Modified Contemporary and Herdmate Comparisons in Sire Summary

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Abstract
The influence of various factors on differences in sire evaluation from herdmate comparison and modified contemporary comparison methodology was examined. The average Predicted Difference for milk of bulls with 10 or more daughters changed -8, 33, 13, -20, and -8 kg in Ayrshires, Brown Swiss, Guernseys, Hosteins, and Jerseys; standard deviations of change ranged from 76-118 kg. The population from which the bulls were selected (pedigree group means) accounted for 49-73% of the change. Genetic average of herdmates'' sires accounted for 26-39% of the change. For bulls with repeatabilities of over 80%, the genetic average of herdmates'' sires accounted for 41-83%. New repeatabilities averaged 1.6-1.9 less than those from the old method but were more variable. Correlations of Predicted Difference for milk by modified contemporary comparison with information from daughters added after the Oct., 1974 summary were from .04-.15 higher than Predicted Differences by herdmate comparison. The Predicted Difference by modified contemporary comparison had the greatest advantage over that from the herdmate comparison when bulls had small numbers of daughters. Correlation of Predicted Difference by modified contemporary comparison with additional daughter information was .13 higher than that of the herdmate comparison when fewer than 10 daughters were in the summary and was only .04 higher when more than 100 daughters were in the summary.