Uterine involution is characterized by expulsion of the fetal membranes and contraction of the uterus, cervix, and broad ligaments to normal nongravid dimensions. To achieve maximal reproductive efficiency, a broodmare must produce a foal every year. Horses have an average gestation length of ~340 days. Therefore, to maintain a 12-month foaling interval, the mare must be bred again within 25 days of foaling. Mares can be bred on “foal heat,” which is the first postpartum estrus that occurs 5–11 days after foaling in most mares. However, mares that experienced dystocia or retained membranes and metritis should not be bred on foal heat. Foal heat pregnancy rates are higher for mares bred after 10 days postpartum.
The fertility of the first breeding may be increased if breeding is delayed and PGF2α (dinoprost, 1 mg/45.5 kg, IM, or cloprostenol, 0.55 mcg/kg, IM) administered ~5 days after the foal heat (first) ovulation. The mare can then be bred at the ensuing estrus just before the second postpartum ovulation.