Características termofisiológicas e do ambiente térmico de bezerros mestiços leiteiros em região tropical

Abstract

Initially, the aim was to understand the relationships between the thermal environment and the thermophysiological responses of crossbred dairy calves raised in a tropical region through canonical correlation analysis (CCA), and to determine the most important thermophysiological and environmental variables for this animal category. Thirty-seven females, aged 20 to 60 days, were evaluated. Respiratory rate (RR), rectal temperature (RT), surface body temperature (BST), air temperature (Tair), globe temperature (Tg), relative humidity (RH), and wind speed (Ws) were measured in the morning and afternoon. Cutaneous evaporation (Ec) and respiratory evaporation (Er) as well as the main thermal stress indices were calculated. Through CCA, five sets of physiological variables (U) and thermal environment variables (V) were identified, with the first two (U1V1 and U2V2) being the most important. In the morning, the highest canonical correlations were observed between U1 and Er (-0.911), U2 and BST (-0.966), V1 and RH (0.978), and V2 and the temperature-humidity index (THI) (-0.766). In the afternoon, the highest correlations were between U1 and Er (-0.904), U2 and BST (0.640), V1 and RH (0.996), and V2 and the black globe and humidity index (BGHI) (0.707). The Er variable showed a strong correlation with radiation (S) in the morning and with the thermal comfort index for dairy cows (TCIdc) in the afternoon. Meanwhile, BST was strongly correlated with THI and Tair in the morning and with the heat load index (HLI) and BGHI in the afternoon. The second study aimed to identify the thermophysiological variables, thermal environment variables, and thermal stress indices that contribute the most to assessing the heat stress effect in crossbred dairy calves raised in a tropical environment, as well as propose an adjustment for the THI equation for this animal category. Calves aged 15 to 60 days were evaluated during the morning and afternoon periods. RR, RT, and BST were quantified. Tair, Tg, Ws were measured, and the main thermal stress indices were calculated. In the principal component analysis (PCA), the thermophysiological variables that exhibited the five highest factor loadings in principal component 1 (PC1) were: SBT (0.719), RR (0.697), and RT (0.679). The thermal environment variables were: Tair (0.914), THI (0.837), and respiratory frequency index (0.804). In the multiple linear regression analysis for adjusting the THI for crossbred dairy calves raised in a tropical environment, it was observed that Tair and RH were highly significant, and the new equation showed an R² of 0.98. It is concluded that the thermophysiological and thermal environment variables are strongly associated, and THI is a recommended equation for evaluating the effect of heat stress in crossbred dairy calves raised in a tropical environment.