Breeding Season Manipulation (Book Excerpt)

Editor's Note: This is an excerpt from Breeder's Guide to Mare, Foal, and Stallion Care by Christine Schweizer, DVM; Christina Cable, DVM; E.L. Squires, PhD. This book is available from

Left to her own devices, a mare cycles more or less between April and October. Each estrous cycle (or the length of time between ovulations) lasts approximately 21 days, and the mare ovulates 24 to 48 hours before the end of her behavioral estrous period after a variable number of preceding days in heat. Unfortunately, these patterns are not always the most convenient for humans imposing their own schedules on the breeding process. Equine reproductive physiology lends itself to manipulation in a number of ways, and the mare's seasonality, ovulations, and interestrous interval all can be manipulated to suit manmade demands. 

Because the mare's gestation length is approximately 11 months, the equine breeding season begins during late spring and ends in early to mid-autumn. Over time, natural selection pressures favored individuals born during the warmer seasons, and, consequently, the genetics of those mares that cycled during May, June, and July in the Northern Hemisphere. However, at some period during man's Western development, January was chosen as the start of our calendar year. Many stud books once recorded a horse's actual day of birth as its official birthday. But for racehorses in particular, this manner of measuring a horse's age started to cause considerable confusion as horses might compete as three-year-olds one week, then turn around and run as four-year-olds the next. As a matter of convenience, it was decided that all Standardbreds and Thoroughbreds should have the same designated birthday, and January 1st became the chosen date. It is unfortunate that the decision-makers did not stop to think the matter through a little better.

In order for a mare to foal on or a little bit after January 1, she needs to conceive during February. Left alone, mares are still in late anestrus or spring transition at this time (not the optimal time to be trying to breed a mare!). There are strong incentives for trying to breed mares so that they foal as close to January 1 as possible. A foal born in January will be older and thus more mature than a foal born in June. The older foal will have developmental advantages when the two race against one another as two- and three-year-olds in particular. Older foals generally will be larger when it comes time to sell them as weanlings and yearlings and might command a higher price at the sales. Other breed registries also adopted a January 1 birthday, so racehorses are not the only horses placed under this kind of winter birthing pressure. Breeds, such as Morgans and Quarter Horses, which have richly rewarded futurity competitions for weanlings and yearlings, also are under the "bigger is better" dilemma. Any time money is involved, there is obviously a strong incentive to meet the requirements, and so it is with breeding mares in what, in effect, is the non-physiological breeding season. 

About the Author

Multiple Authors

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More

Free Newsletters

Sign up for the latest in:

From our partners