Abstract

During summer, 34 cows received daily injections of placebo or 25 mg of bST and were placed in a thermoregulated or a heat stress environment. Heat stress increased rectal temperatures, respiration rates, and plasma cortisol concentrations and decreased milk yield. Four of 9 bST-treated cows and none of 8 control cows became atactic on the 1st d of heat stress. When exposed to heat stress, cows treated with bST experienced higher rectal temperatures throughout the trials than cows treated with placebo. Nonetheless, bST increased milk yields in both environments. The major effect of heat stress on immune function was decreased migration of leukocytes to the mammary gland after chemotactic challenge. This effect of heat stress was not altered by bST. In summary, hyperthermia induced by heat stress and associated changes were greater for cows treated with bST. Detected effects of heat stress on the immune system were few and were not alleviated by bST. Use of bST during summer in subtropical climate zones requires careful management to avoid overexposure of bST-treated cows to heat stress.