Comparison of different temperature-humidity index as heat stress indicator in crossbred dairy calves in a tropical environment

The Temperature Humidity Index (THI) was first used to evaluate human beings and later it was adapted to be used in production animals, especially dairy cows. The efficiency of this index is not known for dairy calves, especially the crossbred ones. The aim of this study was to compare different equations of temperature-humidity index as heat stress indicator in crossbred dairy calves in tropical environment. Twenty-seven crossbred dairy calves, males and females, housed in Argentine model’s calf, were evaluated to respiratory rate, rectal temperature and surface body temperature on 2^nd, 15^th, 30^th and 60^th days of live, totalizing 108 observations. The environment was evaluated at the moment of each animal data collection and the dry bulb temperature and wet bulb temperature were recorded, and used to estimate relative humidity, partial air vapour pressure and dew point temperature. Nine different equations of temperature-humidity index were calculated. ANOVA followed by Tukey test of mean comparison with 5% of significance and Pearson correlation analysis were performed using the Action program. Respiratory rate, rectal temperature and surface body temperature did not differ between the ages. Only in 4.62% of the observations calves had a respiratory rate between 60 to 80 movements per minute, which represents a moderate heat stress, and in 5% of the observations calves had a rectal temperature above 39.5 °C, indicating that they were not able to maintain homeothermy. In 25% of the observations presented smaller gradients between surface body temperature and environmental temperature, indicating that in 75% of observations the convective heat loss was favored. The average temperature (21 °C) was near the maximum temperature recommended to dairy calves of European origin, which ranges of 5 °C to 20 °C, but the thermal comfort temperature is not known for crossbred dairy calves in a tropical environment. The majority mean values of THI indicated a comfort situation. The correlation of dry bulb temperature and the physiological parameters was positive and significant. The respiratory rate and surface body temperature had a significant and positive correlation with the temperature humidity indexes. The correlations of physiological data with THI sensible heat-based were higher than the others. The THI sensible heat-based is the equation that better represents the comfort situation of crossbred dairy calves in a tropical environment.