Why New Sire Summaries are Needed

Assumptions underlying the herdmate comparison now used in the United States and evidence on their validity under current conditions were reviewed. The main conclusion was that changes to account more adequately for genetic variation in herdmates of different progeny groups would do most to improve accuracy of the herdmate comparison method of evaluating sires. Changes in procedm'es so that comparisons are based more on cows of similar ages when possible would reduce dependence on assumptions concerning adjustment factors for age. Including information on bulls' parentage also might be useful when limited progeny data are available. Herdmate comparisons as now used in the United States are useful indicators of genetic merit, but they should be made more accurate by refinements listed above. When more sophisticated sire evaluation methods replace herdmate comparisons, they should be designed to have minimum dependence on adjustment factors for age and underlying assumptions.