Heat Cycles of Mares & Cushing's Disease

With stud fees and mare care expenses often costing mare owners several thousand dollars per breeding, the goal is to get a mare in foal as quickly as possible. By culturing mares, uterine infections can be treated prior to breeding, thus increasing conception rates. Older mares are usually much harder to get in foal than younger mares, often because of the effects of Cushing's disease on their heat cycles.

  1. Cushing's Disease

    • A horse with Cushing's disease may have trouble shedding in the spring.

      Although called a disease, Cushing's is actually the result of a benign tumor on the pituitary gland. The symptoms of Cushing's most commonly include a failure to shed in the spring with the coat becoming increasingly coarse and wavy. In addition, a horse with Cushing's often drinks and urinates excessively and has fatty deposits over the eyes and sometimes other parts of the body. Chronic laminitis can be a common symptom, as are abscesses and respiratory ailments. Veterinarians use a variety of blood tests to diagnose Cushing's.

    The Pituitary Gland

    • Located at the base of the brain, the pituitary gland regulates the hormonal system. It also works closely with the hypothalamus, with both regulating the production of cortisol. According to InfoVets, horses with Cushing's "have a broken 'off switch' and produce too much cortisol." With many processes in the horse's body, including those of reproduction, regulated by cortisol and other hormones, Cushing's can affect the fertility of both stallions and mares.

    Heat Cycles in Mares

    • Mares may produce into their late 20s if their health remains good.

      A mare's heat cycle is approximately 21 days in length, although it can be slightly shorter or longer. The mare will only be receptive to the stallion for about five days of the period, with ovulation occurring 24 to 48 hours before its end. Mares usually experience heat cycles from March to October while the days are longer and can be bred well in to their late 20s if their health remains good. However, as mares get older, they are more likely to be affected by Cushing's disease.

    Cushing's and the Heat Cycle

    • Since the pituitary and hypothalamus control hormone production, this syndrome can be responsible for changes in a mare's heat cycle. Cushing's may lead to heat cycles that are much longer than normal or much shorter and, in some cases, may cause a cessation of heat altogether. Some mares have even become sterile as the disease progresses. In addition, mares have been known to produce milk even though they are not pregnant. Mares with Cushing's disease that do become pregnant often have higher incidences of early embryonic death.

    Future Research

    • Cushing's disease may keep some mares from becoming pregnant.

      Since a mare may become harder to get into foal as she ages, it is important to determine whether this difficulty is the result of Cushing's or other factors. Equine veterinarian Mark Pierce of Topeka, Kansas, notes that few studies have been conducted on the long-range effects of Cushing's disease on broodmares, so there may be other problems than those listed. Since more horses are being diagnosed with this syndrome due to longer life spans, this is an area that requires future study.