In horses, a breeding is only as effective as your ability to predict a mare’s impending ovulation. Fortunately, veterinarians have plenty of tools at their disposal to do so.

At the 2014 American Association of Equine Practitioners Convention, held Dec. 6-10 in Salt Lake City, Utah, Patrick McCue, DVM, PhD, Dipl. ACT, Iron Rose Ranch professor of theriogenology at Colorado State University’s Department of Clinical Sciences, in Fort Collins, reviewed current parameters for predicting ovulation in mares. They include:

1. Reproductive history Many broodmares ovulate a follicle of similar diameter (important because a particular follicle size indicates impending ovulation) each cycle, said McCue. For these predictable mares, veterinarians can often use data from previous cycles to estimate ovulation.
2. Follicle growth pattern McCue explained that a 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.
3. Follicle diameter But how do you know what diameter a preovulatory follicle is going to reach? McCue said veterinarians can predict this based on mare size and breed. The smaller the breed, for instance, the smaller the follicle. Friesians, he said, develop notoriously large follicles.
4. Follicle shape Developing follicles are spherical during most of estrus, McCue said, and most lose their shape around 12-24 hours prior to ovulation. At this time the follicle might form a “stigma” (cone or point) before becoming even more irregular in the hours preceding ovulation.
5. 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 McCue said occur about 77% of the time. During ovulation itself, the wall collapses.
6. 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.
7. Endocrine markers McCue said endocrine markers, such as estrogens and luteinizing hormone (LH), fluctuate during estrus but, unfortunately, are not practical for predicting a mare’s ovulation. “Estrogen levels increase in relation to the increase in follicle diameter during estrus, peak approximately two days prior to ovulation, and decline near the day of ovulation,” he explained. “Luteinizing hormone concentrations increase gradually during estrus—as opposed to a ‘spike’ of LH that occurs in other species—and reach peak levels near the day of ovulation.”
8. 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, McCue said. Ovulation occurs as this edema declines or disappears.
9. 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.
10. Ovarian pain Some mares experience pain associated with follicle development, McCue said. More commonly, mares exhibit discomfort when the veterinarian palpates the site of a fresh ovulation.
11. Number of days in estrus The average equine estrous cycle is 20.6 days, and the average duration of estrus is 5.7 days. McCue noted that “46%, 32%, and 12% of mares ovulated within 24, 48, or 72 hours, respectively, prior to the end of estrus, while 10% of mares were out of estrus before ovulation occurred.”
12. Interval from prostaglandin administration The duration from prostaglandin administration (used to manipulate mares’ estrous cycles) to the next ovulation is typically seven to 12 days. Mares with smaller follicles, however, take longer to ovulate post-prostaglandin administration than mares with larger follicles, McCue said.
13. Interval from ovulation-inducing agent administration If the veterinarian has administered an ovulation-inducing agent such as hCG or deslorelin, 85-95% of mares will typically ovulate within 24 to 48 hours if they’ve been in heat for two to three days.

“There’s no one predictive criterion, but a combination of parameters is useful,” to predict a mare’s ovulation or impending ovulation, McCue said. To further enhance ovulation prediction, he suggested administering hCG or deslorelin acetate at the appropriate time of the estrous cycle.