. Furthermore, SBM production is strongly associated with land use and deforestation in certain countries, sensitiveness of supply chains to global trade, climate and price volatility [11,12,13].

In response to these challenges, researchers and commercial innovation companies in the last decade (2015–2025) have focused on finding alternative and novel feed resources that can be integrated into both broiler and layer diets. Candidate ingredients include insect meals [14], agro-industrial co-products such as distiller’s dried grains with solubles (DDGS) [15], oilseed cakes/meals [16], micro- and macroalgae [17] and fermented feedstuffs [18]. These materials offer potential to valorize waste streams, improve local circularity and provide functional ingredients (i.e., proteins, antioxidants, lipids, pigments). However, challenges still remain, especially in terms of nutrient variability, digestibility limitations, higher processing costs and still-evolving regulations which constrain mainstream adoption of these alternatives.

Although literature data presents a growing body of works documenting chemical composition and short-term applications in poultry diets, several gaps still exist, which limit strong recommendations for large-scale production and practice. There is no consensus on optimal inclusion rates in broilers and layers, there is compositional variability between suppliers and processing methods are insufficiently characterized while dose–response studies are limited and heterogeneous in experimental design. In this context, this review aims to explore recent literature data on the potential utilization of alternative and sustainable poultry feeding ingredients, specifically insect meals, distiller’s dried grains with solubles, brewery spent grains, micro- and macroalgae and fermented by-products. The paper’s focus is on the chemical composition reported for the reviewed alternative feed ingredients and their potential to sustain or improve poultry production performances and animal-origin food quality.

2. Methodology of Article Searches

The literature search was carried out from 2005–2025, using international scientific databases, like Web of Science, Scopus, PubMed, CAB Abstracts, ScienceDirect, SpringerLink and Google Scholar. Searches targeted peer-reviewed studies evaluating the chemical composition and poultry applications of alternative protein sources, specifically insect meals, microalgae, DDGS and BSG as agro-industrial by-products and fermented oilseed cakes/meals. Relevant keyword combinations were used, such as “insects meal poultry”, “microalgae poultry feed”, “macroalgae hens”, “fermented feed broilers”, “DDGS poultry”, “brewers’ spent grains chickens”, “oilseed meals poultry”, “poultry nutrition”, “laying hens”, “broiler chickens”, “production performances” as well as additional terms related to poultry meat quality and egg quality. The searches included peer-reviewed articles, as well as reviews, book chapters and technical reports considered relevant to the topic of the current review which reported analytical data. The inclusion criteria were based on: studies in the English language; studies that reported at least chemical composition of the ingredients; included controlled poultry feeding trials; provided quantitative outcomes (performance, meat/egg quality). The exclusion criteria: studies without a control group; missing or incomplete data on chemical composition; studies on non-poultry species; studies with unclear methodology; unclear diet formulation reported or studies published without open access.

3. Challenges of Conventional Poultry Feed Ingredients

Maize and SBM remain the predominant energy and protein sources in commercial poultry diets because of their high metabolizable energy, favorable amino acid balance (lysine and methionine) and global supply chains. However, continuing reliance on these commodities generates multiple sustainability, economic and supply-risk concerns for both meat (broiler) and egg (layer) production [19]. For example, recent farm-level analyses from Europe reported for broiler feed shares of 69.60% to 76.10% of the total cost [6] while feed tonnage and feed-cost pressure are especially acute in Asia-Pacific and Africa where input prices and supply constraints have been largest. This heavy cost weighting means that price volatility in maize and SBM markets rapidly transmits to producer margins and, for layers, to egg retail prices. Environmentally, SBM expansion has driven large land-use changes in South America, where it more than doubled (from 26.4 Mha to 55.1 Mha) between 2000 and 2019 and direct soybean-driven deforestation totaled roughly 3.4 Mha between 2001 and 2016 [20], with broader analyses indicating that SBM replacement accounted for Mha of forest conversion globally over the early 21st century. However, over the thirty-four-year period (1990–2024) maize (corn) and SBM exhibited substantial volatility in prices (Figure 1). It was reported that global average corn prices increased from about USD 166/ton in 2020 to a peak near USD 318/ton in 2022 before easing to approximatively USD 190/ton in 2024 [21]. For SBM the global price showed increases from USD 350/ton in 2020 to USD 560/ton in 2022 then moderating to approximatively USD 405/ton in 2024 according to recent data reports [22]. These significant changes have been driven by a combination of extreme weather events, trade disruptions and macroeconomic shocks. The first sharp increase in corn and soybean prices began in 2008, when global demand for cereals and oilseeds increased rapidly due to China’s expanding imports [23], while United States corn was directed to ethanol production, under the 2007–2008 energy policies, tightening global supply [24]. A second wave followed in 2011, driven by China’s soybean imports exceeding 55–60 million tonnes annually and by the continued use of about 40% of United States corn production for ethanol, further reducing availability for food and feed [25]. The 2012 severe US drought lowered corn output by roughly 13%, escalating global prices and raising feed costs, especially for SBM, while export restrictions imposed by several countries from 2010–2012 further sustained high price levels [25]. Later, the prices of these important grains returned toward 2007 levels, but the post-COVID period and the 2022 global commodity shock triggered by the war in Ukraine generated a new surge with global effects. Small- and medium-scale producers in sub-Saharan Africa and parts of South Asia have experienced sharper real increases in feed costs [26], whereas feed production and tonnage continued to expand overall in Asia-Pacific and Latin America [27,28]. All these factors, the cost exposure, environmental factors and supply vulnerability inherent to maize–soy-based rations, create strong incentives to evaluate and adopt locally available, lower-impact and more circular alternatives, particularly solutions that preserve or improve egg quality traits and broiler performance while reducing land, water and carbon footprints.

4. Alternative Sustainable and Future Sources for Poultry Feeding

4.1. Insects as Alternative Feed Sources for Poultry

Insects are one of the most promising alternative protein sources for sustainable poultry nutrition. A wide range of taxa, such as Orthoptera (crickets, grasshoppers, locusts), Lepidoptera (silkworms, armyworms), Diptera (housefly larvae) and Annelida (earthworms) have been investigated for their nutritional potential. These organisms can efficiently convert low-value organic matter into high-quality biomass rich in proteins, lipids and bioactive compounds. Of these, Acheta domesticus (house cricket) and Locusta migratoria (migratory locust) have shown significant (50–65%) CP levels, with balanced essential amino acid profiles suitable for poultry diets [29]. Silkworm pupae (Bombyx mori), often available as a by-product of silk production, can also represent a valuable feed component, providing up to 50–55% of CP, when fed with Rhodotorula glutinis yeast [30]. Similarly, larvae of lepidopteran species such as Spodoptera frugiperda contain 47.23% CP and have shown potential to replace a significant proportion of SBM in broiler diets without adversely affecting growth or meat quality [31]. Dipteran species such as the common housefly (Musca domestica) are also interesting because they can be mass-reared on organic waste substrates, which contributes to nutrient recycling. Housefly meal contains about 38% to 76% CP and has been shown to improve BW, FCR and intestinal health in broilers [32]. Similarly, Eisenia foetida (earthworm) can serve as a high-CP (62.98%) supplement, although large-scale production is still a challenge [33].

Within this broad spectrum, four species stand out for their research interest and scalability. Black soldier fly larvae (BSFL; Hermetia illucens), yellow mealworm (YMW; Tenebrio molitor) and cricket meal (CM) (Acheta domesticus and Gryllus bimaculatus) are the most investigated insect ingredients for poultry diets, and a rapidly expanding body of scientific work provides quantitative estimates of their nutritive value and production effects. These alternatives provide high CP levels, digestible lipids and balanced amino acid profiles that allow partial replacement of conventional SBM. Current scientific evidence indicates that these insect meals can be integrated into both broiler and layer diets at low to moderate inclusion rates with neutral to positive effects on production performance, feed efficiency and some product quality parameters. Consequently, they currently represent the benchmark for insect-based feed research and application.

The European Union has created a legislative framework (Regulation EU 2017/893) that allows the use of certain insects in the feed of monogastric animals, while also establishing restrictions on the substrates allowed for insect feed and reproduction [34]. To date, there are 13 types of insects that have been confirmed to be incorporated in poultry feeding, of which only half were approved by the European Regulation to be used as feed ingredients as presented by the EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) as well as in the text of the EU 2017/893 regulation (annexes I and IV to Regulation EC 999/2001 of the European Parliament and annexes X, XIV and XV to the regulation EU 142/2011) as regards the provisions on processed animal protein [34,35]. This legislative framework has encouraged projects and demonstrations aimed at incorporating insects into food chains, with a focus on food safety, traceability and circular economy. Of these insects, BSFL, YMW and CM have been the most studied (Regulation EU 2017/893) [34]. This approach also contributed to reducing the pressure on agricultural land and GHGe associated with conventional protein crops, offering a sustainable alternative to substitute SBM. In Asia, particularly in China, Indonesia and Malaysia, pilot farms and demonstration plants have integrated BSFL and YMW production with the valorization of agri-food waste. The studies reported considerable reductions in the volume of treated organic waste and improvements in nutrient use efficiency when larvae are used as animal feed [36,37]. However, regional life cycle assessments (LCAs) showed results sensitive to substrate choice and energy consumption. In some scenarios, bioconversion significantly reduces land use and waste, but although the CO₂ footprint of poultry could be decreased by 30% to 50% compared to SBM-based diets the GHGe can be higher if the infrastructure is energy-intensive [38]. In several African countries (Kenya, Zimbabwe and others), the introduction of BSFL in poultry feed has been associated with real economic savings for smallholder farmers, providing a local source of protein that reduces dependence on expensive protein imports [39]. Cost–benefit assessments and practical reports show that local production of BSFL can reduce feed costs for smallholder farmers, generating up to a 40% reduction in feed costs and more efficient use of resources [40]. Results vary depending on the scale of production and local raw material prices. LCA reports commissioned by industrial insect protein producers in North America reported substantial reductions in CO₂ emissions (40% to 50%) compared to certain reference ingredients in their functional scenarios, as well as large water and land savings for certain insect products. These results come from specific LCAs, with their own functional limits and assumptions, however, independent analyses published in the literature indicate results in certain configurations and for certain production systems [41]. In South America, particularly in Brazil and Colombia, pilot projects growing BSFL on agro-industrial residues (fruits, crop residues) have demonstrated the maintenance of productive performances at moderate inclusions (5% to 10%) and the potential for reducing the local food footprint when the systems are well-sized [42]. Regional LCAs indicate land savings compared to SBM, but the magnitude of the emission reduction (by 25% to 35%) depends on the allocations accounted for in the analysis [42,43,44]. While most available LCAs of alternative feed ingredients are currently region-specific or pilot-scale, comprehensive global-scale LCAs integrating feed production, processing, transport and market dynamics remain limited, highlighting an important area for future research.

4.1.1. Black Soldier Fly Larvae (BSFL)

The nutritional composition of BSFL varies considerably with processing treatments, influencing macronutrient and mineral profiles (

Table 1. Nutritional composition of BSFL under the effect of different treatments (as dry matter).

Components	Untreated	Spray-Dried	Oven-Dried	Whole	Defatted	Flour	Full-Fat
Nutritional composition (%)
CP	49.10	48.20	47.76	45.82	56.18	37.30	42.30
EE	36.60	25.69	28.43	25.78	4.86	-	31.60
CF	-	9.96	9.48	-	-	-	-
Ash	-	8.27	8.19	6.85	11.39	9.20	10.50
Moist	65.90	7.10	3.21	4.14	6.46	5.20	4.20
Chitin	18.10–22.30	-	31.0–35.0	-	6.90		3.87–6.70
Mineral composition (converted to mg/kg)
Ca	185,000	21,176	26,515	20.31	20.58	15,600	35,700
Fe	1500	689.24	300.75	0.25	0.98	916	14,000
Mg	28,700	3616	3310	53.56	509.08	3040	3400
Mn	1200	134.93	140.12	20.03	54.79	124	33,500
K	69,100	13,156	11,256	254.60	509.73	12,400	9200
Na	-	4084	5028	354.72	47.56	900	15,600
Zn	1100	1209	303	0.24	80.72	92.40	9000
Main fatty acid composition (%)
SFA	64.50	46.69	74.83	-	75.98	-	75.0
MUFA	12.30	11.03	18.02	-	10.55	-	8.60
PUFA	19.60	42.28	6.83	-	9.87	-	12.91
n-3	1.48	1.99	0.32	-	0.54	-	1.01
n-6	18.10	40.29	6.83	-	8.10	-	11.90
References	[49,50]	[45]	[45,51]	[45]	[45,52,53]	[54]	[47,48,53]

CP—crude protein; EE—crude fat; CF—crude fiber; Moist—moisture; Ca—calcium; Fe—iron; Mg—magnesium; P—phosphorus; K—potassium; Zn—zinc; Na—sodium; Mn—manganese; SFA—saturated fatty acid; MUFA—monounsaturated fatty acid; PUFA—polyunsaturated fatty acid.

). Protein levels ranged from 37.30% in BSFL flour to 56.18% in defatted larvae, indicating that lipid extraction effectively concentrates crude protein (CP) [45]. Contrarily, crude fat (EE) content decreases in defatted BSFL (4.86%) compared with untreated (36.6%) and full-fat samples (31.60%) [46,47,48]. The most abundant essential amino acids reported by [48] were leucine (from 27.8 g/kg to 78.3 g/kg), lysine (from 23.0 g/kg to 68.2 g/kg) and valine (ranging from 28.2 g/kg to 67.9 g/kg). These three amino acid contents are higher than those of SBM. This aspect confirms the efficiency of defatting in reducing lipid fractions while enhancing protein density in BSFL. The highest mineral levels were generally recorded in dried samples. Ca is mostly concentrated in spray- and oven-dried BSFL, exceeding that in untreated BSFL [45]. The same trend was observed for Mg, suggesting that these treatments retain or even concentrate essential minerals due to water loss. Literature data showed that the Fe content varies significantly from 0.25 mg/kg in the entire insect and 300.75 mg/kg in oven-dried BSFL to 916 mg/kg in BSFL flour, while full-fat BSFL (14,000 mg/kg) contained even higher Fe contents [47,48], possibly due to matrix effects or measurement differences. K and Na contents are also influenced by the processing method. Spray-dried BSFL contained higher contents of these elements, while defatted BSFL showed lower levels, likely reflecting lipid extraction and associated mineral loss. Zn and Mn followed similar trends, decreasing in defatted BSFL but remaining higher in dried or full-fat BSFL, supporting their potential as mineral-rich feed components. The fatty acid composition is also treatment-dependent. Oven drying resulted in the highest proportion of SFA (74.83%), while spray drying enhanced PUFA (42.28%), especially n-6 fatty acids (40.29%) [45]. This suggests that spray drying better preserves UFA by minimizing oxidation during processing. These differences should guide processing choices depending on the intended nutritional application, with high-protein and mineral-rich defatted BSFL for feed fortification or lipid-balanced spray-dried BSFL for broader nutritional use.

4.1.2. Yellow Mealworm (YMW; Tenebrio Molitor)

YMW is another insect species extensively studied for poultry nutrition, with a growing body of evidence documenting its nutrient composition and production effects (

Table 2. Nutritional composition of YMW obtained under the effect of different treatments.

Specification	No Treatment	Sun-Dried	Oven-Dried	Freeze-Dried	Microwave	Vacuum-Dried	Pulsed Electric Field
Nutritional composition (%)
CP	53.53	50.96	51.51	41.21	49.5	52.23	45.18–48.16
EE	27.13	27.26	26.23	20.82	31.0	29.57	17.79–24.71
CF	6.47	6.20	6.11	-	-	6.83	-
Ash	3.27	4.15	4.15	4.17	3.50	3.40	4.49–5.34
Moist	62.87	3.37	21.82	6.07	3.10	1.70	3.09–4.39
Chitin	3.90–8.40	-	4.72	7.60–8.50	-	-	-
Mineral composition (converted to mg/kg)
Ca	-	275.01	294.77	654.5	-	-	-
P	7970	6899.82	7484.15	-	-	-	-
Fe	67.6	50.00	46.45	49.4	-	-	-
Mg	2823	2220.10	2458.60	2607.9	-	-	-
Mn	11.6	11.75	11.07	11.9	-	-	-
K	8007	8201	7244	11,735.2	-	-	-
Na	2066	1080.12	1089.22	1532.6	-	-	-
Zn	95.6	6899.82	7484.15	171.6	-	-	-
Main fatty acid profile (%)
SFA	23.00	23.45	23.22	24.21	24.66–26.44	-	20.22–22.08
MUFA	41.00	37.54	36.62	56.05	40.09–42.83	-	41.17–42.58
PUFA	35.10	7.50	40.11	18.15	29.70–32.26	36.47	34.05–37.21
n-6	33.30	37.25	38.54	17.52	-	34.99	32.25–35.49
n-3	1.60	1.56	1.54	0.63	-	1.48	1.72–7.84
References	[51,60,61]	[57]	[57,62]	[55,63]	[58]	[61]	[63]

CP—crude protein; EE—crude fat; CF—crude fiber; Moist—moisture; Ca—calcium; Fe—iron; Mg—magnesium; P—phosphorus; K—potassium; Zn—zinc; Na—sodium; Mn—manganese; SFA—saturated fatty acid; MUFA—monounsaturated fatty acid; PUFA—polyunsaturated fatty acid.

). The CP content ranged from 41.21% in freeze-dried larvae [55] to over 53% in untreated and sun-dried YMW samples [56,57]. This aspect indicates that some thermal treatments may promote protein denaturation or concentration effects due to moisture removal. Both oven- and vacuum-dried YMW maintained relatively high CP levels (51.51% and 52.23%, respectively), suggesting efficient preservation of nitrogenous compounds under controlled heating. The EE values were also influenced by processing, with microwave-dried YMW showing the highest EE content (31%) [55], possibly due to partial lipid oxidation or concentration during rapid dehydration. Moisture content varied significantly, from 62.87% before treatment to 1.70% in vacuum-dried samples [56]. This type of treatment suggests that vacuum drying provides the most effective dehydration. Ash content increased after drying (3.4% to 4.17%), likely reflecting the relative enrichment of minerals as water was removed. In terms of mineral profile, studies reported that Ca and P were highest in oven-dried YMW (294.77 and 7484.15 mg/kg), indicating that this method supports mineral stability [57]. Mg and K were also better retained under oven drying (2458.6 and 7244 mg/kg), whereas the freeze-drying method resulted in lower mineral concentrations [58], possibly due to partial sublimation losses. Fe and Mn exhibited minimal variation among treatments, while Zn concentrations were notably higher in oven- and sun-dried larvae. Fatty acid composition was moderately altered among different YMW treatments. The SFA remained within a narrow range (23.20% to 26.40%), while MUFA increased under the freeze-drying method (56.05%) as shown by Bogusz et al. [59]. This might suggest limited lipid oxidation under low-temperature conditions. PUFAs were highest in untreated (35.10%) and vacuum-dried larvae (36.47%) [50,54], which indicated that reduced exposure to oxygen and heat better maintains unsaturated lipid fractions in YMW. These results show that drying techniques have a significant impact on YMW nutritional quality and the selection of drying method should therefore depend on the desired balance between protein concentration, mineral retention and lipid stability, when designing feeding diets for poultry.

4.1.3. Cricket Meal (CM)

Among the different species of crickets, as alternative feed ingredients, Acheta domesticus (AD) and Gryllus bimaculatus (GB) have received more attention than other CM species due to their CP content, favorable amino acid profile and the potential for environmentally more efficient production compared with conventional feedstuffs. The nutritive value of CM depends on species, developmental stage, substrate that was fed, processing and defatting conditions (

Table 3. Nutritional composition of CM obtained under the effect of different treatments.

Specification	Acheta domesticus (AD)				Gryllus bimaculatus (GB)
	Air-Dried	Freeze-Dried	Frozen	Partly Defatted	Air-Dried	Freeze-Dried
Nutritional composition, %
CP	71.70, %	56.80, %	15.10, %	75.34, %	60.70, %	53.40, %
EE	10.40, %	22.80, %	5.86, %	10.18, %	23.40, %	26.40, %
CF	4.60, %	3.30, %	-	-	10.00, %	6.60, %
Ash	5.40, %	-	0.66, %	5.00, %	2.80, %	-
Moist	6.30, %	-	78.83, %	4.82, %	3.00, %	-
Chitin	4.30–7.10, %		7.16, %	6.26, %	2.40, %
Mineral composition, mg/kg
Ca	149.75	-	-	-	105.14	-
P	899.33	-	-	-	702.02	-
Fe	8.83	-	-	-	7.16	-
Mg	136.58	-	-	-	72.94	-
Mn	4.40	-	-	-	3.40	-
K	389.92	-	-	-	321.71	-
Na	101.44	-	-	-	88.84	-
Zn	19.61	-	-	-	14.39	-
Main fatty acid profile, %
SFA	8.145, %	37.29, %	-	-	12.76, %	35.13, %
MUFA	4.14, %	28.22, %	-	-	9.85, %	38.57, %
PUFA	1.46, %	34.49, %	-	-	1.80, %	26.29, %
n-6	1.13, %	32.91, %	-	-	1.55, %	24.33, %
n-3	0.07, %	1.10, %	-	-	0.08, %	1.13, %
References	[51,67]	[72]	[43]	[43]	[51,67]	[72]

CP—crude protein; EE—crude fat; CF—crude fiber; Moist—moisture; Ca—calcium; Fe—iron; Mg—magnesium; P—phosphorus; K—potassium; Zn—zinc; Na—sodium; Mn—manganese; SFA—saturated fatty acid; MUFA—monounsaturated fatty acid; PUFA—polyunsaturated fatty acid.

). In 2024, at the request of the European Commission, the EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) was asked to deliver an opinion on the safety of frozen, dried and powder forms of AD-CM as a novel food pursuant to Regulation EU 2015/2283, to be used in food products [64]. The panel concluded that AD-CM is safe to be used, acknowledging that the true protein content is overestimated when using the nitrogen-to-protein conversion factor of 6.25 due to the presence of non-protein nitrogen from chitin. Phesatcha et al. [65] reported that GB-CM contains important CP (54.10%), CF (6.90%), EE (26.90%) and total digestible nutrient (78.90%), as well as a variety of essential amino acids, including methionine, lysine, histidine, valine and leucine. In terms of fatty acid composition, CM generally contains significant proportions of linolenic (30% to 40%), palmitic (24% to 30%) and oleic (23 to 27%) acids [66]. These reported data suggest that this insect can be comparable or even superior to classic plant protein sources, like SBM. In a comparative study of the two CM species (AD and GB) the CP composition was 71.7% and 60.7%, respectively, while the EE content was double in GB-CM (23.4%) compared to AD-CM (10.4%) [67]. The drying method had a significant impact on the nutritional composition of AD-CM. Lucas-González et al. [68] showed that thermal drying and lyophilization influence the CP (62.68% to 67.48%) and EE content (24.91% to 19.32%). Besides the treatment applied, it was shown that the sex of CM influences the chemical composition. Female AD have lower CP (63.10% to 66.6%) than male CM (69.90% to 71.90%), with a similar profile of PUFA, ranging between 28.88% and 31.28% in females and 29.25% to 31.24% in males [69]. In a recent study, the AD-CM sourced from three farms in Thailand showed similar nutrient contents in terms of CP (59–60%), EE (19% to 20%) and linoleic acid (27.55% to 30.21%) [70]. The authors reported significant contents of minerals with significant variations for K (8424 to 12,095 mg/kg), P (6728 to 9753 mg/kg), Ca (1278 to 1799 mg/kg) and Zn (137 to 215 mg/kg). Moreover, the killing process or processing heat also has an influence on CM nutritional composition. Dobermann et al. [71] showed that, when slaughtering GB by freezing at low (45 °C) or high (120 °C) temperature, the CP and Ca content is increased in insects killed at 45 °C. Using freeze drying or drying at 32, 45, 72 or 120 °C revealed that the freeze-drying method preserved significantly more LC-PUFA than drying at 120 °C. From a regulatory standpoint, insect killing methods are subject to EU feed hygiene legislation (Regulation EU 2017/893) and EFSA safety opinions, which allow practices such as freezing, blanching or heat treatment, provided feed safety is ensured. However, specific animal welfare legislation for insects is currently lacking at the EU level, as insects are not explicitly covered by existing animal welfare frameworks. This regulatory gap may influence processing choices, public perception and the future availability of insect-derived feed ingredients. These results highlight that processing methods have a significant impact on CM nutritional composition, and care should be taken in this process. Although insects as alternative feed ingredients offer nutritional and functional benefits, safety issues need careful attention. Microbial contamination if rearing and processing conditions are not strictly controlled could represent a major issue. Moreover, beyond insect-based ingredients, other biologically derived feed resources have gained increasing attention in poultry nutrition, particularly algae, which differ fundamentally in their production systems, nutrient profiles and functional properties.

4.2. Macroalgae and Microalgae as Alternative Feed Sources

The use of algae, both macroalgae (seaweeds) and microalgae, in poultry nutrition is an expanding field, which offers interesting perspectives but also requires rigorous clarification. Both macroalgae such as Ulva, Ascophyllum, Sargassum and Gracilaria and microalgae like Spirulina, Chlorella and Nannochloropsis are valuable biological sources of nutrients. Although both groups photosynthesize and utilize carbon dioxide, they differ considerably in cell structure, biochemical composition and nutritional value for birds [73]. Global production of farmed algae (seaweed) reached 36.5 million tonnes in 2022, an increase of 4.1% (1.4 million tonnes) from the 2020 production of 35.1 million tonnes. This increase was the result of production expansions led by China, Malaysia, the Philippines, the United Republic of Tanzania, the Russian Federation and a few others, according to FAO [74] reports. On the other side, commercially used microalgae (Spirulina, Chlorella) remain much smaller in production volume. The common estimates for global commercial microalgae production are estimated at around 50 to 60 kilotonnes per year in recent statistics, mainly from Asia [75]. Compared with macroalgae, microalgae are a unique biomass resource that does not need to compete for land and water with other biomass feedstocks because it can be cultivated on low-quality unencumbered land using non-competitive water types including saline and wastewater. In Europe, the production of algae remains modest compared to Asia. The countries with significant activity for macroalgae production are France, Ireland and Spain while, for microalgae, they are Germany, Spain and Italy [76]. Although there is a high potential for mainstreaming these alternatives, some key challenges and limitations exist. Scaling up macro/microalgae production for food and feed purposes requires higher production and processing costs (photobioreactors versus open ponds) which places algae as a high-value-added ingredient rather than cheap feedstock. These alternatives exhibit a significant variability in nutritional composition between species and production batches, and there is a risk of contamination with heavy metals and possible presence of toxins [77,78]. Moreover, regulatory barriers exist in the EU, due to the fact that many species are considered “novel foods” by EFSA and require safety assessments by special panelists before commercialization [77,79,80]. In the context of poultry feed, these alternatives could present significant nutritional advantages (protein, PUFAs, antioxidant pigments) while nutritional studies show benefits for performance, egg quality and gut health. However, the higher production costs hinder their full use as a major substitute for traditional ingredients, but in smaller amounts, functional effects can be achieved [81]. In a previous report published by the FAO the potential of seaweed/microalgae for food security and sustainable development was highlighted, noting the increase in global production (36.5 million tonnes for algae in 2022) but emphasizing the need for risk management and the development of value chains [82]. Similarly, EFSA has specifically evaluated ingredients derived from microalgae and developed guidelines and requirements for the authorization of additives and novel foods in the EU derived from algae [83,84].

4.2.1. Macroalgae

Macroalgae, often called seaweeds, are a group of multicellular marine plants, typically divided into three main groups based on their physical properties. Green algae (Chlorophyta) include species of the Chlorella, Chlamydomonas, Enteromorpha, Spirogyra and Ulva genera. Red algae (Rhodophyta) include species in the Palmaria, Delesseria, Chondrus and Corallina genera and are the largest group of seaweeds (5000 to 6000 species) throughout the world. Brown algae (Phaeophyta), the second largest group (about 1500 to 2000 species), include species of the Laminaria, Saccharina and Fucus genera and Sargassum muticum [85]. While most red algae are used in food products, especially in the orient, the brown ones are mostly used in poultry feeding [86]. The use of macroalgae in poultry feed has evolved over the past decade from a marginal strategy based on local resources to an emerging field of functional nutrition, driven by global pressure for sustainable ingredients and reduced reliance on conventional sources. The advantage of macroalgae in poultry nutrition is given by the relative availability of marine biomass in coastal areas, the rich micronutrient profile, especially pigments (xanthophylls, fucoxanthin) that can improve the color of egg yolk and meat, and the potential to be used in the circular economy [87]. However, there are also some limitations, especially lower CP digestibility, higher CF content, mineral salts, heavy metals and potential antinutritional factors, compared to conventional ingredients [73] (

Table 4. Main nutrients of macroalgae species (as dry matter).

Items	Brown Macroalgae				Green Macroalgae		Red Macroalgae
	Sargassum muticum	Saccorhiza polyschides	Laminaria digitata	Laminaria hyperborea	Codium tomentosum	Grateloupia turuturu	Gracilaria gracilis	Osmundea pinnatifida
Nutritional composition (%)
CP	16.90	14.44	2.88–11.12	2.22–9.98	18.80	22.50	20.20	23.80
EE	1.45	1.10	0.03–1.12	0.34–0.80	3.60	2.20	0.60	0.90
Ash	20.52	28.15	18.96–36.00	18.91–35.64	35.99	20.58	24.80	30.62
Moist	9.64	10.88	-	-	9.00	11.68	7.99	11.77
Mineral composition (converted to mg/kg)
Ca	9110	9110	-	-	640	2650	3440	541
K	57,560	76,540	-	-	1860	16,280	65,100	26,100
Mg	15,040	7970	-	-	2790	6950	1750	4800
P	2280	2320	-	-	260	2810	2260	1730
Fe	190	79	-	-	2020	50	90	370
Zn	25	65	-	-	180	69	25	58
Cu	5	3	-	-	630	3	4	50
Mn	11	8	-	-	940	25	20	58
Main classes of fatty acids (%)
SFA	42.17	36.42	23.98–32.28	19.54–30.49	38.88	42.72	63.54	58.07
MUFA	21.13	29.09	12.13–21.48	13.25–21.64	18.51	11.54	15.24	18.92
PUFA	36.70	34.49	6.46–30.10	13.97–31.04	42.60	45.72	21.22	23.01
n-6	27.46	21.48	3.02–14.58	4.78–13.23	10.99	14.41	20.14	6.68
n-3	8.88	13.21	2.80–15.19	6.43–12.64	21.57	31.56	1.38	16.08
References	[100]	[100]	[91]	[91]	[100]	[100]	[100]	[100]

CP—crude protein; EE—crude fat; CF—crude fiber; Moist—moisture; Ca—calcium; Fe—iron; Mg—magnesium; P—phosphorus; K—potassium; Zn—zinc; Cu—copper; Mn—manganese; SFA—saturated fatty acid; MUFA—monounsaturated fatty acid; PUFA—polyunsaturated fatty acid.

). Regarding heavy metal content in macroalgae, in a recent report by Woźniak and Moya [80], EFSA has drawn attention to the potentially high content of inorganic arsenic and cadmium, which might exceed the safe limits. The potential accumulation of iodine, arsenic and cadmium imposes constraints on inclusion levels and necessitates compliance with regulatory limits, in line with EFSA recommendations [80]. Therefore, careful analyses and safety protocols are required. Some recent studies [88,89,90], highlighted that macroalgae represent a rich source of bioactive compounds with immunomodulatory, antioxidant and antimicrobial potential but also present technical and nutritional limitations that would require a critical approach for large-scale adoption. The chemical composition of species like Ulva and Enteromorpha spp. reflected adaptation to coastal environments with large fluctuations in light and salinity. In these species, the CP content varies from 9% to 27%, being influenced by season, light intensity and developmental stage [91]. It was noted that most of the seaweeds have low EE contents (0.30% to 1.0%), and the insoluble fiber fraction, mainly cellulose and hemicellulose, can exceed 15% to 45%, which affects digestibility in poultry feeding. Compared to brown and red macroalgae, green macroalgae (Chlorophyta) provide a significantly larger range of minerals, especially K, Mg and Zn, due to their ability to accumulate cations from low-saline environments [92,93]. However, their low CP content and low digestibility could limit their potential use as a main ingredient. Red algae, such as Palmaria palmata, Chondrus crispus or Porphyra spp., are valued for their superior nutritional composition compared to green and brown algae [94]. The CP content is significantly higher (20% to 35%), and the essential amino acid profile is more balanced, including considerable levels of lysine and methionine. Their characteristic polysaccharides (carrageenans, agar) have distinct technological and functional properties. For example, carrageenans can modulate intestinal viscosity, influencing transit and microbiota, an aspect still insufficiently explored in poultry farming [95]. Brown macroalgae represent, from a research point of view, the most intensively analyzed group, due to their natural abundance and complex bioactive composition. Species such as Ascophyllum nodosum, Laminaria digitata, Saccharina latissima and Sargassum spp. are characterized by a lower CP content, ranging from 6% to 20%, but they are extremely rich in bioactive compounds like polysaccharides (laminarin, fucoidan, alginate), marine polyphenols, specific carotenoids (fucoxanthin) and minerals, especially iodine [96,97,98]. However, the iodine content is a critical aspect that must be considered carefully, as an EFSA report [83] mentioned that species like Laminaria japonica could exceed 2500 to 4000 mg iodine/kg. Therefore, considering the current great expansion of seaweed consumption by the Western population, specific regulations on this subject should be laid down, and controlled use and prior analysis of the ingredient are mandatory. Nevertheless, unlike terrestrial plant extracts, Negi et al. [99] reported recently that natural pigments (i.e., fucoxanthin, prodigiosin and chlorophyllin) from green and brown algae are more attractive as alternatives to synthetic pigments, due to their stability in processing, superior antioxidant activity and health promoting properties [99]. This aspect makes seaweeds very attractive for the food industry.

4.2.2. Microalgae

Microalgae represent another promising alternative source of nutrients for the poultry industry, due to their nutrient density (proteins, lipids), ability to synthesize bioactive compounds and their potential for sustainable growth. However, despite their advantages, there are major challenges associated with digestibility, processing, dietary inclusion and cost. The microalgae often tested in poultry nutrition are Spirulina (Arthrospira platensis), Chlorella vulgaris, Schizochytrium spp. and other species of Nannochloropsis or Dunaliella [79]. Islam et al. [101] recently reported that these microalgae have an almost complete amino acid profile, which makes them a valuable source of proteins. Spirulina is noted for its high CP content, ranging between 52.40% to 62.80%, and CP digestibility can reach 85% after mechanical or enzymatic pretreatment [102,103]. The amino acid profile is characterized by relatively high levels of lysine, leucine and valine, although methionine content is generally lower than in soybean meal, which may require dietary balancing when used at higher inclusion levels. Chlorella vulgaris has a CP content of 50% to 60% and presents a whole profile of essential amino acids, consisting of lysine (3.5% to 6.0%), methionine (1.0% to 1.6%), threonine (3.0% to 4.2%) and valine (4.0% to 5.5%) [104,105,106]. Compared with Spirulina, Chlorella generally shows a more balanced essential amino acid distribution and a lower fiber content, which may translate into improved palatability and digestibility in poultry diets. Schizochytrium spp., although richer in lipids (27%), contains approximately 15% to 25% CP, but the amino acid profile is highly digestible and complete in terms of essential amino acids [107]. Its nutritional relevance is therefore primarily linked to lipid-derived functional effects, rather than protein supply, differentiating it clearly from protein-oriented microalgae such as Spirulina and Chlorella. These data show that microalgae can constitute an alternative CP supply similar to SBM, particularly for young birds and broilers, where the supply of essential amino acids is important for growth and development. Microalgae are also important sources of natural pigments and bioactive compounds. Astaxanthin is produced mainly by Haematococcus pluvialis, especially in the red stage (81.20%), having strong antioxidant effects [108]. When used in poultry feeding, it can improve yolk pigmentation and protect dietary lipids from oxidation. Spirulina contains phycocyanin (0.335 mg/mL), a blue pigment with antioxidant and immunostimulatory activity, present in a significant proportion [109]. Chlorella contains carotenoids, especially β-carotene (72.36 mg/g), which contribute to the antioxidant activity and natural pigmentation of poultry products [106]. These findings indicate that microalgae differ not only in nutritional composition but also in their functional effects. Spirulina and Chlorella primarily act as combined protein and functional additives, enhancing antioxidant status, immune modulation and product pigmentation, whereas lipid-rich microalgae such as Schizochytrium spp. mainly influence oxidative stability and fatty acid composition. Macroalgae such as kelp contribute predominantly to pigmentation, mineral supply and antioxidant protection but play a limited role in protein nutrition. This functional differentiation is important for defining the intended use of algae in poultry diets, whether as protein supplements, natural pigment sources or antioxidants.

The minerals and vitamins in microalgae are also notable [104,105], including water-soluble vitamins (B1, B2, B6 and B12) and fat-soluble vitamins (A, E and K1) [110]. These micronutrients contribute to the nutritional balance of poultry diets and may reduce the need for synthetic supplements, however, processing, including drying or extraction, can alter CP composition and digestibility. Due to this reason standardization of processes is necessary for effective use in poultry nutrition. Both macro- and microalgae may bioaccumulate heavy metals (arsenic, cadmium, lead) depending on cultivation site and environmental quality. Therefore, sourcing, processing and quality control are critical to minimize risks. Future research should focus on standardized safety protocols and regular monitoring to ensure safe inclusion in poultry diets (

Table 5. Main nutrients in microalgae species (as dry matter).

Ingredients	Spirulina	Chlorella vulgaris	Schizochytrium spp.
Nutritional composition (%)
CP	52.40–62.80	51.58–59.30	15.00
EE	0.63–6.93	3.06–17.40	41.70
Ash	7.47–11.90	0.86–7.20	10.00
CF	8.12–34.20	-	0.10
Bioactive compounds
Polyphenols	51.20, μg/mL	1.27, mg GAE	74.52
β-carotene	2.74, mg/g	72.36, mg/g	-
Carotenoids	4.69, mg/g	98.34, mg/g	-
Main classes of fatty acids (%)
SFA	50.10	32.19–35.90	52.91
MUFA	8.50	19.11–23.20	-
PUFA	40.60	41.00–48.38	46.73
n-6	37.70	30.30–34.24	7.79
n-3	2.90	10.70–14.14	38.94
References	[102,103]	[104,106]	[107,111]

CP—crude protein; EE—crude fat; CF—crude fiber; SFA—saturated fatty acid; MUFA—monounsaturated fatty acid; PUFA—polyunsaturated fatty acid.

).

4.3. Fermented Feeds as Alternative and Novel Sources

In contrast to insects and algae, which introduce novel biomass into poultry diets, fermentation-based approaches primarily aim to enhance the nutritional quality, digestibility and safety of conventional feed materials and by-products. Co- and by-products from vegetable processing constitute rich, valuable resources of bioactive compounds, including phytochemicals (phenolics, flavonoids, carotenoids), antimicrobials, antioxidants, vitamins and dietary fats. These components exhibit favorable technological functionalities as well as nutritional benefits, making them suitable for applications in animal feed [16,112]. However, their direct use in poultry diets is often limited by the presence of antinutritional factors (ANFs), non-starch polysaccharides and other non-fermentable fractions, which can impair nutrient digestibility and utilization. Fermentation represents a biological strategy to overcome these constraints by modifying the chemical structure of the substrate prior to ingestion.

Solid-state fermentation (SSF) has been recognized as an effective and robust strategy for enhancing the nutritional quality of plant-derived protein sources in poultry nutrition [113]. Known as a low-cost bioprocessing method, SSF can significantly improve nutrient absorption and digestibility, reduce the ANFs, suppress gut-pathogenic bacteria and enhance the functional properties of raw materials [114]. These improvements are primarily achieved through microbial enzymatic activity, which leads to partial hydrolysis of complex carbohydrates, degradation of fiber fractions and phytates and predigestion of proteins into smaller peptides and free amino acids, thereby increasing their availability to poultry. In this context, the use of the fermentation process to convert agro-industrial co/by-products into high-quality feed ingredients represents a promising strategy for poultry feed. Generally, agricultural residues are divided into field residues and process residues. These residues may include molasses, bagasse, husks, seeds, stems, leaves, straw, stalk, shell, pulp, stubble, peel and roots [115]. Widely, they are used for animal feeding, soil enhancement, fertilizer products, manufacturing and various other applications. Among these diverse materials, many also serve as suitable substrates for SSF. Commonly used substrates for SSF include cereal grains (rice, wheat, barley and corn), legume seeds, wheat bran, lignocellulosic materials (straw, sawdust and wood shavings), as well as a wide variety of by-products [115]. It is known that the process of SSF involves bacteria growing on different solid raw materials under controlled conditions (moisture, temperature, pH, media composition, mixing, sterilization, aeration, agitation, inoculum density) in the absence of free water [116]. Additionally, the microorganisms used in SSF can be either a single pure culture or a mixed culture. Different microorganisms like bacteria (Bacillus spp., Pseudomonas spp.), yeasts (Saccharomyces spp.) or fungi (Aspergillus spp., Streptomyces spp.) are used successfully in SSF for maximizing production of value-added products [117]. Fungal-based SSF is particularly effective for fiber-rich substrates due to the secretion of cellulases, hemicellulases and proteases. Bacterial and yeast fermentations are associated with improved protein availability, reduction of ANFs and production of organic acids with antimicrobial effects in poultry. In contrast to SSF, liquid fermentation is conducted in the presence of free water, allowing faster microbial growth and more uniform substrate contact, but it is generally associated with higher processing costs and energy requirements and increased risks of nutrient losses during drying. The use of exogenous enzymes represents a distinct approach, whereby specific carbohydrases, proteases or phytases are added directly to diets to target particular ANFs or fiber fractions [113]. While enzymatic supplementation can improve digestibility, it lacks the holistic predigestion and bioactive compound generation achieved through fermentation, and its efficacy depends strongly on enzyme–substrate specificity and gastrointestinal conditions.

The composition and nutritional value of products obtained after SSF can present some variations based on the nature of the feed resource, processing, storage parameters, methods of extraction and type of microorganism used in the SSF process [114]. Based on the literature data,

Table 6. Nutritional composition of agro-industrial by-products before and after fermentation process.

By-Product	Condition	DM, %	CP, %	CF, %	NDF, %	ADF, %	Ash, %	Reference
RSM	Unfermented	92.30	37.10	14.80	2.00–4.00	-	6.00–8.00	[118]
	Fermented	88.80	39.60	16.20	2.00–4.00	20.82	6.00–8.00
LSM	Unfermented	-	35.77	6.05	28.87	20.82	3.57	[119]
	Fermented	-	36.00	6.42	32.15	20.88	3.66
SFM	Unfermented	91.90	40.98	0.70	-	-	6.10	[120]
	Fermented	92.70	42.48	0.70	-	-	7.10
SBM	Unfermented	90.00	45.60	22.00	12.00	7.50	6.40	[121]
	Fermented	-	38.25	-	-	-	7.10
HSC	Unfermented	95.82	37.90	8.42	41.48	29.88	7.00	[120]
	Fermented	93.58	38.00	8.44	43.98	29.96	6.20
PSC	Unfermented	90.91	45.27	15.60	45.28	29.60	6.37	[120]
	Fermented	95.17	44.65	15.05	40.64	30.92	4.56
FSC	Unfermented	90.54	33.35	18.82	27.32	13.99	4.11	[122]
	Fermented	96.44	32.53	19.15	28.22	16.82	4.48
SFC	Unfermented	91.90	24.08	12.30	-	-	7.70	[120]
	Fermented	93.20	23.20	11.40	-	-	6.37
BSG	Unfermented	61.53	9.22	8.40	29.50	10.50		[123]
	Fermented	60.50	10.23	7.20	26.20	9.40
SBC	Unfermented	89.93	45.88	-	15.47	9.22	6.88	[124]
	Fermented	94.5	61.60	6.70	-	-	11.70	[125]
HOC	Unfermented	91.08	58.02	8.41	-	-	6.05	[126]
	Fermented	85.43	64.64	9.19	-	-	7.11

DM—dry matter; CP—crude protein; CF—crude fat; NDF—neutral detergent fiber; ADF, acid detergent fiber; RSM—rapeseed meal; LSM—lupin seed meal; SFM—sunflower meal; HSC—hemp seed cake; PSC—pumpkin seed cake; FSC—flax seed cake; SFC—sunflower cake; SBC—soybean cake; HOC—hazelnut oil cake.

presents the nutritional composition of different agro-industrial by-products before and after application of the fermentation process.

4.4. Agro-Industrial Distiller’s Dried Grains with Solubles (DDGS) and Brewery Spent Grain (BSG) as Alternative Feed Sources

Agro-industrial co/by-products such as DDGS, BSG and oilseed by-products (canola, rapeseed, sunflower, flaxseed, cottonseed, etc.) constitute large-volume, lower-cost feedstuffs that are attractive for poultry diets because they can supply both energy and protein while valorizing waste streams. These ingredients, however, are highly heterogeneous in proximate composition and digestibility, so their effects on broiler and layer performance are context-dependent [3] (Figure 2). DDGS is one of the most widely used bioethanol co-products, and it typically supplies both protein (25% to 30% CP for corn-DDGS) and energy but contains higher CF and variable amino acid profiles, especially lower lysine digestibility, and has uneven mycotoxin risk depending on source and storage [127,128]. Similarly, BSC composition varies significantly in CP (20% to 31%) content, as affected by genetic variation in the crops used, specificity of the brewery production and treatment and pretreatment after beer production [129]. In cold-pressed oilseed by-products, the CP contents vary markedly (20 to 40%) but could contain antinutritional factors (glucosinolates in some Brassica meals; high CF in sunflower or cottonseed cakes) that reduce amino acid availability unless processed or limited in inclusion. Detailed compositional reviews show that proper processing (dehulling, heat treatment, deoiling) and use of enzyme or fermentation treatments markedly improve nutritive value and reduce antinutritional effects in oilseed by-products [16].

In terms of production, global DDGS exports from the USA have significantly increased from 5 million tonnes in 2009 to more than 10 million metric tonnes in 58 countries in 2022/2023, according to the reports from the U.S. Grains & BioProducts Council from 2024 [130]. In Europe, reports showed that over 4 million tonnes were produced in 2024 [131], and this is projected to continuously increase until 2033, according to Statista.

BSG production reports estimated around 35 to 39 million tonnes of BSG is produced globally each year, which means 20 kg/100 L of beer produced, as this industry generates large amounts of BSG [132]. In Europe, according to Eurostat, more than 6.4 million tonnes of BSG waste is produced annually, but the exact figure depends on the year and the inclusion of microbreweries versus large breweries [133].

OB is mostly used in the EU. According to the 2024 CropLife Europe report, rapeseed in co-production with others accounts for up to 42% of the domestic production of alternative oilseeds for protein alternatives in the EU [134]. The report highlights a global export volume of rapeseed waste in 2022 of 12.3 million tonnes, and its consumption in 2025 reached a record level of 14.4 million tonnes. The European Feed Manufacturers’ Federation (FEFAC) reported that the EU has lower import availability for rapeseed meal than for other protein sources, especially because rapeseed meal stands out as the sole meal with relatively low import dependency (25%) in the EU [135]. Another important OB comes from flaxseed, from which the EU produces both flaxseed oil and flaxseed waste. Europe holds 28.7% of the global linseed market according to estimates for 2034. The FEFAC report mentions that flaxseed is one protein ingredient on which the EU is partially dependent [135]. In addition to rapeseed, flax and cotton, CropLife reported that other oilseed meals (depending on the region), such as safflower meal, palm kernel meal and non-GMO SBM, could be used as alternatives to SBM, but rapeseed cake and palm kernel cake are the largest contributors [134]. These by-products represent potential alternative sources of classic ingredients for poultry nutrition, due to their increased availability on the agri-food market, significant nutritional value and essential role in promoting a sustainable production system by reducing dependence on conventional raw materials.

4.4.1. Distiller’s Dried Grains with Solubles (DDGS)

DDGS are the dried residue/co-products remaining after fermentation and distillation of cereals, mostly corn, wheat, sorghum, barley and other mixed grains, for production of ethanol. According to the 2025 USDA report [136] 22.5 million tonnes of DDGS was produced in 2024, an increase of 7% from 2023 (21 million tonnes), of which the largest streams of DDGS originate from countries like the United States, Brazil, the EU and China, where industrial dry-mill bioethanol production is growing. This value reflects the evolution of corn consumption for ethanol and alcohol production on a global scale and is relevant when assessing the global supply of alternative proteins to conventional meal [137]. The proximate composition in these residues is highly variable, mostly due to processing conditions and drying regimens. The macronutrient content varies from 25% to 35% for CP, 8% to 13% EE, 8% to 15% CF and ash around 4% to 6% [138]. The processing steps (drying temperature, syrup addition rate, soluble recycling) strongly affect final composition of the DDGS waste. The higher addition of condensed distiller’s solubles increases EE, minerals and soluble sugars in DDGS, while high drying temperatures risk Maillard reactions and lower reactive lysine [139]. A recent review showed that the amino acid profile in DDGS shows that corn is relatively high in leucine but low in lysine and tryptophan compared with SBM [15]. Sulfur amino acids (methionine, cysteine) are present at moderate levels but usually lower than in other conventional sources like fish meal. One essential mineral for poultry, phosphorus, is moderate, but a significant portion can be phytate-bound depending on the grain and processing. The lipid profile in DDGS tends to be rich in linoleic acid and other unsaturated fatty acids derived from the original grain, which contributes to the energy value of DDGS but also to oxidative stability concerns in storage [15,140]. For these reasons, when compared with conventional poultry protein sources, DDGS occupies an intermediate position, due to an unbalanced amino acid profile, which requires attention when it is used as a principal protein supplement, as reported by others [137]. In terms of metabolizable energy for poultry, corn DDGS delivers more energy than typical cereal co-products because the non-starch fractions (oil and digestible protein) are concentrated, but the CF content and digestibility variability suggests that feed formulators should consider the results of analyzed metabolizable energy, instead of theoretical ones, as indicated by Parsons et al. [140]. However, from a practical feed-formulation standpoint, DDGS is widely available in ethanol-producing regions, at competitive prices, and it offers concentrated CP, P and energy in poultry diets when combined with SBM [137].

4.4.2. Brewery Spent Grain (BSG)

BSG is the most abundant co-product of beer production. Recent literature suggests a generation factor of approximately 20 kg BSG (wet)/hectoliter (hL) of beer produced, equivalent to approximately 4.4 to 6.2 kg dry BSG/hL depending on moisture content and postprocess treatment [141]. The largest streams of BSG are produced by global beer leaders such as China, the United States, Brazil and European beer-producing countries (Germany, Spain, Poland and others) [142]. BSG is heterogenous, with high moisture, and it is rich in CP relative to most cereal co-products, which typically ranges between 18% and 30%. The CF (30% to 60%) depends on the methods used, while the EE varies from 5% to 12% and ash around 2% to 6% [143,144]. However, the CP in BSG is partly cell-wall-associated and contains a relatively good profile of glutamic and aspartic acid, branched-chain amino acids and aromatic amino acids, but it is comparatively low in the limiting amino acids, such as methionine which is an essential nutrient for poultry [144]. The EE fraction in BSG is small but not negligible and often contains unsaturated fats, especially linoleic acid, similar to DDGS, and in small amounts sterols and vitamin E isomers [144]. When compared with conventional poultry CP sources, BSG has a distinct niche rather than serving as a one-for-one replacement for SBM. However, after processing, the CP in BSG is nutritionally closer to SBM, but such processes transform BSG from an inexpensive local co-product to a higher-value processed feedstuff, as recently reported by Osei et al. [145]. Although BSG constitutes a significant source of CP and CF for animal feed it is mostly used in ruminant feeding, and little attention has been given to poultry.

Figure 2. Chemical composition of Distillers Dried Grains with Solubles (DDGS) and Brewery Spent Grain (BSG). Average values after [146,147,148,149].

Figure 2. Chemical composition of Distillers Dried Grains with Solubles (DDGS) and Brewery Spent Grain (BSG). Average values after [146,147,148,149].

5. Effects of Reviewed Alternatives on Poultry Production Performances and Product Quality

5.1. Insect Meals

Across studies, BSFL meal consistently improves intestinal health, immune modulation and nitrogen retention, although high lipid and chitin contents limit inclusion levels [150]. In insect meals, high chitin content in the exoskeleton can reduce protein and energy digestibility by encapsulating nutrients and interfering with digestive enzyme access, particularly in young birds with limited chitinase activity. Processing strategies such as defatting, fermentation, enzymatic supplementation or cell wall disruption are critical determinants of digestibility and explain the wide variability in performance outcomes reported across studies, as discussed in the following sections.

In broiler trials, inclusion of BSFL meal at low to moderate levels (5% to 15%) has produced performance equal to or slightly better than control diets when diets were formulated to meet digestible amino acid requirements (Table 7). Including 5% BSFL meal in broilers finisher diets, no adverse effects on BWG or FCR were reported, while improving the markers of intestinal integrity [150]. The same authors [150] reported that up to 20% full-fat BSFL inclusion improved FCR by 10% and with favorable immune indices, although the authors cautioned that substrate, processing and balancing for amino acids were critical to the outcomes. Similarly, Seyedalmoosavi et al. [151] reported that, after testing whole BSFL at a 10%, 20% or 30% inclusion rate, the 20% diet did not negatively affect growth performance and FCR, whereas a higher proportion was associated with lower protein utilization efficiency. Conversely, when testing high substitution rates by replacing 50%, 75% and 100% of SBM, reductions in BWG, FCR and FI were reported [152,153], indicating possible digestibility or palatability constraints. When 75% or 100% replacement of SBM with full-fat BSFL was tested, results showed that it significantly deteriorates carcass quality and the sensory quality of meat, as reported by the authors [152]. The use of 3%, 6% and 9% BSFL fat in broiler chickens’ diets affected the fatty acid profile of breast muscle, particularly in terms of n-3 deficiency, decreasing its content progressively from 2.75 (control) to 1.95% in the 9% BSFL group [154]. Interestingly, the authors noted a significant increase in PUFA concentration, mostly linolenic acid, while MUFA levels were reduced. This might be a response of MUFA synthesis inhibition in the liver by lowering 9-desaturase activity through high PUFA content. In terms of health status of broilers, no major hematological changes at low inclusion levels (5% to 15%) were reported, however, chitin and other indigestible fractions can affect digestibility and alter gut histomorphology at high inclusion levels [155].

Studies on laying hens likewise show promising but dose-dependent effects. Tahamtani et al. [156] tested the effects of live BSFL to laying hens at 10% or 20% inclusion levels and reported that supplementing hens with live larvae could reduce conventional concentrate use and maintain egg production. The study observed improved animals’ behavior and no major negative welfare outcomes at moderate provision levels. Another study [157] evaluated long-term substitution of 1.5% or 3% fish meal with defatted organic BSFL in laying hen diets and reported no adverse effects on production performances (FCR, FI, laying rate). The BW of hens increased only at the 3% SBM replacement level. In terms of egg quality, the authors reported no significant effect on eggshell parameters (weight, thickness and strength), albumen weight, yolk height, yolk color and Haugh unit. However, both half (1.5% fish meal and 1.5% BSFL) and complete substitution of fish meal increased yolk weight (p < 0.01) and egg weight (p < 0.05). Yolk fatty acid composition shifted in the direction of increased medium-chain fatty acids, consistent with the BSFL lipid profile [157]. In a different experimental design, replacing 100% of soybean cake and oil with defatted BSFL + larvae fat in laying hen diets produced eggs averaging 67 g, with daily egg mass of 66 g, without affecting laying rate or egg quality (shell strength, Haugh unit) [158]. Contrarily, Park et al. [159] noted that, with a higher BSFL inclusion rate (12% or 24%) in laying hens’ diets, there were increased medium-chain fatty acids (C12:0; C14:0 and C16:0) compared with the control group. The n-3 fatty acids were significantly decreased in the 24% BSFL group. Interestingly, the 12% BSFL group deposited more EPA while the 24% BSFL group deposited more DHA in egg yolks [160]. Moreover, the use of 10% BSFL or 10% BSFL + 2% microalgae produced significantly heavier eggs with enhanced eggshell quality and yolk pigmentation, yielding higher albumen protein content and lower lipid concentrations when compared with the control [161]. It was concluded that BSFL and microalgae can be used as functional feed ingredients for improving poultry performance and egg quality in a sustainable production system. Reports of studies with sensory panels and shelf-life assays are not available, however, some evidence indicates that, when insects are included at typical levels used for partial substitution (≤15%), meat sensory traits and egg flavor are not detrimentally affected. At higher inclusion levels changes in fat composition can influence oxidative stability and flavor or necessitate use of antioxidants or adjustments to processing. This gap in the field requires further attention from the scientific community, considering consumers’ acceptance of animal-origin products derived from insect feeds.

Regarding YMW as an alternative protein source for broiler chickens, studies showed that it can replace a portion of conventional protein sources without compromising growth performance when diets are balanced for digestible amino acids [162]. However, to date, the results are contradictory in the literature. Complete replacement of SBM with YMW (29.65%), in slow-growing broiler diets, had no significant effect on most growth performance and carcass traits or chemical and physical properties of meat [163]. More recently, it was reported that the inclusion of 5% and 10% dried YMW, by replacing maize and SBM, improved BWG, carcass yield meat composition and color [164]. In slow-growing male Ross 308 broilers, partial replacement of SBM with 2.5%, 5%, 7.5% and 10% low-fat YMW also showed different effects [165]. Higher inclusion levels were associated with a decrease in breast pH and protein decline, while increasing the L* color parameter, drip loss, cooking loss and fat content in meat samples. SFA and MUFA increased in both breast and thigh, which led to a significant decrease in PUFA. The authors suggested that a 5% YMW inclusion level could be an adequate substitution level without detrimental effects on broilers [165].

In laying hens YMW meal has been evaluated predominantly at lower inclusion levels (1% and 5%). In Bovans White laying hens, 2.5% or 5% YMW significantly improved egg production, egg mass, FCR, FI and BW, with no effect on egg quality characteristics and blood parameters [166,167]. The authors concluded that 2.5% was the optimum level of SBM replacement in their experimental conditions, and it may have a role in improving socioeconomic and environmental outcomes, an important aspect that is aligning with the United Nations’ Sustainable Development Goals. Hy-Line Brown hens receiving 5%, 10% or 20% YMW showed age-dependent responses. In 96-day-old hens, YMW had no significant effects, however, in 236-day-old hens, an 10% YMW addition level optimally improved meat texture, while maintaining antioxidant capacity, and enhanced eggshell strength [168]. Low levels of YMW (2% or 5%) given to Lohmann Brown Classic hens had no effect on laying hens’ morphology or intestinal health, suggesting that up to 5% is safe [169]. However, further studies on laying hens are required to assess its safety and potential benefits when included in higher doses as an alternative to SBM or maize.

For CM, several studies have assessed different inclusion levels in broiler diets, replacing part of SBM. Mustafa et al. [170] showed that, when replacing 4%, 8% and 12% of SBM with AD-CM, the BW, ADG and FCR were better in the group fed 12% CM, compared with the SBM group, in all three growing stages. Similarly, up to 10% CM resulted in improvements in carcass quality without detrimental effects on broilers [171]. Replacing 15%, 30% or 45% of fish meal with AD-CM in Japanese quails’ diets increased BW regardless of the bird’s sex. However, the females recorded a better BW at a 30% replacement rate compared with the control, while the males had great BW in all three cases. The carcass yield and organ development were affected in all cases as reported by Kouatcho et al. [172]. However, regarding GB-CM use in poultry diets, there is no study reporting its effects as an alternative feed ingredient. This leaves a door open for further research areas. Nevertheless, discrepancies among studies may be attributed to differences in insect strains, processing methods, basal diet composition, poultry breeds or experimental conditions, which can affect overall performance and product quality outcomes. Taken together, the comparison across insect-derived ingredients reveals that, across broiler and laying hen studies, low to moderate inclusion levels (5% to 15%) maintained or improved growth performance and feed efficiency. A common pattern among all insect meals is the dose-dependent response, where higher inclusion levels are frequently associated with reduced protein utilization, altered carcass or product quality and changes in lipid profiles. These effects are mainly attributed to increased chitin content, high lipid fractions and amino acid imbalances at elevated substitution rates. In laying hens, a shared outcome across insect meals is the general preservation of egg production and shell quality at low inclusion levels, while lipid-derived modifications in yolk fatty acid composition represent the most pronounced ingredient-specific effect. These observations suggest that insect meals can be strategically used as partial substitutes for conventional protein sources, with formulation strategies tailored to the nutritional profile of each insect species and the targeted production outcome.

Table 7. Insect meal used as alternative feeding source in broiler chicken and laying hen diets.

Inclusion Level	Insect Meal	Poultry Type	Main Findings	Reference
20%	BSFL full-fat	Broiler	Improved FCR by 10% and better BWG when compared to control.	[14]
15%	BSFL defatted	Broiler	No effect on production performances and carcass quality versus a commercial diet.	[173]
20% of feed intake (as larvae)	Live BSFL	Laying hens	Maintained egg production and improved foraging behavior.	[156]
5% to 10%	BSFL defatted	Laying hens	Laying rate and egg weight were not influenced. The yolk FA profile was modified by increasing MCFA.	[157]
15% or 30%	BSFL live	Laying hens	Both diets increased the SFA and PUFA (C18:2n 6 and C18:3 n3), while decreasing the MUFA content in eggs.	[174]
29.65%	YMW	Broiler	The FCR was significantly improved, with no other effects on performances. Meat quality was not influences, but organ length was increased. Apparent ileal digestibility coefficients decreased significantly.	[163]
5% or 10%	YMW dried	Broiler	The 5% level lowered BWG and FI; decreased fat content in breast and thigh; SFA, MUFA and PUFA decreased in breast samples and PUFA-n3 in thigh samples. The breast n-3, thigh n-6, spleen weight and CP in breast and thigh significantly increased. The 10% level improved BWG, FI, PUFA content, color, carcass yield, while decreasing fat content in breast and thigh and n-3 and n-6 in breast muscle.	[164]
2%, 4% or 8%	YMW dried	Broiler	Improved BW and ADG in starter phase at 4%. No significant effects were noted for overall experimental period.	[175]
10.48%	YMW fresh	Broiler	Significantly reduced abdominal fat. No other significant effect for overall experimental period was noted.	[175]
2.5% or 5%	YMW dried	Broilers	Higher BW, but significant only in 2.5% group for starter phase, lower FCR, lower carcass yield in 2.5% group, no effect on organ development or blood parameters.	[166]
2% or 5%	YMW dried	Laying hens	The 2% and 5% levels did not affect the length of villi and microbiome of the cecum. The highest digesta viscosity from the ileum was found in the group with 5% mealworm.	[167]
1%, 2% or 4%	YMW dried	Laying hens	Laying performance remained unaffected, except FCR in the 4% group. No adverse effects were observed on egg weight, shell quality or nutritional composition	[176]
17%, 10% or 7.5%	AD-CM	Broilers	The 17% diet resulted in higher FI and BWG for the control group and lower FCR during starter phase. No effect on grower phase (10%). In finisher phase (7.5%), FI increased.	[177]

BSFL—black soldier fly larvae (Hermetia illucens larvae); YME—yellow mealworm (Tenebrio molitor); AD-CM—Ancheta domesticus—cricket meal; BW—body weight; BWG—body weight gain; FI—feed intake; FCR—feed conversion ratio; FA—fatty acid; MCFA—medium-chain fatty acid; SFA—saturated fatty acid; MUFA—monounsaturated fatty acid; PUFA—polyunsaturated fatty acid.

Table 7. Insect meal used as alternative feeding source in broiler chicken and laying hen diets.

Inclusion Level	Insect Meal	Poultry Type	Main Findings	Reference
20%	BSFL full-fat	Broiler	Improved FCR by 10% and better BWG when compared to control.	[14]
15%	BSFL defatted	Broiler	No effect on production performances and carcass quality versus a commercial diet.	[173]
20% of feed intake (as larvae)	Live BSFL	Laying hens	Maintained egg production and improved foraging behavior.	[156]
5% to 10%	BSFL defatted	Laying hens	Laying rate and egg weight were not influenced. The yolk FA profile was modified by increasing MCFA.	[157]
15% or 30%	BSFL live	Laying hens	Both diets increased the SFA and PUFA (C18:2n 6 and C18:3 n3), while decreasing the MUFA content in eggs.	[174]
29.65%	YMW	Broiler	The FCR was significantly improved, with no other effects on performances. Meat quality was not influences, but organ length was increased. Apparent ileal digestibility coefficients decreased significantly.	[163]
5% or 10%	YMW dried	Broiler	The 5% level lowered BWG and FI; decreased fat content in breast and thigh; SFA, MUFA and PUFA decreased in breast samples and PUFA-n3 in thigh samples. The breast n-3, thigh n-6, spleen weight and CP in breast and thigh significantly increased. The 10% level improved BWG, FI, PUFA content, color, carcass yield, while decreasing fat content in breast and thigh and n-3 and n-6 in breast muscle.	[164]
2%, 4% or 8%	YMW dried	Broiler	Improved BW and ADG in starter phase at 4%. No significant effects were noted for overall experimental period.	[175]
10.48%	YMW fresh	Broiler	Significantly reduced abdominal fat. No other significant effect for overall experimental period was noted.	[175]
2.5% or 5%	YMW dried	Broilers	Higher BW, but significant only in 2.5% group for starter phase, lower FCR, lower carcass yield in 2.5% group, no effect on organ development or blood parameters.	[166]
2% or 5%	YMW dried	Laying hens	The 2% and 5% levels did not affect the length of villi and microbiome of the cecum. The highest digesta viscosity from the ileum was found in the group with 5% mealworm.	[167]
1%, 2% or 4%	YMW dried	Laying hens	Laying performance remained unaffected, except FCR in the 4% group. No adverse effects were observed on egg weight, shell quality or nutritional composition	[176]
17%, 10% or 7.5%	AD-CM	Broilers	The 17% diet resulted in higher FI and BWG for the control group and lower FCR during starter phase. No effect on grower phase (10%). In finisher phase (7.5%), FI increased.	[177]

BSFL—black soldier fly larvae (Hermetia illucens larvae); YME—yellow mealworm (Tenebrio molitor); AD-CM—Ancheta domesticus—cricket meal; BW—body weight; BWG—body weight gain; FI—feed intake; FCR—feed conversion ratio; FA—fatty acid; MCFA—medium-chain fatty acid; SFA—saturated fatty acid; MUFA—monounsaturated fatty acid; PUFA—polyunsaturated fatty acid.

5.2. Macroalgae and Microalgae

The use of macroalgae and microalgae in the nutrition of broiler chickens and laying hens has been largely explored in recent years, providing concrete evidence that, when used correctly (species, processing, inclusion level), these alternative feed materials can positively influence productive performance, poultry product quality and health parameters [105]. Literature data showed that moderate Sargassum macroalgae inclusion levels (1%, 2% or 3%) in Leghorn laying hens increased egg mass and laying rate and stimulated an increase in IgM markers, without detrimental effects (Table 8). However, high inclusion (5%) reduced performances, concluding that 3% could be an optimal level in these conditions [128]. In terms of enrichment with n-3 fatty acids, the microalga Schizochytrium limacinum has been used in low to moderate inclusions (0.25%, 0.5%, 0.75% and 1%) in Hy-Line Brown laying hens [17]. The authors reported significantly increased DHA content, ranging from 0.73 to 4.18 g/100 g content in the yolk, without altering productive performances. The ALA content was slightly improved, while EPA was higher only in the 1% group.

Similarly, in broiler chickens, spirulina (Arthrospira/Limnospira) has shown favorable effects on BWG, FCR, BW and intestinal mucosal integrity when included in low–moderate amounts (1 g, 1.5 g and 2 g/kg of feed), as reported by Khan et al. [178]. Michalak et al. [179] concluded that microalgae usage in broiler chickens diets demonstrates two main beneficial effects. The most noted was the modification of lipid profile, by increasing the DHA proportion in muscle tissue, and the second one was improving oxidative stability through the presence of antioxidant compounds, which reduces the rate of lipid oxidation in meat and extends the shelf-life [16,106]. Similar practical and quantified effects have been reported in eggs, which include enhanced yolk color due to carotenoid presence, increased DHA content and, in some cases, decreased cholesterol content in yolks [17,180]. However, the results regarding the effect on cholesterol content are inconsistent and are dependent on algal species and feeding duration. In terms of health effects, studies reported improvements in immunological markers and antioxidant activity [105,179]. These beneficial effects are given by the fact that algae act as immunomodulators and prebiotics, reducing pathogen load and supporting intestinal integrity, leading to fewer digestive disorders and better FCR. Studies have shown that, in laying hens fed 1%, 2% and 3% Sargassum meal, laying rate and egg mass increased [181], while in broilers fed with Spirulina (0.5% to 4%), BW increased up to 8% and FCR increased [182]. The same authors showed that Spirulina positively influenced meat quality and antioxidant activity by increasing glutathione peroxidase and superoxide dismutase and serum biochemical parameters. Moreover, literature data showed that small doses of DHA-rich microalgae can transform eggs into important DHA sources, leading to designing products biofortified in essential nutrients for consumers [16,180]. For example, Kiran et al. [17] showed that 1% Schizochytrium can significantly increase ALA, EPA and DHA content per egg in the first 2 to 4 weeks of feeding, with a plateau after 30 days. Although beneficial effects are noted, in algae, rigid cell walls rich in non-starch polysaccharides and that are resistant biopolymers could limit nutrient release unless disrupted by mechanical, enzymatic or thermal processing. This indicates that careful planning of poultry diets allows planification of formulations to obtain functional eggs. However, limitations and risks exist. Although macroalgae are high in minerals, like iodine, selenium, iron and calcium, they also bioaccumulate heavy metals like arsenic, cadmium and lead if sourced from contaminated waters [105]. From an economic and operational perspective, high-quality microalgae (Schizochytrium, processed Spirulina) are more expensive than traditional sources, so the practical strategy is to use them in a targeted manner. Microalgae should be used as functional additives (0.5% to 3% in most cases) or as ingredients for premium products (n-3-enriched eggs and meat), not as a primary source of energy or protein [183].

Table 8. Effect of algae on poultry performances and product quality.

Inclusion Level	Algae	Poultry Type	Main Findings	Reference
1% or 2%	Chlorella	Broilers	Increased protein content in thigh samples. The proportions of DPA and DHA acid increased in thigh meat samples as well as vitamin E content. TBARS levels were significantly reduced.	[106]
1% or 2% with 250 ppm vitamin E	Chlorella	Broilers	Higher pH values and protein content and increased lightness in thigh meat samples than control. The concentrations of DPA and DHA acid increased in thigh meat samples by 2.01-fold and by 1.60-fold in the 2% C. vulgaris + vitamin E group. Significantly higher vitamin E content in meat samples. TBARS levels were significantly reduced.	[106]
0.5% or 1.0%	Schizochytrium	Laying hens	Both levels had beneficial effects on egg production, egg weight, yolk color and blood lipid profiles of layers, in comparison with the control group. Enhanced lipid profile of the eggs by increasing the concentrations of DHA and EPA acids in egg and decreased n-6 fatty acid or n-6/n-3 fatty acid ratio.	[111]
0.25%, 0.50%, 0.75% or 1.0%	Schizochytrium	Laying hens	No significant differences were observed in production performances or external egg quality parameters. Significantly increased ALA, EPA and DHA levels while decreasing the n-6/-3 ratio in a dose-dependent manner. It can serve as a substitute for DHA in enhancing eggs with n-3 fatty acids.	[17]
15% with or without enzymes	Spirulina	Broilers	Improved BW and FCR compared to control, increased length of duodenum plus jejunum. Breast and thigh meats from chickens fed with Spirulina, with or without the addition of exogenous enzymes, had higher values of yellowness, total carotenoids and SFA content, while n-3 PUFA and α-tocopherol decreased. All diets except control had increased digesta viscosity, which reduced performances.	[184]
0.5%, 1%, 2% or 3%	Spirulina platensis	Broilers	The 3% level improved BW, FCR, BWG; decreased kidney markers and oxidative stress by lowering MDA levels. Health status improved by increasing lactic acid bacteria from cecal microbiota; immunity. In meat, protein and moisture content increased while decreasing juiciness and tenderness.	[185]
0.1%, 0.3% or 0.5%	Spirulina platensis	Broilers	The 0.5% level improved BWG and FCR; increased bursa weight among the immunological organs; catalase activity, blood total antioxidant capacity, SOD GPX activity were significantly improved.	[186]
0.5%	Spirulina platensis	Broilers	BW, BWG and FCR improved while FI decreased. Lowered aspartate aminotransferase concentration, MDA levels and triglyceride level, with no effect on HDL and LDL levels.	[187]
0.25%, 0.50% or 1%	Spirulina platensis	Laying hens	Improved egg weight and mass, but darker yolk color and better-quality albumen, liver function and protein level in blood serum. Improved shelf-life in terms of quality stability during storage for 21 days. The 1% level improved pro-inflammatory and anti-inflammatory cytokines, chemokines and growth factors and beneficially modulated intestinal microflora.	[188,189]
0.1%, 0.2%, 0.3%, or 0.5%	Spirulina platensis	Laying hens	The 0.3% level increased BWG, FI, egg number per hen, egg weight, egg production, egg mass and FCR. The shell thickness was higher while serum cholesterol, ALT and AST concentration and egg yolk cholesterol concentration decreased.	[190]
0.25%, 0.50%, 0.75%, 1.0%, 1.25%, 1.50%, 1.75% or 2.0%	Chlorella vulgaris	Broilers	No effect on performance parameters or intestinal histomorphology, but improved antioxidant capacity of blood plasma and feed viscosity. There was a positive linear association with ileal viscosity. In terms of meat quality, the redness and yellowness in breast meat were altered.	[191,192]
0.25%, 0.50% or 1%	Chlorella vulgaris	Broilers	Higher FCR and productivity index, by producing higher eviscerated carcass percentage and dressed carcass percentage. No effect on serum antibodies, but improved Bifidobacterium spp. A significant impact on microbial diversity and richness, MDA, SOD and CAD levels.	[193]
0.25%, 0.50% or 1%	Chlorella vulgaris	Laying hens	No effect on production performances. The 0.20–0.50% level increased egg weight, while 1% decreased egg weight. No effects on Haugh units, yolk index, albumen index, egg surface area, specific density and translucency. The shell index and shell thickness decreased, while yolk color significantly increased.	[194]
0.25%, 0.50% or 0.75%	Chlorella by-product	Laying hens	Increased hen-day egg production and FI, egg yolk color and Haugh unit. No noted effects on eggshell strength and eggshell thickness, total cholesterol, triglyceride, AST and ALT levels, but plasma IgG and IgM concentrations increased.	[195]
0.2%	Chlorella vulgaris	Laying hens	Increased egg weight while hen-day egg production decreased. Improved SOD, CAT and GSH serum concentration. Enriched fatty acid content, β-carotene concentration, antioxidant capacity. Yolk color intensity, yolk redness a* color parameter increased with no effect on cholesterol concentration.	[196]
0.2%	Schizochytrium spp.	Broilers	Higher WG, with no effect on FI and FCR. Significant increase in DHA content in thigh meat.	[197]

DHA—docosahexaenoic acid; EPA—eicosapentaenoic acid; ALA—α-linolenic acid; C. vulgaris—Chlorella vulgaris; BW—body weight; BWG—body weight gain; WG—weight gain; FI—feed intake; FCR—feed conversion ratio; SFA—saturated fatty acid; PUFA—polyunsaturated fatty acid; TBARS—thiobarbituric acid reactive substance; MDA—malondialdehyde; SOD—superoxide dismutase; GSH—reduced glutathione; ALT—alanine aminotransferase; AST—aspartate aminotransferase; CAD—catalase activity.

Table 8. Effect of algae on poultry performances and product quality.

Inclusion Level	Algae	Poultry Type	Main Findings	Reference
1% or 2%	Chlorella	Broilers	Increased protein content in thigh samples. The proportions of DPA and DHA acid increased in thigh meat samples as well as vitamin E content. TBARS levels were significantly reduced.	[106]
1% or 2% with 250 ppm vitamin E	Chlorella	Broilers	Higher pH values and protein content and increased lightness in thigh meat samples than control. The concentrations of DPA and DHA acid increased in thigh meat samples by 2.01-fold and by 1.60-fold in the 2% C. vulgaris + vitamin E group. Significantly higher vitamin E content in meat samples. TBARS levels were significantly reduced.	[106]
0.5% or 1.0%	Schizochytrium	Laying hens	Both levels had beneficial effects on egg production, egg weight, yolk color and blood lipid profiles of layers, in comparison with the control group. Enhanced lipid profile of the eggs by increasing the concentrations of DHA and EPA acids in egg and decreased n-6 fatty acid or n-6/n-3 fatty acid ratio.	[111]
0.25%, 0.50%, 0.75% or 1.0%	Schizochytrium	Laying hens	No significant differences were observed in production performances or external egg quality parameters. Significantly increased ALA, EPA and DHA levels while decreasing the n-6/-3 ratio in a dose-dependent manner. It can serve as a substitute for DHA in enhancing eggs with n-3 fatty acids.	[17]
15% with or without enzymes	Spirulina	Broilers	Improved BW and FCR compared to control, increased length of duodenum plus jejunum. Breast and thigh meats from chickens fed with Spirulina, with or without the addition of exogenous enzymes, had higher values of yellowness, total carotenoids and SFA content, while n-3 PUFA and α-tocopherol decreased. All diets except control had increased digesta viscosity, which reduced performances.	[184]
0.5%, 1%, 2% or 3%	Spirulina platensis	Broilers	The 3% level improved BW, FCR, BWG; decreased kidney markers and oxidative stress by lowering MDA levels. Health status improved by increasing lactic acid bacteria from cecal microbiota; immunity. In meat, protein and moisture content increased while decreasing juiciness and tenderness.	[185]
0.1%, 0.3% or 0.5%	Spirulina platensis	Broilers	The 0.5% level improved BWG and FCR; increased bursa weight among the immunological organs; catalase activity, blood total antioxidant capacity, SOD GPX activity were significantly improved.	[186]
0.5%	Spirulina platensis	Broilers	BW, BWG and FCR improved while FI decreased. Lowered aspartate aminotransferase concentration, MDA levels and triglyceride level, with no effect on HDL and LDL levels.	[187]
0.25%, 0.50% or 1%	Spirulina platensis	Laying hens	Improved egg weight and mass, but darker yolk color and better-quality albumen, liver function and protein level in blood serum. Improved shelf-life in terms of quality stability during storage for 21 days. The 1% level improved pro-inflammatory and anti-inflammatory cytokines, chemokines and growth factors and beneficially modulated intestinal microflora.	[188,189]
0.1%, 0.2%, 0.3%, or 0.5%	Spirulina platensis	Laying hens	The 0.3% level increased BWG, FI, egg number per hen, egg weight, egg production, egg mass and FCR. The shell thickness was higher while serum cholesterol, ALT and AST concentration and egg yolk cholesterol concentration decreased.	[190]
0.25%, 0.50%, 0.75%, 1.0%, 1.25%, 1.50%, 1.75% or 2.0%	Chlorella vulgaris	Broilers	No effect on performance parameters or intestinal histomorphology, but improved antioxidant capacity of blood plasma and feed viscosity. There was a positive linear association with ileal viscosity. In terms of meat quality, the redness and yellowness in breast meat were altered.	[191,192]
0.25%, 0.50% or 1%	Chlorella vulgaris	Broilers	Higher FCR and productivity index, by producing higher eviscerated carcass percentage and dressed carcass percentage. No effect on serum antibodies, but improved Bifidobacterium spp. A significant impact on microbial diversity and richness, MDA, SOD and CAD levels.	[193]
0.25%, 0.50% or 1%	Chlorella vulgaris	Laying hens	No effect on production performances. The 0.20–0.50% level increased egg weight, while 1% decreased egg weight. No effects on Haugh units, yolk index, albumen index, egg surface area, specific density and translucency. The shell index and shell thickness decreased, while yolk color significantly increased.	[194]
0.25%, 0.50% or 0.75%	Chlorella by-product	Laying hens	Increased hen-day egg production and FI, egg yolk color and Haugh unit. No noted effects on eggshell strength and eggshell thickness, total cholesterol, triglyceride, AST and ALT levels, but plasma IgG and IgM concentrations increased.	[195]
0.2%	Chlorella vulgaris	Laying hens	Increased egg weight while hen-day egg production decreased. Improved SOD, CAT and GSH serum concentration. Enriched fatty acid content, β-carotene concentration, antioxidant capacity. Yolk color intensity, yolk redness a* color parameter increased with no effect on cholesterol concentration.	[196]
0.2%	Schizochytrium spp.	Broilers	Higher WG, with no effect on FI and FCR. Significant increase in DHA content in thigh meat.	[197]

DHA—docosahexaenoic acid; EPA—eicosapentaenoic acid; ALA—α-linolenic acid; C. vulgaris—Chlorella vulgaris; BW—body weight; BWG—body weight gain; WG—weight gain; FI—feed intake; FCR—feed conversion ratio; SFA—saturated fatty acid; PUFA—polyunsaturated fatty acid; TBARS—thiobarbituric acid reactive substance; MDA—malondialdehyde; SOD—superoxide dismutase; GSH—reduced glutathione; ALT—alanine aminotransferase; AST—aspartate aminotransferase; CAD—catalase activity.

Comparing the effects of macroalgae and microalgae used across poultry studies, it was noted that, in both broilers and laying hens, low to moderate inclusion levels are associated with stable or improved productive performance, whereas higher inclusion levels frequently result in reduced performance or metabolic imbalances. This dose-dependent response represents a common pattern, emphasizing that algae function primarily as functional feed ingredients rather than nutrient sources. A key difference between macroalgae and microalgae is their nutritional role. While microalgae are mainly used for targeted enrichment of poultry products with n-3 fatty acids, pigmentation and improve oxidative stability, macroalgae contribute more to mineral supply and immunomodulation. These differences explain the narrower optimal inclusion range for macroalgae compared with microalgae. Moreover, the high cost, potential heavy metal accumulation and regulatory constraints indicate that algae should be strategically included in poultry diets to exploit their functional properties, rather than as primary protein sources.

5.3. Effects of Agro-Industrial Fermented Oilseed By-Products, Distiller’s Dried Grains with Solubles (DDGS), Brewery Spent Grain (BSG) as Alternative Feed Sources

The use of fermentation treatments for by-products and feeds in poultry nutrition is one of the modern directions in livestock research, due to the ability of fermentation to increase nutrient digestibility, reduce antinutritional factors and beneficially modulate the gut microbiota. These can lead to improved production performances and quality of poultry products. In a recent meta-analysis, which included 16 studies, it was shown that replacing SBM with fermented SBM significantly increased BW, however, the effects on ADG, FCR and FI in broilers are not consistent [18]. These differential effects are more dependent on the type of microorganism used in the fermentation process than on the level of inclusion. Obeidat et al. [198] observed that fermented SBM significantly improved intestinal morphology in broilers but without a major effect on BW or carcass characteristics during the starter phase, suggesting that the integration of fermented ingredients may act primarily on digestion and intestinal health rather than exclusively on productive performance. Supplementation of broiler diets with fermented SBM alone or combined with mannan-oligosaccharides improved digestive enzyme activity, intestinal morphology and hepatic expression and overall production performances [199]. This is similar to the results of Li et al. [127], which reported that replacing 25% to 50% of SBM with fermented SBM leads to improved ADG and FCR and a favorable increase in immunoglobulins (IgA, IgG, IgM), concomitant with a positive remodeling of the cecal microbiota, by an increase in lactobacilli and a decrease in Escherichia/Shigella and Clostridiales populations. Moreover, cotton seed meal (CSM) after gossypol elimination (toxic compound) improved growth rate, carcass quality, CP digestibility and blood lipid parameters [200]. Similarly, fermented rapeseed meal can partly (33.30% and 66.60%) or completely (100%) replace SBM, while improving digestibility, intestinal integrity, volatile fatty acid production and microbiota composition, although growth performance is not always superior to SBM-based diets, as recently reported by Dastar et al. [201]. Regarding the use of fermented alternatives in laying hens, literature data is scarce, which leaves a door open for further research areas. However, a study showed that 10% fermented wheat bran significantly increased the levels of serum biochemical parameters, reproductive hormones, immunoglobulins and anti-inflammatory factors [202]. Although data on direct effects on egg quality are limited, the improvement in intestinal and immunological status suggests the possibility of indirect beneficial effects.

Regarding the use of DDGS as an alternative ingredient, it was reported that 5%, 10% or 15% in the diet of broilers (from 12 days to 35 days) showed no significant effects on FI, BW or CP digestibility, although FCR tended to be affected at 15% [203]. Evaluating seven increasing levels of DDGS (0, 10, 40, 70, 100, 130 and 160 g/kg) in broiler diets, from 22 to 42 days of age, Damasceno et al. [204] reported no effects on BW, FI, slaughter yield or meat quality and potential for significant health benefits. It was suggested that it can be used as an alternative source of protein or energy in the late growth phase (finisher), if the diet is formulated correctly. Dal Pont et al. [205] also suggested that 7% or 14% DDGS in the diet of young broilers (0–28 days), with and without exogenous enzyme supplementation (multicarbohydrase complex + phytase), has no negative effects on performances, while the group with enzymes showed improved intestinal health. Although different levels (5%, 10%, 15%, 20% or 25%) of DDGS can be used, higher replacement without proper processing might compromise FCR or carcass yield. However, a major risk of DDGS is contamination with mycotoxins (e.g., deoxynivalenol), especially if the raw material has been contaminated, as reported more recently [206].

Regarding BSG usage, although there is a theoretical interest in using malt residues in poultry nutrition, published data is scarce. This might be explained by the fact that BSG has a very high fiber content, which is difficult for birds to digest, and high variability in composition depending on the grain and the malting process. However, small farm practitioners often use BSG on a local scale as an occasional supplement (either dried or recycled), but without standardization, which could lead to variable and often uncontrolled results. In terms of oilseed by-products, a lot of research has been conducted in the last two decades, as these alternatives have multiple benefits. They can provide substantial nutrients (n-3 fatty acids, antioxidants, vitamins, minerals) to enrich diets and transfer them to the final products (egg and meat). For example, moderate inclusion (3% to 9%) of flaxseed, rapeseed, hempseed and camelina meals improves eggs and meat with essential fatty acids [207,208,209], while supplementing with antioxidant-rich wastes (grapeseed, rosehip, sea buckthorn meal, carotenoids) improves the antioxidant compounds and shelf-life of products [209,210,211,212], as reported by numerous research and review papers.

Across these alternatives, DDGS displays greater variability, with performance outcomes largely dependent on inclusion level, growth phase and diet formulation. A higher inclusion level is more likely to impair FCR or carcass traits if fiber content and mycotoxin risk are not managed carefully. BSG differs significantly from DDGS as its utilization is primarily constrained by high fiber content and compositional variability, limiting its application to lower levels. However, these findings support the application strategy of fermented oilseed meals as partial SBM substitutes, DDGS as energy and protein sources mainly in finisher growing phases of broilers and BSG as fiber-rich residues, as locally adapted feed components (

Table 9. DDGS and BSG used as alternative feeding source in broiler chicken and laying hen diets.

Inclusion Level	By-Product	Poultry Type	Main Findings	Reference
5%, 10%, 20% or 30%	BSG fermented	Broilers	During the entire experimental period, the ADG, FCR and FI in the 30% BSG group was significantly lower than that in the 0%, 5%, 10% and 20% WFBG groups (p < 0.05). Feeding 20% BSG increased duodenal development, while 30% induced intestinal injuries. The optimal dosage of BSG should not exceed 20%.	[213]
20%	BSG fermented	Broilers	Using 20% significantly improved duodenal development (the duodenal villus height increased by 10.2% and the villus height-to-crypt-depth ratio increased by 27.2%).	[214]
5%, 10%, 15% or 20%	DDGS	Laying hens	Significant differences in digestibility coefficient values of CP, EE, CF. The 5% level significantly increased egg production %, egg number and egg mass. The 15% and 20% levels significantly increased yolk color and shell thickness and significantly decreased egg production, egg number, egg weight and egg mass and affected FCR. No significant effect on semen quality, fertility, hatchability and body weight of chicks when hatching.	[215]
20%	DDGS	Laying hens	Increasing feeding duration has no detrimental effects on productive performance of laying hens. Egg yolk color increases linearly, as well as lutein and zeaxanthin concentrations of egg yolks.	[216]
5%, 10% or 15%	DDGS with Bacillus subtilis probiotic	Laying hens	Improved FI, egg shape index and yolk color. The 15% level significantly decreased egg mass while significantly increasing Haugh units. Bacillus supplementation improved FCR, egg weight and egg mass.	[217]
7% or 14%	DDGS with or without multicarbohydrase complex + phytase	Broilers	Both 7% and 14% levels did not impair broiler performance up to 28 days of age. Enzyme supplementation had better effects on production performances. Improves intestinal histomorphology, especially in the enzyme-supplemented groups. The inclusion of DDGS showed positive effects on microbiota composition due to a reduction in Proteobacteria phylum in the ileum at 28 days and a reduction in the presence of Enterococcaceae family in the ileum at 14 and 28 days.	[205]
7.5%, 8%, 15%, 22.5% or 30%	DDGS	Broilers	No effect on production performances in starter phase (8%). Increasing DDGS in grower phase resulted in a linear decrease in BW gain and liver relative weight. No detrimental effects on production performances or intestinal health.	[218]
5%, 10%, 15%, 20% or 25%	BSG	Broilers	FI was greater in the 5% group. BWG and feed efficiency declined in the 25% group. The ileal digestibility values of CP were significantly increased by BSG inclusion. The 20% level supported overall production performances.	[219]
10%, 20%, 30% or 40%	BSG	Laying hens	No detrimental effect on FCR, egg production or egg quality parameters (albumen, yolk, shell). Feed cost was improved by increasing the level of BSG in the diet. It was concluded that a 40% inclusion of BSG in the diet of layers might be economically profitable.	[220]
15%, 20% or 25%	BSG	Broilers	The growth performances, carcass, breast muscle, thigh, wing, shank, liver and spleen were not significantly different. The 25% level increased the serum biochemical parameters significantly compared to other groups. The 25% and 20% levels were concluded to be safe for use.	[221]
6.5%, 13%, 19.5% or 26%	BSG	Laying hens	Similar egg weight but decreased egg production with increasing BSG level. The 6.5% level increased egg mass, while the others decreased it. FI decreased with increasing levels of BSG, but FCR was greater at 19.5% and 26%. Similar internal egg quality parameters but yolk color score decreased with increasing levels of BSG. Although 26% was economically profitable, it decreased egg production.	[222]

ADG—average daily weight gain; FCR—feed conversion ratio; FI—feed intake; BWG—body weight gain; BSG—brewery spent grains; CP—crude protein; EE—crude fat; CF—crude fiber; DDGS—distillers dried grains with solubles.

).

When evaluated from an integrative perspective, the reviewed alternatives differ substantially in their balance between nutritional value, environmental benefits, scalability and regulatory readiness. Insect meals provide high-quality protein and functional lipids with strong sustainability potential due to efficient biomass conversion and waste valorization. However, their large-scale application is currently constrained by production costs, regulatory heterogeneity, and digestibility limitations related to chitin. Micro- and macroalgae offer unique advantages as sources of long-chain n-3 fatty acids, pigments and bioactive compounds, making them particularly suitable for functional and premium poultry products, yet high cultivation costs, cell wall recalcitrance and mineral accumulation limit their use mainly to low inclusion levels. Fermented feeds and agro-industrial by-products, in contrast, present the highest immediate scalability and regulatory acceptance, as they rely on existing feed resources and processing infrastructure. Their nutritional enhancement depends strongly on fermentation technology and microbial strains, resulting in more variable but economically accessible outcomes. DDGS and oilseed by-products occupy an intermediate position, combining availability and cost-effectiveness with moderate nutritional improvements, while requiring careful quality control to manage fiber content and contamination risks. Overall, no single alternative feed represents a universal solution. Their practical adoption should be context-specific, integrating nutritional objectives, production scale, economic constraints and regulatory frameworks.

6. Conclusions, Future Directions and Limitations

Although the primary focus of this review is on nutritional performance, health outcomes and sustainability, sensory quality and consumer acceptance represent critical factors for the commercial adoption of alternative feeds. Available evidence suggests that, at low to moderate inclusion levels, most alternative ingredients do not adversely affect meat or egg flavor; however, data remain limited and inconsistent across feed categories. Comprehensive sensory evaluations and consumer perception studies fall outside the scope of this review but represent an important complementary research area.

The use of these alternative ingredients in poultry nutrition is part of a global trend to diversify nutritional resources, oriented towards sustainability, circular economy and reducing reliance on conventional sources. However, future research must move beyond proof-of-concept studies and focus on defining standardized inclusion strategies, processing requirements and performance benchmarks under commercial production conditions.

Insects are emerging as a protein source of great interest, especially due to the high biomass conversion, the possibility of growing them on agro-industrial waste streams and the nutritional composition. The optimization of the dechitinization process, standardization of amino acids and potential implementation of standard processing technologies could allow using insect meals in high proportions in poultry nutrition. Future studies should prioritize dose–response trials under commercial conditions, long-term feeding experiments assessing cumulative effects on gut morphology and product quality and comparative evaluations of the effect of different insect species and processing methods on amino acid digestibility and nitrogen utilization. In addition, harmonized protocols for assessing sensory quality and shelf-life of products derived from insect-fed poultry are required to address remaining gaps related to consumer acceptance.

Both types of algae (micro and macro) have remarkable nutraceutical potential, being natural sources of carotenoids, n-3 fatty acids and good-quality proteins. Future development of energy-efficient bioreactors, selection of strains with high protein yield and reduction in cultivation costs will make possible the introduction of algae in large-scale poultry diets. Macroalgae, especially red and brown, can contribute to enriching poultry products with iodine, phenolic compounds and soluble fiber. On the other hand, microalgae can become an alternative source to fish meal for designing eggs and meat products enriched in EPA and DHA fatty acids, without impacting on smell or taste, a particular advantage over fish oils. Future research should focus on defining species-specific inclusion limits (e.g., 0.5–3% for functional use), improving cell wall disruption technologies to enhance protein digestibility and establishing monitoring frameworks for iodine and heavy metal transfer into animal-origin foods. Comparative studies between algae and conventional lipid sources are also needed to quantify cost–benefit trade-offs under realistic market conditions.

Fermented products, subjected to solid or liquid fermentation, will continue to play an important role, due to this process’s ability to reduce antinutritional factors, increase amino-acids digestibility and modulate gut microbiota. Future directions should include the use of customized microbial consortia and integrations of probiotics and postbiotics directly into the fermentation process, generating bioactive ingredients with synergistic effects on poultry performance and health. Importantly, future work should systematically compare solid-state versus submerged fermentation systems using identical substrates, while linking fermentation-induced biochemical changes (e.g., peptide release, enzyme activity, reduction of non-fermentable carbohydrates) with animal performance and gut health outcomes.

Agro-industrial by-products are the heart of the circular economy concept and future research should aim to optimize processing technologies of these secondary streams to increase their nutritional value. For DDGS, possibilities relate to standardization of product quality and optimization of processing lines to reduce variability and mycotoxin exposure. For BSG, the obvious direction is bioconversion through solid fermentation, fungal mycelium or insect larvae, which can convert the fiber-rich and difficult-to-digest material into a stable protein component usable in poultry. Oilseed by-products have the potential to become functional sources to enrich eggs and meat with unsaturated fatty acids, tocopherols and phytosterols, to the extent that technological processing would manage to reduce antinutritional limitations and increase energy digestibility.

Despite considerable potential, the use of insects, algae, fermented products and agro-industrial by-products in poultry nutrition is marked by a number of scientific, technological, economic and legislative limitations. In the case of insects, major challenges include high variability of nutritional composition depending on the growth substrate, high chitin content that can limit the digestibility of proteins, still high production costs compared to conventional protein sources and the lack of uniform international standards on hygiene, microbiological safety and fat stabilization. There are still uncertainties regarding consumer acceptability of poultry products derived from insect diets, which may limit their application on an industrial scale.

In the case of algae, the main limitation is the very high production costs for both microalgae cultures in bioreactors and sustainable harvesting of macroalgae. Digestibility is another issue, as the cell walls of many species are difficult to be broken down by the birds’ digestive enzymes, which reduces protein efficiency and may limit inclusion levels to 1–3% for most commercial diets. Other limitations include the risk of excessive iodine deposition in eggs (especially brown macroalgae), heavy metal loading in coastal areas and variability in pigmentation depending on species and season.

Fermented products, while promising, have limitations related to batch-to-batch variability, the need for controlled fermentation conditions, the possibility of microbial contamination in the absence of rigorous hygiene and large differences in the microorganisms used, making it difficult to generalize results. In addition, not all raw materials react to fermentation in the same way, and some are even positive.