Conversation so far:
Are there any other interesting aspects about the Northern Dancer article?
Because of the prevalence of Northern Dancer's bloodline, a certain degree of inbreeding to him is becoming common.
How does inbreedng hurt race horses?
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.
Were breeders breeding Northern Dancer with his chrildren?
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.
Was there any issues with the breeding of Northern Dancer?
Statistical analysis has shown that inbreeding to Northern Dancer is on average slightly less effective than when stallions of his line are bred to mares who do not have Northern Dancer in their pedigree (referred to as an outcross).

Question: When did he retire from breeding?
Self-contained rewrite: When did Northern Dancer retire from breeding?