The transitional period between anestrus and estrus is officially complete once a mare has had her first heat of the year. Getting some mares to this point, however, and deciding when to breed them can be tricky. | Photo: Anne M. Eberhardt/The Horse

They discussed the topic at the 2018 American Association of Equine Practitioners Convention, held Dec. 1-5, in San Francisco, California. Charlie Scoggin, DVM, Dipl. ACT, from Rood & Riddle Equine Hospital, in Lexington, Kentucky, and Dirk Vanderwall, DVM, Dipl. ACT, professor of equine reproduction at Utah State University, in Logan, co-mediated a Table Talk session about transitioning mares. The format allowed attendees to share their practical experience, as well as related research.

Transitional Mares 101

Most, but not all, nonpregnant mares enter a state of reproductive “winter quiescence” when the days become shorter. This annual event is called anestrus and means the mare isn’t fertile, which (in nature) prevents her from foaling during months of inhospitable weather and limited food supply.

These broodmares subsequently transition back into their regular cycles as day length increases. However, if owners are hoping for an early season foal, this transition might require human intervention in the form of artificial lights or medical treatment