Serum insulin-like growth factor-1 concentration in pigs divergently selected for daily food intake or lean growth rate

Divergent selection over seven generations for daily food intake (DFI) and for two lean growth rate (LGA and LGS) selection objectives in pigs produced correlated responses in serum insulin-like growth factor-1 (IGF-1) concentration. The LGA and LGS selection objectives were for lean growth, but with performance testing on ad libitum (LGA) or on restricted (LGS) feeding regimes. There were 342 animals in the study, with an average of 46 pigs in each selection line and 72 pigs in the control line. All animals were performance-tested from 30 to 90 kg on an ad libitum feeding regime. Selection on DFI increased IGF-1 at 6 weeks of age (158 versus 104 mu g/l, SED 21; control 136 mu g/l) and selection for high LGA increased IGF-1 at 30 kg liveweight (202 versus 161 mu g/l, SED 14; control 185 mu g/l). At 6 weeks of age and at 30 kg of weight, there was no response in IGF-1 with selection on LGS (128 versus 129 and 167 versus 153 mu g/l, respectively) suggesting that IGF-1 may not be a reliable physiological predictor of genetic merit for such a selection objective, which combined rate and efficiency of lean growth. IGF-1 measured immediately prior to the start of performance test may be useful in determining which animals to performance test for a lean growth selection objective, whereas IGF-1 measured at 6 weeks of age may be a useful physiological predictor of genetic merit for daily food intake during performance test.