Effects of Dietary Energy Level on Nutrients Apparent Digestibility and Blood Biochemical Indices of Dairy Cows under Heat Stress

In order to investigate the effects of dietary energy level on milk production,nutrients apparent digestibility and blood biochemical indices of dairy cows under heat stress,a single factor experimental design was used. Twenty-five healthy Chinese Holstein dairy cows in mid-lactation were allocated to 5 groups( 5 cows in each group) corresponding to 5 isonitrogenous dietary treatments with net energy levels [net energy for lactation( NEL),dry matter basis]of 6.15( group 1),6.36( group 2),6.64( group 3),6.95( group 4) and7.36 M J / kg( group 5),respectively. The experiment lasted for 45 days,including a 15-day pre-experimental period followed by a 30-day experimental period. The results showed as follows: 1) the improving of dietary energy level reduced respiratory frequency,groups 4 and 5 were extremely significantly lower than groups 1and 2( P 0.01),and had no significant difference compared with group 3( P 0.05). 2) There was a significant negative correlation between dry matter intake and dietary energy level( R2= 0. 983 3,P = 0. 017),and group 5 was significantly lower than the other groups( P 0. 05 or P 0. 01); milk yield in group 4 was the highest,and was increased by 9% compared with that in group 1( group 1 had the lowest value),the difference was extramely significant( P 0.01). 3) Apparent digestibility of nutrients( excepted calcium,phosphorus and gross energy) reached the maximum in group 1 and the minimum in group 5,the differences were significant or extramely significant( P 0.05 or P 0.01). 4) In group 4,serum cortisol concentration was significantly or extramely significantly lower than that in groups 1,2 and 5( P 0. 05 or P 0. 01),serum lipopolysaccharide concentration was significantly or extramely significantly lower than that in the other groups( P 0.05 or P 0.01),and serum glucose concentration was significantly higher than that in groups 1 and 5( P 0.05); the activity of serum creatine kinase in groups 1 and 4 was significantly or extramely significantly higher than that in groups 2,3 and 5( P 0.05 or P 0.01),and group 4 increased by 32.87% and 25.40% compared with groups 2 and 3,respectively; the activities of serum cereal third transaminase and aspertate aminotransferase showed group 1 group 2 group 4 group 3 group 5. Comprehensive analysis on the effects of dietary energy level on milk production,nutrients apparent digestibility and blood biochemical indices indicates that under conditions in the present study,the optimal dietary NELlevel is 6. 95 M J / kg of dairy cows under heat stress.