Effect of Growth Hormone on Milk Yields and Related Physiological Functions of Holstein Cows Exposed to Heat Stress

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Abstract
Responses of milk production and other physiological functions to daily s.c. injections of bovine growth hormone (16.6 mg/cow) were studied in 6 Holstein cows in midlactation under environmental heat stress. Five days were assigned for each treatment of thermoneutral (18.3.degree. C), heat (28.9.degree. C), heat plus growth hormone and heat2. 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 wt 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 heat2. Feed intake declined by 5.8% with no significant increase of heat production or body wt loss during treatment with bovine growth hormone. Heat increaed somatic cell concentration.