Various Strategies Can Control Estrus

Bruce E. Eilts, DVM, MS
Professor of Veterinary Clinical Medicine
Diplomate, American College of Theriogenology

The signs of estrus (heat) that are displayed by mares during shows or performance events are not well tolerated by horse owners. Horse owners frequently ask veterinarians for methods to prevent estrus in these situations. There are several strategies to eliminate estrous behavior in show and performance mares. Elimination of estrus can be permanent or temporary. Temporary methods can either control the onset of estrus or prevent estrus entirely.

Because signs of apparent estrus in mares can be caused by factors other than the mare actually being in heat, it is important to have the mare examined by a veterinarian before trying to control estrous behavior. Mares can show signs of estrus when they have ovarian abnormalities, the most common of which is a granulosa-thecal cell tumor. This ovarian tumor may cause a mare to appear to be in estrus constantly. A granulosa-thecal cell tumor will enlarge one ovary and cause the other ovary to become very small and inactive. If the ovaries are normal in size, the mare may be a nymphomaniac, and merely show very intense estrual signs during normal estrus periods. If the ovaries are extremely small and the mare shows aggressive estrus-like behavior, then the mare may be a psychologic nymphomaniac, and the estrus-like behavior is unrelated to any hormonal activity on the ovary. Therefore, it is best to have the mare evaluated by a veterinarian to determine if the ovaries are normal before attempts are made to prevent or control estrus. If the ovaries are normal, then understanding the mare's estrous cycle is important to provide temporary methods to optimally control estrus.

A brief review of the mare's estrous cycle demonstrates that the mare is in estrus, or heat, for approximately 5-7 days. The actual duration of estrus, however, varies by time of year and among mares. During estrus, the ovarian follicles produce the hormone estrogen. Estrogen, in the absence of progesterone, causes the outward signs of estrus in the mare. This behavior by the mare is termed 'in' or 'hot' and the mare accepts the stallion's advances by showing outward signs of 'winking' and squatting to urinate. After 5-7 days of estrus, the mare goes out of estrus and enters diestrus for a fairly consistent 14 days. After the mare goes out of estrus, the corpus luteum on the ovary produces the hormone progesterone. Progesterone prevents signs of estrus in the mare. During this period, the mare will not accept advances by the stallion and is considered 'out' or 'cold.' After 14 days, the corpus luteum regresses (dies) and the mare comes back into estrus.

These hormonal events of the estrous cycle provide two different strategies to temporarily control estrus in the mare using drugs. One strategy simply controls when the mare will come into estrus, while the other strategy prevents estrus entirely. Prostaglandin (PGF2%; Lutalyse¨) can control the onset of estrus by causing regression of the mature corpus luteum on the ovary. A mature corpus luteum is present on the ovary about 5 days after the mare goes out of heat. When the corpus luteum regresses, the mare returns to estrus. How can bringing a mare into estrus prevent estrus during an event? Only if the dates of shows are anticipated and the mare's estrous cycle has been closely followed, can estrus be induced long enough before the event to have the mare out of estrus when the event actually takes place. This strategy takes considerable planning and the mare's estrous cycle must be completely understood and monitored closely to be successful. To use this method, a mare must be out of estrus at least 5 days before getting the injection of prostaglandin. The injection will cause regression of the mature corpus luteum so the mare will come into estrus in 1-7 days, be in estrus 5-7 days, and then be out of estrus for around 14 days. Obviously, much planning and predicting are needed to use this method, but it is inexpensive. However, a price must be paid, and that price is planning and knowledge of the mare's estrous cycle.

An alternative strategy prevents estrus by administering progesterone, which keeps the mare from entering estrus as long as it is administered. Progesterone, if given at the proper dosage, will prevent estrus, but will not stop estrus very well once it has already begun. Several different types of progesterone are available. The oldest form is the injectable progesterone in oil. Progesterone in oil must be administered daily to prevent signs of estrus. While effective, daily injections may not be tolerated well for prolonged periods by either mare or owner. Other types of injectable progesterone include a repositol type that has a longer action and supposedly does not need to be given as frequently. Repositol progesterone is not readily available, but must be given weekly to prevent estrus.

Progesterone-like cattle implants are inexpensive and have been used in an attempt to prevent estrus in mares. These progesterone-like implants are surgically placed just under the skin and theoretically should prevent estrus. However, in scientific studies there have been no effects of the subcutaneous implants on changing the mare's estrous cycle. Failure of these implants to prevent estrus is probably due to the type of progesterone they contain, insufficient release of progesterone, and other hormones that are present in the implants. Some other progesterone-like compounds such as CAP or medroxyprogesterone have been suggested for use in estrus prevention in the mare, but are not commonly available or used.

The only drug that is approved by the Food and Drug Administration for preventing estrus in the mare is a progestogen called Regu-Mate¨. A progestogen is a progesterone-like compound that mimics progesterone, but is not actually progesterone. Regu-Mate¨ is given orally, and must be given everyday to prevent estrus in the mare. Figure 1 shows how Regu-Mate¨ can be used to prevent estrus in a performance mare. It has been approved to be administered as long as 88 days to prevent estrus. The biggest drawback in using Regu-Mate¨ is that it is relatively expensive compared with other methods of controlling estrus. Regu-Mate¨ can cost as much as $3.70 per day to administer. Another drawback is that some women can get menstrual-like cramps if the Regu-Mate¨ contacts their skin. The most important aspect about Regu-Mate¨ is that it is the only drug approved for preventing estrus in the mare. Because Regu-Mate¨ is approved by the Food and Drug Administration to prevent estrus in the mare, all other drugs that are not specifically approved to prevent estrus in the mare must be considered unapproved. This means that there are no justified 'off label' or 'extra-label' drugs that can legally be used as a substitute, so all the other progesterone and progesterone-like products are not approved to prevent estrus in the mare. This legality issue can put a horse owner and/or veterinarian in an uncomfortable position when trying to provide a low-cost, reliable method to prevent estrus in the mare.

If permanent estrus prevention is desired, the ovaries can be surgically removed. Obviously, this is not the strategy to be taken for a mare that is someday intended for breeding. There are three basic methods to remove ovaries from a mare. The least expensive, but most risky, is via a colpotomy. During a colpotomy, an incision is made blindly in the mare's vagina after an epidural and an instrument is passed through the vaginal incision and used to remove the ovary. This procedure can incur considerable risk in that it is performed blindly and can result in severe complications, including hemorrhage or infection. Alternative surgical approaches are performed through the mare's flank or abdomen. The flank approach is easier and can be used to remove small ovaries. This procedure has some risks in that it too uses an instrument to blindly remove the ovary. The most precise, and most expensive method to remove the ovaries is through an abdominal incision. An abdominal incision, however, carries the risks of general anesthesia and a major incision on the horse's abdomen. This is costly, but is the only method possible for some large ovaries.

In summary, if a mare is normal, the estrous cycle can be controlled legally using prostaglandin or it can be prevented entirely using ReguMate¨. Surgery is an alternative that can be used to permanently prevent estrus. It is imperative, however, that the mare be evaluated for any abnormalities before a reliable, economical, and manageable strategy is selected to control when estrus will occur in the mare.