Variation in Genetic Merit of Sires of Herdmates of First Lactation Cows

Abstract

A basic condition in sire evaluation by herdmate comparison is that progeny of different bulls are compared with herdmates of equivalent genetic merit. We examined the genetic merit of sires of herdmates of bulls’ progeny by geographical region, by artificial insemination organization, by age of bulls, and by calendar year. Average Predicted Differences of herdmates’ sires in Holsteins varied by 134kg between the highest and lowest region and by 173kg between artificial insemination organizations. Progeny of younger bulls are compared to herdmates with higher transmitting abilities than are progeny of older bulls because of genetic improvements across years, but differences within the same year are not great. These discrepancies are causing systematic errors in sire evaluations computed with herdmate comparison procedures. Although errors are small compared to variation among bulls, they can and should be eliminated by either modifying the herdmate comparison or using alternative sire evaluation procedures that account for genetic variation in herdmates of different bulls.