Simple SummaryThe relevance of the expected value of the proportion of Arabian genes (EV%AG) in different horse breeds participating in the eventing discipline, and the way this factor should be included in equine genetic evaluations for eventing, have not been studied in depth in the current literature. A total of 1089 horses participating in eventing competitions (8862 participation records) were used for this study. The significance of the EV%AG in the different scores obtained was studied, and was found to be significant for the three exercises that make up the discipline of eventing (dressage, show jumping and cross-country). Five genetic models were computed to study the importance of the EV%AG in the pedigree of 10,375 horses. The best-fitted models following the DIC criterion were those including age as covariate, sex, breed, level, EV%AG and event as systematic effects, rider, animal and residual as random effects and variance heterogeneity, concluding that the best way to model the EV%AG effect seems to be by considering the variance heterogeneity. Dressage and show jumping heritability ranged from 0.10 in approach C to 0.21 in D. The estimated heritability for the cross-country trait oscillated less, between 0.07 and 0.01 in approach D.AbstractThe Arabian horse is a generally reliable sport horse, and continues to be a remarkable endurance horse, so the relevance of the expected value of the proportion of Arabian genes (EV%AG) in horses participating in eventing could be a relevant factor. A total of 1089 horses participating in eventing (8866 records) were used. A GLM revealed that the EV%AG was significant in dressage, show jumping and cross-country. A BLUP genetic evaluation was computed with five genetic models (without the EV%AG (0) using as a covariate (A), as a fixed effect (B), with variance heterogeneity, and in genetic groups without (C) and with (D)). Dressage heritability ranged from 0.103 to 0.210, show jumping ranged from 0.117 to 0.203 and cross-country ranged from 0.070 to 0.099. The lowest DIC value was used as a criterion of fitness. The best fits (those which included variance heterogeneity) showed fewer than two points of difference in DIC values. The highest average estimated breeding value in dressage, show jumping and cross-country was found for horses with an expected value of the proportion of Arabian genes of 0%, ≥1% to <25%, and 100%, respectively. Therefore, the best way to model the EV%AG effect seems to be by considering the variance heterogeneity.