Based on the available literature, the most favorable age group for breeding horses is with mares between the ages of 4 and 12. Once the mare surpasses the age of 12—and particularly if it’s a maiden mare—the conventional wisdom dictates that issues are more likely to be present which can make conception and/or pregnancy maintenance more difficult.1
The normal gestation length for a mare is 342 days, however this can vary considerably and can range from 315 to 400 days. It is certainly not uncommon for mares to carry their foals beyond 12 months of gestation.2 The gestation length also varies with season; mares foaling earlier in the season tend to have a longer gestation than those foaling later in the season. In general, the gestation length will be predicated upon individual factors rather than age, with overall health and cycling history contributing to the mix.
The younger mare (under the age of 12) is likely to present the least number of issues in establishment and maintenance of pregnancy. It must be kept in mind that at the younger end of the spectrum, the filly must have started cycling regularly and be showing adequate maturity for motherhood. In general, the maiden mare is likely to present fewer issues in the way of establishing pregnancy.
Upon making a decision to breed the mare, the first thing to consider is the mare’s uterine hygiene and health. Several options are available, producing varying degrees of diagnostic reliability. The most basic evaluation is culture and cytology. These two combined evaluations determine with variable accuracy the presence (or absence) of an organism, and whether an organism, if present, is pathogenic in nature. An endometrial swab culture alone will not reliably produce this information, resulting in a high incidence of either missed organisms or organisms identified as being pathogenic, when that is not the case. A double-guarded device is preferable, and a cytology brush has been found to be superior to a swab for the harvesting of cells for a cytology smear.4
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Breeding or inseminating the mare induces an immediate inflammatory response in the uterus. This is a physiological reaction against foreign material. In most mares this inflammation clears within one or two days. Mares susceptible to mating-induced endometritis are known to accumulate fluid in the uterus as a result of impaired clearance of inflammatory products. Reduced myometrial contractions, poor lymphatic drainage, a large, overstretched uterus and cervical incompetence are predisposing factors for persistent-mating induced endometrititis.3
It is particularly important to recognize and appropriately manage older maiden mares; in many cases these mares are susceptible to post-breeding endometritis even though they have never been bred before. Often, sport or Warmblood mares may not be presented to be bred until they are in their teens. These older maiden mares can be very difficult to get in foal.2
Breeding should occur at the optimal time and, although not all breeders and managers are in agreement on this point, the number of breedings should be restricted to one.4 The mare will need very close monitoring of the estrous period by rectal palpation and ultrasonography. Many breeders strongly recommend the use of human chorionic gonadotrophin (hCG) to ensure that the mare is only bred once. Recently, use of a gonadotrophin releasing analog, deslorelin, has been found as effective in inducing ovulation as hCG.
Prediction of ovulation can also be made easier by not breeding maiden mares before they have begun to cycle regularly. If feasible, the use of artificial insemination can be helpful to reduce (but not eliminate) the inevitable post-breeding endometritis in older maiden mares.4
Ideally, a single breeding should occur 1 to 3 days before the anticipated time of ovulation. Most stallion spermatozoa are viable for at least 48-72 hours after mating, although records will ultimately indicate if the semen from a particular stallion is not viable after 48 hours.4 This early mating allows more time for drainage of fluid via an open estrous cervix and also utilizes the natural resistance of the tract to inflammation during estrus. It allows sufficient time to flush the mare more than once before ovulation if necessary.
In the older maiden mare, treatment for endometritis is ideally performed before ovulation. Progesterone concentrations rise rapidly in the mare and any post-ovulation treatment has an increased risk of uterine contamination.3
On the day after breeding, an ultrasound examination of the uterus should be performed to assess the amount and echogenicity of any intrauterine fluid. In older maiden mares, if more than 2.0 cm of fluid was present in the uterine lumen, some breeding managers advise lavage of the uterus with 1-2 liters of warmed, buffered, sterile saline using a uterine flushing catheter. During lavage, intravenous administration of 25 iu oxytocin should be performed. This is followed by infusion of a low volume (40 ml) of water-soluble, broad-spectrum antibiotics instilled through the cervix into the uterus via a sterile irrigation catheter. It is vital that the antibiotic used does not irritate the endometrium or predispose to overgrowth with fungal organisms.1
Predicting when a mare will foal can be difficult. “The two most commonly used indicators are softening of the ligaments around the base of the tail and “waxing-up”. Waxing is the appearance of beads of wax-like material on the ends of the teats. This wax is actually colostrum which oozes from the teats in the last few days of pregnancy. Waxing occurs in about 95% of mares 6 to 48 hours before foaling, however it can also occur several days before foaling or sometimes not at all.”5
Udder development usually starts 3 to 6 weeks prior to foaling but may occur earlier in maiden mares. Udder development prior to this may indicate that the mare is going to abort the foal.3 Other tests used to predict foaling in mares are testing of the chemical make-up of the milk and a drop in body temperature the day before foaling, although neither of these methods are considered as reliable for accurately predicting the day of foaling.
Nearly all (86%) of mares foal during the night, with most mares foaling between 10.00pm and 2.00am.5 This is probably an evolutionary adaptation ensuring that mares foal at the safest possible time to reduce the risk from predators.
The process of foaling is divided into 3 stages. The first stage is characterized by obvious changes in the mare’s behavior. “She will become restless, often walking the paddock aimlessly, she may show signs of colic (intermittent lying and standing, pawing at the ground, rolling) and may have patchy areas of sweating, especially over the shoulders and flanks. Inside the mare during this time the foal turns from its normal position of lying on its back to one where its backbone is next to the mare’s backbone with its head facing towards the mare’s tail. The cervix begins to dilate and the uterus begins to contract. First stage labor ends when the membranes rupture (“waters breaking”). On average, first stage labor lasts about one hour but can range from 5 minutes to 3 hours. First stage labor lasting more than 3 hours can indicate a problem and the mare should be examined by a vet immediately.”5
The second stage of labor is the period when the foal is actually delivered, and should take less than 20 minutes. It is characterized by strong abdominal contractions and the appearance of the foal’s feet at the vulva. Most mares foal lying down and they should not be forced to remain standing during delivery if it is progressing normally. “The white membrane (amnion) that surrounds the foal should appear at the vulva within 5 minutes of the waters breaking. One foot usually appears 5-10cm in front of the other foot and the soles of the feet should be pointing downwards.”5 Nearly all foals (98.9%) are delivered with the front feet and head first (anterior presentation), 1% of foals are presented in posterior presentation (breech) and 0.1% (1 in 1000) are found in a crossways presentation. Both posterior and crossways presentations are abnormal and veterinary assistance is required immediately.2
The third stage of labor involves the expulsion of the membranes which usually takes between 10 minutes and 3 hours.5 Breeding managers advise that if the membranes are still present 6 hours after foaling, it’s time to call your veterinarian.