Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 6, 2001
Publication Date: N/A
Interpretive Summary: A common assumption of genetic evaluation models is homogeneity of variances and covariances, but this assumption has been shown to be incorrect for conformation (type) traits of dairy cattle. Variances and covariances were investigated for linear type traits and final scores of Jerseys and found to be clearly heterogeneous. The effects that contributed dto this heterogeneity were parity (first or second lactation), size of the contemporary group, and whether final score was above or below the average for the contemporary group. The results conclusively showed the need for an adjustment for heterogeneous variances and covariances. Adding such an adjustment to the genetic evaluation system would provide more accurate evaluations on which to base breeding decisions.
Heterogeneity of (co)variances for Jersey linear and final scores was investigated using data from February 2000 genetic evaluations. First- appraisal scores during first or second lactation from records that included all traits were studied. Three contemporary groups were defined based on number of cows for that herd appraisal date: 5 to 15, 30 to 55, and 100 and more. Contemporary groups were further classified into high (above class mean) and low (below class mean) subsets for final score. The parity, group size, and final score classifications resulted in 12 data sets, which contained appraisal information from 8111 to 23,692 cows. (Co)variance components were estimated using expectation-maximization REML and canonical transformation. Across all traits and independent of herd size, phenotypic variances tended to be higher in low-scoring contemporary groups and during first lactation; mean relative differences in variance were 18% during first and 20% during second lactation. Similar or even larger differences existed for genetic variances, but those differences were not as consistent across traits and contemporary group size. The associated mean relative differences were 26% during first and 31% during second lactation. The different patterns for genetic and phenotypic variances led to significant differences in estimated heritabilities. (Co)variances were found to be similarly heterogeneous: 20% and 23% for first and second lactations, respectively, for phenotypic (co)variance and 32% and 36% for genetic (co)variance. This heterogeneity resulted more from variance heterogeneity than from differences among associated correlation matrices (phenotypic: 11% first and 12% second lactation; genetic: 20% first and 26% second lactation), especially for phenotypic (co)variance.