Impact of inbreeding on fitness-related traits in the highly threatened Sorraia horse breed

• In the current population, average inbreeding (F) is 0.38 (0.22–0.60, SD=0.07).
• An increase in foals’ F value will reduce survival at birth and up to 6 months.
• Stallions and mares F values show a negative influence on fertility.
• Foaling interval and age at 1st parturition are not influenced by mare's F value.
• This endangered breed long-term self-sustaining needs a conservation- breeding plan.

The Sorraia horse population can be regarded as a universal equine genetic resource, most likely the representative of the ancestor of Iberian saddle horses and probably the ancestor of several New World horse breeds. The breed was recovered in 1937 and managed without further additions to the initial founder group of 12 horses, originating an extant population with extremely high inbreeding levels (mean F=0.38). There are only about 300 animals representing the Sorraia horse population worldwide, which places it in a critical risk status and strongly supports the need to establish a conservation-breeding plan aiming at a long-term self-sustaining population. Data on all registered horses in the Sorraia Studbook were used to study the impact of inbreeding on offspring's viability at birth and at 6 months of age, and a sub-sample was used to determine the influence of inbreeding on stallion and mare fertility rates, foaling intervals and age at first parturition. The effect of inbreeding on the analysed traits was only significant for the relationship of mare fertility with mare inbreeding coefficient (P=0.003). The influence of age of the mare was quadratic, with a reduction in foal mortality (both at birth and at six months of age) and an increase in foaling interval as age of mare increased. Stud farms had a statistically significant influence on age at first parturition. Decisive management-breeding plans must be taken to control inbreeding levels in Sorraia horses, and contribute to the conservation of this breed.