Impact of Mixed Rations on Rumen Fermentation, Microbial Activity and Animal Performance: Enhancing Livestock Health and Productivity—Invited Review

Simple Summary

Ruminant animals reared under extensive systems spend a substantial amount of time grazing and selecting vegetative parts, which may be considered less effective in terms of feed efficiency, nutrient partition, milk and meat production. Within this context, total mixed ration (TMR) is a feeding method where all ingredients, such as hay, grains, and supplements, are blended into a single, balanced meal for livestock. This approach provides animals with consistent nutrition, enhances digestion, and boosts productivity, resulting in higher milk yield and improved growth, when compared with rearing animals under extensive systems. Studies show TMR increases feed efficiency, supports healthier rumen function, and reduces food waste by preventing animals from picking only their favorite parts. However, challenges include the risk of digestive issues if the mix is not balanced and potential contamination from mold toxins in poor-quality feed associated with feed formulation and storage. Innovations like fermented TMR can further enhance nutrient absorption and reduce methane emissions, benefiting both farms and the environment. Overall, TMR offers a sustainable approach to enhancing livestock health and animal productivity, but careful management is crucial to avoiding potential pitfalls. Future research should focus on optimizing recipes and reducing environmental impacts.

Abstract

Feeding a balanced diet such as total mixed ration (TMR) is a widely adopted feeding strategy providing a uniformly blended diet of roughages, concentrates, and supplements that enhances ruminant productivity by optimizing nutrient utilization, stabilizing rumen fermentation, and improving microbial activity. Scientific studies have confirmed that TMR increases dry matter intake (DMI), milk yield, and growth performance in dairy and beef cattle, as well as in sheep and goats. TMR’s advantages include consistent feed quality, reduced selective feeding, and improved feed efficiency. A key benefit of TMR is its ability to promote the production of volatile fatty acids (VFAs), which are the primary energy source for ruminants, particularly propionate. This enhances energy metabolism, resulting in higher carcass yields, increased milk production, and economic benefits compared to conventional or supplementary feeding systems. However, TMR feeding is also susceptible to mycotoxin contamination (e.g., aflatoxins, zearalenone), potential effects on methane emissions, and the need for precise formulation to maintain consistency and optimise profitability. Prevention and good practices, including routine inspection of feed for pathogens and vulnerable ingredients, as well as careful management of particle size and forage-to-concentrate ratios, are crucial in preventing subacute ruminal acidosis (SARA) and the development of other subclinical diseases. Mycotoxin binders, such as hydrated sodium calcium aluminosilicate, can also reduce mycotoxin absorption. Another advantage of practicing TMR is that it can support sustainable farming by integrating agro-industrial byproducts, which minimises environmental impact. In conclusion, TMR is a widely adopted feeding strategy that significantly enhances ruminant productivity by optimizing nutrient utilization, stabilizing rumen fermentation, and improving microbial activity, leading to increased dry matter intake, milk yield, and growth performance. It offers key benefits such as consistent feed quality, reduced selective feeding, improved feed efficiency, and enhanced energy metabolism, providing economic advantages and supporting sustainable farming through agro-industrial byproduct integration. However, its implementation requires careful management to mitigate risks, including mycotoxin contamination, potential impacts on methane emissions, and digestive issues like SARA if formulation is not precise. Therefore, for sustainable production, future research should focus on optimizing TMR formulations with alternative ingredients (e.g., agro-industrial byproducts) and precision feeding strategies to enhance livestock health and animal productivity while minimizing environmental impacts.

1. Introduction

The rapidly expanding global population necessitates critical advancements in sustainable livestock production to meet the rising demand for animal proteins [1,2]. Optimal nutrition is paramount for achieving high performance, improved animal health, and economic viability in modern ruminant farming for species such as dairy cows, beef cattle, buffaloes, goats, and sheep [3,4]. In this context, the formulation of balanced diets, specifically the Total Mixed Ration (TMR), has become a cornerstone of modern ruminant nutrition. TMR is a scientifically formulated feeding strategy that combines all dietary components—including forages, concentrates, vitamins, and minerals—into a single, uniform mix [3,5,6]. This approach is designed to ensure the consumed diet is nutritionally complete, minimize selective feeding, and provide numerous benefits for herd health and productivity [7,8,9,10]. The widespread adoption of TMR is driven by its significant benefits. It provides a consistent nutrient supply that stabilizes rumen pH and promotes efficient microbial activity, which is crucial for enhanced nutrient digestibility and feed efficiency [8,11,12]. Studies consistently demonstrate that TMR leads to maximized production outcomes, including higher milk yield and improved milk components (e.g., fat, protein) in dairy cows [13,14,15,16], as well as improved average daily gain (ADG), body weight, and marbling scores in beef cattle [17,18,19,20,21] (Figure 1). Furthermore, TMR systems can reduce feed costs and support sustainable practices by facilitating the efficient use of locally available agricultural by-products while minimizing feed wastage [22]. However, the implementation of TMR is accompanied by significant complexities and challenges that necessitate continuous research and refinement. High-concentration formulations, particularly in pelleted forms, can disrupt rumen pH and increase the risk of subacute ruminal acidosis (SARA) [17]. Animals may still sort feed based on particle size or palatability, undermining nutritional consistency and leading to digestive issues and reduced performance [23,24,25]. Susceptibility to aerobic deterioration and mycotoxin contamination from ingredients like silage also poses substantial animal health risks [25]. Additionally, the environmental footprint of TMR, particularly concerning methane emissions, varies considerably with diet composition and digestibility [24,25,26]. These challenges have spurred innovations aimed at optimizing TMR systems. Variations like Ensiled TMR (ETMR) enhance nutrient preservation [23,24], while Fermented TMR (FTMR) has shown promise in improving nutrient digestibility and rumen fermentation, especially when incorporating agricultural by-products [27]. The use of additives, such as yeast cultures or lactic acid bacteria with high antioxidant activity, is also being explored to enhance fibre breakdown and rumen stability, and mitigate nutrient degradation [28]. Given the complex interplay of formulation, animal physiology, economic viability, and environmental impact, a comprehensive synthesis of existing research is essential. The extensive, and sometimes conflicting, findings across diverse ruminant species and production systems require critical evaluation. Therefore, this review aims to consolidate the current understanding of TMR, focusing on its formulation, benefits, inherent challenges, and recent innovations. By identifying optimal strategies and research gaps, this work provides practical insights for farmers and researchers, guiding future efforts toward more efficient, sustainable, and profitable ruminant production systems. This consolidated knowledge is fundamental for translating scientific insights into practical applications that enhance on-farm success and contribute to global food security.

2. Methodology

This review consolidates current knowledge on the formulation, benefits, and challenges of Total Mixed Ration in ruminant nutrition, with a focus on its impact on productivity, rumen health, and environmental sustainability. To ensure a comprehensive assessment, the literature search was conducted across multiple academic databases, including Web of Science, PubMed, Google Scholar, and Scopus, using targeted search terms such as “Total Mixed Ration AND ruminant performance,” “TMR AND rumen fermentation,” “fermented TMR AND meat quality,” and “TMR formulation AND feed efficiency.” The inclusion criteria prioritized peer-reviewed studies, in vivo studies, and selected in vitro studies, while excluding non-ruminant research and non-scientific sources.

The extracted data encompassed TMR composition (e.g., forage-to-concentrate ratios, inclusion of wet brewers’ grains, yeast cultures, and fermented additives), processing methods (ensiling techniques), and their effects on feed intake, growth performance, milk yield, rumen fermentation parameters (volatile fatty acid profiles, pH stability, microbial diversity), and methane mitigation, as well as economic and environmental considerations. Given the narrative structure of this review, no formal statistical analysis was conducted. Instead, findings were synthesized to provide a balanced interpretation of TMR’s role in modern ruminant production systems, emphasizing practical implications for farmers and researchers. The discussion integrates mechanistic insights, such as how fiber digestibility is influenced by particle size or how microbial additives improve aerobic stability, to clarify the functional basis of TMR’s benefits and limitations. References were selected based on relevance, scientific rigor, and applicability across dairy and beef cattle, sheep, and goats, ensuring a representative overview of global TMR practices.

3. Total Mixed Ration: Composition, Advantages, and Fermentation Technologies

The Total Mixed Ration is a cornerstone of modern ruminant production, designed to deliver a balanced intake of nutrients by homogeneously blending forages, concentrates, and supplements [18,25,29]. This method prevents selective feeding, stabilizes rumen fermentation, and enhances overall feed efficiency, making it particularly beneficial for high-producing dairy cattle and intensively reared beef and sheep [18,30].

3.1. Typical Composition and Formulation

The formulation of TMR is tailored to the animal’s species, production stage, and available feed resources. A typical TMR consists of forages (50–70%) and concentrates (30–50%) on a dry matter basis, with minor inclusions (1–2%) of additives such as yeast or molasses to enhance palatability and digestion [14,31,32] (Figure 2). Dairy cows often receive a forage-to-concentrate ratio between 50:50 and 60:40, while beef cattle and growing lambs may be fed higher forage proportions [33].

3.2. Forage Components

Common forages include corn silage, alfalfa hay, and grass/clover silage, which provide essential structural fiber to promote rumination and saliva production for rumen pH buffering [34,35]. In tropical regions, grasses like Napier (Pennisetum purpureum) and Megathyrsus maximus are frequently used [35,36].

3.3. Concentrate Components

Concentrates typically include energy sources, such as corn meals, and protein supplements, like soybean meals, often supplemented with minerals and vitamins [37]. To improve economic viability, by-products like wet corn gluten feed (WCGF) and distillers’ grains are increasingly incorporated into formulations [21]. Processing methods are critical to TMR effectiveness. Pelleting can improve starch digestibility, while fermentation technologies (e.g., Fermented TMR) enhance preservation and aerobic stability [26,38].

3.4. Key Advantages and Innovations

The primary advantages of TMR include enhanced rumen function, improved nutrient utilization, optimized production performance, and cost-effective sustainability (Figure 3). By preventing selective consumption, TMR ensures a consistent nutrient supply, which stabilizes rumen pH and promotes efficient microbial activity. This leads to higher average daily gain (ADG), improved milk yield (with reported increases of 3–5%), and enhanced feed conversion efficiency [18,30]. Furthermore, TMR minimizes feed wastage and allows for the precision formulation of nutrients using locally available by-products, supporting sustainable farming practices [21]. Innovations continue to optimize the TMR system. Ensiled TMR (ETMR) and Fermented TMR (FTMR) leverage preservation technologies to extend shelf-life, improve nutrient digestibility, and enhance aerobic stability [39,40,41,42]. These approaches create an anaerobic environment that promotes lactic acid fermentation, thereby preserving nutrients and inhibiting the growth of spoilage microorganisms [30,42]. The fermentation process can break down complex fibers and proteins, improving volatile fatty acid (VFA) production and increasing energy availability for the animal [42,43,44]. This not only supports higher dry matter intake and better growth performance but may also reduce methane emissions by shifting rumen fermentation toward propionate production [30,44]. Compact TMR (CTMR) is another innovation aimed at reducing labor costs and improving feed utilization [26].

3.5. Fermentation Quality and Implications

The fermentation quality of silage components is a critical determinant of TMR’s nutritional value and stability. Successful ensiling, driven by lactic acid bacteria (LAB), lowers the pH below 4.2 and results in a high lactic acid content with minimal undesirable byproducts, such as butyric acid or ammonia-N [28,42,43,44,45,46,47,48,49,50,51]. High-quality fermentation enhances palatability, supports a stable rumen environment by maintaining optimal pH, and promotes the growth of cellulolytic bacteria essential for fiber degradation [24,45]. The strategic use of microbial inoculants (e.g., Lactobacillus acidophilus, Saccharomyces cerevisiae) can significantly improve fermentation efficiency by accelerating acid production and suppressing undesirable microbes [43,52]. Additives like molasses provide substrates for LAB, while organic acids (e.g., propionic acid) improve aerobic stability, which is a particular challenge in hot and humid climates [42,53]. Poor fermentation quality has direct negative implications, potentially leading to reduced feed intake, impaired rumen function, and metabolic disorders such as subacute ruminal acidosis (SARA) [17,42]. Conversely, well-fermented TMR silage promotes efficient nutrient utilization, better growth rates, and improved overall health and productivity [45]. Therefore, careful management of factors like compaction, sealing, storage conditions, and particle size is essential to maximize the benefits of TMR systems [26,39,40,41].

4. Effects of Total Mixed Ration on Ruminant Diets

4.1. Effects of Total Mixed Ration on Rumen Fermentation

The composition of TMR plays a crucial role in determining rumen fermentation dynamics, which affect pH stability, VFA production, and microbial diversity (Table 1). One of the most critical aspects of TMR is its influence on rumen pH stability. A well-balanced TMR improves nutrient utilization, enhances animal performance, and reduces metabolic disorders, whereas a poorly formulated TMR can lead to acidosis and reduces feed efficiency. Research indicates that TMR formulations with balanced roughage-to-concentrate ratios help maintain optimal rumen pH (6.0–6.5), reducing the risk of SARA [26]. However, high-concentrate TMR diets (exceeding 60% concentrate) can lead to excessive production of lactic acid, lowering pH and impairing fiber-digesting microbes [17]. This is particularly evident in pelleted TMR, where smaller particle sizes accelerate fermentation, further exacerbating pH fluctuations [54]. Conversely, TMR containing adequate roughage, such as maize silage or alfalfa hay, promotes saliva production during chewing, which buffers rumen acidity and stabilizes pH [45].

The production of VFAs is another key indicator of rumen fermentation efficiency. TMR formulations rich in fermentable carbohydrates, such as grains and molasses, enhance propionate and butyrate production, which are crucial for energy metabolism in ruminants [55]. However, excessive concentrate inclusion can shift VFA profiles toward higher propionate at the expense of acetate, reducing fiber digestibility [26]. FTMR has been shown to improve VFA production by promoting microbial activity through the inclusion of probiotics like Lactobacillus acidophilus and Saccharomyces cerevisiae [42]. Moreover, yeast-supplemented TMR enhances fiber degradation, leading to a more balanced acetate-to-propionate ratio, which supports both energy supply and rumen health [24].

Table 1. Effects of Total Mixed Ration on Rumen Fermentation (pH, VFA, Microbial Diversity).

Species/Breed	TMR Type/Modification	Summary of Results	Reference
Holstein calves	WPCS-based TMR (CTMR)	Higher rumen pH, total VFA, and propionate vs. CSCS (15% WPCS)	[56]
Crossbred lambs	Pelleted TMR (PTMR)	Higher acetate (49.8 vs. 45.7 mmol/L), propionate (24.8 vs. 21.4 mmol/L)	[48]
Hu sheep lambs	High-grain pelleted TMR	Lower pH, Increased lactate, Reduced Fibrobacteres	[26]
Mixed ruminal microbes	TMR + ryegrass pasture	↑ Butyrate/valerate, ↓ Methane, ↑ Microbial biomass N	[57]
Holstein dairy cows	TMR + FF	Stable pH/VFA; ↓ N-NH3 in 50% FF	[58]
Red Chittagong Cows	Maize stover-based TMR	↑ TVFA/NH3-N; stable rumen pH	[16]
Dairy cows (in vitro)	Varied TMR compositions	CP fermentation: 25–60%; CPM synthesis: 677–1778 mg/day	[59]
Red Chittagong cows	Maize stover-based TMR	↑ TVFA and NH3-N	[16]
Buffalo (in vitro)	HFA-supplemented TMR	↑ VFAs, ↓ methane (Shatavari @ 3% most effective)	[60]
Dairy cows (in vitro)	100% TMR (69:31 forage/concentrate)	↑ Total VFA (+16.6 mmol/L), ↑ Acetate/propionate ratio	[33]
German Holstein cows	Pasture transition (from TMR to pasture)	Lower pH in the pasture group (SARA risk in wk 9–10). No adverse LPS effects.	[11]
Holstein-Zebu steers	Fermented TMR (FTMR; pH 3.5)	Stable ruminal pH despite low TMR pH. No acidosis observed.	[61]
Angus beef cattle	High-concentrate TMR	Significantly decreased rumen pH	[18]
Wethers (Sheep)	Pelleted TMR + 1% yeast culture	Higher mean pH, reduced time below pH 5.8.	[24]
Simmental bulls	Multi-silage TMR (MS)	Improved rumen pH stability linked to higher VFA production.	[5]
German Holstein cows	Pasture transition	↓ Acetate (C2%), ↑ butyrate (C4%), ↓ C2/C3 ratio	[11]
Cattle (Bulls)	60:40 Roughage/Concentrate	Highest TVFA production (optimal fermentation)	[62]
Holstein-Zebu steers	FTMR (pH 3.5)	↑ Acetic/butyric acid; ↓ propionate.	[61]
Crossbred lambs	Pelleted TMR (PTMR)	↑ Acetate (49.8 vs. 45.7 mmol/L) and propionate (24.8 vs. 21.4 mmol/L).	[48]
Wethers (Sheep)	Pelleted TMR + 1% yeast	↑ Total VFA, propionate, and n-butyrate.	[24]
Simmental bulls	Multi-silage TMR (MS)	↑ TVFA (62.49 vs. 56.09 mmol/L) and acetate.	[63]
German Holstein cows	Pasture transition	↓ Rumen papillae surface area (recovered by wk 10).	[11]
Angus beef cattle	High-concentrate TMR	↑ Starch-degraders (Bacteroidota), ↓ fiber-degraders (Ruminococcus).	[18]
Crossbred lambs	Pelleted TMR (PTMR)	↑ Prevotellaceae (rumen), ↓ Ruminococcaceae.	[48]
Holstein cows	Fermented TMR (FTMR)	↑ Unclassified_Bacteroidales, ↓ Candida (fungi).	[64]
Wethers (Sheep)	Pelleted TMR + 1% yeast	↑ Fibrolytic bacteria (NK4A214, FD2005).	[24]
Yellow cattle (in vitro)	Fermented TMR (FTMR)	↓ Methane production, ↓ Methanobrevibacter abundance.	[46]
Holstein cows	Fermented TMR (FTMR)	↑ Methanobrevibacter (due to higher H2 availability).	[64]
Holstein (Dairy)	Pelleted TMR	Lower rumen pH (6.10 vs. 6.48), higher propionate	[65]
Cattle (Bulls)	60:40 Roughage/Concentrate	Optimal TVFA, stable pH	[62]
Hanwoo Heifers	Italian Ryegrass TMR	Increased propionate, enriched Ruminococcus bromii	[66]
Holstein dairy cows	Grass silage + concentrate + hay	Higher early GP (2–4 h) with particle-associated inocula (PAL). Declined later.	[67]
Brown Swiss cows	TMR + Saccharomyces cerevisiae (CE/LC)	CE improved asymptotic GP more than LC; low/intermediate doses are most effective.	[68]
Suffolk sheep	Fermented TMR (FTMR)	Higher GP due to enhanced microbial activity from lactic acid fermentation.	[44]
Hanwoo steers	TMR with fermented feed (TMRF)	Improved GP linked to higher acetate/propionate production.	[13]
Holstein cows	TMR with varying particle sizes (5.5–25 mm)	Smaller particles ↑ SCFA in dorsal rumen; ↓ acetate/propionate ratio.	[69]
Suffolk sheep	Fermented TMR (FTMR)	↑ Propionate (392.4 mmol/mol), ↓ butyrate (86.6 mmol/mol).	[30]
Nellore bulls	TMR with pefNDF	Optimal SCFA at 20.5 g pefNDF/kg DM; higher fiber ↑ butyrate.	[70]
Holstein steers	TMR vs. separate feeding	Higher total VFA and propionate at 1.5 h post-feeding.	[9]
Holstein cows	TMR (5.5–25 mm particle size)	5.5 mm: ↓ rumen pH; 11 mm: maintained pH and ↑ protozoa.	[69]
Montbéliarde cattle	TMR (high concentrate)	Lower rumen pH (5.58 vs. 5.87 in control), higher acidosis risk.	[17]
Suffolk sheep	FTMR in varying pH media	Lower pH (5.62–5.66) ↓ CH4 and ↑ propionate.	[44]
Nellore steers	High-concentrate TMR	Faster microbial adaptation in preconditioned cattle.	[71]

TMR = total mixed ration, WPCS = whole plant corn silage, CTMR = corn silage-based TMR, CSCS = corn stover and concentrate supplement, PTMR = pelleted TMR, FF = fresh forage, CP = crude protein, CPM = microbial protein, HFA = herbal feed additive, FTMR = fermented TMR, MS = multi-silage, GP = gas production, SCFA = short-chain fatty acids, pefNDF = physically effective neutral detergent fiber, DM = dry matter, PAL = particle-associated inocula, CE = commercial extract, LC = live culture, TMRF = TMR with fermented feed. ↑ = increase, ↓ = decrease, pH = measure of acidity, VFA = volatile fatty acids, mmol/L = millimoles per liter, N-NH₃ = ammonia nitrogen, NH₃-N = ammonia nitrogen, TVFA = total volatile fatty acids, LPS = lipopolysaccharides, SARA = subacute ruminal acidosis, C2 = acetate, C3 = propionate, C4 = butyrate, H₂ = hydrogen, CH₄ = methane, g/kg = grams per kilogram, h = hour, wk = week.

Table 1. Effects of Total Mixed Ration on Rumen Fermentation (pH, VFA, Microbial Diversity).

Species/Breed	TMR Type/Modification	Summary of Results	Reference
Holstein calves	WPCS-based TMR (CTMR)	Higher rumen pH, total VFA, and propionate vs. CSCS (15% WPCS)	[56]
Crossbred lambs	Pelleted TMR (PTMR)	Higher acetate (49.8 vs. 45.7 mmol/L), propionate (24.8 vs. 21.4 mmol/L)	[48]
Hu sheep lambs	High-grain pelleted TMR	Lower pH, Increased lactate, Reduced Fibrobacteres	[26]
Mixed ruminal microbes	TMR + ryegrass pasture	↑ Butyrate/valerate, ↓ Methane, ↑ Microbial biomass N	[57]
Holstein dairy cows	TMR + FF	Stable pH/VFA; ↓ N-NH3 in 50% FF	[58]
Red Chittagong Cows	Maize stover-based TMR	↑ TVFA/NH3-N; stable rumen pH	[16]
Dairy cows (in vitro)	Varied TMR compositions	CP fermentation: 25–60%; CPM synthesis: 677–1778 mg/day	[59]
Red Chittagong cows	Maize stover-based TMR	↑ TVFA and NH3-N	[16]
Buffalo (in vitro)	HFA-supplemented TMR	↑ VFAs, ↓ methane (Shatavari @ 3% most effective)	[60]
Dairy cows (in vitro)	100% TMR (69:31 forage/concentrate)	↑ Total VFA (+16.6 mmol/L), ↑ Acetate/propionate ratio	[33]
German Holstein cows	Pasture transition (from TMR to pasture)	Lower pH in the pasture group (SARA risk in wk 9–10). No adverse LPS effects.	[11]
Holstein-Zebu steers	Fermented TMR (FTMR; pH 3.5)	Stable ruminal pH despite low TMR pH. No acidosis observed.	[61]
Angus beef cattle	High-concentrate TMR	Significantly decreased rumen pH	[18]
Wethers (Sheep)	Pelleted TMR + 1% yeast culture	Higher mean pH, reduced time below pH 5.8.	[24]
Simmental bulls	Multi-silage TMR (MS)	Improved rumen pH stability linked to higher VFA production.	[5]
German Holstein cows	Pasture transition	↓ Acetate (C2%), ↑ butyrate (C4%), ↓ C2/C3 ratio	[11]
Cattle (Bulls)	60:40 Roughage/Concentrate	Highest TVFA production (optimal fermentation)	[62]
Holstein-Zebu steers	FTMR (pH 3.5)	↑ Acetic/butyric acid; ↓ propionate.	[61]
Crossbred lambs	Pelleted TMR (PTMR)	↑ Acetate (49.8 vs. 45.7 mmol/L) and propionate (24.8 vs. 21.4 mmol/L).	[48]
Wethers (Sheep)	Pelleted TMR + 1% yeast	↑ Total VFA, propionate, and n-butyrate.	[24]
Simmental bulls	Multi-silage TMR (MS)	↑ TVFA (62.49 vs. 56.09 mmol/L) and acetate.	[63]
German Holstein cows	Pasture transition	↓ Rumen papillae surface area (recovered by wk 10).	[11]
Angus beef cattle	High-concentrate TMR	↑ Starch-degraders (Bacteroidota), ↓ fiber-degraders (Ruminococcus).	[18]
Crossbred lambs	Pelleted TMR (PTMR)	↑ Prevotellaceae (rumen), ↓ Ruminococcaceae.	[48]
Holstein cows	Fermented TMR (FTMR)	↑ Unclassified_Bacteroidales, ↓ Candida (fungi).	[64]
Wethers (Sheep)	Pelleted TMR + 1% yeast	↑ Fibrolytic bacteria (NK4A214, FD2005).	[24]
Yellow cattle (in vitro)	Fermented TMR (FTMR)	↓ Methane production, ↓ Methanobrevibacter abundance.	[46]
Holstein cows	Fermented TMR (FTMR)	↑ Methanobrevibacter (due to higher H2 availability).	[64]
Holstein (Dairy)	Pelleted TMR	Lower rumen pH (6.10 vs. 6.48), higher propionate	[65]
Cattle (Bulls)	60:40 Roughage/Concentrate	Optimal TVFA, stable pH	[62]
Hanwoo Heifers	Italian Ryegrass TMR	Increased propionate, enriched Ruminococcus bromii	[66]
Holstein dairy cows	Grass silage + concentrate + hay	Higher early GP (2–4 h) with particle-associated inocula (PAL). Declined later.	[67]
Brown Swiss cows	TMR + Saccharomyces cerevisiae (CE/LC)	CE improved asymptotic GP more than LC; low/intermediate doses are most effective.	[68]
Suffolk sheep	Fermented TMR (FTMR)	Higher GP due to enhanced microbial activity from lactic acid fermentation.	[44]
Hanwoo steers	TMR with fermented feed (TMRF)	Improved GP linked to higher acetate/propionate production.	[13]
Holstein cows	TMR with varying particle sizes (5.5–25 mm)	Smaller particles ↑ SCFA in dorsal rumen; ↓ acetate/propionate ratio.	[69]
Suffolk sheep	Fermented TMR (FTMR)	↑ Propionate (392.4 mmol/mol), ↓ butyrate (86.6 mmol/mol).	[30]
Nellore bulls	TMR with pefNDF	Optimal SCFA at 20.5 g pefNDF/kg DM; higher fiber ↑ butyrate.	[70]
Holstein steers	TMR vs. separate feeding	Higher total VFA and propionate at 1.5 h post-feeding.	[9]
Holstein cows	TMR (5.5–25 mm particle size)	5.5 mm: ↓ rumen pH; 11 mm: maintained pH and ↑ protozoa.	[69]
Montbéliarde cattle	TMR (high concentrate)	Lower rumen pH (5.58 vs. 5.87 in control), higher acidosis risk.	[17]
Suffolk sheep	FTMR in varying pH media	Lower pH (5.62–5.66) ↓ CH4 and ↑ propionate.	[44]
Nellore steers	High-concentrate TMR	Faster microbial adaptation in preconditioned cattle.	[71]

TMR = total mixed ration, WPCS = whole plant corn silage, CTMR = corn silage-based TMR, CSCS = corn stover and concentrate supplement, PTMR = pelleted TMR, FF = fresh forage, CP = crude protein, CPM = microbial protein, HFA = herbal feed additive, FTMR = fermented TMR, MS = multi-silage, GP = gas production, SCFA = short-chain fatty acids, pefNDF = physically effective neutral detergent fiber, DM = dry matter, PAL = particle-associated inocula, CE = commercial extract, LC = live culture, TMRF = TMR with fermented feed. ↑ = increase, ↓ = decrease, pH = measure of acidity, VFA = volatile fatty acids, mmol/L = millimoles per liter, N-NH₃ = ammonia nitrogen, NH₃-N = ammonia nitrogen, TVFA = total volatile fatty acids, LPS = lipopolysaccharides, SARA = subacute ruminal acidosis, C2 = acetate, C3 = propionate, C4 = butyrate, H₂ = hydrogen, CH₄ = methane, g/kg = grams per kilogram, h = hour, wk = week.

Microbial diversity in the rumen is significantly influenced by TMR composition. A well-formulated TMR supports a diverse microbial population, including cellulolytic bacteria (Fibrobacter succinogenes, Ruminococcus flavefaciens) and amylolytic species (Streptococcus bovis) [43]. However, high-grain TMR can reduce microbial diversity by favoring lactate-producing bacteria (Megasphaera elsdenii) over fiber degraders, leading to dysbiosis [17]. Fermented TMR, particularly that incorporating lactic acid bacteria, has been found to enhance microbial stability by suppressing pathogenic bacteria and promoting beneficial microbes [52]. Additionally, the inclusion of wet brewers’ grain (WBG) as a roughage substitute in TMR has been reported to maintain microbial balance while improving nutrient utilization [64].

The implications of these findings are significant for both animal performance and feed efficiency. Stable rumen pH ensures optimal digestion and minimizes metabolic disorders, while balanced VFA production maximizes energy availability for growth and milk production (Figure 4). Furthermore, maintaining microbial diversity enhances feed efficiency and reduces methane emissions, contributing to more sustainable livestock production [55]. However, improper TMR formulation, such as excessive concentrate or insufficient fiber, can lead to digestive disturbances, reduced feed intake, and lower productivity [26].

4.2. Effects of Total Mixed Ration on Nutrient Digestibility

The adoption of TMR in ruminant feeding systems significantly improves the digestibility of nutrients—such as dry matter, organic matter, crude protein, neutral detergent fiber, and acid detergent fiber—by ensuring a balanced and consistent diet (

Table 2. Effects of Total Mixed Ration on Nutrient Digestibility (DM, OM, CP, NDF, ADF).

Species/Breed	TMR Type/Modification	Summary of Results	Reference
Holstein calves	WPCS-based TMR (CTMR)	Lower in vitro DM/CP/NDF digestibility vs. starter (CONS)	[56]
Karakul sheep	40% SS-AF silage TMR	Higher DM, CP, and NDF digestibility	[5]
Fattening lambs	Pelleted TMR + LY	Increased DM (38 g/kg), OM (41 g/kg), and NDF (193 g/kg) digestibility	[72]
Comisana lambs	WM-based TMR	Higher aNDF/ADF digestibility No difference in DM/OM/CP	[73]
Red Chittagong cows	Maize stover-based TMR	↑ DM/CP/NDF digestibility	[16]
Dorper lambs	LB-inoculated PH-TMR	Higher DM intake and nutrient digestibility vs. untreated PH-TMR	[74]
Dairy ewes	Wheat middling-based TMR	↑ NDF digestibility	[73]
Dorper lambs	Cactus pear + cottonseed cake	↑ DMD, OMD, EED in 20–30% cottonseed TMR ↓ Rumination time	[7]
Crossbred cows	FTMR with 25% peNDF	Improved nutrient digestibility (CP, NDF, ADF)	[75]
Red Chittagong Cows	Maize stover-based TMR	↑ Digestibility of DM, CP, OM	[16]
Korean native goats	TMR with varying peNDF	No difference in DM, CF, or other nutrient digestibility	[76]
Dairy cows (in vitro)	16 TMR formulations	OM fermentation: 35–47%; NDF fermentation: 3–28%	[59]
Buffaloes	TMR vs. conventional	↑ DM, OM, NDF digestibility	[77]
Holstein dairy cows	TMR + FF	↑ Nitrogen efficiency in 50% FF ↓ urinary N excretion	[58]
Korean native goats	TMR with varying peNDF	No difference in Nitrogen balance	[76]
Holstein-Zebu steers	FTMR (pH 3.5)	↑ Crude protein digestibility ↓ fat digestibility in silage-TMR.	[61]
Angus beef cattle	High-concentrate TMR	↓ DM, CP, and NDF digestibility	[18]
Yellow cattle (in vitro)	Fermented TMR (FTMR)	↓ NDF/ADF; ↑ lactic acid and soluble carbohydrates.	[46]
Simmental bulls	Multi-silage TMR (MS)	Improved fiber degradation linked to ↑ Prevotella-1.	[63]
Murrah buffaloes	TMR (maize silage/concentrate ratios)	Highest DMI (14.35 kg/d) at 50:50 ratio; ME content ↓ with ↑ silage.	[78]
Hanwoo steers	TMR with fermented feed (TMRF)	Higher DM disappearance (3–12 h) and weight gain (308 kg vs. 284 kg control).	[13]
Suffolk sheep	Fermented TMR (FTMR)	↑ CP, EE, ADF, and GE digestibility vs. non-fermented TMR.	[30]
Montbéliarde cattle	TMR (90% concentrate)	No difference in DMI or digestibility vs. separate feeding.	[17]
Nellore bulls	TMR with pefNDF	Optimal fiber digestion at 20.5 g pefNDF/kg DM.	[70]

TMR = total mixed ration, DM = dry matter, OM = organic matter, CP = crude protein, NDF = neutral detergent fiber, ADF = acid detergent fiber, ME= metabolizable energy, WPCS = whole-plant corn silage, CTMR = corn silage-based TMR, CONS = conventional starter, SS-AF = sweet sorghum-alfalfa, LY = live yeast, WM = wheat middlings, aNDF = amylase-treated NDF, LB = lactic acid bacteria, PH-TMR = pineapple husk TMR, DMD = dry matter digestibility, OMD = organic matter digestibility, EED = energy efficiency of digestion; g/kg = grams per kilogram, ↑ = increased, ↓ = decreased.

). Proper formulation, including optimal roughage-to-concentrate ratios and strategic use of additives, enhances rumen function and overall animal performance. This approach not only boosts productivity but also supports sustainable livestock farming by improving feed efficiency and reducing environmental impacts. Several studies have demonstrated that TMR enhances the digestibility of all nutrients compared to conventional feeding systems. For instance, Arbaoui et al. [17] found that TMR formulations with optimized concentrate-to-roughage ratios significantly improved DM and OM digestibility by ensuring a consistent nutrient supply, which stabilizes rumen fermentation. This is primarily because a finely balanced TMR prevents selective feeding, ensuring that animals consume all dietary components in the intended proportions [54]. Moreover, the inclusion of high-quality roughages such as alfalfa hay and maize silage in TMR has been shown to enhance fiber digestibility (NDF and ADF) by promoting rumen microbial activity [45]. Hamidan et al. [53] reported that diets with adequate NDF content support rumen health by maintaining proper rumen motility and buffering capacity, which in turn improves fiber degradation. However, excessive inclusion of roughage may reduce digestibility due to the increased lignin content, particularly in low-quality forages [70]. Bo Trabi et al. [26] observed that smaller forage particle sizes in TMR increase passage rates but may reduce fiber digestibility due to insufficient rumen retention time for microbial degradation.

Crude protein digestibility is also positively influenced by TMR, particularly when protein sources are well-balanced between degradable and undegradable fractions. Lakhani & Tyagi [78] reported that TMR containing 11–14% CP optimizes microbial protein synthesis while minimizing nitrogen wastage. Additionally, FTMR containing microbial additives, such as Lactobacillus acidophilus and Saccharomyces cerevisiae further enhances CP digestibility by promoting proteolytic activity in the rumen [42]. Wang et al. [24] also demonstrated that yeast supplementation in TMR improves fiber digestibility by stimulating cellulolytic bacteria, leading to better NDF and ADF breakdown. The implications of these findings are significant for both animal productivity and feed efficiency. Improved DM and OM digestibility indicate better energy utilization, which translates to higher milk yield in dairy cows and improved weight gain in beef cattle [79]. Enhanced fiber digestibility ensures optimal rumen function, reducing the risk of metabolic disorders such as SARA. Furthermore, the use of FTMR and additives like wet brewers’ grains [38] or yeast culture [24] offers a sustainable approach to enhancing nutrient utilization while reducing feed costs. Farmers and nutritionists should focus on ingredient quality, forage particle size, and fermentation techniques to maximize digestibility while minimizing metabolic risks.

4.3. Effects of Total Mixed Ration on Growth Performance

Total Mixed Ration represents a significant advancement in ruminant nutrition, integrating forage, concentrates, and additives into a homogeneous mixture that enhances growth performance metrics, including BW, ADG, DMI, and feed efficiency (Table 3). Early research by Lailer et al. [80] established that TMR ensures uniform nutrient delivery, minimizing selective feeding and stabilizing rumen fermentation. This consistency prevents fluctuations in ruminal pH and VFAs production, both of which are critical for efficient nutrient utilization. For instance, Liu et al. [45] reported that lambs fed TMR with optimal oat-to-alfalfa ratios exhibited higher DMI and ADG due to improved palatability and balanced energy-to-protein ratios. Similarly, Nguyen et al. [14] observed increased BW in cattle fed alkaline-treated rice straw-based TMR, attributing it to enhanced digestibility from reduced lignin content, which facilitated microbial access to cellulose.

The scientific basis for improved ADG lies in TMR’s ability to synchronize carbohydrate and protein degradation in the rumen. Wang et al. [5] found that lambs fed TMR with 40% sweet sorghum and 60% alfalfa silage achieved the highest ADG, as the blend provided readily fermentable carbohydrates alongside bypass protein, promoting microbial protein synthesis. Furthermore, Sun et al. [72] demonstrated that pelleting TMR with live yeast increased ADG in lambs by 11%, as yeast metabolites enhanced fibrolytic bacteria activity, boosting neutral detergent fiber digestibility. This aligns with Bo Trabi et al. [26], who noted that smaller forage particles in high-grain TMR accelerated ruminal turnover but cautioned that excessive starch could reduce fiber digestibility and increase the risk of acidosis.

Improvements in dry matter intake under TMR systems are often linked to the physical and chemical properties of the feed. Koch et al. [81] observed that dry TMR formulations incorporating by-products like straw maintained DMI comparable to conventional silage-based rations, as the mix’s density and moisture content reduced sorting behavior. Ferrari et al. [21] further noted that replacing modified distillers’ grains with dry-rolled corn in beef steer TMR linearly decreased DMI, likely due to lower palatability and energy density. However, wet brewers’ grains in TMR countered this by providing soluble fibers that stimulated rumination and saliva production, buffering rumen pH and supporting consistent intake [38].

Feed efficiency improvements stem from TMR’s optimization of nutrient absorption. Tufarelli et al. [73] reported a 15% better FCR in lambs fed wheat middling-based TMR, as the diet’s balanced degradable protein and energy reduced nitrogen wastage and directed nutrients toward muscle deposition. Additionally, fermented TMR with lactic acid bacteria, as studied by Chen et al. [28], increased propionate production through enhanced carbohydrate fermentation, thereby supplying more glucogenic precursors for growth. This was corroborated by Zhang et al. [56] in dairy calves, where TMR enriched cellulolytic bacteria like Rikenellaceae, improving fiber degradation and metabolic energy availability. Challenges persist, however, high-grain pelleted TMR can lower rumen pH and increase lactate production, predisposing animals to subacute acidosis [26]. Martins et al. [25] also highlighted the risks of mycotoxin contamination in TMR, particularly from maize silage, which can suppress feed intake and impair hepatic function. Nonetheless, innovations like yeast supplementation [24] and moisture-stabilizing additives [28] mitigate these issues by enhancing aerobic stability and detoxification. Total Mixed Ration significantly enhances ruminant growth performance by ensuring balanced nutrient intake, stabilizing rumen function, and improving feed efficiency. Its adaptability to incorporate agricultural by-products reduces costs while maintaining productivity, making it indispensable for sustainable livestock farming. Farmers should prioritize consistent mixing, moisture control and mycotoxin management to maximize benefits, as these factors directly influence animal health and economic returns.

Table 3. Effects of Total Mixed Ration on Growth Performance (BW, ADG, DMI, Feed Efficiency).

Species/Breed	TMR Type/Modification	Summary of Results	Reference
Holstein dairy calves	WPCS-based TMR (CTMR)	No difference in BW, ADG, or feed efficiency vs. starter (CONS)	[56]
Karakul sheep	40% SS-AF silage TMR	Highest BW, ADG, and DMI	[5]
Fattening lambs	Pelleted TMR + LY (0.8 g/kg)	11% higher ADG (+36 g/d)	[72]
Jersey cows	60% grass hay + 40% concentrate	Higher DMI (12.82 vs. 10.55 kg/day; ↓ FCR (1.36 vs. 1.72)	[82]
Red Chittagong cows	Maize stover-based TMR (50:50)	Higher DMI in block form (T1) vs. mash (T2)	[16]
Holstein cows	MS or IRS TMR + grazing	Night grazing ↑ grass intake (8.53 vs. 5.65 kg DM/d)	[83]
Finnish Ayrshire cows	Grass silage + concentrate (FF1 vs. FF5)	FF1 ↑ DMI (20.9 vs. 19.9 kg/d) in multiparous cows	[84]
Holstein-Friesian cows	High-starch (27.7% DM) TMR	Starch content ↑ DMI and milk yield	[85]
Holstein dairy cows	100% TMR vs. TMR + fresh forage (FF)	No DMI reduction with ≤29% FF; 8% decrease at 47% FF	[58]
Red Chittagong Cows	Maize stover-based TMR (50:50)	Higher DMI vs. conventional feeding	[16]
Korean native goats	TMR with varying peNDF (grinding speeds)	No DMI differences despite reduced peNDF	[76]
Crossbred lambs	Pelleted TMR (PTMR)	Higher ADG (341 vs. 265 g/d) and ADFI (1.86 vs. 1.44 kg/d)	[48]
Comisana lambs	Wheat middlings (WM)-based TMR	Higher final BW (23.5 vs. 21.9 kg) and daily gain (199 vs. 174 g/d)	[73]
Hu sheep lambs	High-grain pelleted TMR (70% concentrate)	No ADG difference vs. high-grain non-pelleted; lower rumen pH	[26]
Aberdeen Angus cattle	60% grass silage + barley (MC TMR)	Highest carcass gain (967 g/d); Best feed conversion (11.1 kg DM/kg gain)	[34]
Crossbred lambs	FTMR with varying oat/alfalfa ratios	AH-300: Higher DMI, ADG, and total weight gain vs. CK and AH-400	[45]
Dorper lambs	Cactus pear + cottonseed cake (20–30%)	No effect on WG/ADG; all treatments met target ADG (200 g/day)	[7]
Beef steers	Hedge lucerne/leucaena TMR	Higher ADG and FCR vs. control (fresh grass + concentrate)	[86]
Dairy cattle (fattening)	Whole crop rice TMR	Higher BW and ADG in mid/late fattening stages	[87]
Goats (barn-fed)	TMR vs. mountainous pasture	ADG doubled in the TMR group	[88]
Sheep	Pelleted vs. unpelleted TMR	Higher feed intake and ADG with pelleted TMR	[46]
Crossbred lambs	Pelleted TMR (PTMR)	↑ ADG (341 vs. 265 g/d) and carcass yield (54.5% vs. 49.4%).	[48]
Holstein-Zebu steers	Grass silage-TMR (STMR)	↑ Early-phase ADG; better FCR in FTMR later.	[61]
Simmental bulls	Multi-silage TMR (MS)	↑ ADG (1.56 vs. 1.30 kg/day); ↓ FCR (10.96 vs. 12.36).	[63]
Angus beef cattle	High-concentrate TMR	↑ DMI but no improvement in ADG or feed efficiency.	[18]
Hanwoo Steers	T70 (70:30 forage/concentrate)	Compensatory growth in late fattening stage; slower initial growth	[66]
Hanwoo Steers	Fermented TMR	Higher DMI (7.17 kg) Increased BW (615.20 kg), improved ADG (0.56 kg)	[89]
Yak	High-energy TMR	Higher ADG (0.87 kg/day vs. −0.17 kg/day in grazing)	[6]
Karakul Sheep	40%SS–60%AF silage TMR	Higher BW, ADG, and DMI (p < 0.05)	[90]
Naemi Lambs	TMR + alfalfa hay (300 g/3 days)	Increased BW and feed conversion ratio	[10]
Boer Goats	TMR + 7.5% intact rapeseed	Reduced feed-to-gain ratio (improved efficiency)	[42]
Sindhi Crossbred	Alkaline-treated straw TMR	Higher ADG (0.69 vs. 0.46 kg/day) & BW (278.8 vs. 258.2 kg)	[14]
Dorper Lambs	Creep feed (18% CP TMR)	Higher weight gain (22.17 vs. 17.83 kg pre-weaning)	[22]
Hanwoo Steers	TMR + Medicinal Plants (30 g/kg)	Improved ADG & feed efficiency	[27]
Simmental Heifers	TMR vs. Free choice	No difference in ADG	[19]
Holstein (Dairy)	pTMR	Balanced nutrient intake, reduced purchased feeds	[91]
Hanwoo Heifers	Italian Ryegrass TMR	No difference in DMI/FCR; improved nitrogen efficiency	[66]
Dairy cows	Apple pomace TMR	Increased LWG	[15]
Sheep	TMR blocks vs. mash	Higher B:C ratio in TMR blocks	[92]
Hanwoo steers	TMRF (fermented feed)	↑ Weight gain (308 kg vs. 284 kg) and feed efficiency (0.16 vs. 0.12).	[13]
Holstein-Friesian	TMR (maize silage + concentrate)	↑ BW gain (0.54 kg/d vs. loss in control).	[93]

BW = body weight; ADG = average daily gain; DMI = dry matter intake; FCR = feed conversion ratio; TMR = total mixed ration; WPCS = whole plant corn silage; CONS = conventional starter; SS-AF = sunflower silage and alfalfa; LY = live yeast; MS = maize silage; IRS = Italian ryegrass silage; FF = fresh forage; peNDF = physically effective neutral detergent fiber; ADFI = average daily feed intake; PTMR = pelleted total mixed ration; FTMR = fermented total mixed ration; WG = weight gain; STMR = silage-based total mixed ration; CP = crude protein; LWG = live weight gain; B:C ratio = benefit-to-cost ratio. ↑ = increase; ↓ = decrease; vs. = versus; % = percent; g/kg = grams per kilogram; kg/day = kilograms per day; g/d = grams per day; DM = dry matter; p < 0.05 = statistically significant difference.

Table 3. Effects of Total Mixed Ration on Growth Performance (BW, ADG, DMI, Feed Efficiency).

Species/Breed	TMR Type/Modification	Summary of Results	Reference
Holstein dairy calves	WPCS-based TMR (CTMR)	No difference in BW, ADG, or feed efficiency vs. starter (CONS)	[56]
Karakul sheep	40% SS-AF silage TMR	Highest BW, ADG, and DMI	[5]
Fattening lambs	Pelleted TMR + LY (0.8 g/kg)	11% higher ADG (+36 g/d)	[72]
Jersey cows	60% grass hay + 40% concentrate	Higher DMI (12.82 vs. 10.55 kg/day; ↓ FCR (1.36 vs. 1.72)	[82]
Red Chittagong cows	Maize stover-based TMR (50:50)	Higher DMI in block form (T1) vs. mash (T2)	[16]
Holstein cows	MS or IRS TMR + grazing	Night grazing ↑ grass intake (8.53 vs. 5.65 kg DM/d)	[83]
Finnish Ayrshire cows	Grass silage + concentrate (FF1 vs. FF5)	FF1 ↑ DMI (20.9 vs. 19.9 kg/d) in multiparous cows	[84]
Holstein-Friesian cows	High-starch (27.7% DM) TMR	Starch content ↑ DMI and milk yield	[85]
Holstein dairy cows	100% TMR vs. TMR + fresh forage (FF)	No DMI reduction with ≤29% FF; 8% decrease at 47% FF	[58]
Red Chittagong Cows	Maize stover-based TMR (50:50)	Higher DMI vs. conventional feeding	[16]
Korean native goats	TMR with varying peNDF (grinding speeds)	No DMI differences despite reduced peNDF	[76]
Crossbred lambs	Pelleted TMR (PTMR)	Higher ADG (341 vs. 265 g/d) and ADFI (1.86 vs. 1.44 kg/d)	[48]
Comisana lambs	Wheat middlings (WM)-based TMR	Higher final BW (23.5 vs. 21.9 kg) and daily gain (199 vs. 174 g/d)	[73]
Hu sheep lambs	High-grain pelleted TMR (70% concentrate)	No ADG difference vs. high-grain non-pelleted; lower rumen pH	[26]
Aberdeen Angus cattle	60% grass silage + barley (MC TMR)	Highest carcass gain (967 g/d); Best feed conversion (11.1 kg DM/kg gain)	[34]
Crossbred lambs	FTMR with varying oat/alfalfa ratios	AH-300: Higher DMI, ADG, and total weight gain vs. CK and AH-400	[45]
Dorper lambs	Cactus pear + cottonseed cake (20–30%)	No effect on WG/ADG; all treatments met target ADG (200 g/day)	[7]
Beef steers	Hedge lucerne/leucaena TMR	Higher ADG and FCR vs. control (fresh grass + concentrate)	[86]
Dairy cattle (fattening)	Whole crop rice TMR	Higher BW and ADG in mid/late fattening stages	[87]
Goats (barn-fed)	TMR vs. mountainous pasture	ADG doubled in the TMR group	[88]
Sheep	Pelleted vs. unpelleted TMR	Higher feed intake and ADG with pelleted TMR	[46]
Crossbred lambs	Pelleted TMR (PTMR)	↑ ADG (341 vs. 265 g/d) and carcass yield (54.5% vs. 49.4%).	[48]
Holstein-Zebu steers	Grass silage-TMR (STMR)	↑ Early-phase ADG; better FCR in FTMR later.	[61]
Simmental bulls	Multi-silage TMR (MS)	↑ ADG (1.56 vs. 1.30 kg/day); ↓ FCR (10.96 vs. 12.36).	[63]
Angus beef cattle	High-concentrate TMR	↑ DMI but no improvement in ADG or feed efficiency.	[18]
Hanwoo Steers	T70 (70:30 forage/concentrate)	Compensatory growth in late fattening stage; slower initial growth	[66]
Hanwoo Steers	Fermented TMR	Higher DMI (7.17 kg) Increased BW (615.20 kg), improved ADG (0.56 kg)	[89]
Yak	High-energy TMR	Higher ADG (0.87 kg/day vs. −0.17 kg/day in grazing)	[6]
Karakul Sheep	40%SS–60%AF silage TMR	Higher BW, ADG, and DMI (p < 0.05)	[90]
Naemi Lambs	TMR + alfalfa hay (300 g/3 days)	Increased BW and feed conversion ratio	[10]
Boer Goats	TMR + 7.5% intact rapeseed	Reduced feed-to-gain ratio (improved efficiency)	[42]
Sindhi Crossbred	Alkaline-treated straw TMR	Higher ADG (0.69 vs. 0.46 kg/day) & BW (278.8 vs. 258.2 kg)	[14]
Dorper Lambs	Creep feed (18% CP TMR)	Higher weight gain (22.17 vs. 17.83 kg pre-weaning)	[22]
Hanwoo Steers	TMR + Medicinal Plants (30 g/kg)	Improved ADG & feed efficiency	[27]
Simmental Heifers	TMR vs. Free choice	No difference in ADG	[19]
Holstein (Dairy)	pTMR	Balanced nutrient intake, reduced purchased feeds	[91]
Hanwoo Heifers	Italian Ryegrass TMR	No difference in DMI/FCR; improved nitrogen efficiency	[66]
Dairy cows	Apple pomace TMR	Increased LWG	[15]
Sheep	TMR blocks vs. mash	Higher B:C ratio in TMR blocks	[92]
Hanwoo steers	TMRF (fermented feed)	↑ Weight gain (308 kg vs. 284 kg) and feed efficiency (0.16 vs. 0.12).	[13]
Holstein-Friesian	TMR (maize silage + concentrate)	↑ BW gain (0.54 kg/d vs. loss in control).	[93]

BW = body weight; ADG = average daily gain; DMI = dry matter intake; FCR = feed conversion ratio; TMR = total mixed ration; WPCS = whole plant corn silage; CONS = conventional starter; SS-AF = sunflower silage and alfalfa; LY = live yeast; MS = maize silage; IRS = Italian ryegrass silage; FF = fresh forage; peNDF = physically effective neutral detergent fiber; ADFI = average daily feed intake; PTMR = pelleted total mixed ration; FTMR = fermented total mixed ration; WG = weight gain; STMR = silage-based total mixed ration; CP = crude protein; LWG = live weight gain; B:C ratio = benefit-to-cost ratio. ↑ = increase; ↓ = decrease; vs. = versus; % = percent; g/kg = grams per kilogram; kg/day = kilograms per day; g/d = grams per day; DM = dry matter; p < 0.05 = statistically significant difference.

4.4. Effects of Total Mixed Ration on Milk Yield and Composition

Total Mixed Ration has been widely adopted in ruminant nutrition due to its ability to provide a balanced diet, ensuring consistent nutrient intake and improving milk production (Table 4). Several studies have demonstrated that TMR enhances milk yield and modifies milk composition, primarily due to its well-balanced formulation of concentrates, roughages, and additives. Sunarso et al. [79] reported that dairy cows fed TMR exhibited higher milk yields compared to those on conventional feeding systems, likely due to the optimized energy and protein balance in TMR. Similarly, Lailer et al. [80] found that TMR improves feed efficiency, leading to increased milk production by maintaining stable rumen fermentation conditions. The nutritional quality of TMR also influences the composition of milk. A higher concentrate proportion in TMR (30–80%) elevates energy density, which directly impacts milk fat and protein content [26]. However, excessive concentrate inclusion may reduce fiber digestibility, negatively affecting milk fat synthesis due to altered rumen pH and volatile fatty acid production [53]. The fatty acid composition of milk can also be influenced by the high proportion of concentrate in TMR, particularly by reducing the levels of health-promoting fatty acids like vaccenic, rumenic, and alpha-linolenic acids, and increasing the formation of trans-10 fatty acids associated with milk fat depression in dairy ruminants [94].

Conversely, adequate roughage inclusion, such as maize silage or alfalfa hay, ensures sufficient fiber intake for rumen health, promoting optimal milk composition [45]. The inclusion of fermented components, such as molasses and microbial additives like Saccharomyces cerevisiae, further enhances nutrient availability, leading to improved milk yield and quality [24,42].

Moreover, the physical characteristics of TMR, such as particle size, play a crucial role in digestion and milk production. Smaller particle sizes increase feed intake and rumen turnover but may reduce fiber digestibility, potentially lowering milk fat content [54]. On the other hand, proper fermentation of TMR, as seen in FTMR, enhances nutrient preservation and digestibility, leading to better milk yield and composition [52]. The addition of yeast culture in TMR has also been shown to stabilize rumen pH and improve fiber degradation, further supporting milk production [24]. Despite these benefits, challenges such as feed sorting and SARA may arise, particularly with high-grain TMR formulations [17,84]. Proper management, including optimal roughage-to-concentrate ratios and the use of fermentation enhancers, is essential to mitigate these risks. Additionally, environmental factors, such as storage conditions, can affect TMR quality, emphasizing the need for proper ensiling techniques to maintain nutrient integrity [42,61].

The adoption of TMR in dairy farming significantly improves milk yield and composition by ensuring balanced nutrient intake, enhancing rumen function, and optimizing feed efficiency. However, careful formulation and management are necessary to prevent metabolic disorders and maintain consistent milk quality. Farmers should consider incorporating fermented additives and maintaining proper roughage levels to maximize the benefits of TMR while minimizing potential drawbacks. This approach not only enhances productivity but also supports sustainable dairy farming practices.

Table 4. Effects of Total Mixed Ration on Milk Yield and Composition.

Species/Breed	TMR Type/Intervention	Summary of Results	Reference
Lactating Holstein cows	Hay-based TMR (DM-adjusted)	Increased milk yield (26.99 → 27.29 kg/d)	[23]
Jersey cows	60% grass hay + 40% concentrate	↑ Milk yield (9.57 vs. 6.23 kg/day	[82]
Red Chittagong cows	Maize stover-based TMR	↑ Milk yield (T1: 3.6 L/d; T2: 3.49 L/d vs. T0: 3.35 L/d)	[16]
Holstein cows	TMR vs. separate feeding	TMR ↑ milk yield (34.4 vs. 32.7 kg/d in R2X/R4X)	[95]
Finnish Ayrshire cows	Once vs. 5× daily feeding	No difference (32.8 kg/d ECM FF1 vs. 32.5 kg/d FF5)	[84]
Jersey cows	60% grass hay + 40% concentrate	No difference in fat %; No difference in protein %	[82]
Red Chittagong cows	Maize stover-based TMR	↑ Fat % in T1/T2	[16]
Holstein cows	TMR vs. separate feeding	Separate feeding ↓ fat % (2.14–2.31% vs. 3.31%)	[95]
Dairy ewes	Wheat middling-based TMR	↑ Fat % and yield	[73]
Holstein (Dairy)	Confinement TMR	Highest yield (10,000 kg/cow)	[91]
Holstein (Dairy)	Pasture + Concentrate (PC)	Lowest yield (7500 kg/cow)	[91]
Holstein (Dairy)	Full TMR (confinement)	38.1 kg/day > pTMR (32.0 kg/day) > PC (28.5 kg/day)	[12]
Danish Black/White	Multi-group TMR	Higher yield at high feed levels vs. single-group TMR	[96]
Dairy cows	Apple pomace TMR	Increased milk yield	[15]
Holstein (Dairy)	Pelleted TMR	Higher milk protein (3.38% vs. 3.16%), lower fat	[65]
Holstein-Friesian	TMR (maize silage + concentrate)	↑ Milk yield (29.5 kg/d vs. 21.1 kg/d)	[93]
Crossbred cows	FTMR with 25% peNDF	↑ Milk fat % due to ↑ acetate	[75]
Aosta Red Pied cows	TMR vs. separate feeding	No difference in protein %	[97]
Holstein cows	TMR + night grazing	↑ PUFA (CLA, VA, ALA) in milk	[98]
Holstein dairy cows	100% TMR vs. TMR + FF	8.5% higher yield in 100% TMR; ↑ UFA in 50% FF	[58]
Red Chittagong Cows	Maize stover-based TMR	Higher milk yield, fat, and SNF vs. control	[16]
Holstein cows	Cracked cottonseed in FTMR	↑ C18:2 (linoleic acid) in milk	[99]
Dairy cows	Pasture vs. TMR (maize silage + concentrates)	TMR: 33% higher milk yield; No difference in fat/SCC	[100]
Buffaloes	Brewers’ grain + rice straw TMR	Higher milk yield at 1.2% supplement No difference in composition	[101]
Dairy cows	Apple pomace TMR	Increased milk yield and protein; reduced lactose	[15]

TMR = total mixed ration, DM = dry matter, d = day, L = liter, ECM = energy-corrected milk, FF = feeding frequency, pTMR = partial TMR, PC = pasture + concentrate, FTMR = fermented TMR, peNDF = physically effective neutral detergent fiber, PUFA = polyunsaturated fatty acids, CLA = conjugated linoleic acid, VA = vaccenic acid, ALA = α-linolenic acid, FF = fresh forage, UFA = unsaturated fatty acids, SNF = solids-not-fat, SCC = somatic cell count. R2X/R4X refers to different feeding frequencies or rationing schedules for the “separate feeding” group, ↑ = increased, ↓ = decreased, → = change from baseline, vs. = versus, % = percent, kg = kilogram, g = gram, > = greater than.

Table 4. Effects of Total Mixed Ration on Milk Yield and Composition.

Species/Breed	TMR Type/Intervention	Summary of Results	Reference
Lactating Holstein cows	Hay-based TMR (DM-adjusted)	Increased milk yield (26.99 → 27.29 kg/d)	[23]
Jersey cows	60% grass hay + 40% concentrate	↑ Milk yield (9.57 vs. 6.23 kg/day	[82]
Red Chittagong cows	Maize stover-based TMR	↑ Milk yield (T1: 3.6 L/d; T2: 3.49 L/d vs. T0: 3.35 L/d)	[16]
Holstein cows	TMR vs. separate feeding	TMR ↑ milk yield (34.4 vs. 32.7 kg/d in R2X/R4X)	[95]
Finnish Ayrshire cows	Once vs. 5× daily feeding	No difference (32.8 kg/d ECM FF1 vs. 32.5 kg/d FF5)	[84]
Jersey cows	60% grass hay + 40% concentrate	No difference in fat %; No difference in protein %	[82]
Red Chittagong cows	Maize stover-based TMR	↑ Fat % in T1/T2	[16]
Holstein cows	TMR vs. separate feeding	Separate feeding ↓ fat % (2.14–2.31% vs. 3.31%)	[95]
Dairy ewes	Wheat middling-based TMR	↑ Fat % and yield	[73]
Holstein (Dairy)	Confinement TMR	Highest yield (10,000 kg/cow)	[91]
Holstein (Dairy)	Pasture + Concentrate (PC)	Lowest yield (7500 kg/cow)	[91]
Holstein (Dairy)	Full TMR (confinement)	38.1 kg/day > pTMR (32.0 kg/day) > PC (28.5 kg/day)	[12]
Danish Black/White	Multi-group TMR	Higher yield at high feed levels vs. single-group TMR	[96]
Dairy cows	Apple pomace TMR	Increased milk yield	[15]
Holstein (Dairy)	Pelleted TMR	Higher milk protein (3.38% vs. 3.16%), lower fat	[65]
Holstein-Friesian	TMR (maize silage + concentrate)	↑ Milk yield (29.5 kg/d vs. 21.1 kg/d)	[93]
Crossbred cows	FTMR with 25% peNDF	↑ Milk fat % due to ↑ acetate	[75]
Aosta Red Pied cows	TMR vs. separate feeding	No difference in protein %	[97]
Holstein cows	TMR + night grazing	↑ PUFA (CLA, VA, ALA) in milk	[98]
Holstein dairy cows	100% TMR vs. TMR + FF	8.5% higher yield in 100% TMR; ↑ UFA in 50% FF	[58]
Red Chittagong Cows	Maize stover-based TMR	Higher milk yield, fat, and SNF vs. control	[16]
Holstein cows	Cracked cottonseed in FTMR	↑ C18:2 (linoleic acid) in milk	[99]
Dairy cows	Pasture vs. TMR (maize silage + concentrates)	TMR: 33% higher milk yield; No difference in fat/SCC	[100]
Buffaloes	Brewers’ grain + rice straw TMR	Higher milk yield at 1.2% supplement No difference in composition	[101]
Dairy cows	Apple pomace TMR	Increased milk yield and protein; reduced lactose	[15]

TMR = total mixed ration, DM = dry matter, d = day, L = liter, ECM = energy-corrected milk, FF = feeding frequency, pTMR = partial TMR, PC = pasture + concentrate, FTMR = fermented TMR, peNDF = physically effective neutral detergent fiber, PUFA = polyunsaturated fatty acids, CLA = conjugated linoleic acid, VA = vaccenic acid, ALA = α-linolenic acid, FF = fresh forage, UFA = unsaturated fatty acids, SNF = solids-not-fat, SCC = somatic cell count. R2X/R4X refers to different feeding frequencies or rationing schedules for the “separate feeding” group, ↑ = increased, ↓ = decreased, → = change from baseline, vs. = versus, % = percent, kg = kilogram, g = gram, > = greater than.

4.5. Carcass Traits and Meat Quality in Livestock Fed Total Mixed Ration

The evaluation of carcass traits and meat quality in livestock fed TMR reveals significant insights into the nutritional and physiological impacts of this feeding strategy. Table 5 summarizes data from various studies that demonstrate how TMR formulations influence carcass yield and meat characteristics, with variations observed across species, breeds, and TMR compositions. Khy et al. [86] reported that beef steers fed a hedge lucerne-based TMR showed no difference in dressing percentage but had higher chilled carcass weight compared to controls. This suggests that TMR enhances muscle deposition without altering fat distribution, likely due to balanced nutrient intake. Similarly, Huuskonen et al. [34] found that Aberdeen Angus cattle fed a TMR with 60% grass silage achieved superior carcass gain (967 g/d) and conformation, attributed to the optimal fiber-to-energy ratio promoting lean tissue growth.

Marbling, a critical determinant of meat quality [102], was significantly improved in beef cattle fed whole crop rice-based TMR [87]. The higher marbling score indicates enhanced intramuscular fat deposition, which is linked to improved juiciness and flavor [1,103]. However, the absence of significant changes in rib eye area suggests that TMR primarily influences fat metabolism rather than muscle hypertrophy. This is corroborated by Ku et al. [66], who observed that Hanwoo steers fed a 70:30 forage-to-concentrate TMR had the highest intramuscular fat content, alongside favorable shear strength and drip loss. Moreover, the inclusion of fermented components in TMR, such as molasses and Saccharomyces cerevisiae, has been shown to positively influence meat quality by improving tenderness and reducing shear force [42]. This is attributed to enhanced protein metabolism and reduced oxidative stress in muscle tissues, which preserves meat structure during post-mortem aging [4]. Additionally, the use of wet brewers’ grains as a partial replacement for traditional roughages contributes to higher IMF content, which is associated with improved juiciness and flavor [1,55]. However, excessive concentrate inclusion in TMR may lead to rapid fat deposition, which, while increasing marbling, could also result in undesirable fatty acid profiles if not adequately balanced with fiber sources [53]. The lower n-6/n-3 fatty acid ratio in this group further highlights the role of forage-rich TMR in promoting healthier lipid profiles, as omega-3 fatty acids are associated with cardiovascular benefits in consumers [55,103].

In small ruminants, Wang et al. [90] demonstrated that Karakul sheep fed a 40% sweet sorghum-alfalfa silage TMR exhibited increased carcass weight and subcutaneous fat thickness, and improved meat quality parameters such as water-holding capacity (WHC) and CP content. The enhanced WHC is particularly important, as it results in better moisture retention during cooking, which contributes to tenderness [4,55]. Furthermore, Zhang et al. [48] observed that crossbred lambs fed pelleted TMR (PTMR) had higher carcass yield (54.5% vs. 49.4%) compared to unpelleted TMR, likely due to improved feed efficiency and nutrient utilization. The scientific basis for these findings lies in the ability of TMR to synchronize nutrient release in the rumen, optimizing microbial fermentation and energy availability. For instance, Liu et al. [6] reported that yaks fed a high-energy TMR exhibited a 106.43% increase in carcass weight and 57.52% dressing percentage, alongside improved tenderness and reduced cooking loss. These outcomes are driven by the TMR’s balanced energy-to-protein ratio, which supports muscle hypertrophy and fat deposition while minimizing metabolic waste.

On the other hand, Cooke et al. [29] found that beef heifers fed high-concentrate TMR had higher marbling but also an elevated n-6/n-3 ratio, which is less desirable from a human health perspective [3,55]. This highlights the importance of carefully formulating TMR to strike a balance between meat quality and nutritional value. The study by Horcada et al. [20] on Retinta cattle further emphasized that TMR with maize silage increased PUFA content (18.8% vs. 14.3%) and n-3 PUFA (0.47% vs. 0.35%) compared to a high-concentrate diet, suggesting that forage inclusion can enhance the nutritional quality of meat [1,2]. TMR ensures consistent nutrient intake, reducing selective feeding and metabolic disorders. For example, Iraira et al. [19] found that Simmental heifers fed TMR had no differences in meat tenderness compared to free-choice diets. However, the TMR group exhibited longer rumination times, indicating better rumen health. This aligns with the findings of Alhidary et al. [10], where Naemi lambs fed TMR supplemented with alfalfa hay showed improved meat color and reduced shear force, reflecting enhanced muscle fiber structure.

The impact of TMR on meat quality extends to its oxidative stability and shelf life. FTMR has been reported to enhance the antioxidant capacity of meat due to the presence of beneficial microbial metabolites, such as short-chain fatty acids and bioactive peptides [89,90]. This is particularly evident when FTMR includes Lactobacillus acidophilus and Bacillus subtilis, which modulate rumen microbiota and reduce lipid oxidation in meat [43]. On the other hand, improper TMR processing, such as inadequate particle size reduction, may lead to inconsistent nutrient absorption, negatively affecting meat texture and color stability [54]. From a practical standpoint, the adoption of TMR in livestock farming ensures consistent growth performance and meat quality; however, it requires careful formulation to avoid metabolic disorders, such as SARA. The inclusion of NDF at optimal levels is essential for maintaining rumen health and preventing excessive fat deposition, which can compromise carcass yield [6,78,86]. For farmers, TMR offers a practical solution to optimize growth and carcass quality while reducing feed wastage. For scientists, the data highlight the importance of ingredient selection and processing (e.g., pelleting, fermentation) in modulating meat quality. For instance, Chen et al. [42] demonstrated that Boer goats fed TMR with 7.5% intact rapeseed had altered fatty acid profiles, including higher linolenic acid and lower palmitic acid, which are beneficial for human health. Additionally, Santos-Silva et al. [104] found that feeding finishing young bulls a TMR with high forage content supplemented with sunflower seeds (10% DM) improved the meat fatty acid profile by increasing the levels of alpha-linolenic, vaccenic, and rumenic acids, while reducing the proportion of t10–18:1, a fatty acid that can be detrimental to human health and is often present in meat from animals fed high-concentrate diets [105].

Table 5. Effects of Total Mixed Ration on Carcass Traits and Meat Quality.

Species/Breed	TMR Type/Modification	Summary of Results	Reference
Beef steers	Hedge lucerne TMR	No difference in dressing %; Higher chilled carcass weight	[86]
Aberdeen Angus cattle	MC TMR (60% grass silage)	Best carcass gain (967 g/d) and conformation	[34]
Beef cattle	Whole crop rice TMR	Higher marbling score; No difference in carcass weight/rib eye area	[87]
Karakul sheep	40% SS-AF silage TMR	↑ Carcass weight, subcutaneous fat; Improved WHC, CP, EE, and shear force	[90]
Crossbred lambs	Pelleted TMR (PTMR)	Higher carcass yield (54.5% vs. 49.4%)	[48]
Comisana lambs	WM-based TMR	Higher cold-carcass dressing (10.5 vs. 9.7 kg)	[73]
Crossbred lambs	FTMR (AH-300)	Higher backfat thickness, intramuscular fat; Lower shear force	[45]
Hanwoo Steers	T50 (50:50 forage/concentrate)	Comparable carcass weight to control; higher IMF	[66]
Hanwoo Steers	FTMR	Higher marbling score (5.63 vs. 3.13), fat thickness (13.25 mm)	[89]
Yak	High-energy TMR	Increased carcass weight (106.43%), dressing percentage (57.52%)	[6]
Karakul Sheep	40%SS–60%AF silage TMR	Higher carcass weight and subcutaneous fat thickness.	[90]
Beef Cattle	TMR (pre-mixed)	Higher carcass weight (279.5 kg vs. 268.6 kg in control)	[29]
Hanwoo Steers	FTMR	Improved tenderness, juiciness, crude fat (18.39%); no pH/cooking loss differences	[89]
Yak	High-energy TMR	Improved tenderness (↓ shear force), reduced cooking loss (↓ 7.28%)	[6]
Karakul Sheep	40%SS–60%AF silage TMR	Improved WHC, CP, EE; reduced shear force	[90]
Naemi Lambs	TMR + alfalfa hay	Improved meat color (L*, a*, b*); reduced shear force	[10]
Beef Cattle (Retinta)	Maize silage TMR	Higher PUFA (18.8% vs. 14.3%) and n-3 PUFA (0.47% vs. 0.35%)	[20]
Boer Goats	TMR + 7.5% intact rapeseed	↑ Linolenic acid, eicosenoic acid; ↓ palmitic acid	[42]
Beef Cattle	High-concentrate TMR	Higher 18:1, lower 18:3; increased n-6/n-3 ratio (3.83 vs. 2.72)	[29]
Hanwoo Steers	TMR + Medicinal Plants	Improved meat quality grade	[27]
Simmental Heifers	TMR	No difference in meat tenderness	[19]

TMR = total mixed ration, SS-AF = sweet sorghum-alfalfa, WHC = water-holding capacity, CP = crude protein, EE = ether extract, PTMR = pelleted TMR, WM = wheat middlings, FTMR = fermented total mixed ration, MC = mixed concentrate; ↑ = increased, ↓ = decreased % = percentage, kg = kilograms, g/d = grams per day; Carcass yield = (carcass weight/live weight) × 100; Cold-carcass dressing = carcass weight after chilling; Marbling score = intramuscular fat content grading; Shear force = tenderness measurement (lower values indicate more tender meat); Rib eye area = cross-sectional area of longissimus dorsi muscle; Dressing % = (hot carcass weight/live weight) × 100.

Table 5. Effects of Total Mixed Ration on Carcass Traits and Meat Quality.

Species/Breed	TMR Type/Modification	Summary of Results	Reference
Beef steers	Hedge lucerne TMR	No difference in dressing %; Higher chilled carcass weight	[86]
Aberdeen Angus cattle	MC TMR (60% grass silage)	Best carcass gain (967 g/d) and conformation	[34]
Beef cattle	Whole crop rice TMR	Higher marbling score; No difference in carcass weight/rib eye area	[87]
Karakul sheep	40% SS-AF silage TMR	↑ Carcass weight, subcutaneous fat; Improved WHC, CP, EE, and shear force	[90]
Crossbred lambs	Pelleted TMR (PTMR)	Higher carcass yield (54.5% vs. 49.4%)	[48]
Comisana lambs	WM-based TMR	Higher cold-carcass dressing (10.5 vs. 9.7 kg)	[73]
Crossbred lambs	FTMR (AH-300)	Higher backfat thickness, intramuscular fat; Lower shear force	[45]
Hanwoo Steers	T50 (50:50 forage/concentrate)	Comparable carcass weight to control; higher IMF	[66]
Hanwoo Steers	FTMR	Higher marbling score (5.63 vs. 3.13), fat thickness (13.25 mm)	[89]
Yak	High-energy TMR	Increased carcass weight (106.43%), dressing percentage (57.52%)	[6]
Karakul Sheep	40%SS–60%AF silage TMR	Higher carcass weight and subcutaneous fat thickness.	[90]
Beef Cattle	TMR (pre-mixed)	Higher carcass weight (279.5 kg vs. 268.6 kg in control)	[29]
Hanwoo Steers	FTMR	Improved tenderness, juiciness, crude fat (18.39%); no pH/cooking loss differences	[89]
Yak	High-energy TMR	Improved tenderness (↓ shear force), reduced cooking loss (↓ 7.28%)	[6]
Karakul Sheep	40%SS–60%AF silage TMR	Improved WHC, CP, EE; reduced shear force	[90]
Naemi Lambs	TMR + alfalfa hay	Improved meat color (L*, a*, b*); reduced shear force	[10]
Beef Cattle (Retinta)	Maize silage TMR	Higher PUFA (18.8% vs. 14.3%) and n-3 PUFA (0.47% vs. 0.35%)	[20]
Boer Goats	TMR + 7.5% intact rapeseed	↑ Linolenic acid, eicosenoic acid; ↓ palmitic acid	[42]
Beef Cattle	High-concentrate TMR	Higher 18:1, lower 18:3; increased n-6/n-3 ratio (3.83 vs. 2.72)	[29]
Hanwoo Steers	TMR + Medicinal Plants	Improved meat quality grade	[27]
Simmental Heifers	TMR	No difference in meat tenderness	[19]

TMR = total mixed ration, SS-AF = sweet sorghum-alfalfa, WHC = water-holding capacity, CP = crude protein, EE = ether extract, PTMR = pelleted TMR, WM = wheat middlings, FTMR = fermented total mixed ration, MC = mixed concentrate; ↑ = increased, ↓ = decreased % = percentage, kg = kilograms, g/d = grams per day; Carcass yield = (carcass weight/live weight) × 100; Cold-carcass dressing = carcass weight after chilling; Marbling score = intramuscular fat content grading; Shear force = tenderness measurement (lower values indicate more tender meat); Rib eye area = cross-sectional area of longissimus dorsi muscle; Dressing % = (hot carcass weight/live weight) × 100.

5. Methane Emission and Environmental Impact

5.1. Enteric Methane Emissions

Methane (CH₄) is a potent greenhouse gas, and agriculture is a significant contributor to global CH₄ emissions, largely due to enteric fermentation in ruminants. Globally, agriculture accounts for 52% of CH₄ emissions, with 80 million tonnes of this being a product of enteric fermentation in ruminants [93]. This represents a considerable energy loss for the animal, typically ranging from 2% to 12% of gross energy intake [93,106]. Therefore, strategies to mitigate enteric CH₄ are crucial for both environmental sustainability and animal productivity. Methane production is highly dependent on the quantity of feed consumed and the composition of the diet [2,93,106]. Furthermore, methane emissions from ruminants are primarily driven by rumen microbial fermentation, where methanogenic archaea utilize hydrogen (H₂) and carbon dioxide (CO₂) to produce CH₄ [3]. Studies indicate that TMR formulations influence methane production by altering the rumen environment, revealing complex interactions between diet composition and microbial activity. Metzler-Zebeli et al. [67] found that TMR with a high concentrate content (490 g/kg DM) resulted in rapid gas production during the early fermentation stage (2–4 h), followed by a decline in gas production. This suggests that associative effects between feed components (e.g., grass silage, cereal concentrate) may transiently stimulate microbial activity, increasing methane output before stabilizing. Particle size also plays a critical role. Tafaj et al. [69] demonstrated that reducing TMR particle size (5.5 mm vs. 25 mm) lowered rumen pH and shifted VFA profiles toward propionate, which competitively reduces H₂ availability for methanogenesis. Smaller particles increase the surface area for microbial degradation, accelerating fermentation and potentially lowering methane yield due to improved energy utilization. Recently Jairath et al. [51] found significantly lower methane production under in vitro conditions when maize grain in the concentrate portion of TMR was replaced with fermented agro-waste at a 32% level. In contrast, feeding high-forage, low-starch TMR with 10% DM of sunflower seeds to finishing crossbred bulls increased digestive CH₄ emissions compared to a concentrate finishing diet. However, the carbon footprint did not differ between diets, 6.63 vs. 6.51 kgCO₂e/kg LWG [104]).

5.2. Role of Rumen Inoculum and Microbial Populations

The source of rumen inoculum has a significant impact on methane emissions. Metzler-Zebeli et al. [67] observed higher gas production with particle-associated liquid (PAL) inoculum compared to free rumen liquid (FRL), likely due to greater microbial density and fibrolytic activity. This implies that in vitro studies using only FRL may underestimate methane potential, as PAL microbes are more representative of the rumen mat, where fiber degradation is most active. FTMR has shown promise in mitigating methane emissions. Cao et al. [30] reported a 25% reduction in methane when sheep were fed FTMR, attributed to increased propionate production from lactate fermentation. This metabolic shift consumes H₂, diverting it away from methanogenesis (the production of methane). Similarly, Li et al. [46] found that FTMR inoculated with lactic acid bacteria reduced methane by 15–20% in vitro, correlating with higher propionate and butyrate levels.

5.3. Impact of Dietary Composition

High-concentrate TMR typically reduces methane per unit of feed intake but may increase total emissions due to higher dry matter intake. O’Neill et al. [93] reported that dairy cows fed TMR produced more methane (397 g/day) than pasture-fed cows (251 g/day), despite higher milk yields. This reflects the trade-off between productivity and environmental impact, where energy-dense TMR improves efficiency but may elevate absolute emissions. Conversely, high-fiber TMR can lower methane but risk impairing digestibility. Zhong et al. [65] noted that pelleted TMR improved feed efficiency but had no significant effect on methane, likely due to balanced fiber and starch ratios. Including additives like yeast culture [24] or tannins [107] in TMR further reduces methane by inhibiting methanogens or altering fermentation pathways. Methane mitigation is crucial for environmental sustainability, and TMR strategies must simultaneously maintain optimal rumen health [65]. High-concentrate TMR, particularly in pelleted forms, significantly increases the risk of ruminal acidosis, specifically SARA. This occurs because such formulations can lead to excessive lactic acid production, thereby lowering rumen pH (e.g., to 5.58 compared to 5.87 in separate feeding) and impairing the activity of fiber-digesting microbes [17]. Conversely, fermented Total Mixed Ration offers a promising solution by stabilizing rumen pH and simultaneously mitigating methane emissions. FTMR achieves pH stability by promoting the growth of lactate-utilizing bacteria, such as Megasphaera elsdenii, which convert lactic acid into propionate [30,44]. This metabolic shift not only enhances energy availability for the animal but also consumes hydrogen, thereby diverting it away from methanogenesis, leading to a reported 15–25% reduction in methane production. This dual benefit of FTMR, supporting rumen function while reducing environmental impact, makes it a viable strategy for sustainable livestock production [44]. Therefore, sustainable TMR strategies necessitate careful formulation and management to strike a balance between methane reduction, feed efficiency, and rumen stability to meet both environmental and production goals.

5.4. Feed Utilisation

The environmental implications of TMR are closely linked to its composition, processing methods, and efficiency in nutrient utilization. Scientific evidence suggests that TMR reduces feed wastage and improves rumen fermentation stability, which in turn enhances productivity while mitigating methane emissions [17]. This is particularly significant given that methane is a potent greenhouse gas and optimizing feed efficiency can contribute to more sustainable livestock production. One of the key environmental benefits of TMR lies in its ability to incorporate agricultural by-products such as wet brewers’ grains (WBG) and rice straw, reducing reliance on conventional feedstuffs while minimizing waste [45,90]. Furthermore, FTMR has been shown to improve aerobic stability and nutrient retention, which reduces spoilage and further enhances [42]. The inclusion of microbial additives, such as Saccharomyces cerevisiae and Lactobacillus acidophilus, in FTMR not only improves fibre degradation but also stabilises rumen pH, resulting in more efficient digestion and lower methane output [24]. These findings suggest that TMR, particularly when fermented, can play a vital role in reducing the carbon footprint of ruminant production systems (

Table 6. Environmental Impact of Feeding Total Mixed Ration to Ruminants.

Species/Breed	TMR Type/Modification	Summary of Results	Reference
Brown Swiss cows	TMR + Saccharomyces cerevisiae (CE)	CE (0.6 mg/g DM) increased CH4; LC had no effect.	[68]
Suffolk sheep	Fermented TMR (FTMR)	25% lower CH4 compared to the control due to a propionate shift.	[30,44]
Holstein steers	High-concentrate TMR	Higher CH4 (138.5 L/day) vs. separate feeding (118.2 L/day).	[9]
Holstein-Friesian	TMR (maize silage + concentrate)	Higher CH4 (397 g/d) vs. grass diet (251 g/d).	[93]
Holstein (Dairy)	pTMR	Lower N leaching (21 kg/ha) but higher volatilization (98 kg/ha)	[91]
Holstein (Dairy)	Automatic TMR (AFS)	67.5% lower CO2e emissions	[108]
Holstein (Dairy)	Pasture + Concentrate	Highest N volatilization (116 kg/ha)	[91]
Aberdeen Angus	MC TMR (60% grass silage)	Lowest GWP (19.1 kg CO2 eq/kg beef); best feed conversion	[34]

TMR = total mixed ration; CE = Saccharomyces cerevisiae; DM = dry matter; LC = low concentrate; FTMR = fermented total mixed ration; pTMR = precision total mixed ration; AFS = automatic feeding system; MC = medium concentrate; GWP = global warming potential. CH₄ = methane; CO₂ = carbon dioxide; CO₂e = carbon dioxide equivalent; N = nitrogen; L/day = liters per day; g/d = grams per day; kg/ha = kilograms per hectare; kg CO₂ eq/kg beef = kilograms of carbon dioxide equivalent per kilogram of beef.

).

6. Mycotoxin Contamination and Mitigation Costs

Mycotoxin contamination in TMR poses significant risks to ruminant health and productivity, with scientific evidence linking it to poor feed quality, storage conditions, and ingredient sourcing (Table 7). Mycotoxins are toxic secondary metabolites produced by fungi, primarily Aspergillus, Fusarium, and Penicillium species, which can proliferate in feed components under favorable conditions [43].

6.1. Mycotoxins in Total Mixed Ration

The presence of mycotoxins in TMR is particularly concerning because TMR combines multiple ingredients, increasing the likelihood of contamination if even one component is compromised. One of the primary reasons for mycotoxin contamination in TMR is the inclusion of mold-infected roughages such as maize silage, rice straw, and alfalfa hay [45]. These forages are highly susceptible to fungal growth, especially when harvested or stored at high moisture levels [25]. Moreover, the fermentation process in ETMR does not always eliminate mycotoxins, as some fungal strains remain viable and continue to produce toxins even under anaerobic conditions [42]. This is further exacerbated when wet by-products, such as brewers’ grains, are incorporated, as improper drying or storage can introduce additional fungal contamination [37,38].

The primary source of mycotoxins in TMR is often maize silage, which is highly susceptible to fungal colonization. Cogan et al. [109] reported that 90% of maize silage-based TMR samples contained deoxynivalenol (DON) and zearalenone (ZON). These are toxic secondary metabolites produced by fungi, and their presence in TMR, particularly in maize silage-based TMR, is a significant concern. In contrast, grass silage-based TMR showed no detectable mycotoxins. This contrast highlights the role of feed composition in contamination risk. Additionally, González-Jartín et al. [110] observed that fumonisins (74%), DON (42.5%), and ZEN (39.1%) were the most prevalent mycotoxins in maize silage TMR, with co-occurrence increasing health risks due to potential synergistic effects. Rodríguez-Blanco et al. [111] further noted that 58% of TMR samples were contaminated with Fusarium-derived mycotoxins, emphasizing the need for rigorous feed quality control. Sultana et al. [112] found that 100% of TMR samples tested were contaminated with aflatoxin B1 (AFB1) and ochratoxin A (OTA), exceeding European Union regulatory limits. AFB1, a potent hepatotoxin, was detected at 30 ng/g, while OTA averaged 48.5 ng/g, raising concerns about chronic exposure in dairy cattle. Furthermore, 50% of samples contained zearalenone (ZON) at 700 ng/g, a mycotoxin linked to reproductive disorders. These findings reveal the widespread nature of mycotoxin contamination in TMR, particularly in regions with suboptimal feed storage conditions. The implications for animal health are profound. AFB1 is metabolized in the liver to aflatoxin M1 (AFM1), which is excreted in milk and poses risks to both livestock and consumers Mycotoxins like DON disrupt gut integrity and immune function, while ZEN interferes with reproductive hormones, leading to infertility and reduced milk yield. Even at subclinical levels, chronic exposure can impair growth, feed efficiency, and overall herd health [112]. The quality of TMR is further compromised by microbial and endotoxin contamination.

Table 7. Mycotoxin Contamination in Total Mixed Ration for Ruminants.

Species/Breed	TMR Type/Intervention	Summary of Results	Reference
Dairy cattle	Mycotoxin-contaminated TMR	100% AFB1 (30 ng/g, exceeding EU limits); 100% OTA (48.5 ng/g); 50% ZON (700 ng/g)	[112]
Holstein cows	Grass silage-based TMR	No mycotoxins detected	[109]
Holstein cows	Maize silage-based TMR	90% of samples had DON/ZON	[109]
Dairy cows	Maize silage TMR	Fumonisins (74%), DON (42.5%), ZEN (39.1%) prevalent	[110]
Dairy cows	Fusarium-contaminated TMR	58% samples contaminated (FBs 34%, DON 17%, ZEN 16%)	[111]
Dairy cattle (Pakistan)	Commercial TMR	33.3% AFB1 positive (mean 21.97 ppb)	[113]
Holstein cows	Grass silage TMR	Higher enterobacteria in TMR vs. silage; linked to SCC	[109]
Dairy cows	Maize/grass silage TMR	Highest endotoxins in TMR (293.44 EU/mL)	[114]
Holstein cows	Maize silage TMR	Enterobacteriaceae: 3.93 log10 CFU/g in maize silage	[114]
Dairy cows	Maize silage-based TMR	Maize silage = primary source of DON/ZEA; co-occurrence increases health risks	[25]
Aflatoxin Carryover to Milk
Holstein cows	AFB1-contaminated TMR + sequestering agent	47% reduction in AFM1 with optimal SA inclusion	[115]
High-yielding cows	AFB1-contaminated TMR	75% milk samples exceeded EU AFM1 limits (50 ng/L)	[116]
Dairy cows (Portugal)	Mycotoxin-contaminated TMR	RC/BEA/enniatins showed 2–10% carryover to milk	[110]
Mitigation Strategies
Holstein cows	Pelletized SA in TMR	Most effective (0.013 AFM1 excretion)	[115]
Dairy cows	Mycotoxin adsorbents in TMR	Reduced toxin bioavailability	[117]
Dairy cows	Inoculant additives in grass silage TMR	Improved ME content and milk yield	[109]

AFB1 = aflatoxin B1, OTA = ochratoxin A, ZON = zearalenone, DON = deoxynivalenol, ZEN = zearalenone, FBs = fumonisins, SCC = somatic cell count, SA = sequestering agent, AFM1 = aflatoxin M1, RC = roquefortine C, BEA = beauvericin, ME = metabolizable energy. ng/g = nanograms per gram, ppb = parts per billion, EU/mL = endotoxin units per milliliter, log10 CFU/g = logarithmic colony-forming units per gram, ng/L = nanograms per liter.

Table 7. Mycotoxin Contamination in Total Mixed Ration for Ruminants.

Species/Breed	TMR Type/Intervention	Summary of Results	Reference
Dairy cattle	Mycotoxin-contaminated TMR	100% AFB1 (30 ng/g, exceeding EU limits); 100% OTA (48.5 ng/g); 50% ZON (700 ng/g)	[112]
Holstein cows	Grass silage-based TMR	No mycotoxins detected	[109]
Holstein cows	Maize silage-based TMR	90% of samples had DON/ZON	[109]
Dairy cows	Maize silage TMR	Fumonisins (74%), DON (42.5%), ZEN (39.1%) prevalent	[110]
Dairy cows	Fusarium-contaminated TMR	58% samples contaminated (FBs 34%, DON 17%, ZEN 16%)	[111]
Dairy cattle (Pakistan)	Commercial TMR	33.3% AFB1 positive (mean 21.97 ppb)	[113]
Holstein cows	Grass silage TMR	Higher enterobacteria in TMR vs. silage; linked to SCC	[109]
Dairy cows	Maize/grass silage TMR	Highest endotoxins in TMR (293.44 EU/mL)	[114]
Holstein cows	Maize silage TMR	Enterobacteriaceae: 3.93 log10 CFU/g in maize silage	[114]
Dairy cows	Maize silage-based TMR	Maize silage = primary source of DON/ZEA; co-occurrence increases health risks	[25]
Aflatoxin Carryover to Milk
Holstein cows	AFB1-contaminated TMR + sequestering agent	47% reduction in AFM1 with optimal SA inclusion	[115]
High-yielding cows	AFB1-contaminated TMR	75% milk samples exceeded EU AFM1 limits (50 ng/L)	[116]
Dairy cows (Portugal)	Mycotoxin-contaminated TMR	RC/BEA/enniatins showed 2–10% carryover to milk	[110]
Mitigation Strategies
Holstein cows	Pelletized SA in TMR	Most effective (0.013 AFM1 excretion)	[115]
Dairy cows	Mycotoxin adsorbents in TMR	Reduced toxin bioavailability	[117]
Dairy cows	Inoculant additives in grass silage TMR	Improved ME content and milk yield	[109]

AFB1 = aflatoxin B1, OTA = ochratoxin A, ZON = zearalenone, DON = deoxynivalenol, ZEN = zearalenone, FBs = fumonisins, SCC = somatic cell count, SA = sequestering agent, AFM1 = aflatoxin M1, RC = roquefortine C, BEA = beauvericin, ME = metabolizable energy. ng/g = nanograms per gram, ppb = parts per billion, EU/mL = endotoxin units per milliliter, log10 CFU/g = logarithmic colony-forming units per gram, ng/L = nanograms per liter.

Vaičiulienė et al. [114] reported that TMR had the highest endotoxin levels (293.44 EU/mL) compared to silages, likely due to bacterial proliferation during feed mixing and storage. Enterobacteriaceae counts were elevated in maize silage (3.93 log10 CFU/g), suggesting poor hygienic conditions that exacerbate mycotoxin risks. These findings indicate that TMR not only introduces mycotoxins but also creates an environment conducive to secondary microbial challenges.

Mitigation strategies are crucial for reducing mycotoxin exposure and ensuring feed safety in TMR. Effective mitigation requires a combination of rigorous feed quality monitoring, improved storage practices, and the strategic use of mycotoxin binders. One key strategy involves the use of sequestering agents, such as clay-based binders, which have been demonstrated to reduce Aflatoxin M1 excretion in milk by 47% when incorporated into TMR [115]. Pelletizing these agents further enhances their efficacy, underscoring the importance of feed processing in mitigation efforts. Furthermore, improving silage management is vital to minimizing fungal growth [109]. This includes ensuring anaerobic conditions during ensiling and using inoculants, which can indirectly reduce mycotoxin risks by improving the overall hygienic quality of the feed. For instance, farms that utilized silage additives reported higher metabolizable energy content and milk yields, indicating better feed preservation and reduced spoilage [109]. In conclusion, mycotoxin contamination in TMR is a multifaceted issue driven by the quality of feed ingredients, storage practices, and environmental factors. Therefore, adopting integrated approaches, including routine mycotoxin screening, enhanced silage management, and the utilization of binding agents, is essential to ensure animal health and food safety. Addressing these challenges will ultimately enhance the sustainability and productivity of ruminant production systems, as farmers and nutritionists must prioritize mycotoxin management to maintain productive and sustainable livestock systems.

6.2. Mycotoxin Mitigation Costs

A significant economic concern in TMR is mycotoxin contamination, which can reduce feed quality and animal health. Maize silage, a common ingredient in TMR, is particularly susceptible to aflatoxins and Fusarium toxins [111]. Contaminated feed leads to decreased productivity, increased disease incidence, and higher veterinary costs [25]. Although effective mitigation strategies, such as mycotoxin binders and improved silage storage, add to operational expenses but are necessary to prevent losses [115]. Routine monitoring and sourcing high-quality ingredients are essential to minimize these risks. In conclusion, the economic benefits of TMR are well-documented, including improved feed efficiency, labor savings, and enhanced livestock productivity. However, these advantages depend on proper formulation, management practices, and investment in technology. Farmers must balance cost-effectiveness with animal health considerations, such as maintaining adequate fiber levels and preventing mycotoxin contamination. For small-scale operations, partial TMR systems or cooperative feed mixing may offer viable alternatives. Ultimately, TMR represents a scientifically validated approach for optimizing livestock production, but its economic viability must be assessed in the context of farm-specific conditions.

7. Implications of Total Mixed Ration for Animal Health

The formulation and delivery of TMR have profound implications for the metabolic and physiological health of ruminants. Blood metabolite profiles serve as critical indicators of this status, reflecting dietary composition, rumen fermentation dynamics, and subsequent metabolic adaptations.

7.1. Blood Metabolites as Health Indicators

7.1.1. Glucose Metabolism

Blood glucose levels are primarily influenced by dietary energy content and rumen fermentation patterns. TMR formulations rich in fermentable carbohydrates (e.g., maize silage, cereal grains) promote increased propionate production in the rumen, which is a primary precursor for hepatic gluconeogenesis [67]. For instance, Hu et al. [118] observed elevated blood glucose in dairy heifers fed a high-straw fermented TMR (HSF), a response attributed to enhanced volatile fatty acid production. However, excessive concentrate inclusion can lead to subacute ruminal acidosis, which may impair glucose metabolism due to systemic inflammation [18].

7.1.2. Nitrogen Utilization

Blood urea nitrogen (BUN) levels are a key reflection of nitrogen metabolism and protein utilization efficiency. Elevated BUN in TMR-fed animals often indicates an imbalance, such as excessive rumen-degradable protein or inadequate energy synchronization, leading to increased ammonia absorption and subsequent hepatic urea synthesis [63]. Conversely, optimized TMR formulations with balanced protein-to-energy ratios (e.g., combining alfalfa silage and maize silage) can lower BUN by improving microbial protein synthesis and nitrogen capture in the rumen [66]. Furthermore, fermented TMR has been shown to reduce BUN, likely by enhancing microbial efficiency and nitrogen retention [64].

7.1.3. Lipid Metabolism

The TMR composition significantly influences lipid metabolism, as evidenced by alterations in serum triglycerides and cholesterol levels. High-concentrate TMR can increase hepatic lipid mobilization, a consequence of elevated insulin resistance and altered VFA profiles [18]. In contrast, TMR formulations incorporating adequate forage (e.g., alfalfa hay) promote healthier fatty acid profiles and can reduce the saturation of intramuscular fat [10]. The strategic inclusion of specific additives, such as oilseeds (e.g., rapeseed), further modulates lipid metabolism, increasing the proportion of beneficial unsaturated fatty acids in animal products [42].

7.1.4. Oxidative Stress

Markers of antioxidant capacity, such as superoxide dismutase and total antioxidant capacity, indicate the level of oxidative stress. High-grain TMR diets can induce oxidative stress due to the increased production of reactive oxygen species associated with rapid rumen fermentation and inflammatory states [63]. Conversely, FTMR inoculated with lactic acid bacteria has been demonstrated to enhance antioxidant capacity by stabilizing rumen pH and reducing systemic inflammatory responses [64].

7.1.5. Synthesis for Animal Health

Collectively, blood metabolite profiles underscore the necessity of balancing fermentable carbohydrates, physically effective fiber, and protein sources in TMR to maintain metabolic health. For lactating dairy cows, this typically means a TMR with moderate starch (25–30% DM) and adequate fiber (≥30% NDF) to stabilize glucose and minimize SARA risk [67]. In growing cattle, FTMR can enhance nutrient digestibility and improve energy retention [46], while for goats and sheep, TMR particle size and fiber content must be carefully optimized to prevent suboptimal fermentation [76]. Future research should explore precision feeding strategies using real-time metabolite monitoring to dynamically tailor TMR formulations for different production stages and individual animal needs.

7.2. Economic Sustainability and Operational Costs of Total Mixed Ration

The adoption of TMR is a significant management decision with major economic implications, impacting feed efficiency, labor requirements, capital investment, and overall farm profitability.

7.2.1. Feed Efficiency and Economic Gains

A primary economic advantage of TMR is its ability to improve feed efficiency. By blending roughages and concentrates into a homogeneous mixture, TMR prevents selective feeding by animals and ensures a consistent and balanced intake of carbohydrates, proteins, and minerals [69]. This nutrient synchronization optimizes rumen microbial activity, leading to enhanced volatile fatty acid production and superior energy utilization [59]. Studies, such as one by Zhong et al. [65], have demonstrated that pelleted TMR increased dry matter intake and average daily gain in lambs due to improved digestibility. These improvements in performance directly translate into economic benefits, as shown in

Table 8. Economic Impact of Feeding Total Mixed Ration to Ruminants.

Species/Breed	TMR Type/Intervention	Findings vs. Control	Reference
Simmental bulls	Dry TMR (straw-based)	Higher feed costs but similar carcass yield vs. conventional TMR	[81]
Crossbred lambs	FTMR (AH-300)	Improved net income due to better feed efficiency	[45]
Dorper Lambs	16% CP Growing TMR	Lowest feed cost/kg gain (RM 8.94 vs. RM 22.92 control)	[22]
Holstein (Dairy)	Confinement TMR	Highest net return ($738/cow) but greatest risk	[91]
Holstein (Dairy)	Automatic TMR (AFS)	75% lower labor costs, 91% energy reduction	[108]
Hanwoo Steers	TMR + Medicinal Plants	Reduced feed costs, increased carcass price	[27]
Sindhi Crossbred	Alkaline-treated TMR	17.75% higher economic benefit/kg gain	[14]

TMR = total mixed ration, FTMR = fermented total mixed ration, CP = crude protein, AFS = automated feeding system. RM = Malaysian ringgit, $ = US dollar, % = percent, kg = kilogram.

, where TMR feeding led to lower feed costs per kilogram of gain [22] and higher net income [45]. It is critical, however, that TMR is properly formulated to maintain adequate physically effective fiber (peNDF) to avoid metabolic disorders like SARA, which can erode these economic gains [76].

7.2.2. Labor and Operational Cost Savings

The operational efficiency of TMR systems, particularly when automated, offers substantial labor savings. Tangorra and Calcante [108] reported that automatic feeding systems (AFS) used for TMR delivery lowered labor requirements by 75% and reduced energy consumption by 91% compared to conventional mixer wagons. Furthermore, AFS improves feeding accuracy and consistency, minimizing nutrient variation and feed waste. While the initial investment in TMR equipment (mixer wagons, AFS) can be substantial, the long-term savings in labor and improved feed utilization contribute significantly to economic sustainability, especially in large-scale operations.

7.2.3. Productivity and Quality Premiums

TMR feeding is consistently associated with enhanced productivity, which directly boosts income. In dairy systems, TMR-fed herds typically produce more milk with higher fat and protein content compared to those on less controlled diets [93]. In beef production, cattle fed TMR often exhibit improved carcass quality metrics, such as better marbling and meat tenderness, which can command premium market prices [29]. Strategies like incorporating specific additives (e.g., medicinal plants) into TMR have also been shown to reduce feed costs while simultaneously increasing carcass value [27].

7.2.4. Economic Considerations and Barriers

Despite the clear benefits, economic barriers to adoption exist. The high capital investment required for TMR equipment can be prohibitive for smallholder farmers [108]. To mitigate this, partial TMR strategies, which combine TMR with grazing or forage supplementation, have been proposed as a cost-effective alternative for smaller operations [12,91]. Furthermore, the pursuit of maximum productivity via high-concentrate TMR formulations must be balanced against potential externalities, such as increased methane emissions [9], which may have future economic implications through regulatory costs or market preferences. Therefore, the economic sustainability of TMR depends on optimizing formulations for both profitability and long-term operational resilience.

8. Challenges in Feeding Total Mixed Ration to Ruminants

TMR is widely adopted in ruminant production due to its ability to provide a nutritionally balanced diet, improve feed efficiency, and enhance productivity. However, several challenges arise in its implementation, affecting both animal health and farm profitability (Figure 5). One major challenge is ruminal acidosis, particularly with high-concentrate TMR formulations that reduce rumen buffering capacity. Bo Trabi et al. [26] found that pelleted TMR (PTMR) reduces particle size, thereby increasing starch digestibility; however, it lowers rumen pH, which increases the risk of SARA. Similarly, Arbaoui et al. [17] reported that high-concentrate TMR resulted in a lower rumen pH (5.58 vs. 5.87 in separate feeding), thereby increasing the risk of acidosis. This implies that while TMR enhances nutrient utilization, improper forage-to-concentrate ratios can disrupt rumen homeostasis, negatively impacting microbial activity and fiber digestion.

Feed sorting is another concern, where animals selectively consume preferred ingredients, leading to nutrient imbalances. Although TMR is designed to minimize this, studies indicate that compact TMR or FTMR may better prevent sorting by improving homogeneity [60,114]. Furthermore, mycotoxin contamination in TMR, particularly from maize silage, poses health risks. Martins et al. [25] highlighted that mycotoxins, such as aflatoxin B1 and deoxynivalenol, are prevalent in TMR, which can impair liver function and reduce feed intake. This emphasizes the importance of rigorous feed quality control and the use of mycotoxin binders to mitigate these effects.

Methane emissions from TMR-fed ruminants also present environmental challenges. O’Neill et al. [93] observed that TMR-fed dairy cows emitted more methane (397 g/day) than pasture-fed cows (251 g/day), despite higher milk yields. This suggests that while TMR improves productivity, its environmental footprint must be addressed through strategies like FTMR or methane-inhibiting additives [46]. In conclusion, the challenges of feeding TMR include ruminal acidosis, feed sorting, mycotoxin contamination, and methane emissions, highlighting the need for precise formulation, quality control, and innovative strategies such as FTMR or yeast supplementation. Addressing these issues ensures sustainable ruminant production, striking a balance between productivity, animal health, and environmental impact. Future research should focus on optimizing TMR composition and mitigating its associated drawbacks to enhance farm profitability and global food security (Figure 5).

9. Conclusions and Gaps Identified

While TMR offers a precision feeding strategy that significantly enhances ruminant productivity and health, the current body of research identifies several limitations and points towards crucial directions for future investigation. Early studies, such as the one conducted by Lakhani et al. [78], involved a relatively small sample size of Murrah buffaloes (n = 18 for the feeding trial, n = 9 for in vitro and in vivo comparison) which, while demonstrating the reliability of in vitro techniques for predicting metabolizable energy, suggests that broader validation with more extensive data is needed. Moreover, Tafaj et al. [69], when examining the impact of TMR particle size on rumen fermentation, also noted that their findings were based on limited number of observations (n = 3 cows for the main experiment) and emphasized the need for further studies to verify observed trends and better understand the complex relationships between in vitro gas production and in vivo measurements across different digesta compartments.

A significant challenge in TMR research lies in the translation of in vitro findings to in vivo animal performance and livestock productivity as milk and meat. Researchers, such as Mansfield et al. [119], highlighted that rumen simulation systems, like Rusitec, may not perfectly replicate the natural rumen environment due to factors including reduced protozoa numbers, potential shifts in bacterial populations, and limitations imposed by feed enclosures in nylon bags, which can affect microbial accessibility and nutrient recycling. This discrepancy hinders direct comparisons and the drawing of definitive conclusions for the live animal feeding complex, despite some short-term in vitro studies showing similar fermentation patterns to those of the natural rumen. Furthermore, Mendoza et al. [98] observed that the spot sampling technique used for assessing microbial protein flow might lack sufficient sensitivity to detect small differences between dietary treatments, indicating a methodological limitation that could obscure subtle biological responses. Similarly, Zhong et al. [65] cautioned that their findings on rumen pH and risk of acidosis in dairy cows fed pelleted TMR were based on a single sampling over 42 days, emphasizing the need for longer-term evaluations consisting multiple sample collection to establish the sustained effects on lactating performance and health status. Sun et al. [72] also noted that their study’s short duration might have limited the detection of significant differences in carcass traits in fattening lambs supplemented with live yeast.

A notable gap in the current literature, which merits further investigation as detailed below in Section 10 (Future Research Directions), is the predominant focus of studies on dairy and beef cattle. While some research exists on sheep, there is limited research on other ruminants, such as goats. The physical characteristics of TMR ingredients also present limitations. Zhang et al. [48] pointed out that while pelleted TMR can improve growth performance, the fine grinding of roughage often reduces physically effective neutral detergent fiber (peNDF) content, potentially leading to lower rumen pH and compromised fiber degradation. This can sometimes result in minimal or no substantial difference in feed conversion ratio despite increases in average daily gain. Jang et al. [76] found that for goats, traditional peNDF metrics based on particle size distribution might not be as effective in predicting chewing activity or digestibility, suggesting that goats, due to their unique digestive characteristics, may respond differently to feeding pattern and particle size variations, necessitating further research with a broader range of peNDF levels and particle sizes to better identify these effects. Confounding factors can also complicate interpretations of finding in terms of practical on farm applicability. Kronqvist et al. [31] reported that their study design was unable to distinguish between the effects of particle size and dry matter concentration on feed intake, highlighting the need for studies specifically designed to isolate these variables.

Variability in environment and raw material in feed pose additional challenges. Mohammadi Shad et al. [116] reported that aflatoxin B1 (AFB1) contamination in TMR and feed ingredients, particularly cottonseed cake and corn gluten meal, was higher during the rainy season, emphasizing the influence of climatic conditions on feed quality and the need for improved storage and feeding procedures to mitigate mycotoxin occurrence and carryover effect into milk. Furthermore, high-concentration TMR, especially when finely pelleted, can pose a higher risk of acidosis due to rapid fermentation and reduced buffering from saliva if not carefully managed, which may result in reduced feed intake and lower animal performance. An unusually low protein content in the control TMR for one study also highlights the importance of consistent feed quality in comparative trials. These limitations directly inform future research directions.

10. Future Research Directions

This review underpins a need for long-term studies to fully assess the sustained effects of various TMR formulations, including pelleted and fermented TMR (FTMR), on livestock health, animal productivity, and milk composition, particularly at different stages of lactation. Research should focus on optimizing TMR composition, including ideal forage to concentrate ratios, and exploring alternative ingredients, such as sweet sorghum and alfalfa silage mixtures, or novel agricultural by-products, while comprehensively evaluating their impact on rumen function, microbial populations, and digestive enzyme activity. The efficacy of processing methods, such as fermentation, which potentially incorporates lactic acid bacteria and fibrolytic enzymes, in improving nutrient digestibility, reducing methane production, enhancing feed storage quality, and improving aerobic stability, warrants further in vivo validation. Moreover, future investigations should delve deeper into the impact of feed additives, such as yeast cultures for ameliorating rumen pH drops and enhancing fibre degradation, or essential oils and volatile fatty acid solutions as alternative energy sources, with a focus on optimal dosage and long-term effects.

Understanding the intricate interplay between diet, rumen microbiota, and metabolic profiles is crucial for developing precision feeding strategies and identifying markers related to feed efficiency and growth performance. Economic analyses are also important to determine the feasibility of incorporating novel ingredients or feeding strategies, such as the use of wheat middlings in lamb diets or total mixed ration silage in dairy and beef cattle production in tropical countries, to ensure sustainable and profitable livestock farming. Ultimately, continuous research is essential for developing more resilient TMR formulations that can cope with variations in raw material quality and environmental conditions, thereby ensuring optimal animal performance and contributing to global food security in an environmentally responsible manner.