Exogenous growth hormone releasing factors and cottonseed meal improve growth performance and composition of gain in lambs fed lucerne chaff ad libitum

Abstract

Forty-eight crossbred wether lambs (38 kg) were randomly assigned to two dietary groups (LUC, chaffed lucerne ad libitum; CSM, LUC plus 300 g cottonseed meal/lday) and two hormone treatment groups (rGRF, recombinant growth hormone-releasing factor (GRF); sGRF, synthetic GRF) plus controls (CON, excipient only), and slaughtered after treatment for 28 (n = 36) or 30 (n = 12) days. An initial slaughter group (n = 12) was killed on day 0. CSM reduced intake of lucerne chaff, but increased total feed intake. CSM also increased liveweight gain and wool growth, with no effect on feed conversion efficiency. CSM increased carcass weight and decreased subcutaneous fat depth, with no effect on dressing percentage or longissimus dorsi (LD) area. Meat tenderness tended to decrease with CSM supplementation. CSM increased carcass protein accretion ( P < 0.01), with no significant change in fat gain or final composition (P > 0.05). CSM increased weights of the pelt, liver and empty body, and decreased the proportion of digesta in liveweight. Weights of blood, forestomachs and small intestine tended to increase as well (P < 0.10). Both sources of GRF had similar effects. Intakes were not significantly affected by GRF, but feed conversion efficiency improved (P < 0.01) with GRF treatment (6.4 v. 9.2 feed:gain for GRF and CON respectively). Liveweight gains and final weights of the whole body and carcass were increased additively by CSM and GRF. GRF reduced fat depth and increased LD area, with no effect on dressing percentage, carcass length or meat tenderness. These changes reflected increased carcass protein and reduced fat contents, brought about by increased accretion of protein and water and reduced fat gain due to GRF. GRF increased weights of blood and liver beyond the general increase in body size. Cottonseed meal and GRF additively produced significant improvements in growth performance and carcass quality of young wether lambs fed a roughage diet. GRF may be a useful tool to improve livestock production under extensive and intensive conditions.