Because mares have such long pregnancies, often lasting more than 11 months, they give birth later each calendar year if they’re rebred a month or more after foaling. Eventually they’ll have to stay empty a season to catch up. But breeding on the foal heat—estrus occurring within 20 days after foaling—can keep mares on a regular annual foaling cycle, said Patrick McCue, DVM, PhD, Dipl. ACT, professor and equine reproduction specialist at Colorado State University.

However, mares are less likely to conceive on foal heat than subsequent heats, and they’re more likely to lose the pregnancy, said McCue, who spoke during the 2020 American Association of Equine Practitioners Convention, held virtually. So breeders and practitioners should make informed decisions based on the mare, her reproductive health, and her foaling history when deciding whether to breed on a foal heat.

Specifying that he’s not advocating for or against breeding on foal heat, McCue said his research, clinical experience, and scientific review process have allowed him to develop practical recommendations for improving success rates when breeding a mare during her foal heat. For example, mares might be less likely to conceive or maintain a pregnancy if:

• They had a difficult birth (dystocia);
• Their reproductive tract incurred damage during the previous pregnancy or foaling (and hasn’t recovered);
• They had retained membranes for more than three hours;
• They had excessive or abnormal uterine fluid (lochia); and/or
• They’re 15+ years old.

Practitioners should perform a clinical exam six to eight days after foaling to verify the physical status of the reproductive tract and a transrectal ultrasound nine or 10 days after foaling to check follicular development. Laboratory analyses of reproductive tract samples in the first 20 days after foaling yield unreliable results, and scientists do not always agree about their significance, so they might not be useful for determining the mare’s reproductive health at that stage, he said.

In McCue’s practice, if the mare has not yet ovulated by Day 10, and she doesn’t have the issues cited above, he recommends breeding the mare. If she’s already ovulated (because some mares ovulate very early after foaling), or she has issues that make her a poor candidate for foal-heat breeding, the clinician can administer prostaglandins five to six days after that first ovulation to “short cycle” her and get her to ovulate sooner during the first post-foal-heat cycle.

Keeping pregnant mares under artificial lights can also encourage normal return to cycling after foaling, McCue said. About 10% of mares don’t have foal heats, and some have a foal heat but then stop cycling for a few months. Because lactation doesn’t seem to interrupt mare cycles, like it does in many other mammals, these post-foaling periods of anestrus are probably related to the “photoperiod”—the season of the year as determined by the number of daylight hours.

In summary, said McCue, “Careful post-partum mare selection and adherence to sound reproductive principles can lead to an acceptable pregnancy rate while minimizing pregnancy loss in mares bred on the foal heat.”