Repeatability As An Indicator of Stability in Contemporary Comparison Sire Evaluations

Abstract

Repeatabilities of sire summaries by contemporary comparison were a measure of stability of sire evaluation. Modified contemporary procedures of sire evaluation for 1st lactation records only were used to compute predicted differences for all Holstein bulls having 10 or more daughters calving in any calendar year (1964-1973). A subset of 455 bulls having repeatabilities of last available (final) proofs 90% was used to examine relationships between 1st available (initial) predicted difference, initial repeatability and final predicted difference. Sire evaluations with low repeatabilities were more variable with a slightly greater tendency to decline than evaluations with high repeatabilities. The average decline of sire evaluations with low repeatabilities was due to less than expected substantial increases in predicted difference rather than to more than expected substantial declines of predicted difference. Final evaluations increased 1.53 kg/point increase in initial repeatability. Initial repeatability accounted for 1% of the differences in final evaluations while initial predicted difference accounted for 76%. Analysis of semen prices showed a substantial cost for repeatability for bulls of high genetic merit for production and/or type. Cost-benefit ratios indicated that dairymen with intensive sire selection programs often will make most econimical semen purchases at intermediate to low repeatabilities.