How Long Do Horses Ovulate?

Mares cycle when days are long and there is abundant daylight during most of the day, meaning that under normal circumstances, they cycle in the Spring, Summer and Fall. Mares do not cycle in Winter when daylight is short; this is known as anestrus. During anestrus, the uterus is flaccid, and the ovaries are inactive with no significant follicles or corpora lutea. The cervix may be closed but not firm and tight, or it may be thin, short, and dilated.¹

As the length of daylight increases with the onset of Spring, the ovaries become active once again. Their cycle resumes, and the ovaries begin to produce follicles, but ovulation does not occur immediately. "Mares have three or four prolonged intervals of estrus (periods of sexual receptivity to the stallion) during the vernal transition, but ovulation does not occur. The end of vernal transition is marked by a surge of luteinizing hormone and subsequent ovulation. After this ovulation, the first 21-day interovulatory period of that breeding season occurs and a regular estrous cycle is established."¹

Mares have two follicular waves each cycle. Although the mare continues to ovulate regularly every 21 days throughout the breeding season, the length of estrus varies, ranging from two to eight days, and the length of diestrus varies accordingly to maintain a 21-day interval.¹ Early in the breeding season, estrus tends to be longer, whereas around the summer solstice the mare may be sexually receptive for only two to three days.

Predicting Ovulation in the Mare

"In horses, a breeding is only as effective as your ability to predict a mare’s impending ovulation."² That said, there are a plethora of factors that impact the mare's cycle, when she will ovulate, and the approximate lengths of time before, during and after ovulation.

Other than the mare's general health and diet, these factors include:

• Reproductive history - Many broodmares ovulate a follicle of similar diameter (important because follicle size indicates impending ovulation) each cycle. For these predictable mares, veterinarians can often use data from previous cycles to estimate ovulation.
• Follicle growth pattern - The mare’s dominant follicle typically increases in diameter by 2.7-3 mm a day during estrus. This growth ultimately peaks and holds steady for about a day or two prior to ovulation and might even decrease by 2-3 mm within the 12 hours before ovulation.
• Follicle diameter - Veterinarians can predict the diameter a preovulatory follicle is going to reach based on the mare's size and breed. The smaller the breed, for instance, the smaller the follicle.
• Follicle shape - Developing follicles are spherical during most of estrus and most lose their shape around 12-24 hours prior to ovulation.
• Follicle wall changes - A follicle’s wall thickness increases as the interval to ovulation decreases. Other changes also occur as ovulation nears, such as rents or tears in the wall, which occur about 77% of the time. During ovulation itself, the wall collapses.
• Follicle tone - Early in the growth phase, follicles feel firm on manual palpation. In the 12-24 hours before ovulation, they often become noticeably softer.
• Uterine edema - Fluid swelling within the uterine lining develops with the presence of estrogen and the absence of progesterone and peaks about two days prior to ovulation. Ovulation occurs as this edema declines or disappears.
• Cervical relaxation - Also in response to increased estrogen levels and an absence of progesterone, the mare’s cervix relaxes, as detectable via rectal palpation or vaginal exam.
• Ovarian pain - Some mares experience pain associated with follicle development and, more commonly, exhibit discomfort when the veterinarian palpates the site of a fresh ovulation.
• Number of days in estrus - The average equine estrous cycle is 20.6 days, and the average duration of estrus is 5.7 days. 46%, 32%, and 12% of mares ovulate within 24, 48, or 72 hours, respectively, prior to the end of estrus, while 10% of mares were out of estrus before ovulation occurred.²

Finally, if the mare has been given prostaglandins or other agents to induce ovulation, schedules must be adjusted accordingly, since this will bring on earlier ovulation.

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¹Sertich, P., DVM. Reproductive Cycle in Horses, Merck Veterinary Manual, 2019.
²Beckstett, A. 13 Ways to Predict Ovulation in Mares. The Horse, AAEP Convention 2014.

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