Mass Selection in a Pig Population: Correlated Responses in Preweaning Growth1

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Abstract
The intensity and consequences of the indirect selection for piglet weights at birth and weaning that occurred during nine generations of selection for postweaning performance traits was examined for a population of 16,284 pigs born and 11,606 pigs weaned. The cumulative indirect selection was substantial for both preweaning traits, 4.2, 1.7 and 2.9σ for birth weight of the maximum growth (G), minimum fat (B) and index (I) lines, respectively, and 5.8, 1.7 and 4.5σ for weaning weight. A portion of this selection was associated with preweaning survival (heavier birth weights favored), and designation of candidates for test (heavier weaning weights favored), but most of it derived from choice of breeding replacements at test termination. Presumably this reflected positive (in respect of merit) phenotypic correlations between the pre- and postweaning traits. The phenotypic time trends observed did not agree with line differences in the intensity of indirect selection, no phenotypic time trends being evident for the G or B lines while positive trends (P<.05) were observed for weights at birth (19.9 ± 9.5 g/generation) and weaning (.28 ± .07 kg/generation) of the I line. This lack of agreement could not be explained on the basis of line differences in heritabilities, all estimates being near zero for birth weight and nonsignificant for weaning weight; although there may have been line differences in the genetic correlations for pre- vs postweaning traits, the large standard errors of estimate precluded meaningful comparisons. The majority of genetic correlations that were significant, or approached significance, were positive in respect of merit. The results support the conclusion that the selection practiced in these populations was not detrimental to preweaning growth and that both birth and weaning weights were enhanced under index selection. Copyright © 1986. American Society of Animal Science . Copyright 1986 by American Society of Animal Science

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