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

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