The objective of this study was to find optimal traits for inclusion in selection criteria by estimating genetic parameters for direct genetic, maternal genetic, and common environmental effects for growth traits before 60 d of age and for the number of teats under an open breeding population, and to evaluate genetic relationships for traits at 60 d of age. Records of 2,344 male and 2,204 female purebred Berkshire pigs were analyzed. For BW at 14 d of age and for weaning weight, the heritabilities of a direct genetic effect were greater than those of a maternal genetic effect. This result is contrary to previous results showing a gradual decrease in the maternal genetic effect and an increase in the direct genetic effect up to weaning. The positive genetic correlations between direct and maternal genetic effects for BW at 14 d of age and weaning weight are clearly contrary to other reports. This phenomenon seems to be caused by creep feeding begun just after the birth of the piglets and maintained throughout the preweaning period in this Berkshire population.

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