Sensitivity of Methods of Variance Component Estimation to Culling Type of Selection

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Abstract
Two pseudoexpectation methods of variance component estimation were examined for selection bias from culling and were compared with Henderson''s simple method and restricted maximum likelihood. Milk yield data were simulated for 100 herds, 50 sires, and approximately 2000 cows per replicate. Heritability was .25, and repeatability was .50. Each cow had two records. Twenty replicates were made. Two data sets were created; one was unselected and included both records of the cow, and the second excluded second records of cows with first records below the herd mean. Sire, cow, and error variances were estimated according to a mixed model that included fixed herd parity, and random site, cow and error effects. Estimates of sire, cow, and error variances and heritability and repeatability were unbiased and were similar for all methods for unselected data. For selected data, sire and cow variances were overestimated and error variances were underestimated with Henderson''s simple method and the pseudoexpectation methods. Accordingly, heritabilities and repeatabilities were overestimated. Biases were greatest for Henderson''s simple method and differences between the two pseudoexpectation methods were small. With restricted maximum likelihood, sire variances were unbiased but error variances were slightly over estimated and cow variances underestimated. Use of restricted maximum likelihood is preferred with selected data.