Information provided by Dr. Audrey Kelleman, Veterinarian of UF Large Animal Reproduction, Lecturer, DACT, DVM.
Fall transition and spring transition!
What do those confusing terms mean in regards to the mare’s reproductive cycle? Horses are what are known as “seasonal breeders,” which indicates that only a part of the year, a season, is the time when they are having regular ovarian cycles and are most fertile. In mares, day length, the number of hours of light per day, is the “time keeper” for their reproductive cycles. More light stimulates the ovaries, while less light “shuts them down”. In winter, with the shortest days, most mares stop reproductively cycling. In summer, with long days, mares are most fertile.
In the fall, days are getting shorter, and the mares are transitioning from regular summer ovarian cycles to stopping ovarian cycles in winter. Likewise, in spring, the days are getting longer, and the mares are transitioning from winter into the normal ovarian cycles of late spring and summer. Both the fall and spring transition phases can be frustrating for owners. The ovaries during transition are shutting down or ramping up erratically, and as a consequence, the mares then may show erratic reproductive behaviors of “heat” (also known as “estrus”) or sometimes even constant heat behavior. Such behaviors can include urinating frequently, and standing firm and not wanting to respond to commands, or general irritability.
It is important to note that during fall and spring transition that these occurrences can be completely normal physiologically, though frustrating. Once winter is upon us and the days are very short, the ovaries shut down and things quiet down. Likewise, in summer, after the erratic spring transition, the mare will go into regular cycles by about mid-April, when the mare will start having about 1 week of heat and 2 weeks out of heat rhythmically.
If your mare is showing erratic reproductive behaviors, there are hormonal treatments available that can help lessen the signs. Before beginning any such medications, the mare’s ovaries will be examined by ultrasonography in order to confirm that the behavior signs are caused by the transition phases.
Sometimes, other abnormal health problems, such as an ovarian tumor, like the granulosa cell tumor (GCT), can also cause behavioral problems. The mare affected with a GCT, because the tumor makes testosterone, can act like an intact stallion. Other times with GCT, it could be that the mare has erratic behaviors. Treatment for GCT is surgical removal of the tumorous ovary. Usually, such tumors only affect one ovary. Ovarian examinations and hormonal testing are helpful in diagnosing these tumors.
If your mare is showing erratic reproductive behaviors, it could be a normal function of the time of year and the daylight. The simple approach to this may be giving the mare “tincture of time” and waiting for the seasons to change. A reproductive examination can help determine what is going on and provide guidance for potential treatment.