Effects of Dietary Concentrate-to-Roughage Ratio on Rumen Microbiota, Functional Profiles, and Fermentation Characteristics in Yak

This study investigated the effects of different concentrate-to-roughage ratios on the rumen microbial community, functional potential, and fermentation characteristics in yak. Forty Qinghai Plateau-type yaks (8–9 months, 68.725 ± 18.973 kg) were randomly assigned to four dietary groups with concentrate-to-roughage ratios of 80:20 (C80), 65:35 (C65), 50:50 (C50), and 35:65 (C35). After a 15-day adaptation period, animals were fed for 105 days. Rumen contents were analyzed using metagenomic sequencing combined with fermentation parameter measurements. High-concentrate diets (C80 and C65) were associated with increased relative abundance of starch-degrading and propionate-producing bacteria, such as Prevotella and Succiniclasticum, whereas low-concentrate diets (C50 and C35) were associated with higher abundance of cellulolytic bacteria, including Ruminococcus and Fibrobacter. Functional analysis indicated increased relative abundance of genes involved in glycolysis (ko00010), propanoate metabolism (ko00640), and energy-related pathways in high-concentrate groups, while fiber degradation and methane-related pathways were relatively higher in low-concentrate groups. Rumen fermentation parameters showed a significant decrease in pH with increasing concentrate level (p = 0.001), and NH3-N concentrations differed among treatments (p = 0.036). Dietary concentrate-to-roughage ratio significantly influences rumen microbial composition, functional potential, and fermentation characteristics in yak. A moderate concentrate level (approximately 65:35) may contribute to a more balanced rumen microbial and fermentation profile under the conditions of this study.