为了研究饲粮能量水平对热应激奶牛产奶量、营养物质表观消化率和血液生化指标的影响,本试验采用单因素试验设计,选择25头健康的泌乳中期中国荷斯坦奶牛,随机分为5组,每组5头。各组分别饲喂5种不同能量水平的等氮饲粮,泌乳净能(干物质基础)分别为6.15(组1)、6.36(组2)、6.64(组3)、6.95(组4)和7.36 MJ/kg(组5)。试验期共45 d,预试期15 d,正试期30 d。结果表明：1)提高饲粮能量水平降低了呼吸频率,组4、5极显著低于组1、2( P<0.01),与组3差异不显著( P>0.05)。2)试牛干物质采食量与饲粮能量水平呈显著的负相关关系( R2=0.9833,P=0.017),组5显著或极显著低于其他各组( P<0.05或P<0.01)；产奶量最高的组4与最低的组1比较,提高了9.0%,差异极显著( P<0.01)。3)除钙、磷和总能外,其他养分表观消化率均以组1最高,组5最低,差异显著或极显著( P<0.05或P<0.01)。4)组4血清皮质醇浓度显著或极显著低于组1、2、5(P<0.05或P<0.01),血清内毒素浓度显著或极显著低于其他各组( P<0.05或P<0.01),血清葡萄糖浓度显著高于组1和5( P<0.05)；组1、4血清肌酸激酶活性显著或极显著大于组2、3、5(P<0.05或 P<0.01),其中组4与组2、3相比分别提高了32.87%和25.40%；血清谷丙转氨酶和谷草转氨酶活性为组1>组2>组4>组3>组5。综合考虑饲粮能量水平对奶牛产奶量、营养物质表观消化率和血液生化指标的影响,热应激条件下泌乳奶牛的饲粮适宜泌乳净能水平为6.95 MJ/kg。%In order to investigate the effects of dietary energy level on milk production, nutrients apparent di-gestibility 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 lac-tation (NEL), dry matter basis] of 6.15 (group 1), 6.36 (group 2), 6.64 (group 3), 6.95 (group 4) and 7.36 MJ/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 1 and 2 ( P<0.01) , and had no significant difference compared with group 3 ( P>0.05) . 2) There was a signifi-cant 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 differ-ence was extramely significant ( P<0.01) . 3) Apparent digestibility of nutrients ( excepted calcium, phosphor-us and gross energy) reached the maximum in group 1 and the minimum in group 5, the differences were sig-nificant or extramely significant ( P<0.05 or P<0.01) . 4) In group 4, serum cortisol concentration was signifi-cantly or extramely significantly lower than that in groups 1, 2 and 5 (P<0.05 or P<0.01), serum lipopo-lysaccharide 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 high-er 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% com-pared with groups 2 and 3, respectively; the activities of serum cereal third transaminase and aspertate amin-otransferase showed group 1>group 2>group 4>group 3>group 5. Comprehensive analysis on the effects of di-etary energy level on milk production, nutrients apparent digestibility and blood biochemical indices indicates that under conditions in the present study, the optimal dietary NEL level is 6. 95 MJ/kg of dairy cows under heat stress.
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