Variation of Milk Records Within Paternal-Sib Groups

Authors

L. Dale Van Vleck, University of Nebraska-LincolnFollow

Date of this Version

March 1968

Comments

Published in Journal of Dairy Science. Copyright © 1968 American Dairy Science Association. Used by permission.

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_y = μ + s_i + e_iy with σ ²yi = σ ²_s + = σ²_ei. Estimates of the within sire-group variances, σ₁ ²_ei, were heterogeneous (P < .05) for each year of freshening, 1951-64. Average heritability estimates were .24, for both the usual analysis which assumes σ²_ei = σ²_e for all i and the method which estimates oG as σ²_s as σ₁²_s,each as σ²_ei as σ₁²_ei and heritability as 4σ₁²_s/ σ₁²_s + σ₁²_ei). Similar results were obtained for other breeds. Repeatability of σ₁²_eifrom year to year was estimated as .39 if yearly σ₁²_ei 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 σ₁²_ei in tile regression of the group mean on true daughter superiority as compared to using the pooled within group variance in the regression coefficient.