Comparative physiological and biochemical assessment of the heat tolerance of dwarf Vechur, Kasaragod, and standard-size crossbred cattle under humid, hot conditions.

Abstract

Reduced body size is an ecological response to climate change. Differential responses to heat stress in phenotypically diverse bovine lineages may imply a body size-dependent stress response. Heat-tolerant dwarf Vechur, Kasaragod (Bos taurus indicus), and heat-sensitive crossbred (CB, B. t. indicus x B. t. taurus) cattle were tested physiologically, biochemically, and in terms of cellular protein expression. Thirty adult lactating cows (ten Vechur, Kasaragod, and CBs each) were allowed to graze during the summer. The environmental parameters measured included Ta (ambient temperature), RH (humidity), WS (wind speed), and SR (solar radiation intensity). In addition, the temperature humidity index (THI), heat load index (HLI), and accumulated heat load (AHL) were computed. The panting score (PS) was determined using breath characteristics. Numerous physiological (rectal temperature-RT, respiratory rate-RR and pulse rate-PR), haematological, and biochemical (serum cortisol) heat stress markers were identified and validated. RT, RR and PR were evaluated to determine the linear correlation coefficients and predictors. The correlation coefficients in CB were significantly higher than in Vechur and Kasaragod (p < 0.01). Stepwise regressions showed that in CB, Ta alone was the environmental measure that best described the indicator variables PS, the difference between RT recorded at half-hour intervals (RT_diff, R² = 0.925), and white blood cell count (R² = 0.984). Differences in cellular protein expression were also evident. Under heat stress conditions, linear discriminants based on RT, RR, and PR separated dwarf (Vechur and Kasaragod) and crossbred cattle into various clusters, and significant breed-wise grouping was identified based on haematological parameters in pre-stress and heat stress. The study established the variable heat stress response of phenotypically divergent Bos lineages and relevant heat stress markers and thermal indices for measuring heat stress.