The Use of Food Industry By-Products in Pig Diets as a Challenge Option to Reduce the Environmental Footprint

Abstract

The swine industry represents a significant contributor to the global meat supply but also exerts considerable pressure on natural resources through feed production, greenhouse gas (GHG) emissions, and nutrient losses. The integration of food industry by-products into pig diets offers a promising pathway to mitigate these environmental impacts while maintaining productivity and animal welfare. Such by-products can serve as nutritionally valuable feed ingredients, reducing waste streams and supporting the principles of a circular economy. This review synthesizes current knowledge on the nutritional properties, environmental implications, and economic advantages of incorporating food industry by-products into pig feeding systems. It further outlines the challenges related to feed safety, variability in composition, and regulatory aspects. Overall, the sustainable valorization of food processing residues as animal feed represents a challenge option to reduce the environmental footprint of pig production without compromising growth performance or health outcomes.

1. Introduction

The growing global demand for food—driven by population growth, longer life expectancy, and rising living standards—poses major challenges for sustainable agriculture. By 2050, food demand is projected to increase by up to 50%, intensifying pressure on agricultural systems to boost productivity while minimizing environmental impacts [1,2,3]. This pressure is further exacerbated by climate change, urbanization, and land-use transformation, which collectively constrain the availability of natural resources [4,5]. Both crop and livestock production are under growing scrutiny due to their competing demands for land, water, and energy, as well as their contributions to air, water, and soil pollution [6,7]. Global pig production has increased by 140% since the 1960s. The rising world population and the improvement in socioeconomic conditions in many countries have led to higher meat consumption, including pork [8]. Global pork production benefits from cheaper feed and higher productivity, but it faces challenges from geopolitics and ongoing disease risks. Swine production’s role is essential for the global economy and food security; however, its significant impacts—including high use of water, increased feed consumption, land use, greenhouse gas (GHG) emissions, and food loss—are pushing the environment to its limits [9,10]. Modern pig production faces growing economic, environmental, and health-related challenges, including rising feed and energy costs, limited resources, and increased emissions [11].

Feed production represents the largest single cost and environmental input in intensive livestock systems, accounting for 65–85% of total farm-gate value for poultry and pigs [12]. Feed represents the largest share of both economic and environmental costs in pig production, making ingredient selection and formulation critical considerations [13,14,15]. Beyond its effects on animal health, welfare, and productivity, feed composition also influences farm profitability and overall environmental performance, particularly in relation to GHG emissions, land use, fossil energy consumption, and water quality [15,16]. The competition between feed, food, fuel, and fiber production—the so-called 4F challenge—intensifies the need for sustainable alternatives that minimize resource conflicts. Rising demands for land and water, coupled with the expansion of energy crop cultivation, highlight the necessity of more efficient and environmentally sound feed solutions [17,18]. Sustainable practices, such as waste reduction, diet reformulation, and water-efficient cropping, have already shown potential to reduce environmental burdens [19].

Within this framework, reducing the environmental footprint per unit of animal product is essential [17,18,20]. Incorporating alternative feed ingredients with lower ecological costs, higher nutrient availability, and better cost-efficiency has become central to modern pork production systems [21]. While pigs typically have a smaller carbon footprint per kilogram of meat compared to ruminants, due to their better feed conversion efficiency, their total environmental impact remains high because of the extensive scale of global pig production [22]. Pig production systems differ in their capacity to achieve key sustainability objectives, including reduced environmental and climate impacts, minimized land use, economic viability, and enhanced animal welfare [23]. However, the broader understanding of sustainability can give rise to dilemmas that require trade-offs [24]. This complexity is also evident in the Sustainable Development Goals (SDGs) [25], where progress toward one goal may inadvertently have negative effects on others.

Strategies like improving feed efficiency, using precision farming techniques, and incorporating agricultural by-products into pig feeds are becoming more popular [26,27]. Adopting a circular economy approach in livestock production, which utilizes agricultural by-products and co-products, can enable the reduction, reuse, and redistribution of resources [27].

Potential by-products that may be considered for swine diets may be classified from their primary product origin as follows: (a) grain: distilling by-products/co-products, brewing by-products, milling by-products, and baking by-products, (b) animal: milk by-products, meat by-products, and egg by-products, (c) vegetable: potato by-products, cull beans, and field peas, and (d) sugar and starch production: cane, beet and corn molasses, and salvage candy. Former Food Products (FFPs), also known as ex-foods, may offer a valuable opportunity to enhance resilience in animal feeding systems. Given their strong nutritional properties, FFPs show considerable promise for consistent inclusion in animal diets [8,9]. However, their use and potential benefits in animal nutrition are not yet fully explored. Using food industry by-products in pig diets helps address these problems. It diverts waste, reduces environmental stress, improves feed efficiency, and supports a circular economy that keeps resources in use rather than wasting them [28]. Additionally, this practice supports global agricultural and livestock sustainability policies and helps ease pressure on natural ecosystems.

The objective of this review is to comprehensively examine the nutritional values, environmental benefits, and economic challenges of incorporating food industry by-products into pig diets while investigating the future perspectives as a challenge option to reduce the environmental footprint.

Methodology of This Review

The present review was conducted through a structured search of the Scopus, PubMed, and Web of Science databases for peer-reviewed papers published between 2010 and 2025. The search terms combined “pig OR swine”, “by-products OR co-products OR food waste”, and “circular economy OR sustainability OR environmental impact”. Articles were selected based on relevance to nutritional, environmental, and economic aspects of by-product use in pig feeding. Information was synthesized thematically, emphasizing both experimental and modeling studies.

2. Nutritional Value of Food Industry By-Products

Food by-products encompass a wide variety of materials with diverse nutritional properties (

Table 1. Beneficial effects of some by-products used in pig diets.

By-Product	Major Nutrients	Potential Inclusion Rate	Beneficial Effects	Challenges	References
Whey	Lactose. Protein. Minerals.	10–15% of diet	Enhances palatability and digestibility. Rich in essential amino acids.	Life Cycle Assessment (LCA) evaluation of the environmental sustainability of milk production.	[29]
Brewers’ spent grains	Protein. Fiber. Vitamins.	15–25%	Contains beta-glucans. Improves gut microbiota and immunity.	Efficiency of converting by-products into milk, meat, and eggs production.	[30]
Bakery waste	Carbohydrates. Energy.	10–20%	High energy value. Higher ADG/lower FCR. No significant effect on performance, quality behavior characteristics, and welfare.	Identification/quantification of the environmental impacts of conventional pig production. Effects on growth performance, welfare, and behavior indicators in post-weaning piglets.	[10,31]
Fruit/vegetable pulp	Fiber. Antioxidants.	5–15%	Bioactive compounds support antioxidant status.	Environmental and health impacts of converting municipal food wastes into pig feed.	[9]
Oilseed meals	Protein. Fatty acids.	5–20%	Partial replacement of soybean meal. Contributes essential fatty acids.	Evaluation of feed demand landscape and impacts on food security and climate change.	[32]

). These include energy-rich residues, protein sources, and fibrous fractions. Understanding their nutrient composition is crucial for formulating balanced diets. This grouping follows the classification proposed by previous studies, which categorize by-products according to their main processing origin (grain, animal, vegetable, or sugar/starch industry).

FFPs are generally classified into two main groups: those originating from the confectionery sector (e.g., chocolates, biscuits, and sweet snacks) and those from the wearer sector (e.g., bread, pasta, savory snacks, and potato chips), including other high-quality baked goods. These materials are typically high in carbohydrates (starch, simple sugars) and may also contain considerable concentrations of fat [12]. On a dry matter basis, FFPs usually contain around 50–60% starch [33] and about 10% protein, which limits their classification as a significant protein source for livestock [12]. Because FFPs represent a heterogeneous mix of various ex-food types and origins, it is difficult to establish a standardized nutritional composition. Nevertheless, they can serve as an effective source of simple sugars, processed starch, fat, and readily available energy in post-weaning piglet diets. However, their inclusion rates must be managed carefully. Regarding weaners or grower/finisher pigs, further research is required to determine whether higher inclusion levels of FFPs influence growth performance and meat quality [12,31,34,35,36].

Appropriate processing methods, such as drying, pelleting, and ensiling, are vital for ensuring feed safety, stabilizing nutrients, and preventing microbial spoilage [29]. The combination of multiple by-products in the same diet can also improve nutrient diversity and balance, compensating for deficiencies inherent in individual materials.

Studies investigating both productive and environmental outcomes of by-product inclusion employ several complementary methodologies (

Table 2. Approaches to evaluate environmental and productive impacts of by-products in pig diets.

Approach	Evaluated Impacts	References
Life Cycle Assessment (LCA)	Quantification of GHG emissions, energy demand, land and water use, and overall resource efficiency	[10]
Growth performance trials	Average daily gain (ADG), feed conversion ratio (FCR), and carcass characteristics	[30]
Nutritional analysis	Determination of crude protein, amino acid profile, fiber content, and energy digestibility	[29]
Waste diversion metrics	Measurement of the proportion of food residues redirected from landfills to feed use	[9]
Economic assessment	Evaluation of feed cost reduction and profitability associated with by-product utilization	[28]

). These analytical frameworks collectively allow for the evaluation of not only growth and efficiency metrics but also sustainability indicators and economic feasibility.

Several European countries with established swine industries have successfully integrated by-products into feed systems. In France and the Netherlands, for example, incorporating bakery waste led to approximately 15% lower feed-related GHG emissions and partially replaced imported cereals, thus lowering the carbon footprint of production [10]. In Germany, the inclusion of brewers’ spent grains improved fiber intake and gut health while substituting soybean meal with a sustainable, locally sourced alternative [30]. In Denmark, fruit and vegetable pulp replaced 20–25% of conventional feed ingredients, maintained growth performance, and reduced feed costs by roughly 8–10% while also diverting large volumes of organic waste from disposal sites [9].

3. Feedstuffs

The applied strategies that concern generated agro-industrial by-products serve as critical mechanisms in the mitigation of emerging environmental issues, such as global warming, air and water pollution, and GHG emissions. The generated agro-industrial by-products within the European Union (EU) are estimated at 16 million tons, with Germany (3 million tons), the United Kingdom (UK) (2.6 million tons), Italy (1.9 million tons), France (1.8 million tons), and Spain (1.6 million tons) forming the major producers. The livestock sector is responsible for a total emission of 10% of the global GHG [37]. Additionally, it must be noted that nowadays, the commercial prices of cereal grains and soybean products have revealed a remarkable increment, which has a direct impact on the livestock field, resulting in increased production costs [38]. Therefore, it is crucial to discover novel alternative sources capable of offering opportunities with lower operational costs. Agro-industrial by-products form an intriguing case, since they can be adopted by the livestock industry. Specifically, the latter can be characterized as raw materials instead of wastes due to their nutritional composition, since they consist of a plethora of valuable components. The already existing technologies provide us with the ability to recycle and reuse them in various applications, such as feed additives [39].

The Regulation (EC) No. 767/2009 of the European Parliament and of the Council states that “feed materials” can be characterized as the products that are derived from vegetables and animals and provide the ability to fulfill animal nutritional requirements [40]. Furthermore, the latter legislation also includes products that are generated by industrial processing procedures, organic or inorganic compounds with or without feed additives. These products are used (a) for instant oral animal feeding, (b) after processing, (c) for the development of a compound feed, and (d) as premixture agents [41]. It is remarkable that these alternative feeding strategies have gained interest since their final goal concerns the development of enhanced novel feedstuffs with improved quality by incorporating the aforementioned by-products, thus exerting a positive impact on pig health and welfare.

Table 3. Addition of agro-industrial by-products to feedstuffs.

By-Product	Age	Type	Concentration	Effects	Challenges	References
Apple	Finishing pigs	Fermented apple supplement	2% w/w	Improved growth performance and feed quality.	Improvement in growth performance and meat quality.	[42]
	Finishing pigs	Apple pomace	10–20% w/w	Promotion of beneficial bacteria. Reduced volatile fatty acid emissions. Increased FCR *.	Improvement in beneficial bacterial and decreased VFA ** emission in feces. Improvement in production parameters and meat quality.	[43,44]
	Piglets	Apple pomace	3.5% w/w	Beneficial effects on gut microbiota and blood parameters.	Improvement in gut health, blood parameters, and mRNA marker gene expression.	[45,46]
	Finishing pigs	Fermented apple pomace with Lactobacillus plantarum		Enhanced feed efficiency. Lowered ADFI *** without effect on animal’s final BW and back fat thickness.	Enhancement in nutritional composition and antioxidant activities.	[47]
Grape pomace	Finishing pigs	Fermented grape pomace with Lactobacillus plantarum		Improved beneficial bacteria and reduced VFA ** emissions in feces.	Improvement in beneficial bacterial and decreased VFA ** emission in feces.	[43]
Strawberry	Growing pigs	Fermented strawberry pomace with Lentinus edodes		Positive effect on lean tissues.	Effects on blood serum metabolic profiles and the cecal microbiota.	[48]
Mango	Growing pigs	Mango pulp	15% w/w	Improved efficiency of starch. Protein digestion, to a certain extent.	Effects on macronutrient digestion and small intestinal enzyme activity. Improvement in the quality of agro-industry by-products as unconventional feeds.	[49,50]
Tomato	Pigs	Tomato residues	3% or 5% w/w	Slight effect on pork meat attributes.	Improvement in meat quality.	[51]
	Finishing pigs	Tomato silage	30% w/w	Promotion of growth performance.	Improvement in growth performance and meat quality.	[52]
Carrot	Finishing pigs	Carrot wastes	20–25% w/w	Increased FCR *. Improved meat quality.	Improvement in production parameters and meat quality.	[44,53]
Olive	Growing–finishing pigs	Olive cake processing waste	5%, 10% w/w	Reduced backfat thickness and intramuscular fat. Changed their fatty acid content. Increased levels of MUFA and PUFA **** improved quality indices.	Improvement in meat quality traits.	[54]
	Piglets	Fermented mixture of olive mill stone waste and Lathyrus clymenum pericarp with Pleurotus ostreatus	5% w/w	Improved antioxidant blood parameters.	Improvement in antioxidant blood parameters.	[55]
Cordyceps. militaris  spent mushroom substrate	Growing pigs	Cordyceps. militaris spent mushroom substrate	2 g/kg	Improved growth performance and immunoglobulin secretion. Enhanced antioxidant activity. Reduced cholesterol and MDA ***** levels.	Improvement in growth performance, immunity, metabolic profiles, and antioxidant capacity.	[56]
Mulberry	Finishing pigs	Mulberry leaves	3%, 6%, 9%, 12% w/w	Higher loin eye area and increased crude protein levels. Enhanced inosine monophosphate content and amino acids in muscle tissues.	Improvement in growth performance, carcass traits, and meat quality.	[57]
Strawberry	Finishing pigs	Strawberry pomace	10% w/w	Improved fatty acid composition in pork meat.	Improvement in production parameters and meat quality.	[44]

* feed conversion ratio (FCR), ** volatile fatty acids (VFAs), *** average daily feed intake (ADFI), **** monounsaturated fatty acids (MUFAs)-polyunsaturated fatty acids (PUFAs), ***** malondialdehyde (MDA).

describes the incorporation of agro-industrial by-products into feedstuffs.

According to Lee et al. [42], the addition of fermented apple in the finishing pig diet at 2% w/w revealed beneficial effects in some crucial parameters, namely, growth performance, daily feed intake, and feed efficiency, as well as carcass weight. Moreover, apple pomace was incorporated into finishing pig rations at 10 or 20% w/w, promoting the presence of beneficial bacteria and a reduction in volatile fatty acid emissions. It must be noted that the addition of 10% w/w led to enhanced nitrogen retention efficiency [43]. Fermented apple pomace with Lactobacillus plantarum was included in finishing pig feedstuffs, leading to significant results, with the latter’s addition exhibiting a positive impact on some crucial characteristics. Specifically, feed efficiency was found to be improved, and the average daily feed intake was reduced without affecting the animals’ final body weight or back fat thickness [47]. A similar pattern was observed after the addition of fermented grape pomace with Lactobacillus plantarum in the finishing pig diet. This addition improved the presence of beneficial bacteria with a parallel reduction in VFA emissions in feces [43]. Another study examined the incorporation of strawberry by-products in pig conventional feedstuffs. Specifically, Islam et al. [48] included fermented strawberry pomace with Lentinus edodes in the pig diet, and the results revealed a beneficial effect on the lean tissues of grower pigs. Pluschke et al. [49] reported that the addition of mango pulp at a ratio of 15% w/w improved the efficiency of starch and protein digestion to a certain extent. Another study examined the fortification of pig feed with tomato residues at 3 or 5% w/w, revealing a slight effect on pork meat attributes and, more specifically, tenderness [51]. Tomato silage was added at a 30% w/w of dry matter basis in fermentable liquid diets for growing–finishing pigs, promoting their growth parameters without presenting a negative impact on carcass standards [52]. Furthermore, supplementing the pig diet with carrot wastes at 20–25% w/w, where the by-products consisted of 11.3 MJ/kg dry matter metabolizable energy, justified their utilization as ideal animal feed [58]. Liotta et al. [54] enhanced pig feedstuffs by incorporating olive cake processing waste in a ratio of 5 and 10% w/w. The results indicated a reduced backfat thickness, a decreased intramuscular fat level, and a modified fatty acid content. Additionally, the presence of MUFAs and PUFAs was found to be improved [54]. In another research study, a fermented mixture of olive mill stone waste and Lathyrus clymenum pericarp (80–20% w/w) with Pleurotus ostreatus was used as a feed supplement in pig nutrition [55]. Specifically, the authors incorporated the latter mixture in piglet conventional feedstuff at 5% w/w, and the results revealed improved antioxidant blood parameters [55]. Another study in the existing literature examined the effects of Cordyceps militaris spent mushroom substrate as a feed additive in the growing pig diet. The authors stated that the addition of 2 g/kg exhibited a positive impact, since it improved pig performance, facilitated immunoglobulin secretion, and enhanced antioxidant activity. Furthermore, the latter addition reduced cholesterol and MDA presence [56]. Liu et al. [57] studied the fortification of finishing pig feed with Mulberry leaves by 3, 6, 9, and 12% w/w. The obtained results indicated a higher loin eye area and an increased crude protein profile. Moreover, this supplement exerted a positive impact on animal health since it enhanced inosine monophosphate content and amino acids in muscle tissues.

4. Environmental Benefits

This section outlines the major environmental benefits derived from the use of food industry by-products in pig diets, structured under five key domains: greenhouse gas reduction, land and water savings, food waste mitigation, circular economy integration, and impacts on animal health and welfare.

4.1. Greenhouse Gas Emission Reduction

The livestock sector contributes approximately 15% of global anthropogenic GHG emissions, with pork production responsible for roughly 9% of livestock-related emissions [59]. Within the EU, agricultural GHG emissions predominantly stem from enteric fermentation (45%), soil management (38%), and manure management (15%) [60]. Unlike ruminants, pigs generate relatively low emissions from enteric fermentation [61]. Instead, feed and manure represent the most significant sources of environmental impact in swine production systems [62]. Consequently, optimizing feed formulations and manure management practices is critical for reducing the environmental footprint of the sector [63,64].

Replacing conventional feed ingredients with food by-products decreases GHG emissions associated with crop cultivation, fertilizer use, and transport [10]. Moreover, a 10–25% reduction in CO₂-equivalent emissions per kg of pork when by-products are included is indicated. The reduction in emissions is especially meaningful when imported soybean meal is replaced by domestic by-products, as this substitution decreases emissions from transport and associated deforestation [28].

Feeding piglets by-products, in addition to reducing GHG emissions, diverts land, water, and energy away from the production of virgin resources, such as cereals and soybean meal [28,29]. The transformation of food processing waste into high-end feed products enables the food industry to reduce its dependency on virgin resources and deforestation associated with soy production, as well as to improve biodiversity [65]. This approach is also in alignment with the main pillars of the circular economy and promotes a closed-loop system that converts the potential waste into a valuable resource, according to the European Union’s Circular Economy Action Plan [65,66].

On the other hand, from a positive viewpoint, the avoidance of landfill disposal, the cost reduction associated with waste management, and potential carbon credit initiatives could also provide financial incentives for farmers who implement circular feed strategies [28,67]. Furthermore, the usage of by-products positively affects several national and international Sustainable Development Goals (SDGs), such as mitigating climate change, lowering environmental pollution, and promoting more sustainable food production [10].

Further studies on the optimization of nutrient content, retention of bioactive compound content in processing, and feed safety may allow the climate and sustainability benefits of this approach to be further enhanced [68,69,70]. Considering these aspects of integration, the use of food processing waste as pig feed becomes a highly efficient, practical, and environmentally friendly means of tackling the dual issues of livestock production and climate change.

4.2. Land and Water Use

The use of industry by-products for feeding pigs has been found to reduce reliance on arable land and freshwater resources and, subsequently, create more sustainable agricultural systems. Previous research showed a saving of 15–30% of land use and 10–20% of the water footprint, depending on the by-product type and proportion used [9,15,29]. Demand for products, such as cereals and soybeans, whose cultivation demands extensive land and irrigation, could be reduced by repurposing fruit and vegetable pulp, bakery waste, and other agro-industrial residues [10,28,31]. This strategy could have a beneficial effect by liberating arable land for other purposes—reforestation, biodiversity conservation, etc.—but could also alleviate the strain on water resources, which tend to be increasingly limited and highly impacted by climate change.

Decreased land use and water consumption are typically major when by-products are produced and used locally because transportation emissions and the need for extra resources are reduced [66]. Furthermore, by-product integration is consistent with the circular economy initiative promoted by the recycling of energy and nutrients instead of wasting them [65,67]. Current data suggest that between 10% and 25% of conventional feed ingredients can be safely and effectively replaced with food industry by-products under typical commercial conditions [10,28]. This approach not only contributes to the preservation of natural resources but also enhances the resilience and sustainability of animal production in the wake of increasing environmental challenges. As a paradigm, precision feeding techniques combined with high moisture by-products can significantly lessen the environmental footprint of swine production and, in parallel, improve nutrient utilization efficiency [30,69]. The ongoing advances in by-product processing techniques, such as drying, ensiling, and fermentation, contribute to feed stability, the retention of nutrients, and all-year-round availability [68,70].

4.3. Food Waste Mitigation

The redirection of bakery, dairy, and brewing waste from landfills for use as pig feed is a key strategy to address the issue of food waste in the context of climate change adaptation. Methane (CH₄) produced from the anaerobic decomposition of organic waste in landfills also poses a significant concern due to its high potential for global warming, as it is considered more than 30 times more potent than CO₂ over a century [71]. With the conversion of 1 ton of food waste into pig feed, it is estimated that approximately 0.3 tons of CO₂-equivalent methane emissions can be prevented, resulting in a notable decrease in the overall carbon footprint of food production systems [9,10].

Furthermore, approximately one-third of all food produced globally is either lost or wasted, with environmental, economic, and social repercussions [29]. The recycling of these substances not only decreases the volume of waste sent to landfills but also saves nutrients and promotes sustainable resource utilization [28,29]. When food waste residues are consumed instead of traditional crops, like soybeans and grains, which are land-, water-, and energy-intensive, the environmental impact is further mitigated [10,15].

By-products from local sourcing also increase the sustainability of this solution by reducing emissions from transport and by strengthening local circular economies [66]. Also, the addition of residues high in bioactive compounds could have advantages for animal welfare and health without negative effects on growth performance, which would be doubly good for environmental and livestock system efficiencies [68,69,70].

Future work will need to investigate the optimization of food by-products through selection choice, cost–benefit use, and inclusion rates to maximize emission reductions and understand the impacts on long-term animal performance, thus creating equivalent verification or measurement protocols for environmental impacts. There is also a need to consider how to incorporate studies that investigate future processing technologies, such as fermentation or ensiling, to optimize nutrient availability because feed safety and availability 365 days a year are critical and can provide a much larger benefit to global food waste reduction [65,67]. Through the strategic use of by-products from food processing, pork production systems could significantly contribute to the management of waste and waste disposal sustainability, climate change mitigation, and the promotion of a circular economy.

4.4. Circular Economy Integration

Integrating circular bioeconomy strategies—such as manure management for biogas production, alternative feed ingredients, and wastewater recycling—enhances resource efficiency and reduces environmental footprints [72]. Adding food processing by-products into pig diets is a prime example of industrial symbiosis, in which waste streams from one industry are converted into useful inputs for another, creating a low-resource and low-environmental-input closed-loop system. This approach decreases the reliance on fresh feed sources, such as soybean meal or grains, and can also assist in conserving water, saving arable lands, and saving energy while lowering GHG emissions [10,28]. Moreover, this method fosters circularity within the agri-food system [9,65].

Furthermore, this strategy opens avenues for creating new products, such as revalorizing bioactive compounds for the development of functional feed additives that enhance animal welfare and improve meat quality [66,68,69]. The circular economy approach encompasses a comprehensive system that integrates environmental stewardship, animal welfare practices, and practical business strategies in modern pork production.

4.5. Beneficial Effects on Animal Health, Welfare, and Performance

Studies show that the use of by-products has no negative impact on growth performance. For example, the feed conversion ratio (FCR) is not affected or slightly improved (2.4–2.8 compared to 2.5–3.0 for normal feed), and the average daily gain (ADG) is not significantly changed (700–900 g/day) [29,30]. Also, some by-products, like brewers’ grains and whey, have beneficial effects on gut health. They help different types of beneficial microbes grow and outcompete harmful bacteria, which could lead to a decrease in antibiotic use [9]. Finally, meat quality is not affected in terms of parameters such as protein, fat, and taste [15].

In addition, some by-products contain important compounds, like plant-based polyphenols and antioxidants, that can improve animal health [73,74,75,76,77]. Enhancing the immune system of animals can have beneficial effects on overall animal health [77]. Food industry by-products (e.g., fruit and vegetable peels, seeds, cereal, dairy, beer, and pulp) contain important compounds, such as polyphenols, carotenoids, and glucosinolates. For example, grapes, tomato waste, red corn cobs, and olives are enriched with these compounds [70,75,77,78]. These compounds have various beneficial effects on the immune system, redox status, growth performance, and animal health and welfare [79,80,81,82,83]. Recent studies on pigs revealed that plant-based polyphenols benefit growth performance, support the immune system, and protect against oxidative stress damage [79,80].

Modern pig production systems are increasingly required to reconcile productivity goals with enhanced animal welfare and reduced environmental impacts while systematically incorporating circular bioeconomy principles to strengthen resource efficiency and long-term sustainability [72]. Nutrition is a key factor not only in maintaining pig health but also in ensuring high standards of animal welfare. Nutritional interventions that enhance animal welfare can concurrently improve productivity, product quality, and overall profitability. Animal welfare has become an important basis of sustainable production, and improvements in environmental impact are likely to come at the expense of animal welfare, or vice versa, regardless of cost [23]. Feeding strategies aligned with circular bioeconomy principles leverage local ingredients and by-products to maximize resource efficiency, whereas conventional methods may overlook pigs’ changing nutritional needs, resulting in nutrient waste and higher feed costs [84].

4.6. Alignment with the UN Sustainable Development Goals

The evidence synthesized in Section 4.1, Section 4.2, Section 4.3, Section 4.4 and Section 4.5 shows that integrating food industry by-products into pig diets advances multiple sustainability outcomes through three main mechanisms: (i) substitution of high-impact primary crops, (ii) avoidance of emissions and resource use associated with alternative waste management, and (iii) valorization of bioactive fractions that support animal robustness without sacrificing performance—collectively underpinning SDGs 12, 13, 6, and 15. The conceptual framework illustrated in Figure 1 summarizes how the integration of food industry by-products into pig diets contributes simultaneously to resource efficiency, animal welfare, and the United Nations Sustainable Development Goals (SDGs 6, 12, 13, and 15).

Integrating food industry by-products into pig diets contributes to several United Nations Sustainable Development Goals (SDGs). By replacing resource-intensive crops and redirecting food waste streams to animal feed, this practice supports SDG 12 (Responsible Consumption and Production) through nutrient recycling and waste prevention [9,85,86]. It also aligns with SDG 13 (Climate Action) by reducing feed-related greenhouse-gas emissions by approximately 10–25%, mainly via substitution of imported soybean meal and avoidance of landfill disposal [9,28,86]. Moreover, the use of locally sourced by-products decreases irrigation demand and nutrient losses, advancing SDG 6 (Clean Water and Sanitation) through improved water-use efficiency and reduced eutrophication risks [87,88,89]. Finally, reduced dependence on land-intensive feed crops lessens pressure on arable land and mitigates deforestation, consistent with SDG 15 (Life on Land) [28,86,87]. Overall, circular feeding strategies enhance productivity and animal welfare while strengthening environmental and economic resilience across the livestock sector [9,29,30,90,91]. Regarding performance, welfare, and business viability (enablers across SDGs), evidence shows no penalty—and in some cases small gains—in FCR/ADG when diets are formulated isonutritionally with common by-products, while certain bioactive-rich streams support gut health and robustness [9,29,30]. These outcomes facilitate adoption by maintaining productivity, while circular economy governance and safety guidance ensure compliance and trust [90,91].

The suggested indicators for verification in operational tracking can include the following: (i) percentage of ration derived from food industry by-products (Targets 12.2 and 12.5) [28,86]; (ii) greenhouse gas intensity (kg CO₂-eq per kg pork, farm gate) with separate reporting of substitution and avoided-waste credits (Target 13.2) [9,92]; (iii) blue-water footprint per kg pork (Target 6.4) [87,88]; (iv) nutrient excretion intensity (kg N and kg P per kg pork) (Targets 6.3, 6.6) [89,92]; (v) cropland requirement and displacement of the feed basket based on crop-specific land-use intensities and share of verified deforestation-free soy/cereals (Target 15.2) [28,86,87]; and (vi) median transport distance/tonne-km for by-product sourcing (supports SDG 13 co-benefits).

In sum, the practices detailed in Section 4.1, Section 4.2, Section 4.3, Section 4.4 and Section 4.5 provide coherent, evidence-based pathways for pig production systems to advance progress toward SDG 12 (circular resource use and waste prevention), SDG 13 (greenhouse gas mitigation), SDG 6 (water-use efficiency and quality), and SDG 15 (land sparing and biodiversity protection)—without compromising animal performance.

4.7. Economic Implications and Cost–Benefit Considerations

The integration of food by-products can reduce total feed costs by 8–15% depending on local availability and processing requirements [9,72]. Savings arise from substituting imported cereals and soybean meal, whereas additional costs relate to collection, transport, drying, and storage. Net benefits are generally positive when by-products are sourced regionally and used promptly. Moreover, avoided-waste disposal costs and potential access to carbon-credit schemes can further improve profitability [9,19,28].

5. Economic Aspects and Feed Costs

Industrialized production accounts for 55% of the global pork production and 71% of the poultry production. Currently, around 800 million tonnes of cereals—about one-third of the total cereal production—are used for animal feed, and this is projected to exceed 1.1 billion tonnes by 2050 [93]. With the expansion of the monogastric sector, demand for maize and coarse grains is expected to represent nearly half of all grain production by 2050 [94]. In 2000, 78% of feed grains were allocated to pigs and poultry in regions dominated by intensive industrial systems [95]. By 2013, the monogastric sector consumed 155 million tonnes of feed protein, and an additional 52 million tonnes of feed protein is projected to be required by 2030 to meet demand [96]. Addressing these feed-related challenges could be facilitated by utilizing food-not-feed resources (human-inedible materials), including novel feed resources that do not compete with human food and forages from grazing lands, and by revisiting proven crop-based technologies and implementing them through sustainable business models [96].

The use of food industry by-products (e.g., bakery meal, co-products from starch/ethanol production such as Distillers Dried Grains with Solubles—DDGS) offers potential to reduce feed cost, enhance resource efficiency, and promote circular economy principles [16].

Table 4. Summary of economic evaluations of food industry by-products in pig diets.

By-product Type	Inclusion Level (% of Diet)	Animal Stage	Main Findings	Economic Outcome	References
DDGS	10–30%	Grower–finisher pigs	Growth performance maintained up to 20%. Mild FCR increase at 30%.	Profit per kg live weight ≈ USD 0.23–0.32/kg; up to 12%. Lower feed cost.	[97]
cDDGS	20%	Fattening pigs	Carcass yield maintained. Low ADG reduction.	Decrease of the total cost of fattening: ↓ 7%. Decrease of the cost per kg BW: ↓ 8%. Increase of the gross margin: ↑ 63%.	[98]
Bakery meal	20%	Grower–finisher pigs	No adverse effect on growth or carcass traits.	Lower ingredient purchase cost Life cycle evaluation showed improved economic and environmental efficiency.	[99]
Bakery waste	25–50%	Weaner piglets	Linear ADG decrease beyond 50% inclusion. Optimal ≤ 25%.	Decrease of the feed cost per kg BW gain ↓ 20.45% at 50% inclusion.	[100]

presents economic evaluations of food industry by-products from the bakery/food waste sector and DDGS in pig diets from previous studies.

By-products from the bakery/food waste sector (e.g., bakery meal) are increasingly studied in pig feeding. A study in Greece examined the inclusion of 20% bakery meal replacing corn, wheat, barley, and soybean meal in growing–fattening pigs, using a life cycle inventory approach; the authors reported cost-savings via lower ingredient purchase costs and waste valorization [99]. Another study found that replacing maize with bakery waste up to 50 % in growing piglets reduced feed costs per kg body weight gain by up to ~20.45 % compared with a conventional concentrate diet [100]. Regarding the use of DDGS in pig diets (0, 10, 20, 30% inclusion levels), a study reported that with 10% DDGS inclusion, the profit per kg of weight gain for finishing pigs reached USD 0.23/kg and for females, it reached USD 0.28/kg; in contrast, 20% inclusion had a profit of USD 0.32/kg compared with a 0% control at USD 0.33/kg [97]. Moreover, the use of cDDGS (corn-based DDGS) to replace a standard soybean–corn mix resulted in a decrease in the total cost of fattening and cost per kg of body weight by approximately 7% and 8%, respectively, when cDDGS replaced part of a conventional concentrate—the direct surplus per pig was about 63% higher for the cDDGS group [98].

While the cost-saving potential of incorporating by-products into animal feed is real, caution is required. Animal performance, such as ADG and FCR, may decline if nutrient composition, digestibility, or antinutritional factors are not properly accounted for. For example, DDGS inclusion in pig diets indicated that high inclusion levels can reduce ADG and gain–feed, particularly when fiber content is high or the digestible amino acid profile is compromised [101]. However, the real economic value also relies on local conditions, such as by-product availability and costs related to collection, processing, transport, moisture content, and stability during storage. Bakery waste, for example, usually needs to be either dried or thermally treated before inclusion in feed, which increases its cost in countries like Greece [99]. In addition, indirect benefits related to avoiding waste disposal economies of scale, together with reduced environmental externalities, may provide value but are often missed in simple feed cost analyses. The addition of LCC or LCA thus provides a more complete picture of economic sustainability [99].

6. Challenges

Despite the numerous environmental and growth performance benefits associated with the inclusion of by-products in pig feed, several obstacles hinder their widespread use. The primary concern is linked to the inconsistency in nutritional value, which can vary greatly depending on its source, the season of production, and processing techniques [29,68]. This inconsistency negatively impacts the energy, protein, fiber, and bioactive compound levels present in feed, which are all critical for animal health and growth.

Microbiological safety is also an important aspect to consider. Improper handling and storage of fruit, vegetable, bakery, or dairy residue from the food industry can result in pathogenic microorganism contamination [10,31,65]. Processing can provide a means to alleviate the risks of pathogenic microorganisms while also maintaining the nutritional and functional properties of the resulting feed material [68,69]. Among the different feed materials, FFPs can be a particularly valuable, nutrient-dense feed. However, studying the “safe” inclusion of these products into pig diets should be carried out inclusively [36]. Processing (e.g., baking, extrusion, frying) applied during the processing of raw materials such as grains or tubers can change the starch structure and glycemic index, causing the carbohydrates to become rapidly fermentable and possibly affect gut physiology or metabolism. Also, due to the varied source materials of FFPs, through a standardized pre-processing procedure, some materials may contain traces of packaging or handling materials that should be removed as well. Developing a traceability and certification scheme for feed handling would improve feed safety and help to formally determine a standard for environments that transport FFP materials. Ultimately, identifying how FFPs can be safely valorized in relation to their environment will affect animal health and guarantee food safety for the human population.

Adequate diet formulation is crucial for achieving amino acid, mineral, vitamin, and energy balance with the incorporation of by-products [102]. Nutrient content fluctuations require advanced formulation tools and ongoing laboratory analysis. Computer surveillance and feed-formulating software can assist in facilitating real-time adjustments so that animals are fed correctly and growth performance, immune status, and overall welfare are maintained [30,103]. Regulatory restraints also present obstacles, particularly for by-products from animals, treated waste streams, or those that are not classified as safe to feed [28,65]. Guidelines, certification programs, and standardized processing protocols must be made available to facilitate compliance with the law and increase farmer confidence.

Although conventional feed ingredients also show variability, by-products tend to display broader fluctuations in nutrient composition because of differing industrial processes and seasonal factors. Therefore, stricter compositional monitoring and standardization are required for consistent performance outcomes.

Regulatory heterogeneity across regions remains a significant constraint. Although Regulation (EC) No 767/2009 [40] and related directives define feed-material categories, national authorities may differ in classifying or approving specific by-products, particularly those containing animal-derived components. Establishing harmonized European guidance on processing standards, traceability, and risk assessment protocols would promote broader market access and enhance farmer confidence.

Resolving such issues requires a multi-step approach, including standardization of by-product quality, severe quality control, cooperative agreements between feed producers, food manufacturers, and government authorities, and investment in processing plants. Research should target the synergistic use of a range of by-products, fortification methods with basic nutrients, and long-term effects on animal health and performance, as well as environmental sustainability [70,75,77].

Although most studies report beneficial effects of by-product inclusion, some have revealed neutral or inconsistent results, often linked to variability in nutrient composition, processing method, or storage stability. For example, certain fruit or vegetable residues demonstrated reduced digestibility or limited palatability when inclusion rates exceeded recommended levels, while bakery waste exhibited batch-to-batch inconsistency affecting the amino acid balance. Such divergent outcomes underline the importance of standardized quality control and further validation under commercial farm conditions [12,36,50].

7. Future Perspectives

The use of food processing by-products as animal feeds provides a viable means to achieve sustainable innovation in livestock production. Processes such as fermentation, pelleting, and ensiling, besides improving nutrient digestibility and shelf life, also help to preserve bioactive compounds and reduce microbial contamination [29,68]. Such innovations promote better feed efficiency, animal health, and resilience in production.

Precision feeding systems and digital monitoring equipment now make it possible to provide real-time evaluation of nutrient intake, growth rates, and metabolic indicators [30,69,79]. The technologies enable adaptive diet composition and continuous optimization of by-product inclusion. Tracking the number of functional compounds also ensures welfare levels and immune system maintenance [27].

Future research directions include the determination of synergistic by-product blends of maximum nutritional and functional quality, supplementation with limiting nutrients, as required, and identification of long-term effects on immunity, performance, and meat quality [70,75,77]. Future research should also determine the stability of bioactive compounds during storage, their effect on meat quality, and the economic feasibility of including such materials in industrial production systems. Combined, these technologies can transform food by-products into the focal point of sustainable pig livestock production.

Both EU and national policies are increasingly guiding the transition toward more sustainable livestock systems by encouraging innovation, implementing emission reduction strategies, and enhancing resource management [104,105]. Policy instruments would accelerate uptake. Incentives such as carbon credits, tax credits, or subsidies for farms that adopt circular feeding strategies can encourage broader industry participation [28,66]. Furthermore, aligning livestock feed policies with national and international food waste reduction targets, sustainability measures, and climate change mitigation efforts can achieve systemic benefits, including reduced environmental impacts and improved animal welfare, and contribute to economic resilience [9,67].

Innovative methods that use multi-objective optimization tools in feed formulation will continue to enhance sustainability in pig production. Recently, introducing an equivalent model into environmental, nutritional, and economic contexts demonstrated that the right balance between performance and the ecological footprint can be calculated based on a combination of feeds [21]. The activation of these optimization models, along with supporting public policy and targeted incentives (carbon credits or sustainability-directed subsidies), can enable the broader implementation of circular feed ingredients in commercial systems. Furthermore, combining this framework with the European Green Deal [106], as well as the Farm-to-Fork Strategy [107], is likely to enhance or accelerate the transition to a more climate-neutral and more resource-efficient livestock sector. A standardized LCA database and regulation guidelines specific to circular feed ingredients would give scientific and legislative assurance for these developments.

Sustainable pork production increasingly depends on careful feed ingredient sourcing, the application of LCA, and consideration of antinutritional factors within multi-objective feed formulation and precision nutrition frameworks. Additionally, advancing and implementing strategies that upcycle nutrients from lower-value by-products and food waste streams into swine feed ingredients are critical to reducing the environmental footprint of pork production. Collectively, these approaches support broader One Health objectives and contribute to the transition toward circular agricultural and food systems [108].

For effective adoption at the farm level, producers should collaborate with feed specialists to evaluate by-product nutrient profiles through laboratory testing, ensure compliance with EU feed safety legislation, and gradually replace traditional ingredients following isonutritional formulation principles. The development of farmer manuals and regional training programs could further enhance confidence in circular feed use.

While the current evidence largely derives from controlled experimental studies, large-scale implementation depends on multiple practical aspects, such as feed logistics, availability and stability of by-products, farmer awareness, and local market incentives. Further pilot studies under commercial conditions are needed to assess the real-world performance, economic feasibility, and scalability of circular feed strategies.

8. Conclusions

Feeding pigs food industry by-products could be a smart, research-backed way to reduce pollution, use fewer resources, and maintain the health, welfare, and productivity of the animals. This practice aligns with efforts to cut down on waste and help combat climate change. When thoughtfully performed, including appropriate processing, careful monitoring, and appropriate regulations, by-products used as feed could be a feasible, long-term solution for the global swine industry.

Together, by tackling these issues and leveraging the ideas described in this study, by-products from the food industry can form the foundation of a sustainable pig production system, influencing improved environmental footprints, animal welfare, and circular, resilient agricultural systems.