Abstract:[Background] Isoacids are branched-chain volatile fatty acids, providing nutrients to the microorganisms and promoting the growth of fiber-degrading bacteria in the rumen. [Objective] This study aimed to investigate the effects of dietary supplementation with mixed isoacids (MI) on the activities of carbohydrate-degrading enzymes, microbiota composition, and functional genes in the yak rumen. In addition, the correlations of the enzyme activities with the relative abundance of rumen microorganisms and functional genes were analyzed. The results are expected to provide a reference for the rational use of MI to improve the fermentation performance in the yak rumen. [Methods] A single factorial experimental design was used and 16 male yaks weighing approximately 200 kg were randomized into four groups with four replicates per group and one yak per replicate. Yaks in the control group were fed with a basal diet, while those in the other groups were fed a basal diet supplemented with 0.1% (T1 group), 0.2% (T2 group), and 0.3% (T3 group) of MI, respectively. The pre-experiment lasted for 10 days, and the formal feeding lasted for 85 days. At the end of the experiment, rumen fluid samples were collected and the rumen microbiome was analyzed by metagenomic sequencing. Activities of carbohydrate-degrading enzymes were measured by and UV spectrophotometry. [Results] Compared with the control group, dietary MI supplementation significantly increased the activities of carboxymethyl cellulase and amylase in the yak rumen. Supplementation with 0.3% MI reduced the microbial richness and increased the similarity of microbiota composition and functional gene composition in the yak rumen. The differential bacteria such as Firmicutes, Erysipelotrichia, Saccharibacteria, and Aeromonadaceae had high relative abundance in the T2 group. The KEGG pathway enrichment analysis showed that the differentially expressed genes encoding phosphotransferase (2.7.1.68), ubiquitin hydrolase (3.4.19.12), aminoacyltransferase (2.3.2.27), and protein serine/threonine kinases (2.7.11.11) had high relative abundance in the T1 group. Rumen amylase activity was positively correlated with Profundibacterium, Aeromonas, Saccharibacteria, and Aeromonadaceae, as well as members of the GH13_20, GH77, and CBM72 gene families. The carboxymethyl cellulase activity in the rumen was positively correlated with members of the GT90 gene family. [Conclusion] Supplementation of MI in yak diets increased the activities of amylase and carboxymethyl cellulase, improved the similarity of rumen microbiota composition and functional gene composition, and increased the relative abundance of bacteria and functional genes in the yak rumen. In addition, rumen microbiome and their functional genes were correlated with the activities of rumen enzymes. The results suggest that MI can improve the rumen fermentation in yaks to a certain extent.
