Department of Animal Sciences, University of Florida, Gainesville 32611.
Department of Animal Sciences, University of Florida, Gainesville 32611.
J Dairy Sci. 2020 Sep;103(9):8587-8600. doi: 10.3168/jds.2020-18381. Epub 2020 Jun 26.
Although dairy calves are more thermotolerant relative to mature cows, they are still susceptible to heat stress, as demonstrated by elevated physiological responses and reduced feed intake under high ambient temperature and relative humidity. However, indicators of heat stress have not been well-characterized in calves. Herein, we evaluated associations between environmental and thermoregulatory and productive animal-based indicators of heat stress in dairy calves exposed to chronic heat stress or continuous cooling in a subtropical climate. Holstein calves were exposed to heat stress (HT; shade of barn, n = 24) or continuous cooling (CL; shade of barn plus 2 fans, n = 24) from 2 to 42 d of age. Environmental indices, including ambient temperature, relative humidity, temperature-humidity index (THI), and wind speed, and animal-based indices, including respiration (RR), heart rate (HR), rectal (RT), and skin temperature (ST) were recorded thrice daily (0900, 1300, and 1900 h). Milk replacer (MI) and grain intakes were recorded daily from 15 to 42 d of age. Using segmented regression models, we then estimated THI thresholds for significant changes in physiological responses. We found a strong, positive correlation between animal-based indicators (except for HR, MI, and grain intakes) and ambient temperature and THI, with the highest correlation obtained with ST and THI (r ≥ 0.72). Ambient temperature and ST and ambient temperature or THI and MI were the only correlations that differed between treatments. The coefficient of determination (R) obtained from regression analyses to model animal-based indicators was substantially improved by the inclusion of environmental indicators, with the greatest improvement achieved with THI. Overall, continuous cooling by fans promoted calf heat loss as CL calves had lower RR, RT, ST, and higher feed intake compared with HT calves. Temperature-humidity index breakpoints could be determined for RT (THI = 67), RR (THI = 65), and MI (THI = 82) in HT calves, and only for RR (THI = 69) in CL calves. Skin temperature variables had no detectable breakpoints in either treatment due to the strong linear relationship to THI. Collectively, our results suggest that ST is appropriate to estimate chronic heat stress and that THI is the best environmental indicator of heat stress in dairy calves raised in a shaded, subtropical environment. At a practical level, calves should be closely monitored when THI reaches 65 to 69 to minimize the risk of heat stress-related impairments.
虽然奶牛犊牛相对于成年奶牛具有更强的耐热性，但它们仍然容易受到热应激的影响，在高温高湿环境下，会出现生理反应升高和采食量减少的情况。然而，在犊牛中，热应激的指标尚未得到很好的描述。在此，我们评估了在亚热带气候下，处于慢性热应激或连续冷却条件下的荷斯坦奶牛犊牛的环境和体温调节以及生产动物相关热应激指标之间的关联。从 2 到 42 日龄，将荷斯坦犊牛暴露于热应激（HT；牛舍遮荫，n = 24）或连续冷却（CL；牛舍遮荫加 2 个风扇，n = 24）。每日记录 3 次（0900、1300 和 1900 h）环境指数，包括环境温度、相对湿度、温湿度指数（THI）和风速，以及动物指标，包括呼吸频率（RR）、心率（HR）、直肠温度（RT）和皮肤温度（ST）。从 15 到 42 日龄，每日记录牛奶代用品（MI）和谷物摄入量。然后，使用分段回归模型，我们估计了生理反应发生显著变化的 THI 阈值。我们发现，动物指标（除了 HR、MI 和谷物摄入量外）与环境温度和 THI 之间存在很强的正相关，与 ST 和 THI 的相关性最高（r≥0.72）。环境温度和 ST，或环境温度或 THI 和 MI 之间的相关性在处理之间是唯一不同的。通过纳入环境指标，从回归分析中获得的动物指标模型的决定系数（R）得到了显著提高，其中 THI 的提高最大。总的来说，风扇的连续冷却促进了犊牛的热量损失，因为 CL 犊牛的 RR、RT、ST 较低，饲料摄入量较高，而 HT 犊牛的 RR、RT、ST、ST 较低，饲料摄入量较高。在 HT 犊牛中，仅可确定 RT（THI = 67）、RR（THI = 65）和 MI（THI = 82）的温度-湿度指数断点，而在 CL 犊牛中，仅 RR（THI = 69）有温度-湿度指数断点。由于 ST 与 THI 之间存在很强的线性关系，因此在两种处理中，皮肤温度变量均没有检测到可检测的断点。总的来说，我们的结果表明，ST 适合估计慢性热应激，THI 是在遮荫的亚热带环境中饲养的奶牛犊牛热应激的最佳环境指标。在实际水平上，当 THI 达到 65 到 69 时，应密切监测犊牛，以最大程度地降低与热应激相关的损伤的风险。
