Effects of homogeneity and heterogeneity of variance components in different levels of herd-year size on genetic parameters for milk yield of Iranian Holsteins

Document Type : Research Paper

Authors

1 PhD Student, Department of Animal Sciences, Faculty of Agricultural Sciences, University of Guilan, Rasht, Iran

2 Associate Professor, Department of Animal Sciences, Faculty of Agricultural Sciences, University of Guilan, Rasht, Iran

3 Professor, Department of Animal Science, Faculty of Agricultural Sciences, University of Guilan, Rasht, Iran

Abstract

This study was conducted to investigate the homogeneity of variance components for milk yield (MY) and to assess the effects of different data transformation methods on the ranking of elite animals in Iranian Holstein cows. Data sets included 245192 records for 1st lactation period, 202078 records for 2nd lactation and 147253 records for 3rd lactation collected from 1983 to 2014 by the Animal Breeding Center and promotion of Animal Products of Iran. Records were classified into three different groups based on herd-year size. Four different data transformation methods including Logarithmic, Arc sin, Square root and Box-Cox were applied and the data were tested for heterogeneity of variance before and after using Bartlett’s test. The results indicated the heterogeneity of variance in all three groups (P < 0.01) before transformation. Yet, data transformation did not result in homogeneity of variance across the herd size classes. The Square root and Box-Cox transformation methods decreased the heterogeneity of variance components in the first lactation period while other methods had no effect in adjusting the heterogeneity of any groups. Heritability and estimated breeding values (EBVs) were obtained for non-transformed data using different methods based on animal model using VCE program. Heritability varied from 0.142 to 0.184 in single trait analysis and 0.143 to 0.221 in multi trait analysis. Some re-ranking of animals occurred after data transformation, but the Box-Cox method had a small effect on overall rankings and Spearman's rank correlations of animals. The applied transformation caused a substantial re-ranking of EBVs of elite sire and dams considering herd size. Data transformation for adjusting heterogeneity of variance caused different proportions of top sires and dams to be excluded from lists when compared to the homogenous variance scenario. Therefore, to increase the accuracy of the evaluation and selection efficiency of milk yield, when evaluating the genetic of Holstein cows it is necessary to consider heterogeneity of variance.

Keywords

References

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