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Method and Effect of Adjustment for Heterogeneous Variance of Holstein Conformation Traits

Centre for Genetic Improvement of Livestock, Department of Animal and Poultry Science, University of Guelph, Guelph, ON, Canada N1G 2W1

Type classification records of Canadian Holsteins were investigated for evidence of heterogeneous variance across herds. Data consisted of records for 1,139,104 cows from 20,226 herds with classifications on 26 conformation traits collected from 1982 through 1992 and 338,046 cows from 9600 herds with classifications on 2 additional traits from 1990 through 1992. Phenotypic standard deviations of herd-round-classifier were fitted to a mixed model that included round, classifier, and region as fixed effects, herd size as a covariant, and herd as a random effect. Estimates of the variance components, solutions of fixed effects, and BLUP estimates for herd were obtained by maximum likelihood procedures. Repeatability of within-herd standard deviation across rounds ranged from 1.4 to 10.3% for the 28 traits. Type classification data were subsequently standardized for phenotypic standard deviations of herd-round-classifier that were derived from estimates of the fixed effects and the BLUP estimate of the herd effect. Genetic evaluations for cows and bulls were produced from adjusted and unadjusted data Correlations between 3754 sire and 1,142,782 cow estimated transmitting abilities obtained from unadjusted and adjusted data were essentially unity. Although some evidence of heterogeneous variance existed across herds for 28 conformation traits, standardization of the classification records had only a minor effect on genetic evaluations.

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