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Restricted Selection Index: an Approach to Selecting for Feed Efficiency^{1 ,2,}

North Carolina State University, Raleigh 27607

Abstract

Bidirectional restricted index selection was conducted for 10 generations in mice. The goal was to maximize genetic change in postweaning weight gain from 3 to 6 weeks of age, while keeping genetic response in feed intake at zero. Realized heritabilities of the restricted index and their nearly unbiased standard errors were .18 ± .026 and .19 ± .023 in the restricted index lines selected for increased (I⁺) and decreased (I^—) postweaning gain, respectively. The paternal half-sib estimate of restricted index heritability was .19 ± .09. The nearly unbiased variances of the realized heritabilities of the restricted index were 1.9 and 2.8 times higher than the variances of the regression coefficients for I⁺ and I^—, respectively. Correlated responses in postweaning gain were significant (P<.01) in the intended direction of selection in both lines, but feed intake did not respond significantly. Little net change accrued in feed intake until genetic change in postweaning gain had reached about one absolute phenotypic standard deviation. Subsequently, genetic change in feed intake tended to follow the direction of response in postweaning gain, but this was not significant. A method was developed for estimating the realized genetic correlation between the restricted and unrestricted traits in a restricted selection index. The pooled realized genetic correlation between feed intake and postweaning gain was estimated to be .36 ± .17, which was not significantly different from the paternal half-sib estimate of 61 ± .19. Positive (P<.01) and negative (P<.01) correlated responses were obtained in I⁺ and I^—, respectively, for feed efficiency, feed efficiency /W^.75 (average metabolic body size from 3 to 6 weeks of age), 6-week body weight and relative growth rate. Neither line exhibited significant correlated responses in feed intake adjusted by covariance for W^.75, feed intake/W^.75, 3-week body weight or litter size. The I⁺ line showed a decrease (P<.01) in percent fertile matings and an increase (P<.01) in days from initiation of cohabitation to parturition.

¹ Paper No. 5077 of the Journal Series of the North Carolina Agricultural Experiment Station, Raleigh. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Experiment Station of the products named, nor criticism of similar ones not mentioned.

² The excellent technical assistance of Ms. Billie Jean Edwards is acknowledged.

³ Department of Animal Science.

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