Time spent in shade and its relationship with physiological factors of thermoregulation in three breeds of cattle

Time spent in shade by a group of 6 Brahman (B), 6 Shorthorn (S) and 6 Brahman × Hereford—Shorthorn cross (BX) steers was studied in an experimental shade-yard and subsequently at pasture. B sought shade least and S the most in both situations; the cooling effect of time spent in shade on S is reported. Time spent in shade was positively correlated with mean radiant temperature and solar radiation in both situation. The amount of time animals spent in shade in an experimental shade-yard was highly correlated with the amount of time spent in shade and in grazing at pasture.Mean values of rectal temperature (T_re), respiration rate (R_r), skin temperature ($\text{T}{}_{\mathrm{sk}}$) and sweating rate (S_r) measured immediately prior to release in the shade-yard were calculated. T_re and R_r were positively correlated with time spent in shade, both in the shade-yard and at pasture, and negatively correlated with time spent grazing. S_r was highly positively correlated with time spent in shade and negatively correlated with time spent grazing. The relative contribution of the physiological parameters to time spent in shade and grazing, were estimated by stepwise multiple regression models. R_r was the best predictor of the time spent in shade in both situations. The only significant improvement in variance accounted for was by the inclusion of S_r in the time spent in shade at pasture. T_re was the best predictor of grazing time and the variance accounted for by the model could not be significantly improved by inclusion of any other physiological factor measured.It is suggested that shade use and respiration rate could be simple and effective methods to select heat tolerant animals in the tropics. Both are easily measured over a wide range of situations and are better correlates of thermoregulatory behaviour than the other physiological factors measured.