Ask Your Veterinarian Presented By Kentucky Performance Products: A Vet Perspective On The Stud Book Limit - Horse Racing News | Paulick Report

Ask Your Veterinarian Presented By Kentucky Performance Products: A Vet Perspective On The Stud Book Limit

Veterinarians at Rood and Riddle Equine Hospital answer your questions about sales and healthcare of Thoroughbred auction yearlings, weanlings, 2-year-olds and breeding stock. If you have a question for a veterinarian, email us info at .

QUESTION: Colts born this year will be the first subject to the newly-announced cap on stud book at the end of their careers. What are your thoughts on that as a veterinarian?

DR. CHARLIE SCOGGIN: In May 2020, The Jockey Club of North America (TJC) announced a rule whereby Thoroughbred colts born in 2020 or later will have their stud book limited to 140 mares once they begin their breeding careers. According to a statement by the stewards of TJC, this limit was established to: “promote diversity of the Thoroughbred gene pool and protect the long-term health of the breed.”

Note that horses born prior to 2020 will not be subject to this rule, thus allowing their book size to be determined by individual stud farms.

This new rule has many layers, and as a scientist and specialist in reproduction, I believe it has a lot of merit. First, consider the notion of whether or not the Thoroughbred gene pool has become narrower and/or shallower during the modern era. This concept appears to be backed by sound science. Using pedigree analyses and cutting-edge cytogenomic methods, independent researchers from around the world have demonstrated a decline in genetic diversity throughout the global population of Thoroughbreds. One of these studies, published in Nature this year, demonstrated an association between certain sire lines and an increase in in-breeding within the population of horses studied. These particular sires were deemed highly influential or “popular” based upon certain characteristics of their progeny that made them elite on the track, in the sale ring, or both. Their popularity led to increased numbers of their progeny but a reduction in genetic diversity. Studies have also indicated in-breeding has accelerated over the past few decades, which has been due to many factors — such as increased access to stallions — and will be covered shortly.

Dr. Charlie Scoggin

Whether this book limit will, as stated by TJC, “protect the long-term health of the breed” assumes the practice of in-breeding could have an adverse effect on the breed. To my knowledge, no definitive studies exist demonstrating a clear association between in-breeding and poor performance in Thoroughbreds. However, there is evidence in other species demonstrating the negative issues encountered when family trees fail to branch properly. There is also credible data in cattle and pigs regarding how deepening and widening their respective gene pools has had measurable benefits on their progeny.  The term hybrid vigor is used to describe the phenomenon of improving genetic diversity and has spawned the widespread use of industry-supported metrics to maximize the genetic potential of the sire and dam. Examples are expected progeny differences (EPDs), which are usually available to all beef and pork producers and supported by their respective industries. Not only do EPDs lend well to maintaining hybrid vigor, but they also provide another level of quality control that translates into a better end product.

Regarding the question of whether modern day breeding practices are adversely affecting Thoroughbreds, I believe they have evolved because of science, especially with respect to continued improvements in our understanding of reproductive physiology and management of common conditions affecting fertility. Nevertheless, nature does have its limits, and certain statistics suggest Thoroughbred stallions may be reaching their limits in terms of book size. As background, book sizes for individual stallions have increased significantly over the past two decades. The following data was gleaned from evaluating the online records from TJC:

  • For all of North American Thoroughbred stallions, the average book size per stallion in 2008 was 16.2 mares/stallion; in 2019, it was 25.7 mares/stallion
  • In Central Kentucky, the average book size per stallion in 2008 was 59.6 mares/stallion; in 2019, it was 78.3 mare/stallion
  • The migration towards larger books is also evident when evaluating the proportion of stallions with relatively large book sizes. In 2005, 1.4% (53/3,711) of all stallions had book sizes > 120 mares; in 2019, this proportion almost tripled to 5.9% (67/1,135) for all stallions with a book size > 120 mares.

As to what impact increased breeding frequency and/or larger book sizes can have on stallion fertility, the science appears to be mixed and dependent on individual stallions. In 2007, a study from the Journal of the American Veterinary Medical Association reported foaling rates of 58, 67 and 72% for stallions with book sizes between 41 and 80 mares, 81 and 120 mares and more than 120 mares, respectively. This report suggested foaling rates improved with increasing book size.

Regarding daily usage rates, a more recent study described a reduction in per-cycle pregnancy rates when stallions performed more than three covers daily for seven consecutive days. As mentioned earlier, access to certain stallions has significantly increased over the past several decades, and not solely because of increasing book sizes. Shuttling to the Southern Hemisphere is another duty some popular stallions will perform, which leads to increased use and, by extension, a greater number of progeny born in a calendar year. The effect of dual-hemisphere matings were evaluated in 2017, and researchers reported higher fertility in stallions bred in the Southern Hemisphere, where average book sizes were numerically lower (120.8 mares/stallion), than in the Northern Hemisphere (155.8 mares/stallion). Also of interest was the trend of reduced fertility in novice stallions as their book sizes increased. These findings were indeed intriguing but certainly not definitive, particularly as it pertained to a limit in book size.

As to what this limit is, TJC has decided it is 140 mares. My perspective as a veterinarian deems this number both fair and reasonable. My rationale is due in a large part to a previous precedent: The United States Trotting Association (USTA) established a 140 mare limit over 10 years ago. While there were grumblings and even a few lawsuits initially, it was my impression most stakeholders moved on to more pressing issues. It is also important to note the USTA permits the use of artificial insemination and embryo transfer, but it is my understanding they only allow one foal to be registered per mare per year.

Perhaps I am comparing apples to oranges, but there is no denying both are fruits. Another reason why I like the number 140 is that it's a harmonic divisor number, which means it can be divided by multiple and different denominators. Consequently, 140 can be broken down into easily divisible numbers as would be needed when establishing the number of shares for a syndicate. For these two reasons, the number 140 seems logical to me. It is also pretty cool to be able to use college mathematics to make an argument, which does not happen very often.

What does happen fairly often is me failing to see another side of an argument or failing to be empathetic. Admittedly, my perspective lacks that of a breeder or bloodstock agent and fails to account for changes in the global and equine markets. It also does not account for The Law of Unintended Consequences that often plays out when restrictions are placed on biological entities. Only time will tell what impact this new rule will have on the Thoroughbred breed, but, as an advocate of the horse and of science, I hope it achieves its objectives. In my opinion, not only will it help sustain the Thoroughbred breed, it will also help evolve it.

Dr. Scoggin holds a master's degree in equine reproductive physiology in addition to his DVM and worked as resident veterinarian at a breeding farm near Paris, Ky., for seven years. He is a Diplomate of the American College of theriogenologists and joined Rood and Riddle's reproduction department in 2015. He is an affiliate faculty member in the clinical sciences department at Colorado State University.

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