​The Use of Hormones in Regulating Mares

She stamps and squeals when other horses pass her stall. She forgets her manners, swishing her tail and trying to nip when you groom and tack her up. And when you ride, she's a total airhead, ignoring your aids, whinnying, jigging and dancing, rubbernecking right and left.

Yes, your mare's in heat, and how you feel about that likely depends on your plans for her. ¹

While the behavior of some mares changes little upon coming into heat, others behave erratically as described above, prompting empathy on the owner's part and, of course, an abundance of caution in handling. Whether this behavior is brought on by heightened physical, emotional sensitivity or both isn't certain, but the behavior set, when present, is very real.

Clinical Signs of Estrus in Mares

A mare in heat typically exhibits gait abnormalities, raises her tail, urinates repeatedly, and interacts unpredictably with people and other horses. This estrous behavior normally lasts five to seven days, becoming more intense as the ovarian follicles increase in size and produce more estrogen. As the mare ovulates, she goes out of heat and structures on her ovaries begin producing the hormone progesterone (which prepares the uterus for pregnancy).

This quiescent period when the mare is not receptive to a stallion is called diestrus, and her behavior can vary from ear pinning and unwillingness to cooperate to kicking, squealing, and striking at other horses. A mare should normally be out of heat for 14 to 15 days; this is the most consistent period in the estrous cycle, so any deviation should alert an owner to have a veterinarian examine the mare. ²

Your mare may hardly change when she comes into heat, or she may behave so erratically that you wish you'd bought a gelding. Most fall somewhere between those extremes. ¹ Again, the course of action toward mitigating troublesome behavior will be contingent upon the owner's plans for the horse. Having established that, there are numerous methods for addressing these behaviors; many are tried-and-true, with new ones coming to the fore as new medications and holistic approaches are developed.

What's Going On Inside

Horses are seasonally polyestrus. This means that their estrous cycles depend on the season. The length of a mare’s natural breeding season is said to vary, but for the northern hemisphere it typically starts in March/April and extends until October/November. A transitional mare’s ovaries typically have multiple small- to mid-sized, grapelike follicles. They might also possess one or more large hemorrhagic (ovulating) follicles. By definition, the mare’s transitional period ends after her first ovulation of the year. ²

Karen Wolfsdorf, DVM, a specialist in equine reproduction at the Hagyard Equine Medical Institute in Lexington, Kentucky, has produced both primers and comprehensive guides to regulating cycling mares for horse owners. In these and in several publications, she explains how horse owners can better understand the equine estrus cycle in general, and influence mares' heat cycle to meet their objectives, whether that's in the interest of maintaining good performance or breeding.

"Ovaries are unfairly blamed for lots of problems, including lameness and training issues, so first rule out other causes," Dr. Wolfsdorf says. She advises that horse owners first discuss behavior problems with veterinarians and trainers at the onset, and to keep a journal of the mare's behavior. Consistently abnormal behavior is more likely to be a sign of underlying problems not related to your mare's cycle. ¹

Dr. Wolfsdorf maintains that if a mare's behavior fits the normal cyclical, seasonal heat pattern, then the odds are that any unusual behavior is heat related. Unless the mare's behavior poses a danger to herself or others, the answer depends on whether it interferes with what the owner or trainer wants to do. "If she's harder to ride or doesn't perform well during estrus, you may decide to just take it easy or give her time off on those days. But if you need to stick to a competition or training schedule, you may decide to control the timing of her heat cycle with hormone therapy." ¹

Hormone Therapy

As we learned in high school Biology class, hormones are the body’s messengers, traveling through the bloodstream to communicate between various glands and organs. They affect and regulate body functions and metabolism. In horses, these can have an influence on behavior, most commonly involving mares in heat.

In mares, estrogen brings the mare into heat and prepares her reproductive tract for breeding, while progesterone keeps her out of heat. In the non-pregnant mare, the interaction of estrogen and progesterone are the main factors that determine her behavior, personality, and pain threshold. Medications to control your mare's cycle are available by prescription from veterinarians. Most of these medications don’t affect long-term fertility and, because most mimic hormones circulating naturally in your mare’s system, would not show positive in a swab. ³

There are several medications (hormones) that are commonly used for regulating mares. Many are available in synthetic form. Among these are progesterone, estradiol and oxytocin. Compounds containing combinations of these medications are also available from veterinary compounding pharmacies. Altrenogest is effective in normally cycling mares for minimizing the necessity for estrus detection, for the synchronization of estrus, and permitting scheduled breeding. Altrenogest is also effective in suppressing estrus expression in show mares or mares to be raced. It is also available in combination with progesterone.

Breeding Mares

In breeding mares, Dr. Wolfsdorf and many other experts maintain that it's all about planning. Wolfsdorf advises that one start by scheduling a breeding soundness exam before the season begins; then, it is necessary to "monitor—or manipulate—your mare's cycle to time her breeding." ¹

If a mare must be "manipulated" into coming into heat through short cycling, it can make planning easier, but is more labor-intensive than waiting for a mare to come into heat.

"This can be especially helpful when you need to coordinate delivery of shipped semen and a visit from your vet for artificial insemination," Dr. Wolfsdorf notes. "You can also try to induce the cycle with hormones. This works best in the transition stage, in early spring. Prostaglandins won't help at this stage if no corpus luteum is present, but a combination of progesterone and estradiol may. Follicle stimulating hormone, gonadotropin-releasing hormone and other hormones are also used." ¹

According to Dr. Wolfsdorf, "There are lots of protocols, which tells you that none works perfectly.

¹Pascoe, E. Managing Mares In Heat, Practical Horseman, Jan. 2012.
²Cook, J., DVM. Mares and Hormones, The Horse, Nov. 2012.
³hiralabs.co.nz.

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