Preventing Estrus In Performance Mares

It has happened to many horse owners. The day of the major event or stakes arrives and the prize mare is ready to compete. The night before her coat glistened, her muscles rippled, and her eyes were aglow with competitive spirit.

This morning, when everything is on the line, there is a profound difference. Instead of bouncing on her toes when led from the stall, her ears are pinned and her tail is switching. Her eyes smoulder with anger instead of sparkling with competitive zeal.

There are other scenarios to this same, basic story. The mare might be in docile good humor--too docile--and might want to squat and urinate whenever she is in the presence of a gelding or stallion.

No matter how it is manifested, coming into estrus can compromise the ability of the best performance or race mares.

To avoid such a compromise, there is only one solution--prevent her from coming into estrus while she is competing.

The best way to do this, says Dave Beckman, DVM, of Anchorage, Ky., is to use the synthetic progesterone altrenogest. It is sold under the trade name of Regu-Mate.

Beckman has had experience with cycling female performance horses at three levels. Early in his career, he was a steeplechase jockey, and, shortly after graduating from veterinary college, was resident veterinarian at a Standardbred farm where horses were bred, trained, and raced. Today, his practice centers on the broodmare.

There are three basic routes one can take in preventing the performance or race mare from coming into season, he says, but the safest approach and the one that holds forth the most promise of the performer ultimately becoming a broodmare is Regu-Mate.

The other two methods involve direct use of progesterone either as an injection or as an implant, in the form of a time-release pellet that is inserted beneath the skin.

Before discussing the pros and cons of these methods, one must first thoroughly understand what occurs within the mare's reproductive system during estrus.

Understanding Estrus

There are two basic stages to the estrous cycle, and they are distinguished by a mare's behavioral response to a stallion. Estrus, or heat, generally lasts from five to seven days, which is reported as being the longest heat period of any domestic animal.

Estrus is characterized by receptivity to the stallion. When in estrus and in the presence of a stallion or even a gelding, the mare, if demonstrating classic behavior, will squat as though to urinate, with legs spread and tail raised. Her vulva will also "wink" to expose her clitoris.

Behavior of mares in estrus is an individual thing. In the very early and very late stages, some are extremely irritable while others are extremely docile, to the point that it is difficult to get them to perform, no matter what the discipline.

Somewhere between 25 and 48 hours after the mare ovulates, she will enter stage two of the cycle known as diestrus. During this 14- to 16-day period, she will show no interest in a stallion or gelding and, if a performance mare, might very well be at the top of her game.

Unfortunately, the natural breeding season for a mare occurs in late spring and summer, the same time as many major jumping, eventing, hunt, and race competitions. During this time of year, mares are cycling and ovulating on a regular basis.

During the winter months, the mare's reproductive system pretty much shuts down as it goes into a season-long diestrus. If one were to use her as a performance horse during this stage of her annual reproductive cycle, there is a good chance that one would have no more estrus-induced behavioral problems than with a gelding.

One of the most erratic periods, from an estrus point of view, occurs between diestrus and regular estrous cycles. It is called the transition period. During this time, the sexual behavior of the mare tends to be erratic. Estrous cycles during the transition period might be long and irregular and are not always accompanied by ovulation.

Normal cyclic activity appears to be strongly influenced by light, with temperature and dietary intake having secondary effects. The process that occurs with longer days is described like this by Edward L. Squires, PhD, of Colorado State University, who has both written on the subject and discussed it at seminars:

The increasing amount of light perceived by the retina of the eye inhibits secretion of the hormone melatonin from the small pineal gland in the brain. Removal of the inhibitory effects of melatonin allows the hypothalamus in the brain to release a hormone called gonadotropin releasing hormone (GnRH). This hormone stimulates the pituitary gland at the base of the brain to secrete the gonadotropic hormone follicle-stimulating hormone (FSH).

FSH stimulates the growth of one or more ovarian follicles, the structures within the ovary that contain the eggs or ova. Estrogen acts on behavioral centers in the brain to stimulate estrual behavior in the mare. The hormone also causes relaxation of the cervix, which facilitates the entrance of spermatozoa into the uterus and increases contractions of the smooth muscles in the reproductive tract, which facilitates the transport of both sperm and ovum.

Finally, estrogen (and possibly other unknown factors) inhibit the further secretion of FSH by the pituitary gland and stimulates the release of a second gonadotropic hormone, luteinizing hormone (LH).

Luteinizing hormone causes the ovarian follicle to mature and induces ovulation. After ovulating, the follicle is replaced by a reddish-yellow mass called the corpus luteum (yellow body). The corpus luteum secretes the hormone progesterone, which initiates a period of the mare being unreceptive to the

After 12 to 14 days, if pregnancy has not occurred, the uterus secretes the hormone prostaglandin, which causes regression of the corpus luteum. As the corpus luteum regresses, progesterone secretion declines. The reduction in the serum progesterone level allows the mare to return to estrus and the cycle is

Thus, we know that progesterone is the key hormone in stopping the cycle of hormonal activity that culminates with the mare being in season and, as a result perhaps, being out of sorts for racing or

Breaking the Cycle

One of the ways in which the mare can be kept from coming into estrus is by giving direct shots of progesterone, says Beckman. The main drawback to this approach is that mares must receive a progesterone shot at least every four days, or, in some cases, every day. Administration of progesterone by needle, says Beckman, can result in soreness at the injection sites.

However, he says, it provides the most immediate result and can be effective if an owner is coming to a performance competition and realizes at the last minute it will coincide with the mare coming into heat. The progesterone normally takes effect in one day to prevent estrus.

Another downside, Beckman believes, is that it might have a more long-term deleterious effect on the mare's hormonal system than does the synthetic Regu-Mate. This is an important consideration, he says, if the performer is to go into the broodmare band one day.

The same, he believes, is true of the progesterone implants.

In addition, he says, implants for equines have not been approved in the United States, although some horse owners have been able to purchase them in European countries where they have been approved. Still other horse owners have successfully used implants that are approved for cattle.

Regu-Mate appears to be the safest way to go when seeking to prevent a mare from coming into heat, Beckman says, although it too, has a downside--it is expensive. Generally speaking, one can expect to pay $3 per day for Regu-Mate that is administered orally, either by top-dressing feed or the more efficient method of squirting it directly into the mouth with a syringe.

It takes Regu-Mate a bit longer to act on the hormonal system than does a direct shot of progesterone.

"You need to get the mare on it five or six days before the event or race, then keep her on it throughout the competition for it to prevent her from cycling," Beckman says.

Some trainers and owners, he says, will keep the performing mare on Regu-Mate continuously through a competitive season, while others will remove it if the mare has a down time between perform-ances of three weeks or more. Normally, he said, about nine days after Regu-Mate is no longer administered, the mare will cycle.

Then, five or six days before the next performance she is put back on the synthetic hormone to prevent the next series of cycles.

If the owner eventually planned a broodmare career for the performing equine, Beckman says, the on-again, off-again approach to Regu-Mate would be preferred to keeping her on it constantly.

Do the majority of mares have their performance capability compromised when in heat?

Beckman explains his views and approach this way: "Whenever I'm doing a pre-purchase examination of a performance mare, I always give the prospective owner my 'female speech.' How she will react when in estrus is not something I can objectively evaluate during a pre-purchase exam, so I make certain they know that there are some mares that when they come into heat, are a definite behavioral problem and they will not perform well, be it in a race, jumping, eventing, or whatever. It is like a big, rolling ball of hormones within their reproductive system and you don't know what to expect.

"Now, there are some mares that humiliate me. I give that speech, the person buys the mare, and she never causes them a problem. There are a handful of great, athletic mares out there that will perform at their peak without Regu-Mate or anything else. However, in my experience, the majority of them are some sort of problem during the heat period."

When the cavalry depended on horses, said Beckman, it was reported to be a common practice to geld all stallions and spay all mares.

So, why aren't more performance mares spayed?

There are two problems involved with spaying mares, he feels. The first is quite obvious. If the ovaries are removed, the mare will never reproduce.

The second factor is the risk involved with the surgery as well as its cost, which can run between $500 and $1,000, depending on the procedure. The two most common ways to remove the ovaries are to go in through an incision in the vagina near the cervix or to go in through an incision in the flank.

In ovary removal there is always the risk of hemorrhage or side effects.

"I contend," Beckman says, "that if you had a good performance mare you didn't want to breed, and we had a simpler method for spaying that wouldn't cause problems, it would be an excellent way to go."

Removal of the ovaries, he says, basically shuts down the mare's hormonal cycle in the same way that removal of testicles from a stallion eliminates his sex drive.

When Performance Ends

Knowing when to administer hormone therapy early in the career of a performing mare is pretty much a matter of observation, Beckman says. Normally, one wouldn't administer Regu-Mate until the mare had conclusively demonstrated that she was going to be a problem during estrus. In most cases, that would show up during the training regimen.

Frequently when the performance mare ends a long and illustrious career, with her hormonal cycle short-circuited on a regular basis during that time frame, the owners decide it is time for her to produce a foal.

What are the chances for success in such a scenario?

"If the mare has been on Regu-Mate," Beckman says, "I think there would be a pretty positive prognosis, depending, of course, on the mare. Sometimes we have 10-year-old maiden mares brought to us that may also have uterine problems. That compounds the situation. You must make sure that you are dealing with a healthy reproductive system and that everything is in good order nutritionally."

The key to success in changing the performer to broodmare status, Beckman says, is time.

He would like to begin dealing with the retired performance or race mare that is now to become a broodmare several months before the breeding season begins. She must acclimate to no longer being a performance horse, he explains, and to becoming a broodmare.

"It is always a big problem when such a mare is sent to the breeding barn in mid-season," he says. "I always tell people that it is a crapshoot if they send a performing or race mare to me in March or April and expect her to start cycling.

"We are just kidding ourselves if we think we can stop this cycle for several years, then cease hormonal therapy and expect her to start cycling immediately. You've messed with Mother Nature for a couple of years and often it takes a while for that mare's reproductive system to come around and have normal estrous cycles once again."

Some performance and race mares, Beckman says, also have been given anti-inflammatory drugs and steroids that can bring about an adrenal shutdown, which also causes reproductive problems.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at or by calling 800/582-5604.

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