Growth hormone-releasing factor stimulates milk production and sustains growth hormone release in Holstein cows

Serum growth hormone was determined in lactating cows following repeated intravenous injections of growth hormone-releasing factor. A given dose was injected every 4 h for 24 h in a 4 (cow) X 4 (d) Latin square. Growth hormone increased similarly above controls after 10, 20, or 40 micrograms releasing factor/100 kg body weight. In another experiment the effects on lactational performance and growth hormone responses of cows to repeated injections of releasing factor for 10 d were determined in a 2 (cow) X 2 (period) Latin square crossover. Administration of 20 micrograms releasing factor/100 kg body weight to 16 Holstein cows (lactating 4.5 to 7.5 mo) every 4-h for 10 d increased milk yield from 25.4 to 27.7 kg/d and increased total fat, protein, and lactose 11% above controls. Releasing factor did not affect milk composition or feed intake. Peak growth hormone response to releasing factor was similar between d 1 (19.9 ng/ml) and 10 (24.4 ng/ml). Exogenous growth hormone-releasing factor administered to lactating Holstein cows at the doses tested: 1) increases growth hormone consistently, although the response is not dose dependent, 2) is galactopoietic, 3) causes an apparent increase in feed to milk conversion, and 4) increases growth hormone to at least the same magnitude on d 10 as on d 1.