Comparative gut microbiome composition and predicted microbial functions in captive and free-range yaks (Bos grunniens)

Background and Aim: The gut microbiota is essential for nutrient digestion, immune function, and environmental adaptation in ruminants, particularly high-altitude species like yaks (Bos grunniens). Different husbandry practices (captive vs. free-range) can potentially alter the microbial communities and affect the yak health. However, comparative data on how these systems affect yak gut microbiomes remain limited, with most studies focusing on taxonomy rather than functional implications. This study aimed to compare gut microbiome composition, diversity, and predicted functional profiles between captive (CY) and free-range (FY) yaks using a 16S rRNA gene metabarcoding approach. Materials and Methods: Fecal samples were collected from healthy ~2-year-old yaks (n=5 CY, n=5 FY) in Litang County, Ganzi Prefecture, Sichuan, China, during summer. DNA was extracted, and the V4 region of the 16S rRNA gene was sequenced on Illumina NovaSeq 6000. Bioinformatic analyses included quality filtering, Operational taxonomic units (OTU) clustering (97% similarity), taxonomic annotation (SILVA database), α- and β-diversity analysis. The microbial function was predicted using PICRUSt2 (KEGG pathways), BugBase (community phenotypes), and FAPROTAX (ecological functions). Statistical comparisan used Welch’s t-tests, Wilcoxon rank-sum tests, principal coordinates analysis (PCoA), and Analysis of similarities (ANOSIM) with significance set at p < 0.05. Results: α-Diversity indices (e.g., Shannon p = 0.5476) showed no significant differences between CY and FY. However, β-diversity revealed distinct community structures (PCoA: PC1 30.52%, PC2 12.25%; ANOSIM R = 0.976, p = 0.007), with FY samples more homogeneous. At the genus level, CY were enriched in Ruminococcaceae bacterium UCG-005, Streptococcus, Escherichia-Shigella, Treponema, Christensenellaceae R-7, and Clostridium sensu stricto 1 (many fermentative or potentially opportunistic). FY showed higher abundances of Bacillus, Arthrobacter, Rhodococcus, Candidatus Saccharimonas, Prevotellaceae UCG-001, and Paenibacillus. Predicted functions indicated FY had greater capacities for carbohydrate/amino acid metabolism, DNA repair, fatty acid biosynthesis, and vitamin B pathways, while CY favored fermentation and reductive acetogenesis. BugBase highlighted higher anaerobic phenotypes in CY. Conclusion: Husbandry practices profoundly influence yak gut microbiome structure and inferred metabolic potential, with free-range systems promoting, homogeneous communities suited to natural high-fiber diets while captive systems promotes fermentative and opportunistic shifts. These microbiome differences suggest opportunities for probiotic interventions to enhance yak health, productivity, and sustainability in high-altitude pastoral systems. Future metagenomic and metabolomic validation is needed.