Abstract: Two experiments were conducted to evaluate the effects of different levels of complex enzyme on rumen fermentation and milk production in early lactation dairy cows. The first experiment evaluated rumen fermentation in vitro with total mixed ration (TMR) as substrate. There were four treatments, with complex enzyme supplementation at concentrations of 0, 0.10%, 0.15% and 0.20%. Each treatment had 9 replicates and each replicate consisted of 0.500 g accurately weighed substrate. The degradation rate of nutrients in the substrate and in vitro fermentation parameters were determined with an in vitro fermentation gas automatic recording device within 48 h. Results showed that the complex enzyme significantly increased total volatile fatty acids and acetic acid in fermentation fluids (P<0.05). The degradation rates of neutral detergent fiber (P<0.05) and crude protein (P<0.01) in the supplemented treatments were significantly higher than in the control. The second experiment was conducted using a randomized complete block design with thirty-six Holstein dairy cows of similar body weight, parity, days in milk and milk yield. Complex enzyme was supplied to four treatments at concentrations of 0, 0.10%, 0.15% and 0.20% respectively. Each treatment had 9 replicates and the experiment lasted for 8 weeks. Milk yield, milk composition content and 3.5% fat-corrected milk were measured. The results showed that complex enzyme significantly increased 3.5% fat-corrected milk (P<0.05). For the 3.5% fat-corrected milk, the 0.10%, 0.15% and 0.20% treatments were 3.88, 4.27 and 2.26 kg·d^-1 more than the control respectively. The milk fat rate in the 0.15% treatment was significantly higher than the control (P<0.05), with a 12.7% increase. In conclusion, adding complex enzyme can affect rumen fermentation and production performance. The results suggest that the optimal compound enzyme level in TMR is 0.15%.

Key words: complex enzyme, in vitro rumen fermentation, milk yield, milk quality, dairy cows