Abstract
In a first experiment growth hormone was determined in serum of lactating cows infused intravenously with bovine growth hormone-releasing factor for 24h. Concentrations of growth hormone increased 5 times above placebo after 3.125, 6.25, 12.5, 25.0, or 50.0mg releasing factor/cow per 24h. Releasing factor increased the pulsatile nature of growth hormone release. In a second experiment we determined effects of intravenous infusions of releasing factor for 20 d on growth hormone and lactation in primiparous cows (n = 5/treatment; 144 ± 7 d after parturition). Milk yield of placebo-infused cows averaged 26.5 ± 1.7 kg/d during d 6 to 20 of infusion. Infusions of 1 and 3mg releasing factor/cow per 24h increased milk yield 11 and 23%, respectively. Releasing factor had only small effects on milk composition, net energy intake, and BW. Releasing factor did not affect energy efficiency of milk production (energy output in milk per unit of net energy intake) when corrected for BW changes. Average growth hormone across d 1, 10, and 19 of infusion was 2.1, 4.1, and 6.8ng/ml serum for placebo, 1, and 3mg releasing factor, respectively. Infusions of bovine growth hormone-releasing factor: 1) increased mean and pulsatile release of growth hormone during 24h and 20 d, 2) increased milk yield in a dose-dependent manner, and 3) did not affect BW-corrected energy efficiency of milk production.
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