Efficient implementation of multitrait random regression test-day models with external information for dairy cattle genomic evaluations.

ABSTRACT.- Random regression models (RRM) combined with single-step genomic best linear unbiased prediction (ssGBLUP) are widely used for genomic evaluations in dairy cattle. This study aimed to efficiently implement RRM with ssGBLUP for national dairy cattle evaluations. Data from the Czech Holstein population were used, including 30 million test-day records for milk yield across 3 lactations. The pedigree included 2.5 million animals, of which 54,000 were genotyped.