The effect of 3-nitrooxypropanol, a potent methane inhibitor, on ruminal microbial gene expression profiles in dairy cows

Metagenomic insights into the mechanistic differences of plant polyphenols and nitrocompounds in reducing methane emissions using the rumen simulation technique

Methane (CH4) emissions from ruminants contribute significantly to greenhouse gas levels and also result in considerable feed energy losses. Plant polyphenols and nitrocompounds are two typical types of methane inhibitors. The study investigates the mechanistic differences between 2-nitroethanol (NE) and proanthocyanidins (PAC) in reducing methane emissions from ruminant livestock using the rumen simulation technique (RUSITEC) combined with metagenomic analyses. The experiment was performed as a complete randomized block design with 3 runs. Run was used as a blocking factor. The treatments included a control (CON) with no additive, NE at 0.5 g/kg dry matter (DM), and PAC at 20 g/kg DM, all incubated in vitro for 24 h (h) with eight replicates per treatment. The results showed that NE significantly reduced CH4 production by 94.9 % (P < 0.01) and total volatile fatty acid (TVFA) concentration by 11.1 % (P < 0.05) compared to the control. NE also decreased the acetate-to-propionate ratio (A/P) from 1.93 to 1.60 (P < 0.01), indicating a shift towards more efficient fermentation. In contrast, PAC reduced methane production by 11.7 % (P < 0.05) and decreased the A/P (P < 0.05) while maintaining microbial diversity and fermentation stability, with no significant impact on TVFA concentration (P > 0.05). Metagenomic analysis revealed that NE markedly suppressed the abundance of key genera involved in carbohydrate metabolism, including Prevotella and Bacteroides, leading to reduced acetate and butyrate pathways. NE also selectively inhibited methanogenic archaea, particularly Methanobrevibacter spp., which are integral to the hydrogenotrophic pathway (P < 0.01). On the other hand, PAC showed selective inhibition of Methanosphaera spp., targeting the methylotrophic pathway (P < 0.01). These findings provide valuable insights into the distinct microbial and metabolic pathways modulated by NE and PAC, offering potential strategies for developing effective dietary interventions to mitigate methane emissions in ruminant livestock.

Harnessing meta-omics to unveil and mitigate methane emissions in ruminants: Integrative approaches and future directions

Methane emissions from enteric fermentation present a dual challenge globally: they not only contribute significantly to atmospheric greenhouse gases but also represent a considerable energy loss for ruminant animals. Utilizing high-throughput omics technologies to analyze rumen microbiome samples (meta-omics, i.e., metagenomics, metatranscriptomics, metaproteomics, metabolomics) holds vast potential for uncovering the intricate interplay between diet, microbiota, and methane emissions in these animals. The primary obstacle is the effective integration of diverse meta-omic approaches and their broader application across different ruminant species. Genetic variability significantly impacts methane production in ruminants, suggesting that genomic selection could be a viable strategy to reduce emissions. While substantial research has been conducted on the microbiological aspects of methane production, there remains a critical need to delineate the specific genetic interactions between the host and its microbiome. Advancements in meta-omics technologies are poised to shed light on these interactions, enhancing our understanding of the genetic factors that govern methane output. This review explores the potential of meta-omics to accelerate genetic advancements that could lead to reduced methane emissions in ruminants. By employing a systems biology approach, the integration of various omics technologies allows for the identification of key genomic regions and genetic markers linked to methane production. These markers can then be leveraged in selective breeding programs to cultivate traits associated with lower emissions. Moreover, the review addresses current challenges in applying genomic selection for this purpose and discusses how omics technologies can overcome these obstacles. The systematic integration and analysis of diverse biological data provide deeper insights into the genetic underpinnings and overall biology of methane production traits in ruminants. Ultimately, this comprehensive approach not only aids in reducing the environmental impact of agriculture but also contributes to the sustainability and efficiency of livestock management.

Unraveling the genetic basis of methane emission in dairy cattle: a comprehensive exploration and breeding approach to lower methane emissions

Ruminant animals, such as dairy cattle, produce CH4, which contributes to global warming emissions and reduces dietary energy for the cows. While the carbon foot print of milk production varies based on production systems, milk yield and farm management practices, enteric fermentation, and manure management are major contributors togreenhouse gas emissions from dairy cattle. Recent emerging evidence has revealed the existence of genetic variation for CH4 emission traits among dairy cattle, suggests their potential inclusion in breeding goals and genetic selection programs. Advancements in high-throughput sequencing technologies and analytical techniques have enabled the identification of potential metabolic biomarkers, candidate genes, and SNPs linked to methane emissions. Indeed, this review critically examines our current understanding of carbon foot print in milk production, major emission sources, rumen microbial community and enteric fermentation, and the genetic architecture of methane emission traits in dairy cattle. It also emphasizes important implications for breeding strategies aimed at halting methane emissions through selective breeding, microbiome driven breeding, breeding for feed efficiency, and breeding by gene editing.

Microbiome-informed study of the mechanistic basis of methane inhibition by Asparagopsis taxiformis in dairy cattle

Copious amounts of methane, a major constituent of greenhouse gases currently driving climate change, are emitted by livestock, and efficient methods that curb such emissions are urgently needed to reduce global warming. When fed to cows, the red seaweed Asparagopsis taxiformis (AT) can reduce enteric methane emissions by up to 80%, but the achieved results can vary widely. Livestock produce methane as a byproduct of methanogenesis, which occurs during the breakdown of feed by microbes in the rumen. The ruminant microbiome is a diverse ecosystem comprising bacteria, protozoa, fungi, and archaea, and methanogenic archaea work synergistically with bacteria to produce methane. Here, we find that an effective reduction in methane emission by high-dose AT (0.5% dry matter intake) was associated with a reduction in methanol-utilizing Methanosphaera within the rumen, suggesting that they may play a greater role in methane formation than previously thought. However, a later spike in Methanosphaera suggested an acquired resistance, possibly via the reductive dehalogenation of bromoform. While we found that AT inhibition of methanogenesis indirectly impacted ruminal bacteria and fermentation pathways due to an increase in spared H2, we also found that an increase in butyrate synthesis was due to a direct effect of AT on butyrate-producing bacteria such as Butyrivibrio, Moryella, and Eubacterium. Together, our findings provide several novel insights into the impact of AT on both methane emissions and the microbiome, thereby elucidating additional pathways that may need to be targeted to maintain its inhibitory effects while preserving microbiome health and animal productivity.

IMPORTANCE

Livestock emits copious quantities of methane, a major constituent of the greenhouse gases currently driving climate change. Methanogens within the bovine rumen produce methane during the breakdown of feed. While the red seaweed Asparagopsis taxiformis (AT) can significantly reduce methane emissions when fed to cows, its effects appear short-lived. This study revealed that the effective reduction of methane emissions by AT was accompanied by the near-total elimination of methane-generating Methanosphaera. However, Methanosphaera populations subsequently rebounded due to their ability to inactivate bromoform, a major inhibitor of methane formation found in AT. This study presents novel findings on the contribution of Methanosphaera to ruminal methanogenesis, the mode of action of AT, and the possibility for complementing different strategies to effectively curb methane emissions.

Invited review: Advances in nutrition and feed additives to mitigate enteric methane emissions

Methane, both enteric and from manure management, is the most important greenhouse gas from ruminant livestock, and its mitigation can deliver substantial decreases in the carbon footprint of animal products and potentially contribute to climate change mitigation. Although choices may be limited, certain feeding-related practices can substantially decrease livestock enteric CH4 emission. These practices can be generally classified into 2 categories: diet manipulation and feed additives. Within the first category, selection of forages and increasing forage digestibility are likely to decrease enteric CH4 emission, but the size of the effect, relative to current forage practices in the United States dairy industry, is likely to be minimal to moderate. An opportunity also exists to decrease enteric CH4 emissions by increasing dietary starch concentration, but interventions have to be weighed against potential decreases in milk fat yield and farm profitability. A similar conclusion can be made about dietary lipids and oilseeds, which are proven to decrease CH4 emission but can also have a negative effect on rumen fermentation, feed intake, and milk production and composition. Sufficient and robust scientific evidence indicates that some feed additives, specifically the CH4 inhibitor 3-nitrooxypropanol, can substantially reduce CH4 emissions from dairy and beef cattle. However, the long-term effects and external factors affecting the efficacy of the inhibitor need to be further studied. The practicality of mass-application of other mitigation practices with proven short-term efficacy (i.e., macroalgae) is currently unknown. One area that needs more research is how nutritional mitigation practices (both diet manipulation and feed additives) interact with each other and whether there is synergism among feed additives with different mode of action. Further, effects of diet on manure composition and greenhouse gas emissions during storage (e.g., emission trade-offs) have not been adequately studied. Overall, if currently available mitigation practices prove to deliver consistent results and novel, potent, and safe strategies are discovered and are practical, nutrition alone can deliver up to 60% reduction in enteric CH4 emissions from dairy farms in the United States.

Metataxonomic and metabolomic profiling revealed Pinus koraiensis cone essential oil reduced methane emission through affecting ruminal microbial interactions and host-microbial metabolism

BACKGROUND

Pinus koraiensis cone essential oil (PEO) contains functional compounds such as monoterpene hydrocarbons, and the administration of PEO reduced methane (CH4) emissions during growing phase of goats. However, the mode of action of PEO driven CH4 reduction is not known, especially how the administration of PEO can affect rumen microbiota and host metabolism in goats during the fattening phase. This study aimed to elucidate the potential microbial and host responses PEO supplementation in goats using metataxonomics (prokaryotes and protozoa) and metabolomics (rumen fluid and serum).

RESULTS

Ten fattening Korean native goats were divided into two dietary groups: control (CON; basal diet without additives) and PEO (basal diet + 1.5 g/d of PEO) with a 2 × 2 crossover design and the treatment lasted for 11 weeks. Administration of PEO reduced CH4 concentrations in the exhaled gas from eructation by 12.0-13.6% (P < 0.05). Although the microbial composition of prokaryotes (bacteria and archaea) and protozoa in the rumen was not altered after PEO administration. MaAsLin2 analysis revealed that the abundance of Selenomonas, Christensenellaceae R-7 group, and Anaerovibrio were enriched in the rumen of PEO supplemented goats (Q < 0.1). Co-occurrence network analysis revealed that Lachnospiraceae AC2044 group and Anaerovibrio were the keystone taxa in the CON and PEO groups, respectively. Methane metabolism (P < 0.05) was enriched in the CON group, whereas metabolism of sulfur (P < 0.001) and propionate (P < 0.1) were enriched in the PEO group based on microbial predicted functions. After PEO administration, the abundance of 11 rumen and 4 serum metabolites increased, whereas that of 25 rumen and 14 serum metabolites decreased (P < 0.1). Random forest analysis identified eight ruminal metabolites that were altered after PEO administration, among which four were associated with propionate production, with predictive accuracy ranging from 0.75 to 0.88. Additionally, we found that serum sarcosine (serum metabolite) was positively correlated with CH4 emission parameters and abundance of Methanobrevibacter in the rumen (|r|≥ 0.5, P < 0.05).

CONCLUSIONS

This study revealed that PEO administration reduced CH4 emission from of fattening goats with altered microbial interactions and metabolites in the rumen and host. Importantly, PEO administration affected utilizes various mechanisms such as formate, sulfur, methylated amines metabolism, and propionate production, collectively leading to CH4 reduction. The knowledge is important for future management strategies to maintain animal production and health while mitigate CH4 emission.

In-Silico and in vitro Studies Revealed that Rosmarinic Acid Inhibited Methanogenesis via Regulating Composition and Function of Rumen Microbiota

Inhibition of methyl-coenzyme M reductase can suppress the activity of ruminal methanogens, thereby reducing enteric methane emissions of ruminants. However, developing specific and environmentally friendly inhibitors is a challenging endeavor. To identify a natural and effective methane inhibitor that specifically targets methyl-coenzyme M reductase, molecular docking technology was employed to screen a library of phytogenic compounds. A total of 52 candidate compounds were obtained through molecular docking technique. Rosmarinic acid (RA) was one of the compounds that could traverse a narrow channel and bind to the active sites of methyl-coenzyme M reductase, with a calculated binding free energy of -9.355 kcal/mol. Furthermore, the effects of rosmarinic acid supplementation on methane production, rumen fermentation, and the microorganism's community in dairy cows were investigated through in vitro rumen fermentation simulations according to a random design. Supplementation of RA resulted in a 15% decrease in methane production compared with the control. In addition, RA increased the molar proportion of acetate and propionate, whereas the sum of acetate and butyrate divided by propionate was decreased. At the bacterial level, the relative abundance of Rikenellaceae RC9 gut group, Christensenellaceae R7 group, Candidatus Saccharimonas, Desulfovibrio, and Lachnospiraceae FE2018 group decreased with RA supplementation. Conversely, the addition of RA significantly increased the relative abundance of DNF00809 (a genus from Eggerthellaceae), Denitrobacterium, an unclassified genus from Eggerthellaceae, an unclassified genus from Bacteroidales, and an unclassified genus from Atopobiaceae. At the archaeal level, the relative abundance of Methanobrevibacter decreased, while that of Methanosphaera increased with the RA supplementation. These findings suggested that RA has the potential to be used as a novel natural additive for inhibiting ruminal methane production.

Review: Biological consequences of the inhibition of rumen methanogenesis

Decreasing enteric CH4 emissions from ruminants is important for containing global warming to 1.5 °C and avoid the worst consequences of climate change. However, the objective of mitigating enteric CH4 emissions is difficult to reconcile with the forecasted increase in production of ruminant meat and milk, unless CH4 production per animal and per kilogram of animal product are decreased substantially. Chemical compound 3-nitrooxypropanol and bromoform-containing red algae Asparagopsis are currently the most potent inhibitors of rumen methanogenesis, but their average efficacy would have to be increased to mitigate enteric CH4 emissions to contain global warming to 1.5 °C, if the demand for ruminant products increases as predicted. We propose that it may be possible to enhance the efficacy of inhibitors of methanogenesis through understanding the mechanisms that cause variation in their efficacy across studies. We also propose that a more thorough understanding of the effects of inhibiting methanogenesis on rumen and postabsorptive metabolism may help improve feed efficiency and cost-effectiveness as co-benefits of the methanogenesis inhibition intervention. For enhancing efficacy, we examine herein how different inhibitors of methanogenesis affect the composition of the rumen microbial community and discuss some mechanisms that may explain dissimilar sensitivities among methanogens to different types of inhibitors. For improving feed efficiency and cost-effectiveness, we discuss the consequences of inhibiting methanogenesis on rumen fermentation, and how changes in rumen fermentation can in turn affect postabsorptive metabolism and animal performance. The objectives of this review are to identify knowledge gaps of the consequences of inhibiting methanogenesis on rumen microbiology and rumen and postabsorptive metabolism, propose research to address those knowledge gaps and discuss the implications that this research can have for the efficacy and adoption of inhibitors of methanogenesis. Depending on its outcomes, research on the microbiological, biochemical, and metabolic consequences of the inhibition of rumen methanogenesis could help the adoption of feed additives inhibitors of methanogenesis to mitigate enteric CH4 emissions from ruminants to ameliorate climate change.

Effects of type of substrate and dilution rate on fermentation in serial rumen mixed cultures

Forages and concentrates have consistently distinct patterns of fermentation in the rumen, with forages producing more methane (CH4) per unit of digested organic matter (OM) and higher acetate to propionate ratio than concentrates. A mechanism based on the Monod function of microbial growth has been proposed to explain the distinct fermentation pattern of forages and concentrates, where greater dilution rates and lower pH associated with concentrate feeding increase dihydrogen (H2) concentration through increasing methanogens growth rate and decreasing methanogens theoretically maximal growth rate, respectively. Increased H2 concentration would in turn inhibit H2 production, decreasing methanogenesis, inhibit H2-producing pathways such as acetate production via pyruvate oxidative decarboxylation, and stimulate H2-incorporating pathways such as propionate production. We examined the hypothesis that equalizing dilution rates in serial rumen cultures would result in a similar fermentation profile of a high forage and a high concentrate substrate. Under a 2 × 3 factorial arrangement, a high forage and a high concentrate substrate were incubated at dilution rates of 0.14, 0.28, or 0.56 h-1 in eight transfers of serial rumen cultures. Each treatment was replicated thrice, and the experiment repeated in two different months. The high concentrate substrate accumulated considerably more H2 and formate and produced less CH4 than the high forage substrate. Methanogens were nearly washed-out with high concentrate and increased their initial numbers with high forage. The effect of dilution rate was minor in comparison to the effect of the type of substrate. Accumulation of H2 and formate with high concentrate inhibited acetate and probably H2 and formate production, and stimulated butyrate, rather than propionate, as an electron sink alternative to CH4. All three dilution rates are considered high and selected for rapidly growing bacteria. The archaeal community composition varied widely and inconsistently. Lactate accumulated with both substrates, likely favored by microbial growth kinetics rather than by H2 accumulation thermodynamically stimulating electron disposal from NADH into pyruvate reduction. In this study, the type of substrate had a major effect on rumen fermentation largely independent of dilution rate and pH.

Distinct microbial hydrogen and reductant disposal pathways explain interbreed variations in ruminant methane yield

Ruminants are essential for global food security, but these are major sources of the greenhouse gas methane. Methane yield is controlled by the cycling of molecular hydrogen (H2), which is produced during carbohydrate fermentation and is consumed by methanogenic, acetogenic, and respiratory microorganisms. However, we lack a holistic understanding of the mediators and pathways of H2 metabolism and how this varies between ruminants with different methane-emitting phenotypes. Here, we used metagenomic, metatranscriptomic, metabolomics, and biochemical approaches to compare H2 cycling and reductant disposal pathways between low-methane-emitting Holstein and high-methane-emitting Jersey dairy cattle. The Holstein rumen microbiota had a greater capacity for reductant disposal via electron transfer for amino acid synthesis and propionate production, catalyzed by enzymes such as glutamate synthase and lactate dehydrogenase, and expressed uptake [NiFe]-hydrogenases to use H2 to support sulfate and nitrate respiration, leading to enhanced coupling of H2 cycling with less expelled methane. The Jersey rumen microbiome had a greater proportion of reductant disposal via H2 production catalyzed by fermentative hydrogenases encoded by Clostridia, with H2 mainly taken up through methanogenesis via methanogenic [NiFe]-hydrogenases and acetogenesis via [FeFe]-hydrogenases, resulting in enhanced methane and acetate production. Such enhancement of electron incorporation for metabolite synthesis with reduced methanogenesis was further supported by two in vitro measurements of microbiome activities, metabolites, and public global microbiome data of low- and high-methane-emitting beef cattle and sheep. Overall, this study highlights the importance of promoting alternative H2 consumption and reductant disposal pathways for synthesizing host-beneficial metabolites and reducing methane production in ruminants.

Methane mitigation in ruminants with structural analogues and other chemical compounds targeting archaeal methanogenesis pathways

Ruminants are responsible for enteric methane production contributing significantly to the anthropogenic greenhouse gases in the atmosphere. Moreover, dietary energy is lost as methane gas without being available for animal use. Therefore, many mitigation strategies aiming at interventions at animals, diet, and microbiota have been explored by researchers. Specific chemical analogues targeting the enzymes of the methanogenic pathway appear to be more effective in specifically inhibiting the growth of methane-producing archaea without hampering another microbiome, particularly, cellulolytic microbiota. The targets of methanogenesis reactions that have been mainly investigated in ruminal fluid include methyl coenzyme M reductase (halogenated sulfonate and nitrooxy compounds), corrinoid enzymes (halogenated aliphatic compounds), formate dehydrogenase (nitro compounds, e.g., nitroethane and 2-nitroethanol), and deazaflavin (F420) (pterin and statin compounds). Many other potential metabolic reaction targets in methanogenic archaea have not been evaluated properly. The analogues are specifically effective inhibitors of methanogens, but their efficacy to lower methanogenesis over time reduces due to the metabolism of the compounds by other microbiota or the development of resistance mechanisms by methanogens. In this short review, methanogen populations inhabited in the rumen, methanogenesis pathways and methane analogues, and other chemical compounds specifically targeting the metabolic reactions in the pathways and methane production in ruminants have been discussed. Although many methane inhibitors have been evaluated in lowering methane emission in ruminants, advancement in unravelling the molecular mechanisms of specific methane inhibitors targeting the metabolic pathways in methanogens is very limited.

Evolving understanding of rumen methanogen ecophysiology

Production of methane by methanogenic archaea, or methanogens, in the rumen of ruminants is a thermodynamic necessity for microbial conversion of feed to volatile fatty acids, which are essential nutrients for the animals. On the other hand, methane is a greenhouse gas and its production causes energy loss for the animal. Accordingly, there are ongoing efforts toward developing effective strategies for mitigating methane emissions from ruminant livestock that require a detailed understanding of the diversity and ecophysiology of rumen methanogens. Rumen methanogens evolved from free-living autotrophic ancestors through genome streamlining involving gene loss and acquisition. The process yielded an oligotrophic lifestyle, and metabolically efficient and ecologically adapted descendants. This specialization poses serious challenges to the efforts of obtaining axenic cultures of rumen methanogens, and consequently, the information on their physiological properties remains in most part inferred from those of their non-rumen representatives. This review presents the current knowledge of rumen methanogens and their metabolic contributions to enteric methane production. It also identifies the respective critical gaps that need to be filled for aiding the efforts to mitigate methane emission from livestock operations and at the same time increasing the productivity in this critical agriculture sector.

Invited review: Rumen modifiers in today's dairy rations

Our aim was to review feed additives that have a potential ruminal mechanism of action when fed to dairy cattle. We discuss how additives can influence ruminal fermentation stoichiometry through electron transfer mechanisms, particularly the production and usage of dihydrogen. Lactate accumulation should be avoided, especially when acidogenic conditions suppress ruminal neutral detergent fiber digestibility or lead to subclinical acidosis. Yeast products and other probiotics are purported to influence lactate uptake, but growing evidence also supports that yeast products influence expression of gut epithelial genes promoting barrier function and resulting inflammatory responses by the host to various stresses. We also have summarized methane-suppressing additives for potential usage in dairy rations. We focused on those with potential to decrease methane production without decreasing fiber digestibility or milk production. We identified some mitigating factors that need to be addressed more fully in future research. Growth factors such as branched-chain volatile fatty acids also are part of crucial cross-feeding among groups of microbes, particularly to optimize fiber digestibility in the rumen. Our developments of mechanisms of action for various rumen-active modifiers should help nutrition advisors anticipate when a benefit in field conditions is more likely.

3-Nitrooxypropanol supplementation of a forage diet decreased enteric methane emissions from beef cattle without affecting feed intake and apparent total-tract digestibility

Supplementation of ruminant diets with the methane (CH4) inhibitor 3-nitrooxypropanol (3-NOP; DSM Nutritional Products, Switzerland) is a promising greenhouse gas mitigation strategy. However, most studies have used high grain or mixed forage-concentrate diets. The objective of this study was to evaluate the effects of supplementing a high-forage diet (90% forage DM basis) with 3-NOP on dry matter (DM) intake, rumen fermentation and microbial community, salivary secretion, enteric gas emissions, and apparent total-tract nutrient digestibility. Eight ruminally cannulated beef heifers (average initial body weight (BW) ± SD, 515 ± 40.5 kg) were randomly allocated to two treatments in a crossover design with 49-d periods. Dietary treatments were: 1) control (no 3-NOP supplementation); and 2) 3-NOP (control + 150 mg 3-NOP/kg DM). After a 16-d diet adaption, DM intake was recorded daily. Rumen contents were collected on days 17 and 28 for volatile fatty acid (VFA) analysis, whereas ruminal pH was continuously monitored from days 20 to 28. Eating and resting saliva production were measured on days 20 and 31, respectively. Diet digestibility was measured on days 38-42 by the total collection of feces, while enteric gas emissions were measured in chambers on days 46-49. Data were analyzed using the mixed procedure of SAS. Dry matter intake and apparent total-tract digestibility of nutrients (DM, neutral and acid detergent fiber, starch, and crude protein) were similar between treatments (P ≥ 0.15). No effect was observed on eating and resting saliva production. Relative abundance of the predominant bacterial taxa and rumen methanogen community was not affected by 3-NOP supplementation but rather by rumen digesta phase and sampling hour (P ≤ 0.01). Total VFA concentration was lower (P = 0.004) following 3-NOP supplementation. Furthermore, the reduction in acetate and increase in propionate molar proportions for 3-NOP lowered (P < 0.001) the acetate to propionate ratio by 18.9% as compared with control (4.1). Mean pH was 0.21 units lower (P < 0.001) for control than 3-NOP (6.43). Furthermore, CH4 emission (g/d) and yield (g/kg DMI) were 22.4 and 22.0% smaller (P < 0.001), respectively, for 3-NOP relative to control. Overall, the results indicate that enteric CH4 emissions were decreased by more than 20% with 3-NOP supplementation of a forage diet without affecting DM intake, predominant rumen microbial community, and apparent total-tract nutrients digestibility.