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

The genotype with feeding regime interaction was examined by testing p igs form four selection groups on both ad libitum and restricted feedi ng regimes. Within each selection group, there were high, low and cont rol 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 Landran ce-Wye (LR) populations. There were 1187 LW pigs and 768 LR pigs in th e 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 signific antly 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 simil ar for growth rate, daily food intake and food conversion ratio, but b ackfat 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 r atio nad mid-back fat depth were broadly similar to those with ad libi tum feeding, except for the high LFC line. In the LR population, the h igh LGS, LGA and LFC lines did not differ significantly in growth rate , daily food intake, food conversion ratio and backfat depth within ei ther 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 hig h LGS line compared with the high DFI line. In the LW population, the genetic correlation between feeding regimes for growth rate was signif icantly less than one (0.4 (s.e 0.20)), but was not significantly diff erent from unity (0.8 (s.e 0.14)) for backfat depths. Based on the per formance test results, selection for lean growth with testing on a res tricted feeding may be preferable to testing animals on an ad libitum feeding regime.