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 (R²) 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.

尽管相对于成年母牛而言，乳牛更耐高温，但它们仍易受热应激，这在较高的环境温度和相对湿度下表现为生理反应增强和采食量减少。但是，犊牛的热应激指标尚未被很好地表征。在本文中，我们评估了在慢性热应激或在亚热带气候中持续冷却的奶牛犊中，环境和温度调节以及以生产性动物为基础的热应激指标之间的关联。荷斯坦犊牛在2到42 d的年龄中暴露于热应激（HT;谷仓阴影，n = 24）或连续冷却（CL;谷仓的阴影加2个风扇，n = 24）。环境指数，包括环境温度，相对湿度，温湿度指数（THI）和风速，以及基于动物的指数，每天三次（0900、1300和1900 h）记录呼吸（RR），心率（HR），直肠（RT）和皮肤温度（ST）。每天记录15至42 d年龄的代乳品（MI）和谷物摄入量。然后使用分段回归模型，估算生理反应的显着变化的THI阈值。我们发现以动物为基础的指标（HR，MI和谷物摄入量除外）与环境温度和THI之间存在很强的正相关性，其中ST和THI的相关性最高（r≥0.72）。环境温度，ST和环境温度或THI和MI是唯一的不同处理之间的相关性。测定系数（R 每天记录15至42 d年龄的代乳品（MI）和谷物摄入量。然后使用分段回归模型，估算生理反应的显着变化的THI阈值。我们发现以动物为基础的指标（HR，MI和谷物摄入量除外）与环境温度和THI之间存在很强的正相关性，其中ST和THI的相关性最高（r≥0.72）。环境温度，ST和环境温度或THI和MI是唯一的不同处理之间的相关性。测定系数（R 每天记录15至42 d年龄的代乳品（MI）和谷物摄入量。然后使用分段回归模型，估算生理反应的显着变化的THI阈值。我们发现以动物为基础的指标（HR，MI和谷物摄入量除外）与环境温度和THI之间存在很强的正相关性，其中ST和THI的相关性最高（r≥0.72）。环境温度，ST和环境温度或THI和MI是唯一的不同处理之间的相关性。测定系数（R 和谷物摄入量）以及环境温度和THI，与ST和THI的相关性最高（r≥0.72）。环境温度，ST和环境温度或THI和MI是唯一的不同处理之间的相关性。测定系数（R 和谷物摄入量）以及环境温度和THI，与ST和THI的相关性最高（r≥0.72）。环境温度，ST和环境温度或THI和MI是唯一的不同处理之间的相关性。测定系数（R²通过将环境指标包括在内，从回归分析获得的基于动物的指标模型的回归分析得到了显着改善，其中THI的改善最大。总体而言，与HT犊牛相比，CL犊牛的RR，RT，ST较低，而采食量较高，因此风扇持续冷却会增加犊牛的热量损失。HT犊牛的RT（THI = 67），RR（THI = 65）和MI（THI = 82）的温度-湿度指数断点可以确定，而CL犊牛的RR（THI = 69）只能确定。由于与THI的线性关系密切，因此两种方法的皮肤温度变量均无可检测的断点。总的来说，我们的结果表明ST适合估算慢性热应激，而THI是在阴凉，亚热带环境中饲养的奶牛犊中热应激的最佳环境指标。在实践上，