 Whenever this happens, there is a concern that the inbreeding will weaken the breed, making horses more prone to injury and potentially leading to inbreeding depression.Statistical studies have shown that inbreeding has increased by a small but significant amount in the last 50 years, partly because of the larger number of foals that a successful stallion will now sire each year compared to in the past.For example, Northern Dancer sired 645 foals in 20 years at stud, whereas his grandson Danehill sired 2,499 foals in 14 years at stud.