Many factors might contribute to a broodmare safely exceeding her foaling date

by Bryan M. Waldridge, D.V.M., M.S., Mary Beth Stanton, D.V.M., and Tricia Salazar, D.V.M., M.S.

THE HORSEMEN'S dogma that it takes a mare 11 months, a week, and a day from breeding date to foaling date often causes anxiety for an owner when a mare does not foal when expected. But the length of gestation depends on the individual mare, and when a mare exceeds her due date, it generally is not cause for alarm if she has experienced a normal, properly managed pregnancy.

Several factors can affect a mare's length of gestation. Average gestation generally is cited as 335 to 342 days. Therefore, it is important not to use ballpark figures such as 11 months or 330 days when calculating a mare's expected foaling date.

The point at which gestation is considered prolonged varies. It has been reported to range from greater than 350 days to greater than 360 days. Pregnancies lasting up to 400 days have resulted in normal foals.

Approximately 1% of equine pregnancies might be considered prolonged, or greater than 350 days. One Thoroughbred farm in the Rood & Riddle practice has had up to 18% of its pregnancies exceed 350 days, and we have seen normal foals born to mares as late as 13 months of gestation without difficulty.

Consider factors

Many physiologic and management factors influence the length of gestation in otherwise normal pregnancies:

•Gender. Mares tend to carry colts an average of two to three days longer than fillies;

•Breed. Draft horses tend to have longer pregnancies than light breeds;

•Individual. Particular stallions or mares might influence gestational length. In a study of 500 Standardbred pregnancies, gestational lengths were consistently less than 340 days for certain sires and consistently greater than 350 days for other sires. Exactly what factors in a particular stallion influence the length of gestation are unknown, but prolonged pregnancy could be related to the size of the foal.

In one study, gestation in mares in their fifth or later pregnancy tended to be two days longer. Another study of 21 mares that had exceeded their due date reported that twin pregnancies carried to term tended to be an average of nine days longer. However, twins are an important cause of noninfectious abortion, and not all mares pregnant with twins will have prolonged pregnancies. In fact, most mares pregnant with twins will abort or have a shortened pregnancy;

•Time of the year. In the Standardbred study, gestation decreased by approximately 2.5 days per month in mares bred during April through June, which is the natural equine breeding season. For example, mares that foaled in March carried their foals an average of 349 days, while mares foaling in June were pregnant for an average of 340 days.

Mares that foal in the winter and early spring tend to have pregnancies approximately ten days longer than mares foaling in the summer. Mares bred early in the breeding season tend to have longer pregnancies than mares bred later in the season.

This effect can be circumvented by placing mares under lights using the same protocols to promote early season return to cyclicity. Usually this involves exposing mares to 16 hours of light per day or adding three to four hours of artificial light at the end of daylight. Artificial light should be bright enough to read a newspaper easily wherever mares are kept;

•Nutrition. Severe undernourishment in the latter half of gestation will increase gestation by an average of four to ten days. Overfeeding mares in the later part of pregnancy could reduce gestational length by a few days. Mares with higher body-condition scores averaged a four-day shorter pregnancy in one study. Ideally, broodmares should be maintained at a body-condition score of five (the ribs can be felt easily but are not visible).

Mares in the first two trimesters of pregnancy can meet their nutritional requirements easily without additional supplementation. Fetal growth is greatest during the last three months of pregnancy. During this time, mares will need additional energy, protein, and minerals;

•Delayed development of the embryo. Called embryonic diapause, the condition occurs in other large animals and is thought to occur in the mare, as well. Embryonic diapause is believed to occur before 35 days of gestation and the early formation of the placenta. The period of diapause could last approximately three to four weeks. It is unknown whether a shortened embryonic diapause of only a few days is common in horses; and

•Exposure to endophyte-infected tall fescue. The presence of as little as 5% of infected tall fescue in pasture grass or hay can be enough to cause reproductive problems in mares. Gestational lengths up to 13 to 14 months have been reported in mares grazing endophyte-infected tall fescue.

Fescue poisoning

The endophytic fungus (Neotyphodium coenophialum) that affects certain varieties of tall fescue produces several toxins that result in many reproductive difficulties, including:

•Prolonged pregnancy;

•Placental abnormalities;

•Decreased or absent milk production;

•Dystocia (difficulty foaling); and

•Reduced chance the foal will survive.

Affected foals are born very large and, despite their size, they appear immature with slick hair coats, poor muscling, and weak tendons. These foals also have decreased thyroid and steroid hormone levels.

Foals born to mares with fescue toxicity have a poor chance of survival because they are hormonally immature and at high risk for neonatal bacterial infections; their dams lack colostrum and milk; and they often are faced with a difficult foaling. One study found that neonatal foal losses in mares exposed to endophyte-infected tall fescue were three times higher than for mares grazing other forages.

Dystocia is common with fescue toxicity because the foals usually are larger than normal, and the mares have poor relaxation of their pelvic ligaments. Mares with extremely large foals might require a Caesarean section to deliver the foal, controlled vaginal delivery under general anesthesia, or fetotomy (dissection of the dead fetus and extraction of the pieces).

Increased thickness of the placenta might not be observed in mares grazing endophyte-infected tall fescue pastures until eight hours before foaling. This increased thickness occurs due to swelling of the placenta and could accompany premature placental separation or "red bag" deliveries. Retained placenta is also more common in mares grazing endophyte-infected tall fescue.

Pregnant mares should be removed from endophyte-infected tall fescue pastures and hay during the last trimester of gestation and no later than the last 30 days of gestation to avoid toxicity. If fescue toxicity is suspected, administration of drugs such as domperidone could aid in the production of colostrum and offset some of the other expected problems.

Several laboratories are able to test fescue plants or fescue seed to determine if they contain the endophytic fungus. Consult your local agricultural extension service to obtain the contact information for laboratories in your area that provide this service.

Begin with mare's history

A complete reproductive history of the mare can help identify risk factors in the current pregnancy and allow you and your veterinarian to plan for management of any possible or expected foaling problems.

A thorough physical examination of the mare and examination of the fetus and the mare's reproductive tract by means of rectal palpation should be performed. An examination using a vaginal speculum can be useful in some mares if placentitis (inflammation or infection of the placenta) is suspected or to assess relaxation of the cervix. Ultrasound examinations, measurement of milk electrolyte concentrations, hormone assays, and occasionally fetal electrocardiography (ECG) are useful and readily available diagnostic tests to assess the general health of the fetus and intrauterine environment.

Unfortunately, confidently determining the well-being of the foal in the uterus and its readiness for birth can be difficult. Ultrasound examinations allow the determination of fetal heart rate and measurement of the thickness of the uterus and placenta. This is helpful to determine if fetal stress or placentitis are present. A persistently elevated fetal heart rate can indicate fetal stress, while a persistently slow fetal heart rate can indicate a severely compromised fetus. The heart rate should increase briefly with increased fetal movement.

Amniocentesis, insertion of a needle into a mare's abdomen to withdraw a sample of the amniotic fluid for testing, has been associated with a high risk of abortion in mares and currently cannot be recommended as a diagnostic test for fetal health or maturity and intrauterine infections. Experimentally, amniocentesis in mares has been used to diagnose fetal equine herpesvirus-1 infections, a common infectious cause of abortion.

Measurement of milk electrolyte concentrations (calcium, sodium, and potassium) aids in predicting fetal maturity and is useful to determine when mares should be observed for impending labor. As pregnancy progresses, the concentration of sodium in the milk decreases, and the concentrations of potassium and calcium increase.

Water-hardness tests can be used to measure increasing concentrations of calcium carbonate in the mare's milk, which signals that foaling is approaching. However, these tests have been shown to be better at predicting when a mare will not foal, rather than when a mare will foal within 24 hours.

With the exception of exposure to endophyte-infected tall fescue hay or pasture, most prolonged pregnancies will result in a live, healthy foal, although the youngster might be smaller than expected. Prolonged gestation probably also is caused by several yet-undetermined factors.

After analyzing the results of diagnostic tests, veterinarians must decide whether to intervene in the pregnancy, and how. In most situations, the foal benefits by remaining in the uterus even if the environment is somewhat hostile, because premature induction of foaling can be disastrous.

 Bryan M. Waldridge, D.V.M., M.S., is a practitioner at Rood & Riddle Equine Hospital in Lexington. Mary Beth Stanton, D.V.M., is a visiting assistant professor of theriogenology at Auburn University's Large Animal Teaching Hospital. Tricia Salazar, D.V.M., M.S., is a resident in large-animal medicine at Auburn.