Effect of solar radiation on the heat load of dairy heifers.

Abstract

Thermoregulatory responses of Holstein heifers were studied under direct solar radiation and shade conditions during summer and autumn periods in southern Japan. The data were used to predict the contribution to effective temperature (ET) of solar radiation. The amount of solar radiation, including both direct and indirect sources, was estimated from black globe temperature (GT). There was no effect of solar radiation on the rate of heat production nor heart rate of the heifers. Rectal temperature increased significantly under no shade conditions during the summer period. Respiration rate (RR) and mean skin surface temperature paralleled changes in dry bulb temperature (DBT) and GT. The ET in the test situations, using RR and mean body temperature (Tb) as physiological indices, was predicted as: ET(DBT, GT) = 0.24DBT + 0 76GT. This equation indicates that solar radiation as measured by black globe temperature contributes substantially more to the heat load on animals than does dry bulb temperature.