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Effect of exogenous growth hormone on lactational performance in high yielding dairy cows.

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.