SELECTION FOR COMPONENTS OF EFFICIENT LEAN GROWTH-RATE IN PIGS .3. RESPONSES TO SELECTION WITH A RESTRICTED FEEDING REGIME

Responses to four generations of divergent selection in pigs for lean growth rate (LGS) with restricted feeding were studied. The selection criterion was designed to obtain equal correlated responses in growth rate and carcass lean content, measured in phenotypic s.d. Animals were to be performance tested in individual pens with a mean starting weight of 30 kg for a period of 84 days. Daily food intake was equal to 0.75 g/g of the daily food intake for pigs offered food ad-libitum. In the high, low and control lines, there were 1250 Large White-Edinburgh (LW) pigs and 875 British Landrace-Wye (LR) pigs. Each selection line consisted of 10 sires and 20 dams, with a generation interval of 1 year. After four generations of selection, cumulative selection differentials were 5.9 and 4.8 phenotypic s.d. for LW and LR populations, with similar responses, 1.8 (s.e. 0.17) phenotypic s.d. Mean weight at the end of test, growth rate and backfat depths at the shoulder, mid back and loin were 89 kg, 712 g/day, 26, 13 and 13 mm for LW and for LR pigs were 87 kg, 683 g/day, 28, 10 and 10 mm. High line pigs were heavier at the end of test (4.3 (s.e.d. 1.4) kg and 4.0 (s.e.d. 1.6) kg) for LW and LR populations, with corresponding responses in growth rate (54 (s.e.d. 16) g/day and 47 (s.e.d. 18) g/day). Responses in the three backfat depths were -4.1 (s.e.d. 1.2) mm, -2.6 (s.e.d. 0.7) mm and -2.9 (s.e.d. 0.7) mm for LW and -2.2 (s.e.d. 0.05) mm, -2.2 (s.e.d. 0.4) mm and -2.4 (s.e.d. 0.5) mm for LR populations. Responses in weight off test and backfat depths were symmetric about the control lines. Heritabilities for LGS were 0.34 and 0.28 (s.e.d. 0.5) for the LW and LR populations, when estimated by residual maximum likelihood. Common environmental effects for LGS were 0.11 (s.e. 0.03) for LW and 0.17 (s.e. 0.04) for LR. Heritabilities for growth rate and average backfat depth were similar for LW and LR populations (0.17 and 0.29, s.e. 0.05), as were common environmental effects (0.10 s.e. 0.04). Average phenotypic and genetic correlations between growth rate and backfat, for LW and LR populations, were small (0.15 (s.e. 0.03) and -0.06 (s.e. 0.16), respectively). Responses to selection and genetic parameter estimates demonstrate that there is substantial genetic variation in growth and fat deposition when pigs are performance tested on restricted feeding.