Circadian Patterning of Infrared Thermographic Body Surface Temperature in Karan Fries, Sahiwal, and Murrah Heifers during Hot Dry Season and Influence of Shelter Provision

Abstract

The pre-monsoon hot dry period (May to June) in the Indo-Gangetic plains subjects dairy genotypes to a combination of high ambient temperature, low humidity, and intense direct solar radiation that is thermally more demanding than the preceding thermoneutral spring. This study used digital infrared thermography (IRT) to profile body surface temperature at five anatomical sites, namely the forehead, muzzle, left flank, dewlap, and ear, in Karan Fries (KF), Sahiwal, and Murrah heifers during the hot dry season, and to examine how shelter provision modifies the diurnal course of these temperatures. Thirty-six heifers (twelve per breed) were divided equally into a Sheltered Group (SG, n = 6 per breed) housed in a partially open, cross-ventilated shed, and a Non-Sheltered Group (NSG, n = 6 per breed) maintained in an adjacent open paddock under direct solar exposure. Thermographic images were recorded with a Testo 868 infrared camera (emissivity 0.96) at eight three-hourly intervals across the 24-hour cycle over 30 consecutive days. The mean temperature-humidity index during the study period ranged from 74 to 78, corresponding to a mild-to-moderate heat load. All five body regions displayed a pronounced 24-hour temperature rhythm in both groups, with values rising through the morning to a daytime maximum and falling to an early morning minimum, a pattern consistent with an underlying circadian thermoregulatory rhythm. Breed differences in surface temperature were not statistically significant within the sheltered group at nearly all time points, but became significant during the warmer daytime hours in the non-sheltered group, where KF recorded the highest values, Murrah intermediate values, and Sahiwal the lowest. Direct solar exposure amplified the daily temperature range at every site relative to the sheltered group, most strikingly at the ear, where non-sheltered KF heifers ranged from approximately 20 degrees Celsius before dawn to approximately 41 degrees Celsius in the mid-afternoon. These findings indicate that the interaction between genotype and shelter availability, rather than genotype alone, governs peripheral thermal load during the hot dry season, and they provide genotype-specific thermographic reference values that may assist in refining shelter management during this season.