Impact of Hair Color on Thermoregulation of Dairy Cows to Direct Sunlight

Abstract

Summary:The effects of hair coat color on the thermoregulatory responses of heat-stressed cows exposed to direct solar radiation were observed. During August 2000, three white and three black lactating Holstein cows in Hawaii were exposed to 1.5 hr of sunlight for four trials in the morning and four trials in the afternoon with or without spray cooling. Solar radiation ranged between 495 and 971 watts/m 2 and THI ranged between 78 and 81 during the experimental trials. Cows with white hair coat absorb about 66% of the short wave radiation, while cows with black hair coat absorb about 89% of the short-wave radiation. Placing the cows under direct sunlight increased the surface temperature of the black cows by 4.8C and the white cows by 0.7C. Rectal temperatures of black cows increased 1.3C/hr, while the rectal temperatures of white cows increased 0.8C/hr. Increasing solar load on the skin surface increases the local rate of sweating. Spray wetting alone is not sufficient to cool cows exposed to high solar loads.