Dietary L-leucine supplementation improves ruminal fermentation parameters and epithelium development in fattening Angus beef cattle

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Abstract Background In this study, the effects of L-leucine (Leu) on rumen fermentation parameters, rumen epithelium development, amino acid composition, rumen bacterial communities and rumen metabolites in beef cattle were investigated. Twenty-four fattening Angus females of similar initial weight (575.5 ± 22.1 kg) were randomly assigned to 2 treatments with 4 replicate pens (3 cattle per pen). They were fed either a basal diet or a basal diet supplemented with 6.0 g L-Leu/100 kg BW/d for 120 d. Results (1) Leu increased the ruminal concentrations of total volatile fatty acid (VFA) ( P = 0.017), propionate ( P = 0.023), isovalerate ( P = 0.001), and branched-chain volatile fatty acid (BCVFA) ( P = 0.01) at 4 h post-feeding. It also tended to increase acetate ( P = 0.083) and decrease the ammonia-N (NH 3 -N) concentration ( P = 0.055), but it did not affect ruminal pH ( P > 0.1). Leu also increased microbial crude protein (MCP) ( P = 0.026) at 4 h post-feeding, but decreased MCP at 8 h post-feeding ( P = 0.010). (2) Supplementation with L-Leu increased the ruminal concentrations of phenylalanine ( P = 0.011), lysine ( P = 0.034), and tyrosine ( P = 0.033), while decreasing the cystine concentration ( P = 0.010). (3) Leu increased the thickness of the stratum spinosum and basal ( P < 0.05), while decreasing the thickness of the stratum granulosum ( P < 0.05). (4) Leu upregulated the relative mRNA abundance of genes involved in tight junction proteins ( P < 0.05) and VFA absorption and metabolism ( P < 0.01) in the rumen epithelium. This upregulation was positively correlated with the concentrations ruminal isovalerate and BCVFA ( P < 0.01). (5) L-Leu did not affect the diversity and richness of ruminal microbes ( P > 0.05), but differential bacterial biomarkers (LEfSe, LDA > 2) were either positively or negatively correlated with ruminal MCP, NH 3 -N, and BCVFA concentrations ( P < 0.001). Additionally, differential bacterial metabolites (OPLS-DA, VIP > 1.5) were primarily enriched in the amino acid metabolism pathway and the cofactors and vitamins metabolism pathway ( P < 0.05). Conclusions Dietary supplementation with L-Leu altered rumen fermentation parameters and patterns, improved rumen epithelial morphology, and enhanced the expression of genes related to VFA absorption and metabolism in the rumen epithelium of beef cattle.

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