Stallions and mares can be a challenge to have on the farm, and both come with their fair share of problems. Thankfully, we do have certain medications that can make handling the difficult, sometimes fractious horses a little easier.

When it comes to behaviour, mares have a certain reputation in regards to attitude (especially if you believe the folklore behind the chestnut ones!). I cannot count the number of times I hear owner complaints about mares having behavioural problems, whether it is seasonal or not.

Whenever an owner approaches me about a certain problem with a mare, my first questions usually try to investigate if the behavioural problems are coming from pain or discomfort, rather than hormones. If we can rule out lameness or other issues such as those that can be solved through different training or handling methods, there are medications that can be used to suppress estrus, or heat. Estrus is the recurring cycle of sexual receptivity of mammals. Mares are seasonally polyestrous; this means they ovulate many times over the long-day seasons‒ spring and summer‒ and transition into anestrous, where no cycling occurs during the winter when days are shorter. The average mare will ovulate every 21 days, but it is considered normal to have some cycles last a few days shorter, or longer. The goal of most medications used to control their behaviour by modulating hormones circulating in the body.

The most common medication used to tone down “mare-ish” behaviour in females is Regumate or Altrenogest. Both are synthetic progestins that suppress the receptive behaviour of estrus by mimicking the hormone progesterone. Progesterone is considered to be the hormone of pregnancy, and in simple terms, suppresses estrogen, which promotes growth follicles on the ovary, and ovulation of an egg. Estrogen is the hormone responsible for the unwanted mare behaviour. Without estrogen, no follicles develop, and no egg is released from the ovary, and the mare cannot become pregnant. In general terms, it stops her from cycling.

Altrenogest can be used continually, or in a well-timed manner if there is a big competition coming up where the mare needs to perform at her best. It is easy to administer and is given orally, so no extra training needs to be given for owner administration.

Progestins are also used to help manipulate the mare’s reproductive cycle to allow for a more precise breeding date, especially if an early-in-the-year breeding is beneficial. There are many studies proving that the use of Regumate will not inhibit the mare’s ability to have a foal in later years. There are some risks involved with using Regumate, as the same hormone that regulates a mare’s cycle is also involved in the human reproductive cycle. For example, women who are pregnant, or trying to get pregnant, should not handle or administer Regumate. It is important to speak with your veterinarian before starting this medication to ensure you are aware of all the risks involved.

There are a couple less-common ways to suppress estrus in mares, and one of them involves implanting sterile marbles in the uterus. The theory behind the marbles is that they mimic a fetus moving around in the uterus in early pregnancy which, in theory, prevents true reproductive cycling. This technique has fallen out of favour, because there can be serious complications including broken marbles and uterine infection.

The final option for suppressing estrus in mares is using the hormone oxytocin. Oxytocin is commonly referred to as the ‘love hormone’ and is involved in bonding between mother and offspring. It is also important in stimulating very strong contractions to enable proper parturition of mammalian offspring. There has been recent research suggesting that chronic oxytocin administration in mares prevents the mare from coming into heat. The mechanism in which oxytocin works is also related to the follicle on the ovary. Once ovulation occurs, the follicle on the ovary transitions to become a corpus luteum, which is responsible for producing progesterone soon after ovulation, and is responsible for maintaining early pregnancy in the mare. Oxytocin is thought to prevent the death of the corpus luteum, which enables the continued production of progesterone in the body.

The difficult part of oxytocin administration is that specific timing is required to ensure it works properly. Oxytocin needs to be administered daily on days 8-14 after ovulation. In order to determine when ovulation occurs, an ovarian ultrasound needs to be completed by your veterinarian. Currently, there is only an intramuscular form of oxytocin available, so the owner would need to be comfortable administering injections to avoid a significant veterinary bill.
As more research is completed on this option, it may be a good alternative to administering a synthetic hormone daily, as long as the caretaker is competent in administering IM injections.

Modulation of stallion behaviour revolves around suppressing testosterone and other sex hormones that induce aggressive behaviour. It is important to note that manipulating hormones in stallions can affect their ability to perform reproductively and should be used as an absolute last resort when changes in management, feed and environment don’t improve behaviour.

There are two main options when it comes to modulating stallion hormones. The first, and probably easiest way to reduce testosterone levels is through off-label use of Regumate. Off label means the medication was not developed for use in stallions, and hasn’t been approved by the FDA. As mentioned above, Regumate is a progestin, which, in males inhibits the release of Luteinizing Hormone (LH). LH stimulates leydig cells in testicles to produce testosterone. So when Regumate is given, LH is suppressed and testosterone is not produced, or produced to a lesser degree.

There have been studies that have shown that testosterone does decrease after progestin administration; however once progestin administration has stopped, return to previous levels of testicular size, and testosterone production may not ever be reached. This is important to remember if the stallion in question is required to have a breeding career later in life.

Another way that is being used to decrease testosterone production is through a vaccine that targets Gonadotropin Releasing Hormone (GnRH). GnRH is released from the pituitary gland and is involved in controlling the release of both female and male sex hormones; for example, LH is released only in response to GnRH releases. The vaccine stimulates the horse’s immune system to attack GnRH molecules, effectively inhibiting the release of LH and other sex hormones and subsequently reducing testosterone production. Similar to administration of Regumate, return to reproductive function is not guaranteed and the vaccine should be used with extreme caution if the stallion is expected to breed later in life.

The GnRH vaccine has also been used in mares and has been shown to effectively suppress estrus. There are no GnRH vaccines currently available in Canada or the USA, but some may become available as more research is completed.

To conclude, there are options available in the use of hormones to modulate behaviour in both mares and stallions, but all options come with risk either to humans administering it, or horses receiving the medication or vaccine. If you are interested in using any of these products it is extremely important to talk to your veterinarian in detail about what are the best options for you, the horse, and your breeding operation.