Dietary Supplementation of Concentrate Improves Lactation Performance and Immune Function in Grazing Lactating Yaks Through Changes in Rumen Microbial Population and Metabolites.

Grazing yaks often face protein deficiency due to low-quality pasture, which limits milk production. This study aimed to investigate the effects of varying protein levels in concentrate supplementation on lactational performance, immune function, and rumen microbial and metabolites in grazing lactating yaks. Thirty-six lactating Qinghai Plateau yaks (172.78 ± 11.70 kg) were assigned to four treatments for 70 d (10 d adaptation + 60 d trial): grazing only (CON) or grazing plus 1.50 kg/d concentrate containing 15.09% (CP15), 17.00% (CP17), or 18.98% CP (CP19). Concentrate supplementation significantly increased average daily gain (ADG; 0.22 vs. 0.72-0.90 kg/d; p < 0.001) and milk yield (622.18 vs. 1094.25-1385.73 g/d; p < 0.001), and milk yield showed a linear increase with higher dietary protein levels (p < 0.001). Milk protein yield (29.99 vs. 56.00-68.60 g/d; p < 0.001) and milk lactose yield (40.71 vs. 79.85-93.53 g/d; p < 0.001) were also increased. Milk composition, including fat, protein, and lactose, also improved across supplementation groups, with the greatest enhancement observed at the CP17 group (p < 0.05). Rumen pH and volatile fatty acids did not differ between treatments, while microbial protein increased with supplementation (10.88 vs. 12.72-15.00 mg/dL; p = 0.041) and showed a linear response to dietary CP level (p = 0.033). Concentrate supplementation significantly altered the rumen microbial structure (ANOSIM, p = 0.036), enriching Succinivibrionaceae_UCG-002, Fibrobacter, Ruminobacter, and Succinimonas and reducing Saccharofermentans in CP17 yaks (p < 0.05). Untargeted metabolomics further indicated a marked shift in the ruminal metabolite profile. Compared with CON, CP17 yaks exhibited higher levels of calcium propionate, 2-nitrofuran, curvulalide, and 2,5-dihydroxybenzoic acid, but lower levels of 4-pyridoxic acid, L-carnitine, bitocholic acid, and taurodehydrocholic acid. Pathway enrichment analysis identified vitamin B6 metabolism as significantly enriched. Collectively, these findings suggest that moderate protein concentrate supplementation (CP17) may enhance lactation performance by modulating rumen microbiota and metabolism in lactating yaks.