EFFECTS OF BOVINE SOMATOTROPIN ON INSULIN-LIKE GROWTH FACTOR-I, INSULIN, GROWTH AND CARCASS COMPOSITION OF LAMBS

The effect of different doses (0.0, 0.05, 0.15, 0.25 and 0.55 mg/kg/day) of recombinant bovine somatotrophin (BST) on plasma concentrations of growth hormone (GH), insulin-like growth factor-I (IGF-I), insulin, plasma metabolites, growth and body composition were studied by treating five groups (n = 8 /group) of pasture-fed wether lambs (6-7 months of age) with daily subcutaneous injections of saline or BST for 62 days. Lambs (n=4/group) were bled from 09.00 to 06.00 h on day 59-60. Plasma GH concentrations rose to a maximum (914 +/- 184 ng/ml) by 2 h following administration of an injection of BST (0.55 mg/kg/day) and after 24 h had decreased to 30-40 ng/ml GH in 0.25 and 0.55 mg/kg/day BST groups or to control concentrations in 0.05 and 0.15 mg/kg/day BST groups. Long-term BST treatment increased (P<0.001) plasma concentrations of IGF-I, insulin, nonesterified fatty acids and glucose. Body and carcass weights were also increased (P<0.05) by BST and there was a major (P<0.01) effect on body and carcass components, particularly fat depot weights. The results from this study suggest that the growth/compositional responses to BST treatment are related to increases in plasma IGF-I and insulin concentrations.