Effects of Concentrates with Different Energy and Protein Levels on Growth Performance, Blood Metabolism, and Rumen Microbiota of Housed Pregnant Yaks During the Warm Season

The nutritional management of pregnant yaks is critical yet remains understudied, particularly under housed conditions. This study investigated the effects of concentrate supplementation at varying nutritional levels on growth performance, blood metabolism, and rumen microbiota in housed pregnant yaks during the warm season. Eighteen mid-gestation yaks were allocated to low- (LP), medium- (MP), and high-nutrition (HP) groups and fed for 110 days. Growth performance, serum metabolites, hormones, and rumen microbiota composition were assessed. No significant differences were observed in final body weight, average daily gain, or dry matter intake among the groups (p > 0.05). However, the serum malondialdehyde concentration was significantly higher in the MP group (3.03 nmol/mL) than in the LP group (1.30 nmol/mL) (p < 0.05). Concentrations of glucose and lipid metabolites tended to increase with higher nutritional levels, whereas circulating levels of growth-related hormones (GH, INS, IGF-1, and IGF-2) showed a declining trend (p > 0.05). Rumen alpha diversity was numerically highest in the MP group. Specific taxa (norank_o_Clostridia_UCG-014) were enriched in the MP and LP groups (p < 0.05). The microbial composition in the MP group was positively correlated with blood TG, TC, and HDL-C (p < 0.05). In conclusion, within the constraints of the present experimental design, medium- or low-nutrition concentrate supplementation maintained growth performance and modulated the rumen microbiota associated with lipid metabolism. However, the elevated MDA in the MP group warrants caution, and the observed trends should be interpreted as preliminary and require validation in future studies with larger sample sizes.