Department of Animal Sciences, The University of Arizona, Tucson, AZ 85721, USA.
J Dairy Sci. 2010 Feb;93(2):644-55. doi: 10.3168/jds.2009-2295.
Heat stress has an enormous economic impact on the global dairy industry, but the mechanisms by which hyperthermia negatively affect systemic physiology and milk synthesis are not clear. Study objectives were to evaluate production parameters and metabolic variables in lactating dairy cows during short-term heat stress or pair-fed conditions coupled with bST administration. Twenty-two multiparous Holstein cows were subjected to 3 experimental periods: 1) thermoneutral conditions with ad libitum intake for 7 d (P1); 2) heat stress (HS) with ad libitum intake (n=10) or pair-fed (PF) in thermoneutral conditions (n=12) for 7 d (P2), and 3) 7 d of HS or PF in conditions as described in P2 with recombinant bovine somatotropin administered on d 1 (P3). All cows received an intravenous glucose tolerance test (GTT) on d 5 of each period. Heat stress conditions were cyclical and temperatures ranged from 29.4 to 38.9 degrees C. Rectal temperatures and respiration rates increased during heat stress (38.6-40.4 degrees C and 44-89 breaths/min, respectively). Heat stress reduced dry matter intake by 30% and by design PF cows had similar intake reductions (28%). During heat stress and pair-feeding, milk yield decreased by 27.6% (9.6kg) and 13.9% (4.8kg), respectively, indicating that reduced feed intake accounted for only 50% of the decreased milk production. Milk yield increased with recombinant bovine somatotropin in both HS (9.7%) and PF (16.1%) cows. Cows in both groups were in positive energy balance (3.95 Mcal/d) during P1 but entered negative energy balance during P2 and P3 (-5.65 Mcal/d). Heat stress and pair-feeding treatments decreased (9.3%) basal glucose concentrations. Heat stress conditions had no effect on basal NEFA levels during P2; however, PF cows (despite a similar calculated energy balance) had a 2-fold increase in basal NEFA concentrations. Both groups had increased plasma urea nitrogen levels during P2 and P3 compared with P1. Basal insulin levels increased (37%) during P2 and P3 in HS cows but did not differ between periods in PF cows. During P2 and compared with P1, PF cows had a decreased rate of glucose disposal, whereas HS cows had a similar disposal rate following the GTT. During P2 and compared with P1, PF cows had a reduced insulin response whereas HS cows had a similar insulin response to the GTT. In summary, reduced nutrient intake accounted for only 50% of heat stress-induced decreases in milk yield, and feed intake-independent shifts in postabsorptive glucose and lipid homeostasis may contribute to the additional reduction in milk yield.
热应激对全球奶牛养殖业有巨大的经济影响，但高热如何对全身生理学和牛奶合成产生负面影响的机制尚不清楚。本研究的目的是评估泌乳奶牛在短期热应激或限饲条件下与 bST 给药联合应用时的生产参数和代谢变量。22 头经产荷斯坦奶牛经历了 3 个实验期：1）热中性条件下自由采食 7 天（P1）；2）热中性条件下自由采食（n=10）或限饲（n=12）的热应激（HS）7 天（P2），和 3）P2 中描述的条件下的 7 天 HS 或 PF 与重组牛生长激素（rBST）在第 1 天给药（P3）。所有奶牛在每个时期的第 5 天接受静脉葡萄糖耐量试验（GTT）。热应激条件呈周期性变化，温度范围为 29.4 至 38.9 摄氏度。直肠温度和呼吸频率在热应激期间升高（38.6-40.4 摄氏度和 44-89 次/分钟）。热应激使干物质摄入量减少 30%，而设计的限饲奶牛的摄入量减少相似（28%）。在热应激和限饲期间，产奶量分别下降 27.6%（9.6kg）和 13.9%（4.8kg），表明减少的采食量仅占产奶量减少的 50%。rBST 使 HS（9.7%）和 PF（16.1%）奶牛的产奶量增加。在 P1 期间，两组奶牛均处于正能量平衡（3.95 Mcal/d），但在 P2 和 P3 期间进入负能量平衡（-5.65 Mcal/d）。热应激和限饲处理降低了（9.3%）基础葡萄糖浓度。在 P2 期间，热应激条件对基础非酯化脂肪酸（NEFA）水平没有影响；然而，限饲奶牛（尽管计算出的能量平衡相似）基础 NEFA 浓度增加了 2 倍。与 P1 相比，两组在 P2 和 P3 期间血浆尿素氮水平升高。在 P2 和 P3 期间，HS 奶牛的基础胰岛素水平升高（37%），但 PF 奶牛在各期之间没有差异。在 P2 期间，与 P1 相比，限饲奶牛的葡萄糖清除率降低，而 HS 奶牛在 GTT 后葡萄糖清除率相似。在 P2 期间，与 P1 相比，PF 奶牛的胰岛素反应降低，而 HS 奶牛的胰岛素对 GTT 的反应相似。总之，营养摄入减少仅占热应激引起产奶量下降的 50%，而吸收后葡萄糖和脂质稳态的摄食独立变化可能导致产奶量的进一步下降。
