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Effect of Exogenous Growth Hormone on Lactational Performance in High Yielding Dairy Cows¹

Department of Animal Science, Cornell University, Ithaca, NY 14853

Bovine growth hormone (bGH) was administered to high-yielding Holstein cows fed a complete mixed ration ad libitum. Commencing on day 74 of lactation, 10 cows averaging 34.4 kg milk per day were divided into two groups and received a daily subcutaneous injection of bGH (51.5 IU/day) or a placebo. Injections were continued for an 11-day period and differences in lactational performance, nitrogen balance and estimated energy balance between the two groups were compared for the last 5 days of the preinjection and injection periods. Growth hormone resulted in increases of 9.5% in milk yield, 22.7% in milk fat yield, 14.5% in milk lactose yield and a 17.1% increase in milk energy secretion. Feed intake was slightly reduced (–4.3%, nonsignificant) while milk protein secretion and nitrogen balance were unchanged. Serum growth hormone levels in the bGH group were maintained at the higher concentrations of the normal physiological range during the injection period. By 48 hours following the last injection, declining bGH concentrations approached control values, and milk production decreased to preinjection values. Serum prolactin levels and plasma concentrations of free-fatty acids were slightly increased during the injection period in the bGH group. Growth hormone clearly enhances milk synthesis in the high-yielding dairy cow.

KEY WORDS: • growth hormone • milk composition • milk yield

¹ Supported in part by Cornell University Agricultural Experiment Station, The Upjohn Company and USDA/SEA-AR Dairy Forage Cluster. Portions of this paper were presented at the 75th Annual Meeting of the American Dairy Science Association in Blacksburg, VA, 1980, abstract no. P49, and at the Cornell Nutrition Conference in Syracuse, NY, 1980.

² On leave from Victoria Department of Agriculture, Ellinbank Dairy Research Institute, Warragul, Victoria 3820, Australia.

³ To whom reprint requests should be sent.

Manuscript received 23 February 1981.