ناهمگنی اجزای واریانس مقدار شیر در سطوح متفاوت تولید گاوهای هلشتاین ایران

This research was carried out to investigate the heterogeneity of milk yield variance components in different production levels of holstein cattles. The first lactation milk yield records of 95945 Holstein cattles, which had calved in 651 herds through years 1991 to 2000, were used in this research. Data was collected by the Animal Breeding Center of Iran and adjusted for two-time milking per day. Records were classified into three classes: low (less than 5275 kg), average (5275-6874 kg), and high (more than 6874 kg) based on the level of production. Variance components and genetic parameters were estimated using single and three-trait animal models for each class and three classes with Derivative Free Restricted Maximum Likelihood methods (DF-REML). Bartlett test showed significant differences for variance components between classes. Milk yield heritability was estimated as 0.205, 0.276, and 0.196 for low, average, and high class, respectively, in both single and three-trait analysis. The gentic correlation coefficient between classes ranged from 0.80 to 0.85, Phenotypic correlation coefficients between classes were low and ranged from 0.17 to 0.19. The highest rank correlation coefficient between estimated breeding value by single and three-trait model was obtained in average class. The average changes in the rank of common animals in the top 1% of cows were greather than the top 5% of bulls. Adjustment for heterogeneity of variance components suggested selecting more top cows from herds with high level of production.