When a mare exhibits unexpected or undesirable behavior that cannot be explained by illness or injury, the owner might suspect it is related to the mare's estrous cycle. He or she might notice the mare is more difficult to train or ride periodically, she might act aggressively toward other horses, wring her tail, urinate frequently, or exhibit pain or sensitivity in her flanks. Owners often ask veterinarians for options to keep mares from coming into heat, expecting that this will alleviate the undesirable behavior.

Before employing an estrus (heat) suppression program, the veterinarian should first establish that the observed undesirable behavior is actually related to the mare's cycle. In most mares, once the veterinarian investigates the behavior closely, he or she determines that it is not associated with estrus or the estrous cycle at all. The owner should begin keeping a detailed journal of the mare's behavior to help with this assessment. A mare's estrous cycle is approximately 21-22 days long, with the mare exhibiting signs of heat for five to seven days of that cycle. Close observation and documentation can reveal if the behavior is truly cyclical and, when combined with a reproductive tract exam, they can help determine if the behavior is related to the mare's cycle.

Once the veterinarian determines that the mare's behavior is linked to her estrous cycle, the appropriate treatment option for estrus suppression can be chosen. The most common treatment for estrus suppression is the administration of some form of progesterone. Oral altrenogest (Regu-Mate), a synthetic progestin, will reliably keep mares in diestrus and prevent them from coming into heat. There are also injectable forms of progesterone that will suppress estrus in mares. Unfortunately Regu-Mate requires daily administration, and some mares develop muscle soreness after progesterone injections. Other forms of progesterone, such as Depo-Provera (medrox