The experiment was conducted to evaluate the effects of two dietary energy levels (TDN: 70 and 76% DM) and two rumen-degradable protein levels (RDP: 7.7 and 9.4% DM) on rumen fermentation, nutrients apparent digestibility and blood biochemical constituents of Chinese crossbred yellow bulls. Four ruminally-fistulated Charolais×Nan yang yellow bulls, about 540±23kg live weight, were randomly assigned to a 2×2 factorial arrangement in a 4×4 Latin Square design to receive four dietary treatments. The treatments were as follows: low energy and high protein (LEHP; TDN: 70%, RDP: 9.4%), high energy and high protein (HEHP; TDN: 76%, RDP: 9.4%), low energy and low protein (LELP; TDN: 70%, RDP: 7.7%) and high energy and low protein (HELP; TDN: 76%, RDP: 7.7%). Ruminal pH and total volatile fatty acids were not different among treatments. The acetate concentration was lower and propionate concentration was greater (P<0.01) for bulls fed HE diet compared with LE diet. The higher ammonia nitrogen (NH₃–N) concentration was found for bulls fed HP diet compared to bulls fed LP diet (P<0.01). Total apparent digestibility of dry matter (DM), crude protein (CP), organic matter (OM) and N utilization were greater for bulls fed HE vs LE diet (P<0.05). Bulls fed HP diet had increased N retention than those fed LP diet (P<0.05). Blood biochemical constituents were not different among all dietary treatments, except plasma urea nitrogen (PUN) which was higher in HP dietary treatment (P<0.05). These findings suggest that high energy and high protein (energy: 76%, RDP: 9.4%) treatment is the best for high performance of yellow bulls without affecting their health.

Key words: Apparent digestibility, Blood biochemical constituents, Energy, RDP, Rumen fermentation