Abstract: The study aimed to explore the effects of different dietary nutrition levels on production performance, slaughter indexes and serum biochemical indexes of yaks. Sixty healthy male yaks with the similar age and weight, the average weight was (269.75±35.46) kg, and the yaks were randomly divided into 3 groups, each 20 replicates, and each repeating 1 yak. According to the energy level of the diet, they were divided into high nutrition level (HN) group, medium nutrition level (MN) group, and low nutrition level (LN) group. The test period was 105 days, the pre-test period was 15 days, and the normal test period was 90 days. During the trial period (90 days), the total net energy (NE_mf) of the diet was 5.51, 6.22, 6.94 MJ·kg^-1, respectively. The crude protein level of the diet during the whole trial period was 16.98% for each group. Production performance was detected at the end of the fattening trial. 8 yaks from each gorup was randomly selected for the jugular vein to collect blood, the serum separated quickly, determined the blood biochemical indicators, and the slaughter performance and meat quality of each yak was determined after slaughter. The experiment redults showed that the dietary nutrition level had no significant effect on the average daily gain (ADG) of the yak (P>0.05). However, with the increase of the feed nutrition level, the serum glucose content of the HN group was the highest, which was compared with the LN group and the MN group, increased by 38.86% and 29.69%, respectively (P<0.05). The MN group had the lowest serum urea content, which was 34.31% lower than the LN group (P<0.05), but the difference was not significant compared with the HN group. The content of total protein in serum decreased with the increase of energy level (P<0.05). The LN group had the lowest total cholesterol content, which was 10.98% lower than the MN group. With the increase of energy level, the pre-slaughter live weight, carcass weight and eye muscle area of the yak in the HN group were extremely significantly higher than those in the MN and LN groups (P<0.01); the slaughter rate in the HN group was significantly higher than that in the MN and LN groups (P<0.05). The meat color L^* value was the lowest in the LN group (P<0.05), which was 17.84% and 25.65% lower than the MN and HN groups, respectively. There was no significant difference in water lossing rate and cooking loss among different groups (P>0.05). Under the conditions of this experiment, adding energy is feasible to improve growth performance and meat quality of yak in house. The comprehensive growth performance, slaughter performance, meat quality and serum biochemical indexes are obtained. When the comprehensive net energy level of the diet is 6.94 MJ·kg^-1, the nutritional level of the diet is more suitable for cold-season house feeding and fattening, which can provide the energy level required for high-quality yak meat.

Key words: nutrition levels, yak, production performance, slaughter performance, serum biochemical index