Age, Breed, Trait, and Regional Variation in the Regression of Daughters on Herdmates

A total of 418,139 mature-equivalent (ME) lactation records representing cows of five dairy breeds calving from October, 1964, to September, 1965, was used to estimate the intrasire regression of daughters on herdmates. Differences in the regressions from breed, lactation number, trait, and region were studied. Regressions for first lactations were generally lower than those for later lactations. Guernseys and Holsteins had the highest regressions for milk and fat yield, followed by Ayrshire, Jerseys, and Brown Swiss. Holsteins and Ayrshires had the highest regressions for fat per cent. Regional differences in the Holstein regressions were substantial. New England, the South, and California had the highest regressions for yield. The South and California had the highest regressions for fat per cent, but the Northeast was intermediate. The accuracy of the adjustment of sire evaluations for performance of herdmates can be improved by considering these differences. Regressions for firstlactation fat per cent were around 0.7, indicating that the deviation of herdmates from breed average should be considered in evaluating sires.