Citing concerns about a growing lack of diversity in the gene pool of Thoroughbred racehorses, the Jockey Club, which is the registrar of the breed in the United States, in 2020 promulgated a new rule.  Stallions foaled in 2020 and thereafter are limited to breeding 140 mares per season.  In 2021, three prominent breeding farms—Ashford, Spendthrift, and Three Chimneys—sued to overturn the decision.

An article in the Wall Street Journal by Jo Craven McGinty (“Most U. S. Dairy Cows are Kissing Cousins,” May 29-30, 2021) pertained precisely to the sort of genetic diversity concern that prompted the Jockey Club rule.  She began the article by asking:

“Trick question: How many Holsteins…are in the U. S.?  (a) Nine million; (b) Fewer than 50; (c) Both a and b.”

The answer: “…selective breeding…has led to so much inbreeding that virtually all Holsteins in the U. S. and abroad descend from just two bulls [born in the 1960s].  So, while there are roughly nine million Holsteins in the U. S., the breed’s effective population—a measure of genetic diversity—is just 43, according to an estimate published last year in the peer-reviewed Journal of Dairy Science.”  To put this finding in perspective, wild animals with an effective population of less than 50 are considered to be in danger of extinction “because of higher risks of miscarriages, stillbirths, and genetic abnormalities.”

On the positive side, inbreeding optimizes milk production as well as “boosts longevity, mobility, leg and foot health.”  Yet inbreeding is also “responsible for the proliferation of some diseases.”  Specifically, “at least 15 genetic disorders …adversely affect Holsteins.”

Since genomic selection was introduced in the United States in 2008, inbreeding of Holsteins has accelerated and both bulls and cows have been bred at earlier and earlier ages.  The percentage of inbreeding of genotyped Holstein bulls (by birth year) rose from 2 percent in 1980 to 12% recently.

Geneticists said that the Holstein breed might be able to tolerate the inbreeding, but “the worry is that the dwindling diversity of Holsteins could permanently undermine the breed’s fitness.”

All Thoroughbred horses are descended from three foundation sites. Inbreeding has become increasingly prevalent, as more and more mares have been bred to fewer and fewer popular stallions, especially stallions from the Northern Dancer line. Jockey Club records show that in 2007, 9.5% of all mares bred were sent to stallions covering more than 140 mares. This percentage soared to 27% in 2019.

It is not possible–lacking a longitudinal experimental design—to say for certain why the number and frequency of starts for the modern Thoroughbred racehorse pale in comparison to the same statistics for racehorses of 50 years ago and longer.  But inbreeding is a leading suspect for declining durability, resulting in fatalities in training and races that threaten the viability of horse racing as a humane sport.

As with Holstein cattle, the end result of inbreeding in Thoroughbred racehorses is unknowable.  But the potentially negative outcomes of “dwindling diversity” are too great to risk the future of the breed.  Hence the Jockey Club’s 140-mare per stallion rule is precautionary and far-sighted.

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