Weighting Factors of Sire Daughter Information in International Genetic Evaluations

摘要

International genetic evaluations of dairy bulls are current1y based on national genetic evaluation results. Total number of daughters in a country is used to weight national information, but may not optimally re- flect the precision of a sire's daughter contribution to international genetic evaluations. This study investi- gates the impact of alternative weighting factors on international evaluation results. A conventional prog- eny test scheme was simulated for two dairy cattle pop- ulations, with semen exchange at a fixed rate after each generation. True breeding values for both populations were generated as bivariate normal deviates. Each cow had three lactation records in one country only. After 10 generations of selection, all records were used in national breeding value prediction. National breeding values of bulls were used as input to international eval- uations. Seven different weighting factors were evalu- ated: 1) total number of daughters; 2) total number of lactations; 3) as (one) also adjusted for finite contempo- rary group size; 4) as (three) also adjusted for distribu- tion of daughters over contemporary groups; 5) effective daughter contribution considering finite contemporary group size and correlation between repeated records; 6) as (five) also considering the reliability of the daugh- ter dam evaluation; and 7) as (five) also considering the reliability of the daughter female ancestors' evalua- tions. Using the last two weighting factors yielded em- pirically unbiased estimates of sire variance. Using to- tal number of daughters overestimated genetic vari- ance by up to 7/100. In general, international breeding values were marginally affected by choice of weighting factor. The effect was larger when different national evaluation models had been applied in the tWo coun- tries. International reliabilities for the last tWo weighting factors were close to expectation, whereas using total number of daughters resulted in 1 to 4/100 negative bias. In practice, different countries apply a wide range of national evaluation models, and genetic ties may be weak between some populations, thereby increasing the potential effect of weighting factors on international comparisons. The weighting factor devel- oped in this study, which considers contemporary group structure, correlation between repeated records, and reliability of dams of daughters, should replace total number of daughters in international genetic evalua- tions of dairy sires.