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

Background Leucine can play a crucial role in regulating rumen fermentation, rumen bacterial composition, and nutrient degradation, however, most of these findings are derived from in vitro rumen fermentation results. In this study, the effects of L-leucine (Leu) on rumen fermentation parameters, rumen epithelium development, amino acid metabolism, rumen bacterial communities and metabolites in beef cattle were investigated. Twenty-four Angus cows of similar initial weight (575.5 ± 22.1 kg) were randomly assigned to 2 treatments with 6 replicate pens (2 cattle per pen). They were fed a basal diet or a basal diet supplemented with 6.0 g/100 kg BW per day of L-Leu for 120 days. Results (1) Leu increased the ruminal concentrations of total volatile fatty acid (VFA) (P = 0.017), propionate (P = 0.023), iso-valerate (P = 0.001), branched-chain volatile fatty acid (BCVFA) (P = 0.01) at 4 h post-feeding, and tended to increase acetate (P = 0.083) and decrease the ammonia-N (NH₃-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 < 0.05). (2) Supplementation with L-Leu increased the ruminal concentrations of phenylalanine (P = 0.011), lysine (P = 0.034), and tyrosine (P = 0.033), and decreased the cystine concentrations (P = 0.010). (3) Leu increased the thickness of stratum spinosum and basal (P < 0.05), while decreased the thickness of stratum granulosum. (4) Leu up-regulated the relative mRNA expression of genes involved in tight junction proteins (P < 0.05) and VFA absorption and metabolism (P < 0.01) in the rumen epithelium, and this upregulation was positively correlated with ruminal isovalerate and BCVFA concentrations (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 positively or negatively correlated with ruminal MCP, NH₃-N, and BCVFA concentrations (P < 0.001), and differential bacterial metabolites (OPLS-DA, VIP>1.5) were primarily enriched in the amino acid metabolism pathway (P < 0.05). Conclusions Dietary supplementation with L-Leu improved rumen fermentation parameters and patterns, promoted epithelial development, and enhanced rumen epithelium VFA absorption and metabolism in beef cattle.