Inbreeding depression and durability in the North American Thoroughbred horse

The proportion of the genome containing runs of homozygosity (ROH) affects production traits in livestock populations. In European and Australasian Thoroughbreds inbreeding, quantified using ROH (F_ROH ), is associated with the probability of ever racing. Here, we measured F_ROH using 333 K SNP genotypes from 768 Thoroughbred horses born in North America to evaluate the effect of inbreeding on racing traits in that region. Among North American horses, F_ROH was not associated (p = 0.518) with the probability of ever racing but was significantly associated with the number of race starts (p = 0.002). Among raced horses, those with a 10% higher F_ROH than the mean inbreeding coefficient were predicted to have 3.5 fewer race starts compared to horses with a mean inbreeding coefficient. Considering the trend of increasing inbreeding and a decline in the average number of race starts per runner in North America, mitigating inbreeding in the population could positively influence racing durability.