Heterogeneity of (Co)Variance and Heritabitity in Different Levels of Intraherd Milk Production Variance and of Herd Average

A multitrait sire model was used to estimate sire (co)variance and residual variance components in different levels of intraherd production variance, and in different levels of production average. An expectation-maximization algorithm of REML method was used on 305-d, mature equivalent. first lactation milk records of 366,202 cows that freshened from 1976 through 1987 in the northeastern US. Within each of the three 4-yr periods, records were stratified into three levels of intraherd-year variance in one case, three groups of intraherd variance in another, three groups of herd-year production and three groups of herd production in the other two cases.