EFFECTS OF CORRECTION FOR HETEROGENEITY OF VARIANCE ON BIAS AND ACCURACY OF BREEDING VALUE ESTIMATION FOR DUTCH DAIRY-CATTLE

Data on 305-d milk yield from the Dutch dairy evaluation were used to obtain breeding value estimates from an animal model for 1984 to 1992. Changes in sire evaluations were investigated for bias and realized a ccuracy. Evaluations based on progeny records were generally lower tha n their expectation based on parent average. The average decrease was 157 kg for Black and White bulls and 73 kg for Red and White bulls. Ev aluations based on test daughters changed about -50 kg when second and third lactations became available, but estimates for breeding value c hanged insignificantly when records on daughters from the breeding per iod of the bull were used. The standard deviation of changes from eval uations for first to second batch was about 14% larger than expected f rom population parameters. Breeding values of imported bulls, based on Dutch data, decreased 75 kg when more information became available in subsequent evaluations. Heterogeneity of variance was estimated by a quasi-likelihood approach with a model that accounted for sampling var iance on estimates of variances within herd. The coefficient of variat ion of the variances within herd-year was 31%. A simple method for sta ndardization of variances within herd-year decreased bias of parent av erages by about 20%, and fluctuations of breeding values were within t he expected range. A correction for heterogeneity of variance within h erd may not remove all bias of parent averages, but a general improvem ent of bias and accuracy of breeding values can be expected.