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Variation of Milk Records Within Paternal-Sib Groups

Department of Animal Science, Cornell University, Ithaca, New York

ABSTRACT

Differences between daughter-dam and paternal sib estimates of heritability of first lactation milk yield have not been explained for New York data. Deviations of first lactation records from herdmate means of 155,070 artificially sired Holsteins were analyzed to examine the effect of unequal within group variances on the paternal sib estimate of heritability. The statistical model used to describe the data was y_ij = µ + s_i + e_ij with ²y_i = ²s + ²e_i. Estimates of the within sire-group variances, ₁²e_i, were heterogeneous (P < .05) for each year of freshening, 1951–64. Average heritability estimates were .24, for both the usual analysis which assumes ²e_i = ²e for all i and the method which estimates ²s as ₁²s, each ²e_i as ₁²e_i and heritability as 4₁²s/(₁²s + ₁²e). Similar results were obtained for other breeds. Repeatability of ₁²e_i from year to year was estimated as .39 if yearly ₁²e_i was expressed as a fraction of the average within group variance for that year and as .58 if yearly changes in average variance were ignored. Estimates of genetic value changed little when using ₁²e_i in the regression of the group mean on true daughter superiority as compared to using the pooled within group variance in the regression coefficient.

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