One of the major problems with using mares in performance events is that some of them exhibit strange and unusual behaviors when they are in heat. Whether the mare actually has a training or performance problem or the owner or trainer just thinks the mare has a problem is difficult to determine. Regardless, many mare owners have used multiple options to suppress estrous to prevent the unwanted behavior. The most common method of preventing mares from coming into heat is a progesterone drug called Regu-Mate that works well, but it must be used daily and is fairly expensive. A compounded, injectable form of the drug is also available that lasts for 30 days but since it is compounded, it is not approved by the FDA. Another option is to insert a 35 mm glass ball or marble into the uterus. This therapy has been shown to be effective in some mares while others do not respond. Disadvantages include the glass ball breaking in the uterus, and it is difficult to know if the ball is still in the uterus without a rectal exam.
The newest method for preventing mares from coming into heat is to inject 60 units of oxytocin in the muscle twice a day on days seven through 14 after ovulation. This treatment has been shown to prevent mares from coming into heat for an extended period, and it is inexpensive. However, it is important to know when the mare ovulates, which requires a veterinary exam because the first oxytocin injection must be given on day seven after ovulation and be continued for one week. A recent report has shown that once daily administration of oxytocin is as effective as twice daily administration. The length of time mares are out of heat is variable, but this seems to be an effective option.