Reproductive Problems in the Draft Horse

Draft mares usually make excellent mothers, providing a large amount of milk and rarely rejecting a foal. Because of these traits, draft mares make good nurse mares; some mares can even support and feed two foals. Also, the tendency for multiple ovulations makes draft mares good candidates for embryo transfer. However, draft horses have some reproductive traits—lower fertility rates, twinning, retained placentas and other foaling difficulties—that lead to problems. In addition, one common draft breed possesses a gene for a devastating birth defect.

Epitheliogenesis imperfecta (EI) is a congenital defect seen in newborn Belgian foals. These foals have a defect in the gene responsible for forming the attachment of the skin to the underlying tissue. They are born with patches of skin missing from their extremities and body. Sometimes, an entire limb will be skinless. Other abnormalities include teeth that are present at birth, ulcers of the mouth and ulcers of the coronary band. Foals will have difficulty nursing due to the oral ulceration and the hooves will eventually slough off. Although the severity of the disease can vary at birth, the disease is progressive and there is no treatment. Affected foals should be euthanized as soon as a diagnosis is made.

This defect is an inherited disease. Currently, it is classified as an autosomal recessive which means that the sire and dam must both be carriers of the disease for a foal to born with EI. In the Belgian breed, there is a certain line of breeding that has been identified as carriers. Research is underway to develop a test to identify carriers of this defect.

Getting a foal on the ground in the first place can be a challenge in the draft breeds. One reason is that, overall, both stallions and mares tend to have lower fertility rates than their light horse counterparts. Since draft stallions mature slower, many two-year-olds have poor semen quality and therefore a low conception rate. Also, the testicles of a draft stallion are small in relation to their body size compared to light horses. Typically the draft stallion tends to have a low sperm concentration, high semen volume and a large amount of gel in the ejaculate. Because of this, the semen often must be centrifuged to maintain proper concentration and sperm quantity for optimum reproductive efficiency in artificial insemination. These factors make it more difficult and time consuming to successfully transport cooled and frozen semen from the heavy horses.

Twinning is most common in thoroughbreds and draft horses. Twins result when a mare ovulates and conceives on two follicles during a heat cycle. Up to 25% of draft mares will double ovulate or ovulate on two follicles. They have even been known to ovulate with three follicles and conceive with triplets. Due to a draft mare’s large uterus, they are more likely to carry twins to late gestation or even foaling. This makes it very important to have draft mares sonogrammed by a veterinarian14-15 days after ovulation to detect unwanted twins. As with light mares, one embryo may be crushed during this early phase of pregnancy, allowing the other to mature.  Unfortunately, the higher tendency for the development of multiple follicles does make predicting ovulation more difficult in draft mares. This becomes a factor when breeding a mare with cooled, transported semen.

The large foals desired by draft horse breeders may create additional reproductive dilemmas such as dystocia (difficult birth), difficulty rebreeding and retained placentas. Although draft mares do not have a higher incidence of dystocia than the light mares, when they do have problems during foaling, the foal’s large size complicates delivery further. For example, even if the foal is in the proper position, many mares need assistance to deliver due to inadequate uterine muscle tone and contractions given the size of the foal.

The stress of delivering these large foals makes rebreeding difficult. Because draft mares have poor uterine muscle tone, the uterus of a draft mare after foaling is slow to contract down, retaining a large amount of fetal fluids. This fact coupled with the large amount of semen volume introduced into the uterus from a natural breeding make these mares difficult to get back in foal. It is beneficial to sonogram draft mares during heat to detect fluid accumulation and aid in its removal. Artificial insemination is a useful technique to decrease the volume of seminal fluid used during breeding.

Finally, retained placentas are seen more commonly in draft mares than light mares. A placenta that is still attached after two hours should be considered retained. Aggressive veterinary treatment is warranted for this condition to prevent uterine infection (metritis), systemic infection (septicemia) and laminitis. “Foal founder” in any type of mare is a very serious condition, but it is compounded in the draft mare due to her size.

Reproductive problems common in draft mares, coupled with the fact that newborn draft foals are less hardy and take longer to stand and nurse, make it very important that all births are attended. It is strongly recommended that a reliable foal monitoring system be used.

By Lisa Hale, DVM, AAEP Member
Courtesy of AAEP