EXOGENOUS HUMAN GROWTH HORMONE-RELEASING FACTOR AND OVINE SOMATOTROPIN IMPROVE GROWTH-PERFORMANCE AND COMPOSITION OF GAIN IN LAMBS

The objectives of this study were 1) to compare intermittent subcutaneous administration of human growth hormone-releasing factor (hGRF) at two doses with a similar regimen of ovine somatotropin (oST) for effects on growth and composition of gain in growing lambs and 2) to determine whether increasing the dietary amino acid availability enhances response to oST or hGRF. Eight crossbred ewe and wether lambs (25.5 kg live weight) were assigned randomly in pairs to receive four daily injections of excipient, 40-mu-g oST/kg BW, 5-mu-g hGRF/kg BW or 10-mu-g hGRF/kg BW for 42 (n = 80) or 56 (n = 40) d. Doses were adjusted weekly for BW. Mean plasma oST concentrations increased from 2.03 ng/ml prior to treatment to 20.64, 4.80 and 5.45 ng/ml with oST, 5 and 10-mu-g/kg hGRF doses, respectively. Lambs did not become refractory to hGRF. Cumulative gain increased approximately 18% with 7 wk of treatment with oST and the low dose of hGRF (both P < .01), and feed efficiency improved 21% with oST and 18% with both doses of hGRF (both P < .05). Carcass lipid accretion rate decreased 22% to 30% (P < .001), and carcass protein accretion rate increased 30% to 36% (P < .001) with hGRF and oST treatment, respectively. Addition of fishmeal to the diet at 4% to replace an equal amount of soy protein improved gain 8.5%; it improved efficiency 14.2% (P < .05) across all treatments, and it significantly enhanced the effects of oST on feed efficiency (interaction P < .12) and hind leg muscle weights.