HETEROGENEITY OF (CO) VARIANCE AND HERITABILITY 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. Stratifying records by intraherd-year variance proved to be the most effective way to produce heterogeneous estimates of variance components and heritability. The criterion of intraherd variance was not as effective and was followed by the criterion of herd-year production averages; the criterion of herd production averages was the least effective. The sire component of variance in the high group of intraherd-year variance was more than six times greater than that in the low group, the residual variance was three times greater, and the heritability estimates were 50 to 140% greater. When data were split by herd production levels, sire and residual variances components at high production were less than twice as great as those at low production, whereas heritability estimates of high group production were only slightly higher. In each of the herd stratification methods, the estimates of sire covariance components between groups were greater when higher production was involved, especially when intraherd-year variance was the criterion, but all genetic correlations between groups were close to unity. Therefore, sires were expected to be ranked similarly in different intraherd-year variance groups, but the magnitude of sire differences was much greater in the high than in the low group. All of these tendencies were similar in the three time periods. © 1990, American Dairy Science Association. All rights reserved.