Heat stress responses and thermoregulatory dynamics in Murrah buffalo calves assessed by infrared thermography

The study investigated thermoregulatory dynamics in Murrah buffalo calves under heat stress using infrared thermography. Six calves (8-12 months old; weighing 125 ± 15.35 kg) were exposed in a psychrometric chamber to temperatures of 25, 28, 31, 34, 37, and 40 °C, corresponding to temperature-humidity indices (THIs) ranging from 73 to 90. Each exposure lasted 12 h daily (0600-1800 h) for 10 consecutive days, with animals kept at 25 °C and 65% humidity (THI: 73.49) during the remaining 12 h. On the 10th day, respiratory rate (RR), pulse rate (PR), rectal temperature (RT), feed intake and water intake were recorded and infrared thermography was done to record body surface temperatures (BST) at head, neck, eye, ear, croup, and muzzle besides blood sampling. Significant increases (p < 0.05) in RT, PR, RR, and water intake were first observed at 34 °C, while feed intake declined significantly (p < 0.05) at 34 °C. BSTs increased significantly (p < 0.05) at 28 °C compared to 25 °C and rose progressively thereafter, though values at 28 and 31 °C, and 34 and 37 °C, were similar except at muzzle. An approximate 5 °C rectal-to-surface temperature gradient was associated with effective thermoregulation under cyclic heat stress. Hematological changes included leukocytosis (p < 0.05) at 34 °C, along with lymphocytosis, granulocytopenia (p < 0.05) and elevated plasma cortisol (p < 0.05) at 31 °C. Among anatomical sites, muzzle temperature exhibited strongest correlation with ambient temperature (p < 0.01), while ear and neck temperatures correlated most strongly with RT (p < 0.01). Correlation analysis further revealed that all studied physiological and hematological parameters, except erythrocytic indices and granulocyte percentage, were significantly (p < 0.05) associated with BST at one or more body sites. In conclusion, progressive cyclic heat exposure induced staged thermophysiological adjustments in Murrah buffalo calves, with 31-34 °C representing a transitional range of systemic activation. Maintenance of an approximate 5 °C rectal-to-surface temperature gradient reflected effective thermoregulatory reserve, whereas its narrowing indicated increasing thermal strain. Ear and neck surface temperatures showed strong associations with rectal temperature, supporting their potential as non-invasive indicators of thermal status. Infrared thermography may therefore aid precision-based heat stress monitoring in buffalo.