Comparison of thermoregulatory mechanisms in heat sensitive and tolerant breeds of Bos taurus cattle

Six Angus steers (319±8.5kg) were assigned to one of two groups (hot or cold exposure) of three steers each, and placed into two environmental chambers initially maintained at 16.5–18.8°C air temperature (Ta). Cold chamber Ta was lowered to 8.4°C, while Ta within the hot chamber was increased to 32.7°C over 24 hours. Measurements included respiration rate, and both air and body (i.e., rectal and skin) temperatures. Skin temperature was measured at shoulder and rump locations, with determination of sweat rate using a calibrated moisture sensor. Rectal temperature did not change in cold or hot rooms. However, respiration rate nearly doubled in the heat (P<0.05), increasing at a Ta above 24°C. Skin temperatures at the two locations were highly correlated (P<0.05) with each other and with Ta. In contrast, sweat rate showed differences at rump and shoulder sites. Sweat rate of the rump exhibited only a small increase with Ta. However, sweat rate at the shoulder increased more than 4-fold with increasing Ta. Increased sweat rate in this region is supported by an earlier report of a higher density of sweat glands in the shoulder compared to rump regions. Sweat rate was correlated with several thermal measurements to determine the best predictor. Fourth-order polynomial expressions of short-term rectal and skin temperature responses to hot and cold exposures produced r values of 0.60, 0.84, and 0.98, respectively. These results suggest that thermal inputs other than just rectal or skin temperature drive the sweat response in cattle.