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Differences between times of day for the basic physiological traits of Nellore bulls exposed to solar radiation

ABSTRACT The aim of this study was to evaluate the basic physiological traits of Nellore bulls exposed to solar radiation at different times of the day. Sixteen Nellore bulls with an average age of 3 years were used. Air temperature (Ta, °C), relative humidity (RH, %), wind speed (U, m/s) and solar radiation (Rs, W/m2) were the meteorological variables evaluated during the study. Respiratory rate (Fr, breaths/min), rectal temperature (Tr, °C), body surface temperature (Ts, °C) and expired air temperature (Texp, °C) were evaluated as physiological traits. The highest Fr and Ts averages were observed from 11:00 to 14:00 (36.34 breaths/min and 37.62 °C, respectively). Texp presented increasing values throughout the day, with its highest average (33.35 °C) between 15:00 and 17:00. The Tr did not present significant variation between the times of day. It is concluded that intense Rs and high Ta during the hottest times of day induce Nellore bulls to trigger thermoregulatory mechanisms, such as the respiratory system, to maintain body temperature within narrow limits.