Forced heat loss from body surface reduces heat flow to body surface

Heat stress is commonly relieved by forced evaporation from body surfaces. The mode of heat stress relief by heat extraction from the periphery is not clear, although it reduces rectal temperature. Radiant surface temperature (Ts) of the right half of the body surface was examined by thermovision in 4 lactating Holstein cows (30 kg of milk/d) during 7 repeated cycles of forced evaporation created by 30 s of wetting followed by 4.5 min of forced airflow. Wetting was performed by an array of sprinklers (0.76 m3/h), and forced airflow (>3 m/s velocity) over the right side of the body surface was produced by fans mounted at a height of 3 m above the ground. Sprinkling wetted the hind legs, rump, and chest, but not the lower abdomen side, front legs, or neck. The animals were maintained in shade at an air temperature of 28°C and relative humidity of 47%. Coat thickness was 1 to 2 mm, so Ts closely represented skin temperature. Mean Ts of 5 × 20 cm areas on the upper and lower hind and front legs, rump, chest, abdomen side, and neck were obtained by converting to temperature their respective gray intensity in single frames obtained at 10-s intervals. Little change occurred in Ts during the first wetting (0.1 ± 0.6°C), but it decreased rapidly thereafter (1.6 ± 0.6°C in the fifth wetting). The Ts also decreased, to a smaller extent, in areas that remained dry (0.7 ± 1.0°C). In all body sites, a plateau in Ts was reached by 2 min after wetting. The difference between dry and wet areas in the first cooling cycle was approximately 1.2°C. The Ts of different body areas decreased during consecutive cooling cycles and reached a plateau by 3 cooling cycles in dry sites (front leg, neck, abdomen side), by 5 cooling cycles in the hind leg, and 7 cooling cycles in the rump and chest. The reduction in mean Ts produced by 7 cycles was 4.0 to 6.0°C in wetted areas and 1.6 to 3.7°C in sites that were not wetted. Initial rectal temperature was 38.9 ± 0.1°C; it remained unchanged during first 5 cooling cycles, decreased by 0.1°C after 7 cooling cycles, and decreased to 38.4 ± 0.06°C after 8 to 10 cooling cycles, with no additional subsequent decrease. The concomitant reduction in Ts in dry and wet areas suggests an immediate vasoconstrictor response associated with heat extraction and later development of a cooler body shell. The reduction in rectal temperature represents a response involving transfer of heat from the body core to the body shell. This response mode requires consideration in settings of heat stress relief.