LAUSR

Fertility is a key factor in the economic success of horse farms. However, it has received little attention due to the difficulty of measuring fertility objectively. Since its studbook creation (1912), the Pura Raza Española (PRE) breed has been a closed population and become high in-bred resulting in inbreeding depression (poor phenotypic values). Nevertheless, heterogeneous effects of inbreeding depression have been detected among founders and non-founders. The aims of this study were (1) to analyse the genetic parameters for reproductive traits in mares of the PRE horse breed, and (2) to estimate, for the first time, the inbreeding depression load associated with common ancestors of the breed. A total of 22,799 mares were analysed. Heritability estimates ranged from 0.05 (interval between first and second foaling) to 0.16 (age at first foaling), while inbreeding depression load ratios ranged from 0.06 (parturition efficiency at 6 th foaling) to 0.17 (age at first foaling), for a partial inbreeding coefficient of 10%. While heritability is related to the variability expressed in the population, inbreeding depression load ratios measure the potential variability, whether expressed in the population or not. Most correlations between additive and inbreeding depression load genetic values were significant (P-values <0.001), and of low to moderate magnitude. Our results confirm that individual inbreeding depression loads allow us to select horses that have a genetic value resistant to the deleterious effects of inbreeding.