The Jockey Club Considers Limiting Book Size by 2021
In an interesting development, The Jockey Club board of stewards, concerned with narrowing diversity of the Thoroughbred gene pool, has announced that it's considering rules to limit the annual breeding of individual stallions starting with the 2021 breeding season.
The Jockey Club, established in 1894, is the keeper of the American Stud Book and maintains the Principal Rules and Requirements of the American Studbook in order to ensure the welfare of the Thoroughbred breed. The size of the North American foal crop has diminished significantly, from 37,499 in 2007 to 20,500 estimated for 2020.
On the mare side, in 2007, 5,894 mares (9.5% of the total) were bred by stallions that covered more than 140 mares. By 2019, 7,415 mares (27% of the total) were covered by stallions with books of more than 140, a threefold increase.
The combination of these changes has resulted in a substantial increase in the percentage of foals produced by a discreet segment of stallions signaling a worrisome concentration of the gene pool.
The stewards will continue to study the decreasing diversity of the Thoroughbred gene pool and its cause and potential effects over the course of time. As more data and analyses become available, the stewards may revise The Jockey Club's approach to protecting the breed's health and welfare.