EFFECT OF A GROWTH HORMONE-RELEASING FACTOR ANALOG ON GROWTH HORMONE, INSULIN-LIKE GROWTH FACTOR 1 AND MILK PRODUCTION IN DAIRY COWS

In a first experiment, 21 cows (98 d in lactation, 609 kg BW) were randomly used to determine the dose-response release of growth hormone (GH) to [desamino-Tyr1, D-Ala2, Ala15] growth hormone-releasing factor (1–29)NH2 analog (GRF-A; 0, 0.12, 0.37, 1.11 and 3.33 μg kg−1 BW) or to human growth hormone-releasing factor (1–29)NH2 (GRF; 3.33 and 10.00 μg kg−1 BW) after s.c. administration. The interaction (P < 0.01) between the two releasing factors and the doses on GH concentrations indicated that the analog was more potent than the original molecule; at the 3.33 μg kg−1 BW dose, the analog elicited a 3.5 times higher GH response than GRF. In a second experiment, the effects of daily s.c. injections for 10 d of either saline, GRF (10 μg kg−1 BW) or GRF-A (0.6 or 1.8 μg kg−1 BW) on GH concentrations and lactational performance were determined using 32 cows (n = 8 per treatment) averaging 91 d in lactation and subjected to a random block design. The three releasing factor treatments similarly increased GH concentrations measured during the 8 h following the injection on d 1 and 10. GRF at the dose of 10 μg kg−1 BW and the analog at the doses of 0.6 and 1.8 μg kg−1 BW increased milk yield by 9.6, 11.7 and 17.6%, respectively, when compared to the control; both releasing factors had no effect on milk composition. Insulin-like growth factor I concentrations were increased 8 h after the last injection, from 73.4 ng mL−1 in control cows to an average of 107.6 ng mL−1 in treated cows. Results show that the [desamino-Tyr1, D-Ala2, Ala15] human growth hormone-releasing factor (1–29)NH2 analog is more potent than human growth hormone-releasing factor (1–29)NH2 in stimulating GH release and milk production in dairy cows. Key words: Growth hormone-releasing factor, analog, growth hormone, milk production, insulin-like growth factor 1, cows (dairy)