INSULIN-LIKE-GROWTH-FACTOR (IGF)-I BUT NOT IGF-II PROMOTES LEAN GROWTH AND FEED-EFFICIENCY IN BROILER-CHICKENS

The efficacy of exogenous IGFs to stimulate growth and modulate protei n and fat deposition was examined in a number of broiler chicken lines . From around 600 g body weight the chickens received a continuous inf usion of vehicle (0.1 M acetic acid), human recombinant IGF-I or [Gly( 1)]IGF-II at 300 mu g/kg body weight per day, or a combined infusion o f 150 mu g/kg/day of each IGF for 2 weeks. Experiment 1 used commercia l broiler female chickens and included measurements of nitrogen balanc e, N-tau-methylhistidine excretion and muscle protein synthesis rates. in Experiment 2 the same treatments were applied to three experimenta l lines of chickens selected for high food consumption (relatively fat ), high food utilisation efficiency (relatively lean), or at random (c ontrol). IGF-I, but not IGF-II, significantly increased growth rate an d food utilisation efficiency by around 10-15% in each experiment, an effect which was consistent across all genotypes. Nitrogen balance was significantly increased by IGF-I in Experiment 1 as was carcass nitro gen content in Experiment 2, indicating that the increased growth was in lean tissue. Carcass fat was consistently reduced in chickens recei ving IGF-I and was related to the levels of circulating IGF-I (r(2) = 0.30, P < 0.01) but not triiodothyronine. Protein synthesis rates were unaffected by treatment and could not account for increased growth ra te. However, there was a significant reduction in N-tau-methylhistidin e excretion indicating a reduced rate of muscle protein breakdown in I GF-I-heated chickens (1.56%/day vs 2.05%/day for IGF-I-treated vs cont rols, P < 0.05). The efficiency of feed utilisation was inversely rela ted to the rate of protein breakdown (r(2) = 0.25, P < 0.01). In concl usion, these experiments are the first to report an enhancement of gro wth and food utilisation efficiency by broiler chickens receiving exog enous IGF-I. Our results show that IGF-I may be important in controlli ng the growth and efficiency of food utilisation of young chickens at least in part by modulating the rates of protein breakdown. (C) 1998 A cademic Press.