PREDICTION ERROR VARIANCES FOR INTERBREED GENETIC EVALUATIONS

A table for adjusting expected progeny differences (EPD) to a base yea r and breed basis depends on analyses of records of progeny of bulls o f different breeds in a common environment and requires that those ref erence bulls also have other progeny to provide within-breed EPD. Curr ently, the. germ plasm evaluation project at the Meat Animal Research Center (MARC) provides such a common environment for reference bulls o f several breeds for estimation of breed differences for the reference sires. Reference sire estimates of breed differences are adjusted by the difference between average EPD of reference bulls and average EPD for the base year for that breed. Two related questions are as follows : 1) What are confidence ranges for the adjustments and 2) What are ac curacies of interbreed EPD? Application of statistical principles and algebra shows that 1) apparent confidence ranges for breed adjustments are small, 2) apparent confidence ranges are substantially underestim ated when random sire effects within breed are ignored, 3) correct con fidence ranges also are small, 4) usual measures of accuracy cannot be applied to interbreed comparisons, and 5) standard errors of predicti on used in calculating confidence ranges for interbreed comparisons ar e much less affected by variance of the adjustment factors than by wit hin-breed accuracies for two bulls being compared except for bulls wit h accuracies of near unity. Alternatives of predicting differences bet ween bulls of the same or different breeds or between a bull of any br eed and an average bull of a base breed are discussed in terms of conf idence ranges. Although most theoretically correct, a major educationa l effort would be required to explain confidence ranges on expected di fferences in progeny of two bulls of different breeds. Confidence rang es on expected difference in progeny of a bull and an average bull of a base breed for a base year can be explained with only a slight exten sion of principles currently taught.