Components of Variance and Covariance in Purebred and Crossbred Swine

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Abstract
Components of variance and covariance were estimated from records on 8039 purebred and crossbred pigs sired by 76 boars of the Duroc and Yorkshire breeds. The data were analyzed separately for each breeding system-sex subgroup. Traits studied were weight at birth and 154 days, litter size at birth, weaning and 154 days and backfat probe adjusted to 72.5 kg. bodyweight. Heritability values from paternal halfsib correlations were essentially zero for birth weight, and of moderate magnitude for backfat probe. Those for 154-day weight were 0.70, 0.81 and 0.65 for purebred boars, barrows and gilts, and 0.03 and 0.00 for crossbred barrows and gilts, respectively. A comparison of the components of variance in the two breeding groups led to the suggestion that nonadditive gene action was involved. Discussion on the effectiveness of selection concluded that selection on the basis of purebred performance would not be effective for improving crossbred performance and that selection for specific combining ability should provide a better means for utilizing heterosis experienced in crosses. Heritability estimates computed from daughter-dam regressions (245 pairs) were 0.05, 0.19 and 0.29 for litter size at birth, weaning and 154 days, respectively. Genetically, weight was negatively correlated with backfat probe, and litter size at weaning was positively correlated (0.99) with 154-day litter size. In general, the associations among the traits were of such magnitude and direction as to suggest that simultaneous improvement in all traits would be feasible. Copyright © 1967. American Society of Animal Science . Copyright 1967 by American Society of Animal Science