Heritabilities and Repeatabilities of Milk and Milk Fat Production by Lactations

Abstract

Data from two sources containing 67,729 and 60,788 Holstein lactation records were used to estimate the relationship between various pairs of lactations of the same cow and to estimate the heritability of first through fourth lactation. All records were expressed as deviations from their regressed adjusted herd-year-season average. Estimates of the relationship between pairs of lactations indicated that these relationships were not constant from one lactation to the next. The relationship between consecutive lactations increased gradually as the animals got older, and the relationship between nonconsecutive lactations decreased gradually as the lactations became more separated in time. Estimates of the heritability of first lactation were higher than estimates for second lactation. These differences were not significant. It was concluded that weighting the first and second lactations separately would increase the accuracy of selection. This amounted to an increase in accuracy of about 1.5% (0.615 vs. 0.606 and 0.607 vs. 0.598) for both groups of data. When the first three lactations were considered, this increase in accuracy almost doubled. © 1968, American Dairy Science Association. All rights reserved.