Identification of Thermoneutral Zone in Sahiwal Zebu Calves in Subtropical Climate of India.

This study aimed to determine the thermoneutral zone (TNZ) in Sahiwal zebu calves under controlled environmental conditions. The experiment was conducted in the psychrometric chamber in two phases on six calves aged 8 to 11 months and weighing 120 to 150 Kg at the beginning of the experiment. In the first phase, to determine the upper critical temperature (UCT), calves were kept for six hours per day over 10 consecutive days at six different increasing temperature ranges from 24 to 39 °C with corresponding temperature humidity indexes (THIs) between 67 and 93. In the second phase, the same calves were exposed to decreasing temperatures (24 °C to 9 °C) to determine the lower critical temperature (LCT). On the 10th day of each temperature exposure, physiological parameters were recorded, and blood sampling was done. Using segmented regression analysis (SegReg standard version software), the breakpoints in linear regressions for different parameters with respect to exposure temperatures and THI in both phases were separately determined and considered to be the critical temperatures and threshold THIs, respectively. The LCT and UCT were arranged on a temperature scale. The temperature range between the highest LCT and the lowest UCT with respect to different studied parameters was considered as the thermoneutral zone (TNZ). The highest LCT was observed for granulocyte % at 18.15 °C, whereas the lowest UCT was observed at 30.10 °C (THI: 82.35). It was found that the LCT and UCT varied with respect to different physiological parameters. A subset of parameters displayed identifiable LCT and UCT values, while some did not exhibit clear breakpoints. The respiration rate (RR), rectal temperature (RT), total leukocyte count (TLC), granulocyte%, aspartate amino-transferase (AST), Alanine amino-transferase (ALT), cortisol, IL6, and HSP90 were the sensitive parameters for both cold stress and heat stress, whereas pulse rate (PR), triglyceride, and urea were only sensitive to cold stress, and erythrocytic parameters and lymphocyte % were sensitive only to heat stress. Based on heat stress responses, the UCT for zebu calves was identified at approximately 30.10 °C (THI: 82.35), whereas based on cold stress responses, the LCT for zebu calves was identified at approximately 18.15 °C. Thus, the TNZ for zebu calves can be proposed to be between 18.15 and 30.10 °C. These findings can inform climate-adaptive housing and management strategies for improving calf welfare and productivity in subtropical environments.