Responses to four generations of divergent selection in pigs for lean growth rate with restricted feeding (LGS) were studied in populations of Large White (LW) and British Landrace (LR). Totals of 1250 LW and 875 LR pigs were performance tested in Edinburgh and Wye, respectively. After four generations of selection, cumulative selection differentials were 5.9 and 4.8 phenotypic s.d. for LW and LR pigs, respectively, with similar responses, 1.8 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 and 4.0 kg) for LW and LR pigs, with corresponding responses in growth rate (54 and 47 g/day). Responses in the shoulder, mid-back and loin backfat depths were -4.1, -2.6 and -2.9 mm for LW and -2.2, -2.2 and -2.4 mm for LR pigs. Responses in weight off test and backfat depths were symmetric about the control lines. Heritabilities for LGS were 0.34 and
0.28 for LW and LR, when estimated by residual maximum likelihood. Common environmental effects for LGS were 0.11 for LW and 0.17 for LR. Heritabilities for growth rate and average backfat depth were similar for LW and LR pigs (0.17 and 0.29, as were common environmental effects (0.10. Phenotypic and genetic correlations between growth rate and mid-back fat depth were small (0.16 and -0.06, 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.

N. D Cameron, M. K Curran, J. C Kerr

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 19. Selection and quantitative genetics; growth; reproduction; lactation; fish; fiber; meat., , 87–90, 1994
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