Re-ranking of Imported Angus Sires Following Integration of Foreign Genotypes into a National Genomic Evaluation

Abstract

In South Africa, imported Angus sires are frequently introduced into the national genetic evaluation with little to no local progeny records, limited pedigree depth, and typically without own or progeny performance data recorded under local production conditions. Historically, interim evaluations incorporated country-of-origin genetic merit information, expressed as percentile rankings and blended them into the South African evaluation until sufficient local progeny records became available. This avoids an initial underestimation of their EBVs. Such approaches may not fully account for genotype-by-environment interactions, population-specific allele frequencies, differences in genetic parameters, or divergent selection pressures between source and recipient populations. Single-step multiple-trait genomic BLUP (MT ssGBLUP) enables direct integration of SNP genotypes, allowing imported sires to be evaluated relative to a locally genotyped reference population. This study compared EBVs from the national pedigree-based BLUP evaluation incorporating percentile adjustments with EBVs from MT ssGBLUP without percentile adjustments for 194 imported Angus sires across pre-weaning traits. Pearson correlations between evaluation methods were 0.92 for Birth Weight Direct (BWD), 0.80 for Weaning Weight Direct (WWD), and 0.71 for Weaning Weight Maternal (WWM). Despite the high correlation for BWD, individual re-ranking averaged 21 positions. Greater re-ranking was observed for WWD (maximum −117 to +139 positions) and WWM (−145 to +147 positions), likely reflecting lower data density and the delayed accumulation of maternal information. Across traits, incorporation of genomic information resulted in substantial redistribution of ranking, with most imported sires improving relative position, while others declined depending on genomic relationships with locally measured and genotyped animals. These results indicate that MT ssGBLUP does not uniformly inflate evaluations but reallocates merit according to realized genomic similarity within the South African reference population. Strengthening genomic connectedness between imported sires and locally measured animals is therefore essential to reduce uncertainty during early evaluation stages and to improve the reliability of selection decisions under production environments distinct from those of origin.