Department of Animal Sciences, The University of Arizona, Tucson 85721, USA.
Domest Anim Endocrinol. 2010 Feb;38(2):86-94. doi: 10.1016/j.domaniend.2009.08.005. Epub 2009 Sep 18.
To differentiate between the effects of heat stress (HS) and decreased dry matter intake (DMI) on physiological and metabolic variables in growing beef cattle, we conducted an experiment in which a thermoneutral (TN) control group (n=6) was pair fed (PF) to match nutrient intake with heat-stressed Holstein bull calves (n=6). Bulls (4 to 5 mo old, 135 kg body weight [BW]) housed in climate-controlled chambers were subjected to 2 experimental periods (P): (1) TN (18 degrees C to 20 degrees C) and ad libitum intake for 9 d, and (2) HS (cyclical daily temperatures ranging from 29.4 degrees C to 40.0 degrees C) and ad libitum intake or PF (in TN conditions) for 9 d. During each period, blood was collected daily and all calves were subjected to an intravenous insulin tolerance test (ITT) on day 7 and a glucose tolerance test (GTT) on day 8. Heat stress reduced (12%) DMI and by design, PF calves had similar nutrient intake reductions. During P1, BW gain was similar between environments and averaged 1.25 kg/d, and both HS and PF reduced (P<0.01) average daily gain (-0.09 kg/d) during P2. Compared to PF, HS decreased (P<0.05) basal circulating glucose concentrations (7%) and tended (P<0.07) to increase (30%) plasma insulin concentrations, but neither HS nor PF altered plasma nonesterified fatty acid concentrations. Although there were no treatment differences in P2, both HS and PF increased (P<0.05) plasma urea nitrogen concentrations (75%) compared with P1. In contrast to P1, both HS and PF had increased (16%) glucose disposal, but compared with PF, HS calves had a greater (67%; P<0.05) insulin response to the GTT. Neither period nor environment acutely affected insulin action, but during P2, calves in both environments tended (P=0.11) to have a blunted overall glucose response to the ITT. Independent of reduced nutrient intake, HS alters post-absorptive carbohydrate (basal and stimulated) metabolism, characterized primarily by increased basal insulin concentrations and insulin response to a GTT. However, HS-induced reduction in feed intake appears to fully explain decreased average daily gain in Holstein bull calves.
为了区分热应激（HS）和干物质采食量（DMI）减少对生长肉牛的生理和代谢变量的影响，我们进行了一项实验，其中将 6 头热中性（TN）对照组（n=6）进行配对喂养（PF），以匹配营养摄入与热应激荷斯坦公牛犊（n=6）。饲养在气候控制室内的公牛（4 至 5 月龄，体重 135 公斤）经历了 2 个实验期（P）：（1）TN（18 至 20 摄氏度）和自由采食 9 天，和（2）HS（循环日温度范围为 29.4 至 40.0 摄氏度）和自由采食或 PF（在 TN 条件下）9 天。在每个时期，每天采集血液，所有小牛在第 7 天进行静脉内胰岛素耐量试验（ITT），并在第 8 天进行葡萄糖耐量试验（GTT）。热应激减少（12%）DMI，并且通过设计，PF 小牛具有相似的营养摄入减少。在 P1 期间，环境之间的体重增加相似，平均为 1.25 公斤/天，HS 和 PF 都降低（P<0.01）P2 期间的平均日增重（-0.09 公斤/天）。与 PF 相比，HS 降低（P<0.05）基础循环葡萄糖浓度（7%），并且倾向于（P<0.07）增加（30%）血浆胰岛素浓度，但 HS 或 PF 均未改变血浆非酯化脂肪酸浓度。尽管在 P2 时没有治疗差异，但 HS 和 PF 均增加（P<0.05）与 P1 相比，血浆尿素氮浓度（75%）。与 P1 相反，HS 和 PF 均增加（P<0.05）葡萄糖处置，但与 PF 相比，HS 小牛对 GTT 的胰岛素反应更大（67%；P<0.05）。时期和环境都没有急性影响胰岛素作用，但在 P2 期间，两种环境中的小牛都倾向于（P=0.11）对 ITT 的整体葡萄糖反应迟钝。独立于减少的营养摄入，HS 改变了吸收后碳水化合物（基础和刺激）代谢，主要表现为基础胰岛素浓度增加和对 GTT 的胰岛素反应增加。然而，HS 引起的采食量减少似乎完全解释了荷斯坦公牛犊的平均日增重降低。
