Written by: Jay Altman

You have been working long hard hours training your mare, getting her ready for that one big show of the year. Then with only days to go before the big show, your mare start showing signs of going into heat. You know her heat cycle interferes greatly with her training and will really be detrimental going into a show. This is an all too common scenario with performance horses. There are many mares that go through heat cycles that significantly interfere with their training and showing. The signs attributed to performance problems include tail swishing, attitude changes, difficulty training, squealing, excessive urination, kicking, and colic-like discomfort associated with ovulation. There are some therapies that can help. First, a thorough examination of the mare needs to be done to ensure that are no other conditions causing the problem. Care must be taken in determining that the mare’s estrus cycle is truly causing the performance problem before potentially initiating unneeded therapy.

There are several methods to prevent or decrease the degree of interruption that occurs when female horses go through their estrus cycles. Some methods last longer than others but may not completely control the heat cycle, while other methods have short duration of action but completely control the heat cycle. The ideal treatment in any medical situation would be a treatment that is safe, effective, easy to administer, and cost effective. The pros and cons of the most commonly used methods to control the mare’s estrus cycle will be discussed in regards to the ideal treatment.

The hormone produced by the mare that is responsible for suppression of estrus, is progesterone. Progesterone is produced in the mare when she is pregnant and is essential to maintain pregnancy. There are a few hormone products that we can administer to the mare to suppress estrus. Natural progesterone can be injected once a day into the muscle. This can only be used for a short period of time due to the muscle soreness and swelling that it can cause. A synthetic progesterone drug can also be given daily, in an oral formulation. This drug, Regumate®, is safe for the horse, but problematic to administer, and is expensive to be given on a regular basis. It may be given everyday while in training or it may be given in interval, as in a few days prior to and the days of the shows to correspond with the performances. This allows times when mares can go through their natural heat cycles, still be ridden during shows and not need the drug consistently on a daily basis saving the client some expense. Women must use precautions with the drug when handling it and wear latex gloves, because Regumate® is an oil-based drug and the drug can be absorbed through the skin and have adverse affects on a women’s menstrual cycle.

Another hormonal treatment is to implant progesterone pellets under the mare’s skin in discrete places. This drug is safe, effective, economical, and can last several months depending on the horse. This type of estrus suppression has been tested in a controlled clinical trial which showed that the pellets did not suppress estrus behavior when a stallion teased the mare nor interrupted the cycle when checked on ultrasound examinations. What the trial did not study was if there was a difference in a mare’s attitude while in training or during performance work once implanted. However over the years in prescribing this type of therapy, I have noted how pleased owners and trainers have been with the results they have seen in their implanted mares.

There are some other non-hormonal types of therapies that have been prescribed with varying success. One type is placing sterile glass marbles in the uterus of the mare. This therapy is based on simulating pregnancy by putting a foreign object in the uterus, which results in the mare producing progesterone. A clinical trial showed only 41% effectiveness with the marbles, which is a bit unreliable for good effectiveness for performance horses. Although relatively inexpensive and safe and easy to administer, the effectiveness of this therapy is just not reliable enough to be considered. Other therapies that have been tried are herbal supplements. None of these have been scientifically studied, or tested and the active ingredients are not standardized to a known dosage. In addition, the American Horse Show Association forbids the use of herbal or natural products to affect the performance of a horse. The last therapy to consider is the surgical removal of the ovaries. This is not without the risk and expense of a surgical procedure; it is a permanent and irreversible procedure.

The problem of estrus cycles affecting performance horses is nothing new. That is one reason why geldings are preferred and often chosen for use in competitions. However no matter the reason why we show our horses, we need to the make the most of the time we have with them. In our hectic days, our time has become more valuable. This makes our time to train and show our horses even more valuable and we need to make the most of this time. Understanding this, one can select the hormonal control system which is right for you and your horse.