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 Dystocia in Horses Most causes of dystocia in the mare are due to abnormal presentation, position, or posture. A dead or compromised fetus often is not properly positioned in the pelvic canal. Dystocia due to... read more or retained membranes Retained Fetal Membranes Normally, the membranes are ruptured by the foal over the avillous cervical star region of the chorioallantois. The chorion (red velvety surface) and allantois (shiny surface containing many... read more and metritis Mares: Several specific diseases are associated with metritis or endometritis. These include brucellosis (Veterinary.see page Brucellosis in Large Animals), leptospirosis (Veterinary.see page Leptospirosis)... read more 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.
Also see pet health content regarding the breeding and reproduction of horses Breeding and Reproduction of Horses Mares reach puberty at about 18 months of age and undergo an estrous, or heat, cycle. Mares go into heat repeatedly during the breeding season, which usually continues while day length is long... read more .