Effect of crossbreeding on genetic improvement of growth and reproduction in pigs.

Nine-year data from 3426 pigs were analyzed to present a retrospective evaluation of purebred, 2-breed crossing, and 3-breed rotational crossbreeding in the sixties and seventies. No significant heterosis effect was observed for litter size and the average number of pigs per litter (10) remained constant as crossbreeding intensified and the management system improved. Parental crossing did not influence sex ratios and no significant deviation from the 50:50 sex ratio occurred throughout the study. Among purebreds, Hampshire pigs were heavier (P less than .05) at birth and weaning than Duroc pigs. Crossbred dams sired by Duroc or Yorkshire boars produced offspring which were comparable in birth and weaning weights, but weighed less (P less than .05) than those sired by Landrace boars. Effect of birth weight on weaning weight of crossbred and purebred pigs was consistently positive whereas litter size influenced weaning weight negatively. Survival of the pigs was not dependent on parental crossing and did not change over the years when crossbreeding intensified. Yearly environmental variation influenced birth weight and weaning weight, but not litter traits. Male pigs were heavier (P less than .01) at birth than females, but the two groups were comparable at weaning.