International genomic evaluation methods for dairy cattle

Traditional MACE assumes independent residuals because each daughter is measured in only one country. Genomic MACE could account for residual correlations using daughter equivalents from genomic data as a fraction of the total in each country and proportions of bulls shared. MACE methods developed to combine separate within-country genomic evaluations were compared to direct, multi-country analysis of combined genotypes using simulated genomic and phenotypic data for 8,193 bulls in nine countries.Reliabilities for young bulls were much higher for across-country than within-country genomic evaluations as measured by squared correlations of estimated with true breeding values. Gains in reliability from genomic MACE were similar to those of multi-trait evaluation of genotypes but required less computation. Sharing of reference genotypes among countries created large residual correlations, especially for young bulls, that are accounted for in genomic MACE.International genomic evaluations can be computed either by modifying MACE to account for residual correlations across countries or by multi-trait evaluation of combined genotype files. The gains in reliability justify the increased computation but require more cooperation than in previous breeding programs.Today, selection in many countries uses genotypes in addition to phenotypes and pedigrees [1,2]. More than 50,000 dairy cattle worldwide have been genotyped for 50,000 markers. Breeders can select globally from the best animals if national evaluations with similar properties can be compared fairly and accurately. Changes from genetic to genomic evaluations for dairy cattle at the national level will require corresponding changes to international evaluations.Phenotypes are collected, stored, and evaluated independently by each country, and the resulting estimated breeding value (EBV) files are exchanged and combined by Interbull. Multi-trait across-country evaluations (MACE) for nearly 30 traits are provided routinel