Abstract: The purpose of this study was to investigate the effects of different ratios of enzyme preparation in diet on performance and economic benefit of dairy cows under external heat stress. Sixty Holstein lactating cows with similar lactation times,milk yield and parity were randomly divided into control group,test I group,test II group and test III group with 15 in each group,the changes of milk yield and main milk components in heat stressed dairy cows were measured by adding 0%,0.5%,1.0% and 1.5% enzyme preparation into the basal diet,and the effects of enzyme preparation on the economic benefit of dairy cows were analyzed. The results showed that the average daily milk yield of the three groups was higher than that of the control group,the test II group was significantly higher than the control group（p<0.05） and the milk protein and lactose contents of the test II groups were significantly higher than that of the control group（p<0.05）,increased by 5.38% and 14.25% respectively. The economic benefit of test II group was 6.58 yuan per day more than that of the control group. Under the condition of this experiment,adding compound enzyme preparation to the diet was helpful to improve the performance and economic benefit of dairy cows,and the effect was better when adding 1.0%.

Key words: compound enzyme preparation, heat stress, production performance, economic benefit