The impact of direct-maternal genetic correlations on international beef cattle evaluations for Limousin weaning weight

In beef cattle maternally influenced traits, estimates of direct-maternal genetic correlations (r(dm)) are usually reported to be negative. In international evaluations, r(dm) can differ both within countries (r(dm)_(WC)) and between countries (r(dm)_(BC)). The r(dm_BC) are difficult to estimate and are assumed to be zero in the current model for international beef cattle evaluations (Interbeef). Our objective was to investigate re-ranking of international estimated breeding values (IEBVs) in international beef cattle evaluations between models that either used estimated values for r(dm) or assumed them to be 0. Age-adjusted weaning weights and pedigree data were available for Limousin beef cattle from ten European countries. International EBVs were obtained using a multi-trait animal model with countries modeled as different traits. We compared IEBVs from a model that uses estimated r(dm)_(BC) (ranging between -0.14 and +0.14) and r(dm)_(WC) (between -0.33 and +0.40) with IEBVs obtained either from the current model that assumes r(dm)_(BC) to be 0, or from an alternative model that assumes both r(dm)_(BC) and r(dm)_(WC) to be 0. Direct and maternal IEBVs were compared across those three scenarios for different groups of animals. The ratio of population accuracies from the linear regression method was used to further investigate the impact of rdm on international evaluations, for both the whole set of animals in the evaluation and the domestic ones. Ignoring r(dm)_(BC), i.e., replacing estimated values with 0, resulted in no (rank correlations > 0.99) or limited (between 0.98 and 0.99) re-ranking for direct and maternal IEBVs, respectively. Both r(dm)_(BC) and r(dm)_(WC) had less impact on direct IEBVs than on maternal IEBVs. Re-ranking of maternal IEBVs decreased with increasing reliability. Ignoring r(dm)_(BC) resulted in no re-ranking for sires with IEBVs that might be exchanged across countries and limited re-ranking for the top 100 sires. Using estimated r(dm)_(BC) values instead of considering them to be 0 resulted in null to limited increases in population accuracy. Ignoring both r(dm)_(BC) and r(dm)_(WC) resulted in considerable re-ranking of animals' IEBVs in all groups of animals evaluated. This study showed the limited impact of the current practice of ignoring r(dm)_(BC) in international evaluations for Limousin weaning weight, most likely because the estimated r(dm)_(BC) was close to 0. We expect that these conclusions can be extended to other traits that have reported r(dm) values in the range of r(dm)_(WC) values for weaning weight in Limousin.