Effect of growth hormone on milk yields and related physiological functions of Holstein cows exposed to heat stress.

Responses of milk production and other physiological functions to daily subcutaneous injections of bovine growth hormone (16.6 mg/cow) were studied in six Holstein cows in midlactation under environmental heat stress. Five days were assigned for each treatment of thermoneutral (18.3 degrees C), heat (28.9 degrees C), heat plus growth hormone, and heat. Each period was preceded by at least 3 days of adjustment. Feed and water were available ad libitum. Feed intake, milk production, and rectal temperature were measured twice daily. Blood, energy metabolism, and body weight were measured every 1st, 3rd, and 5th day of each period. Administration of growth hormone under heat stress increased milk production by 3.8 and 12% and fat yield by 9.5 and 12.7% over heat and heat. Feed intake declined by 5.8% with no significant increase of heat production or body weight loss during treatment with bovine growth hormone. Heat increased somatic cell concentration.