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U.S. Department of Agriculture, Beltsville, MD 20705
Abstract
Response of preweaning traits to selection for both purebred and crossbred performance in a strain formed by crossing non-inbreds of three breeds (BS) and a strain formed by crossing three inbred lines (LS) was compared with selection for purebred performance alone in two control strains (BC, LC) derived from the same base populations
Selection in BS and LS was on an index including preweaning litter performance (litter size at birth, 21 and 56 days and litter weight at 21 and 56 days), post-weaning gain and carcass yields (only for crossbred pigs), whereas selection in the BC and LC strains was almost exclusively on an index for preweaning litter performance.
Average standardized selection differentials in the BS and LS strains for weight of crossbred litters weaned were identical for tested sires (.48) and more than twice as large for tested dams (.64) than the corresponding differentials for weight of litters in which they were born. Selection differentials per generation for weight of dam's litter in the BS and LS strains (.34 for boars, .37 for gilts) differed little from those in the BS and LS strains (.48 for boars, .31 for gilts), although cumulative total selection on dam's record was nearly 75% larger for BC and LC
Absolute genetic trends (1957 to 1967) for litter weight at 56 days were negative but not significant in the BS and LC strains (–1.1 kg per year), whereas in the other types of mating trends were generally small and positive (1.2 kg for BS and LS, .3 kg for LSBS and BSLS crosses, and .8 kg for crosses involving LSBS and BSLS dams) but also not significant. Genetic trends of deviations from BC and LC for litter weight at 56 days were generally positive (1.4 kg per year for BS and LS; 1.0 kg for LSS and BCLC crosses, and .9 kg for crosses involving LCBC and BCLC dams) but also not significant. Moreover, there was no advantage for crosses over parents. However, in an evaluation phase (1968 to 1971) heterosis in F1 crosses between the BS and LS strains generally substantially exceeded that in F1 crosses between the BC and LC strains. Although the advantages shown by F2 over F1 litters from BC x LC crosses generally exceeded those shown by F2 over F1 litters from BS x LS crosses, F2 (BS x LS) litters slightly exceeded F2 (BS x LS) litters in most traits.
Thus, reciprocal selection was at least moderately superior to pure-strain selection in exploiting nonadditive gene effects. However, final assessment of the value of the reciprocal selection practiced must await analysis of the post-weaning growth and carcass results.
1 The authors wish to express appreciation to Mr. W. H. Peters who was in charge of the breeding herd, and to Dr. B. T. Weinland for his assistance in the analysis of the data.
2 Animal Physiology and Genetics Institute, ARS.
3 Data Systems Application Division, ARS
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