Evaluation of thermal indices based on their relationships with some physiological responses of housed lactating cows under heat stress.

Thermal indices as environmental risk indicators have been used to assess heat stress of dairy cows. The present study aimed to evaluate the predictive performance of the typical cattle-related thermal indices by comparing their prediction to heat stress levels and associations with some physiological responses. The study was conducted from August to September 2019 in a naturally ventilated barn in Jiangsu, China. Nine typical cattle-related thermal indices, i.e., temperature-humidity index (THI), black globe temperature index (BGHI), equivalent temperature index, effective temperature (ET) for dairy cows, respiratory rate predictor (RRP), adjusted temperature-humidity index (THIadj), heat load index (HLI), comprehensive climate index (CCI), and equivalent temperature index for cattle (ETIC), were evaluated. Respiration rate (RR) and body surface temperature (BST) were collected twice per day from a total of 287 lactating cows, 18 of which were continuously measured vaginal temperature (VT). Over the experimental period, the average daily RR, VT, and BST were 55.8 breaths/min, 38.7 °C, and 32.3 to 36.4 °C that depend on body positions, respectively. The study found that the prediction of THI, BGHI, THIadj, and CCI was closer to the actual heat stress conditions which were mild to moderate heat stress. Correlation analyses showed that RR, VT, and BST correlated most closely with effective temperature (r = 0.580; P < 0.05), BGHI (r = 0.642; P < 0.05), and CCI (r = 0.849; P < 0.05). In this evaluation, based on the comprehensive performance of CCI in the relatively accurate prediction to heat stress level and duration, detection on environmental differences between standing and lying zone, and correlations with some physiological responses, CCI is seemingly the promising thermal index to assess heat stress of housed dairy cows.