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A fully detailed discussion of the reproductive biology and management of reproduction in horses is beyond the scope of this book. However, an overview of key aspects of the horse's reproductive cycle, pregnancy, and foaling are provided in this section.
Reproductive Cycle and Breeding Management
Most mares only have estrous cycles during the seasons of the year when day length is long. During winter, the ovaries are inactive, and the reproductive hormones are at baseline levels in the bloodstream. As the days get longer in the spring, mares go through a transitional stage as the reproductive tract starts to prepare for the breeding season. During this transition, mares will have 3 to 4 episodes of sexual receptivity due to the development of waves of large follicles on the ovaries. Mares are not fertile at this time of year because the ovaries do not release eggs. After 8 to 10 weeks, the level of luteinizing hormone will be high enough to cause one of the follicles to ovulate. Once this occurs, the mare will then establish a normal estrous cycle with regular ovulation. Mares ovulate every 21 days throughout the breeding season, with periods of estrus (receptivity to breeding with a stallion) lasting 2 to 8 days. During estrus, follicles on the ovary enlarge. One of these will become dominant and release a mature egg. The follicle becomes a corpus luteum and produces the hormone progesterone, which prepares the uterus for implantation of the egg. If the mare breeds and the egg is fertilized, the egg passes into the uterus and begins a pregnancy. Otherwise, the corpus luteum is destroyed and another estrous cycle begins.
After the period of winter inactivity and transition, mares naturally begin estrous cycles in April in the northern hemisphere. Because the changes in cycling are stimulated by increasing amounts of daylight, it is possible to hasten the onset of cycles by exposing the mare to increased amounts of artificial light during the winter. The timing of ovarian cycles can also be manipulated by giving injections of various reproductive hormones. This is typically done to facilitate breeding appointments and to allow mares and stallions to remain in competition during much of the breeding season. Your veterinarian can advise you on how this is done.
Detecting Estrus
A successful breeding program revolves around good estrus detection. The mare should be presented to a stallion (teaser) daily or every other day during the breeding season, and an accurate interpretation and record of her response should be made. A mare in estrus (the receptive phase of the cycle) raises her tail, squats, urinates, and presents her rear to the stallion. She will also allow the stallion to mount and copulate. A mare in diestrus (the nonreceptive phase of the cycle) usually squeals, kicks, bites, and rejects the stallion's attempts to sniff the mare or mount. Adequate exposure to the teaser stallion is necessary to determine the mare's receptivity. Prolonged and irregular periods of estrus are common at the beginning and end of the breeding season.
A mare in estrus may not appear receptive at first due to nervousness or inexperience. Some mares with a foal at their side will appear less receptive because they are protective of the foal. The mare's behavior should be consistent with the condition of her reproductive tract as determined by a physical examination. The response to teasing can determine if estrus has begun and indicate when a mare should be bred. Failure to return to estrus 2 to 3 weeks after breeding may suggest that the mare is pregnant. Pregnancy can be confirmed by several methods. Ultrasonography of the uterus (through the rectum) allows the earliest detection. Examination by rectal palpation (feeling a bulge in the uterus) can detect pregnancy from as early as day 28 in some mares. Hormonal tests are most accurate after about day 60 of pregnancy.
Health Programs During Pregnancy
Proper health care of pregnant mares is important to help ensure delivery of a healthy foal. Consult your veterinarian to make sure that your horse's vaccination and deworming programs are up to date.
Parasite Control
Most horse dewormers are safe for use throughout pregnancy, but your veterinarian's recommendations should be followed. In general, mares should not be given antiworming medications during the first 2 months of pregnancy, or during the last few weeks before foaling. Otherwise, mares should be dewormed every 6 to 8 weeks. It is recommended that the specific medication used be switched periodically to prevent development of resistance in the parasites. Mares can also be dewormed 1 or 2 days after foaling to reduce the number of small strongyle worms passed to the foal (see Digestive Disorders of Horses: Small Strongyles). Foals should be dewormed at 6 to 8 weeks of age on the same day as the mare and again at weaning.
Vaccinations
Immunizations should follow an annual schedule based on local health problems. Vaccination against rhinopneumonitis should be given at 3, 5, 7, and 9 months of pregnancy. Vaccinations that require annual boosters should be given 30 days before the estimated foaling date of the mare (see Reproductive Disorders of Horses: Sample Vaccination Schedule for Broodmares ). This will allow the mare to produce protective antibodies that are passed to the foal in the colostrum (see Routine Care and Breeding of Horses: Vaccinations).
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| Sample Vaccination Schedule for Broodmares |
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Vaccine
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Timing
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Equine rhinopneumonitis
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3, 5, 7, and 9 months of pregnancy and after foaling
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Tetanus
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4 to 6 weeks before foaling
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Equine influenza
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4 to 6 weeks before foaling; every 2 to 3 months during pregnancy for mares exposed to new horses
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Eastern and Western equine encephalomyelitis
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Usually given to mares in late spring or early summer before the onset of insect season; depends on location; if foaling late in the season, should be given again 4 to 6 weeks before foaling
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Rabies
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4 to 6 weeks before foaling; annual if rabies occurs regularly in the region
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Botulism
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Initially 3 injections at 1‑month intervals, then annual booster 4 to 6 weeks before foaling
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Equine viral arteritis
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Viral titer should be checked before vaccination; pregnant mares should not be vaccinated; mares should be vaccinated before breeding to a positive stallion that is shedding the virus; mares must be isolated from other horses for 3 weeks after vaccination; positive titers may cause problems if the mare is to be shipped internationally; stallions should also be vaccinated 3 months before breeding
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Strangles
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Not routinely given; used only if warranted for a specific mare and situation; occasionally causes problems with abscesses and sore muscles; questionable effectiveness
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West Nile virus
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Initially 2 injections at a 3- to 6-week interval, then annual booster 4 to 6 weeks before foaling
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Foaling
General preparations for managing delivery and caring for the newborn foal are described below (see Routine Care and Breeding of Horses: Foal Care).
Preparation
The mare should be taken to a foaling location 3 to 4 weeks before the expected foaling date so she can produce antibodies to any disease-causing organisms present in that environment. These antibodies will be passed to the foal via the colostrum (the first milk produced by the mare after foaling).
Foaling box stalls should be large (at least 12 by 16 feet [4 by 5 meters]). The foaling area should have good ventilation and be well bedded with clean, dry straw. The walls should be solidly constructed and free of sharp edges. Observation of the mare should be possible without disturbance.
Although certain signs occur before delivery, they do not allow any accurate prediction of the time delivery will occur. The mammary glands (udder) start developing 2 to 4 weeks before foaling and distend with colostrum in most mares 1 to 3 days before delivery. Colostrum drips from the teats and dries to form a waxy material at each teat opening. This “waxing” develops in almost all mares 6 to 48 hours before foaling, but in some cases it occurs much earlier or not at all.
Stages of Delivery
It is critical to understand the normal progression of events during delivery of a foal. This allows you to know if something is going wrong and whether intervention is needed. Delivery is divided into 3 stages.
Stage I is characterized by signs of abdominal pain and restlessness due to contractions of the uterus. Patches of sweat usually appear on the neck and flank and behind the elbows a few hours before foaling. The uterine contractions increase in frequency and intensity, causing the fetus to move into the pelvic canal. This causes the cervix to open. The fetus changes from a pelvis-up to a pelvis-down position prior to delivery. Mares may roll during this stage, which is thought to assist with the rotation of the fetus. Increasing pressure in the uterus causes the fetal membranes to bulge out of the opening cervix. Rupture of the fetal membranes and release of the fetal fluids (sometimes referred to as “water breaking”) marks the end of the first stage of delivery.
Stage II starts when the fetal membranes rupture and ends when the foal is delivered. An uncomplicated labor usually takes 10 to 30 minutes. The pressure of the fetus on the cervix stimulates abdominal contractions in the mare. The fetal membrane that normally appears between the lips of the vulva is a white, fluid-filled structure. The straining efforts of the mare consist of 3 or 4 strong contractions, followed by a short period of rest. The mare usually lies on her side with her legs extended during labor. The foal is normally delivered head-first, with the head, neck, and forelegs extended. One front hoof usually precedes the other, allowing the elbows and shoulders to pass through the pelvic canal more easily. The foal is usually born with the umbilical cord intact. The white amnion is usually intact, but is easily torn open by the movements of the mare or foal. If the membranes remain covering the nose, they should be removed by an attendant to prevent suffocation. If left undisturbed, the mare may lie for a few minutes with the foal's hindlegs in her vagina. If the foal has not been delivered within 30 minutes of the rupture of the fetal membranes and release of the tea-colored amniotic fluid, veterinary assistance is warranted.
Stage III involves the expulsion of the afterbirth (fetal membranes). Normally, the afterbirth is passed within 3 hours of birth of the foal. The weight of the membranes helps them separate from the inner surface of the uterus. Powerful contractions of the uterus complete the separation of the membranes from the uterus. The mare will stand with the fetal membranes hanging from the vulva. The membranes may extend past the level of the hock. If the mare kicks, which can endanger the foal, the membranes should be tied above the hocks. You should never manually pull on the membranes, as this can tear the membranes or cause damage to the uterus. If the fetal membranes have not been passed by 3 hours after delivery of the foal, your veterinarian may decide to give injections of the hormone oxytocin at 15- to 30-minute intervals until they have been completely expelled (see Reproductive Disorders of Horses: Retained Placenta in Horses).
If the fetal membranes separate from the uterus too early, then the fetal membranes that are first seen at the vulva in Stage II of labor will appear bright red and velvety instead of pale and white. This means the separation has occurred before the foal is able to breathe air on its own. The fetal membranes must immediately be ruptured and the foal delivered manually, or it will not receive enough oxygen. Depending on the degree of oxygen deprivation, the foal may not survive or may have permanent brain damage.
The Early Postdelivery Period
The uterus contracts and returns to its nonpregnant size soon after delivery of the foal and fetal membranes. Horses have an average pregnancy length of about 340 days (about 11 months). It is possible to breed a mare during the “foal heat” that occurs 5 to 11 days after delivery. However, mares that have had a difficult birth, retained fetal membranes, or metritis should not be bred on the foal heat. Foal heat pregnancy rates are higher for mares bred at least 10 days after delivery.
Last full review/revision July 2011 by Robert O. Gilbert, BVSc, MMedVet, DACT, MRCVS; Fabio Del Piero, DVM, DACVP, PhD; R. J. Erskine, DVM, PhD; Paul Nicoletti, DVM, MS; Jerome C. Nietfield, DVM, PhD, DACVP; Donald Peter, DVM, MS, DACT; Patricia L. Sertich, MS, VMD, DACT; Katrin Hinrichs, DVM, PhD, DACT; Brad E. Seguin, DVM, MS, PhD DACT
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