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Effect of Human Growth Hormone-releasing Factor (1–29)NH₂ on Growth Hormone Release and Milk Production in Dairy Cows¹

Agriculture Canada Research Station, Lennoxville, Quebec J1M 1Z3

P. Dubreuil and J. Morisset

University of Sherbrooke, Sherbrooke, Quebec J1K 2R1

P. Gaudreau, Y. Couture and P. Brazeau

University of Montreal, Montreal, Quebec H2L 4M1

² Reprint requests.

ABSTRACT

Twelve cows (209 d in lactation, 642 kg BW) were used in an experiment conducted over four 10-d periods (one preinjection, one injection, and two postinjection). Gelatine (n = 6) or 10 mg of growth hormone-releasing factor in gelatine (n = 6) was injected subcutaneously at 1000 h every day on d 11 to 20. Data were averaged for the last 5 d of each period. During the injection period, milk, fat, and protein yields increased by 3 kg·d^–1 (14.3%), .14 kg·d^–1 (16.7%), and .12 kg·d^–1 (15.4%), respectively. Moreover, milk, fat, and protein yields for the treated cows remained higher than for the control cows until the last postinjection period. Growth hormone response was evaluated from blood samples withdrawn from 2 h prior to 8 h postinjection on d 11, 15, and 20 and on d 40. After growth hormone-releasing factor injection, peaks and area under the curve were 24.5, 30.8, and 47.0 ng·ml^–1 and 2475, 3979, and 3741 ng·ml^–1 · min^–1 on d 11, 15, and 20, respectively. On d 40, there was no difference in growth hormone concentrations in blood between control and treated cows. These results demonstrate that 10 d of daily injection of a growth hormone-releasing factor increases milk production by 14.3% (3 kg·d^–1) and still induces growth hormone release at the end of the injection period without any sign of refractoriness.

¹ Lennoxville Research Station Contribution Number 206.