Short communication: Upper critical temperature-humidity index for dairy calves based on physiological stress variables

摘要

In this study, upper critical values of the temperature-humidity index (THI) were determined in Holsteinbull calves (n = 16) based on noninvasive physiologicalvariables. Meteorological and animal-based data wererecorded for a 4-d period following a 24-h habituation.The estimated upper critical THI values based on theassessment of respiratory rate, rectal temperature, eartemperature, heart rate, and salivary cortisol concentrationswere 82.4, 88.1, 83.0, 78.3, and 88.8, respectively.We inferred that welfare of young calves may becompromised above a THI of 78 and that calves experiencesignificant heat stress above a THI of 88. Basedon the present findings, upper critical THI should beconsidered to minimize the duration of impaired welfareduring summer heat stress episodes.