摘要The study investigated the metabolic responses of yaks to dietary net energy for gain(NEg)and metabolizable protein(MP),and explored potential biomarkers for average daily gain(ADG)in serum metabolites.A total of 40 three-year-old yaks(initial body weight=255±8.04 kg)were assigned to a 2×2 factorial treatment arrangement based on dietary NEg(4.18 vs 4.81 MJ/kg DM;low NEg[LE]vs high NEg[HE])and MP(72.0 vs 82.8 g/kg DM;low MP[LP]vs high MP[HP]),comprising 4 treatments with 10 replicates each(1 yak per replicate).The yaks were fed these diets for 55 days including a 15-day adaptation period.Yaks on HP diets had greater ADG(P=0.013)and a reduced feed-to-gain ratio(P=0.019)compared to those on the LP diet,and those on the LE diet had greater dry matter intake(P＜0.001)but unaffected ADG compared to those on the HE diet(P＞0.05).Dietary NEg interacted with MP in relation to the apparent digestibility of dry matter(P=0.028)and crude protein(P＜0.001).The apparent digestibility of gross energy(GE),organic matter,and neutral detergent fiber in yaks on the HE diet was greater than those in yaks on the LE diet(P＜0.05).The digestibility of GE in yaks on the HP diet was lower than those on the LP diet(P=0.011).An significant interaction between dietary NEg level and MP level was observed on serum total bilirubin concentration,catalase concentration,and superoxide dismutase concentration(P＜0.05).Yaks fed HE diets exhibited higher serum glutathione peroxidase protein concentration(P＜0.001),but lower serum triglyceride concentration(P=0.012)compared to those fed LE diets.Serum level of alanine transaminase was higher in yaks on the HP diet compared to those on the LP diet(P=0.038).Untargeted metabolomics identified novel biomarkers linked to dietary NEg and MP levels,revealing that the HE diet enhanced adenosine triphosphate production through acetyl-CoA synthesis and affected amino acid,fat,and carbohydrate pathways(P＜0.05).The HP diet altered the synthesis of aromatic amino acids and vitamins(P＜0.05).Additionally,random forest analysis identified N-(9-oxodecyl)acetamide and biliverdin,as biomarkers for predicting ADG.These findings provide a theoretical foundation for the effective feeding and nutritional management of yaks.