GENOTYPE WITH FEEDING REGIME INTERACTION IN PIGS DIVERGENTLY SELECTED FOR COMPONENTS OF EFFICIENT LEAN GROWTH-RATE

The genotype with feeding regime interaction was examined by testing pigs form four selection groups on both ad libitum and restricted feeding regimes. Within each selection group, there were high, low and control lines, which had been selected for lean food conversion ratio (LFC), daily food intake (DFI) or lean growth rate on ad libitum (LGA) or on restricted (LGS) feeding, in Large White-Edinburgh (LW) and Landrance-Wye (LR) populations. There were 1187 LW pigs and 768 LR pigs in the study, with 344 LW and 133 LR pigs tested on the alternative feeding regime. In the LW population, pigs in the high LGS line grew significantly faster than the high LGA and LFC lines with ad libitum feeding (919 v. 347 and 786 (s.e.d. 31) g/day), but had similar food conversion ratios adn backfat depths. The high LGS and high DFI lines were similar for growth rate, daily food intake and food conversion ratio, but backfat depth was significantly lower in the high LGS line than in the high DFI line (12.0 v. 15.9 (s.e.d. 0.7) mm). On restricting feeding, the rankings of the selection lines for growth rate, food conversion ratio nad mid-back fat depth were broadly similar to those with ad libitum feeding, except for the high LFC line. In the LR population, the high LGS, LGA and LFC lines did not differ significantly in growth rate, daily food intake, food conversion ratio and backfat depth within either the ad libitum or restricted feeding regimes. Growth rate of the high LGS line was similar to the high DFI line on ad libitum feeding (828 v. 836 (s.e.d. 40) g/day), but significantly higher on restricted feeding (704 v. 636 (s.e.d. 23) g/day). On both feeding regimes, food conversion ratio nad backfat depth were significantly lower in the high LGS line compared with the high DFI line. In the LW population, the genetic correlation between feeding regimes for growth rate was significantly less than one (0.4 (s.e 0.20)), but was not significantly different from unity (0.8 (s.e 0.14)) for backfat depths. Based on the performance test results, selection for lean growth with testing on a restricted feeding may be preferable to testing animals on an ad libitum feeding regime.