Distinct physiological and immunoendocrine responses in indigenous dairy heifers during cold-humid (THI-50) vs. hot dry (THI-76) climatic conditions

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Abstract
Thermal stress threatens dairy productivity in tropical climates, even for heat-adapted indigenous Sahiwal cows (Bos indicus). Temperature extremes combined with humidity impair thermoregulation, immune function, and welfare amid climate change. This study evaluated physiological and immune endocrine responses in nulliparous Sahiwal heifers (n = 6) aged 12-18 months across THI levels of 50, 59, 67, and 76. Repeated measurements on the same animals generated 12 observations per THI level. Physiological parameters (rectal temperature, respiration, and pulse rates) and plasma biomarkers (cortisol, IL-6, IL-10, and TNF-α, via ELISA) were measured at each THI level. Escalating THI from 50 (cold-humid) to 76 (hot-dry; dry-bulb 29.52 °C, wet-bulb 23.27 °C) elicited graded responses. Rectal temperature increased modestly (P < 0.05) to 39.47 °C at THI-76 (+ 2.3% vs. thermoneutral THI-67). Respiration rate showed the greatest sensitivity, surging + 18.7% at THI-76 and declining - 18.3% at THI-50. Pulse rate rose + 13.6% under hot-dry stress. Cortisol followed a biphasic pattern, dropping at THI-67 before rebounding 1.8-fold at THI-76 (+ 84.3% vs. THI-67), with even greater elevation (+ 127.6%) at cold-humid THI-50. Cytokines displayed stress-specific shifts: IL-6 fluctuated, anti-inflammatory IL-10 surged + 39.6% at THI-76, and TNF-α declined then mildly rose. Respiration rate and cortisol emerged as prime stress indicators. Both cold and heat disrupt Th1/Th2 balance, underscoring the need for season-specific microclimate management in tropical dairy systems.
Funding Information
  • Department of Biotechnology, Ministry of Science and Technology, India (BT/PR46654/AAQ/1/864/2022)

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