Circadian and Diurnal Variations in Infrared Thermographic Body Surface Temperature of Karan Fries, Sahiwal, and Murrah Heifers under Thermoneutral Conditions

Abstract

Establishing thermoneutral surface temperature baselines for Indian dairy breeds is critical for designing precision early-warning systems for heat stress. The present study employed digital infrared thermography (IRT) to characterise circadian and diurnal variations in body surface temperature at five anatomical regions, namely the forehead, muzzle, left flank, dewlap, and ear in Karan Fries (KF), Sahiwal, and Murrah heifers under thermoneutral conditions (March–April; temperature–humidity index [THI], 68–72). Thirty-six heifers (12 per genotype) were equally assigned to a Sheltered Group (SG; n = 6 per genotype) and a Non-Sheltered Group (NSG; n = 6 per genotype). Thermographic images were captured using a Testo 868 infrared camera (emissivity 0.96; distance 1.5 m) at eight time points over 24 h at 3-hour intervals for 30 days. A consistent and significant (p < 0.05) circadian rhythm was documented across all body regions and genotypes, with peak surface temperatures recorded at 15:00 h and minimum values at 06:00 h. No significant inter-genotype variation in surface temperature was observed within either management group during thermoneutral conditions; however, significant differences (p < 0.05) were detected between the SG and NSG at warmer diurnal time points for all body regions, with a reversal of this pattern during nocturnal hours. Across all body regions, KF consistently recorded the highest surface temperatures, followed by Murrah, whereas Sahiwal recorded the lowest; this ordering reflects the differential heat-dissipation efficiency and adaptive thermoregulation of these genotypes. The thermoneutral thermographic profiles established here provide preliminary reference data for genotype-specific heat stress threshold delineation and precision livestock management under subtropical conditions.