Incremental Changes in Inbreeding When Comparing Stallion Pedigrees from the Years 2000 and 2020

This paper has been written in response to the Jockey Club’s proposal to limit the number of mares bred by any individual Thoroughbred stallion. The paper has three goals: 1) To provide a statistical analysis of the degree to which inbreeding in Thoroughbreds has increased over the last twenty years. 2) To review recent literature by equine geneticists who research and/or discuss inbreeding trends and genetic diversity and the advantages and drawbacks of inbreeding. 3) To discuss how the specifics of the original Jockey Club mare limit proposal could be altered to better serve the industry. The results of our statistical analyses on inbreeding trends are consistent with those of Binn et al (2012), and McGivney et al (2020) that there has been an increase in inbreeding in Thoroughbreds in recent decades. Geneticists point out that there are drawbacks (mutational load) and advantages (the concentration of beneficial genetic characteristics) arising from inbreeding. We outline an alternative to the Jockey Club’s September 2019 140-mare limit proposal that will result in a decrease in the size of large books for the majority of stallions that will provide flexibility to breeders utilizing seasons from stallions of proven quality