PREPARING OPEN MARES FOR THE BREEDING SEASON (H39-0201CM)
Cindy McCall, Ph.D.
Extension Horse Specialist

As the weather becomes warmer and daylight hours increase, many mare owners begin dreaming about the upcoming breeding season. Breeding a mare can be an expensive procedure even when the mare conceives quickly and maintains the pregnancy. If the mare has a problem getting in foal or maintaining the pregnancy, expenses and frustration can mount rapidly. However, mare owners can assure that their mare has the best possible chance of becoming pregnant through some simple management practices.
First the mare owner must understand the mare’s normal reproductive cycle. Most mares will not exhibit estrus (heat) during the winter months. Typically, most mares in Alabama will begin having regular estrous cycles as the daylight hours increase in the spring (mid March to mid April). During the estrous cycle, the mare will be in heat for 3 to 7 days then out of heat (diestrus) for 15 to 19 days. The mare will not be receptive to the stallion during diestrus. This cycle repeats throughout the spring, summer and early fall if the mare does not become pregnant. The mare ovulates (release of the egg from the ovary) approximately 24 to 48 hours prior to the end of the 3 to 7 day heat period. After ovulation the egg has a relatively short lifespan for breeding purposes (about 12 hours), but the sperm can live in the mare’s reproductive tract for 48 hours or longer. So the best option for fertilization of the egg is to inseminate the mare 36 hours before and up to the time of ovulation. Because the heat period is variable both between mares and within the same mare, predicting ovulation time can be a problem without the help of a knowledgeable veterinarian who can either palpate the ovaries through the mare’s rectal wall or use an ultrasound image to predict probable ovulation time.
Mares which have not been under an artificial light regimen beginning in November (which tricks their reproductive system into thinking it is spring) will not have regular estrous cycles until at least mid March, so there is no point in getting them to the breeding farm at an earlier date. However, there are a lot of things the owner can do in fall and winter to optimize the open mare’s chances of pregnancy. Barren mares (those that were bred the previous year but either failed to conceive or to maintain a pregnancy) and older mares which have never been bred should have a reproductive exam by a veterinarian. This exam can help identify reproductive problems and predict the mare’s probability of conceiving and carrying a foal. Performing the reproductive exam early provides time to correct fixable problems before the breeding season begins, and if the problem cannot be corrected prevents a needless stallion contract.
Throughout fall and winter, mares should be fed a balanced diet which keeps them in good flesh. Research has shown that mares in a thin body condition have more trouble conceiving and maintaining a pregnancy than those in a moderate to fleshy condition (ribs can be felt but not seen). The nutritional requirements of open mares are the same as
any other mature horse. Many mares which are not exercised heavily can maintain a fleshy body condition with free access to good quality hay or pasture. Mares which cannot stay in a fleshy condition on just hay or pasture should be supplemented with a grain ration formulated for mature horses at maintenance or light work. There is evidence that fungus-infected fescue reduces the mare’s chances of conceiving and increases early embryonic death losses, so open mares which are scheduled for breeding should be pulled off fungus-infected fescue at least 30 days prior to breeding and kept off fescue for at least 30 days after breeding.
Routine health care procedures should be performed on open mares. Open mares should remain current on their vaccination and deworming schedule. Mare owners should check with their veterinarian and with the breeding farm manager about any additional vaccinations or health care procedures that they recommend. Also if the mare is going to be shipped out-of-state for breeding, she will need a health certificate and current Coggins test. Mare hoof care should not be neglected because lame mares are under stress and may have difficulty breeding. The mare’s teeth also should be checked annually for dental problems which may reduce her ability to graze and chew feeds effectively.
If the mare has been used recently in an activity which requires a high level of fitness, such as racing, endurance or a heavy show schedule, it is advisable to give the mare some “let down” time before immediately switching her to the broodmare role. Fall and winter should give the owner ample time to slowly adjust the mare to a forage based diet and pasture life.
The owner should inquire about the intended stallion’s breeding ability and the farm management. The stallion’s conception rate during the previous breeding season, the average length of time a mare remains at the breeding farm and the average number of estrous cycles per conception are all indications of the stallion’s ability to produce pregnancies and the management’s expertise. The physical facilities of the farm and the condition of the horses on the farm should be inspected. If the farm does not care for their own facilities and horses, they may not provide good care for outside mares. If shipped semen is going to be used in the breeding program, the mare owner must have a good relationship with an equine veterinarian who is willing to work with the owner on a daily basis until the mare is inseminated. Also the owner must determine the logistics and additional expenses of handling shipped semen in their breeding program.
Finally before breeding the mare, the owner should determine the overall goals of his or her breeding program. The desired type, quality and marketability of the resulting foal should be evaluated before breeding the mare. Mares which cannot make a significant contribution to producing desirable traits in their foals should not be bred. Performance and production records of the mare and her close relatives, the predicted reproductive abilities of the mare, conformation, pedigree, disposition and sometimes color should all be considered in the decision to breed the mare. Mare owners should remember that it is usually more economical to buy a young replacement animal than to try to breed and raise one. However, if the decision is made to breed the mare, keeping her healthy throughout the year will increase her chances of conceiving and producing a healthy foal.