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Managing Mares In Heat

Influence your mare"s cycle for performance-related reasons or for horse breeding.

Some mares hardly change when they come into heat; others may behave erratically, sometimes due to sensitivity in their backs and sides.
Some mares hardly change when they come into heat; others may behave erratically, sometimes due to sensitivity in their backs and sides. © susanjstickle.com

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. If this is the year you plan to breed her, you may welcome the signs. If she's days away from an important competition, not so much.

Timing is everything, either way. In this article Karen Wolfsdorf, DVM, a specialist in equine reproduction at the Hagyard Equine Medical Institute in Lexington, Kentucky, explains how you can influence your mare's heat cycle to meet your goal—whether that's top performance this year or a new foal at her side next year. For an overview of exactly what goes on when she cycles, see "The Heat ­Cycle" below.

Unwelcome Behavior
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. You may notice that she's increasingly distracted. She may develop a passionate attachment to one of her barn buddies and whinny constantly if that horse is out of sight. She may squeal and kick out at the slightest provocation. She may also ­become sensitive in her back and sides—and throw hissy fits when you apply leg pressure. Some mares are unusually spooky as estrus approaches.

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How much of your mare's behavior is due to hormone swings? "Ovaries are ­unfairly blamed for lots of problems, ­including lameness and training issues, so first rule out other causes," Dr. Wolfsdorf says. Discuss the problems with your veterinarian and your trainer. Keep a journal of your mare's behavior.

Consistently abnormal behavior is more likely to be a sign of underlying problems not related to your mare's cycle, Dr. Wolfsdorf says. For example, ovarian tumors occasionally occur in mares. The most common type, the granulosa-cell tumor, can produce testosterone and other hormones that trigger aggression or "studdish" behavior, such as teasing other mares. Your mare may appear to be constantly in heat or never in heat. (Your veterinarian can find these tumors with rectal palpation, ultrasound and hormonal testing, and they can be surgically removed.)

If you find that your mare's behavior fits the normal cyclical, seasonal heat pattern—she's great for two weeks and a basket case the next week—odds are it's heat-related. How big is the problem? ­Unless her behavior is a danger to herself or others, the answer depends on whether it interferes with what you want 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.

Hormone Therapy
Medications to control the cycle are available by prescription from your veterinarian, who can help you figure out what's best for your mare and your situation. Most of these medications don't affect long-term fertility and, because most mimic hormones circulating naturally in your mare's system, shouldn't run afoul of competition drug rules. ­"Every mare is different," Dr. Wolfsdorf notes, "so some approaches may work better than others for yours."

Progesterone. This is the hormone that dominates during diestrus and keeps the mare from coming into heat. There are various types, with varying effectiveness. These medications should be used cautiously in mares with a history of uterine inflammation or infection because they can make that condition worse.

Regu-Mate® ­(altrenogest) is a liquid oral medication that provides a synthetic source of progesterone. If you start it during diestrus, it will prevent your mare from returning to estrus as long as she continues to get a daily dose. When you stop, she'll probably come into heat within five to 10 days. It's easy to ­administer and works well in most mares.

  • You can use Regu-Mate as a temporary fix to be sure your mare won't be in heat for a big competition, or keep her on it spring through fall.
  • At about $2.50 a day, keeping a mare on this medication can be expensive.
  • Regu-Mate is usually given with a dose syringe (it can be mixed in feed). Because the hormone can be absorbed through the skin, you need to wear nonporous gloves and handle it carefully. If you're pregnant, get someone else to do it.

Progesterone injections are given into the muscle and usually take effect within 24 hours. The injections delay heat anywhere from five days to a month or so, Dr. Wolfsdorf says, ­depending on the type of progesterone, the administration vehicle (the substance mixed with the active ingredient) and the individual mare's ­response. Medoxyprogesterone acetate—the human contraceptive

Depo-Provera—is sometimes given, but studies have shown it to be less reliable than other forms of progesterone as a way to suppress estrus in mares.

  • For season-long control, progesterone injections may be more convenient and less expensive than the oral medication, depending on how often your mare needs them.
  • It's important to know the duration of activity of the product you use in your mare, so you know how long an injection will keep her out of heat.
  • Swelling at the injection site may be a side effect, depending on the dose and the administration vehicle.

P&E. Progesterone combined with estradiol 17B (a form of estrogen) may help when progesterone alone doesn't solve behavior problems. This happens in some mares, Dr. Wolfsdorf says, because progesterone delays the onset of heat but doesn't stop ovarian activity—so follicles can still develop in the ovaries. Adding estradiol suppresses follicular activity.

  • P&E is available by prescription from compounding pharmacies as an intramuscular injection, in a short-acting formulation given daily or in slow-release formulation that acts for about 10 days.
  • Progesterone and estradiol are ingredients in implants (Synovex) designed to promote weight gain in cattle—but these implants aren't effective in controlling ­estrus. Inserted under the skin, they release low doses of the hormones over 100 days or more. Researchers at Colorado State University found that even multiple implants didn't keep mares from coming into heat.

Oxytocin. This naturally occurring hormone is also used to suppress heat. It can delay estrus for 30 days or more, Dr. Wolfsdorf says.

  • Given at the right time and according to a specific protocol, oxytocin prolongs the life of the corpus luteum so that it continues to produce progesterone, keeping the mare out of heat.
  • The treatment has few side effects, and it may be cheaper than other hormone therapies.
  • Work with your vet to follow the ­protocol, which calls for intramuscular ­injections of small amounts of the ­hormone twice daily on days 7 to 14 after ovulation.

More Options
Beyond hormones, there are few good options for controlling heat-related behavior.

"Herbal supplements will not keep your mare from coming into heat, but they may help calm her down," Dr. Wolfsdorf says. These supplements don't work for every mare, and some respond better to certain products than to others. If you compete, pay close attention to ingredients and check with the association governing your sport to be sure the product won't violate medication rules. Herbs such as valerian, vervain and passionflower, used in some of these products, are on the International Equestrian Federation (FEI) list of forbidden substances.

Ovariectomy—spaying—is a last resort. The surgery can be done standing with casino con postepay ­local anesthetic and sedation. But, Dr. Wolfsdorf says, "We don't recommend it as a ­behavioral fix. It's usually done for ­tumors or other pathology." It may not solve behavior problems, she explains, because taking a mare's ovaries leaves her with no source of progesterone, the dominant hormone that keeps her on an even keel during diestrus. You'll most likely need to give her supplemental progesterone.

New treatments may be on the horizon. In a chance discovery, British researchers found that infusing small amounts of coconut oil into a mare's uterus 10 days after ovulation can extend diestrus in much the same way that oxytocin injections do. The researchers think that the mix of fatty acids in the oil is responsible. "The method isn't used much yet, but it may be something for the ­future," says Dr. Wolfsdorf.

The Heat Cycle

Length of day plays a major role in the mare's heat cycle by influencing the hormones that control it. Horses are long-day seasonal polyestrous breeders, and mares typically come in heat about every 18 to 21 days from April to October. This is nature's way of ensuring that winter weather won't greet foals born 11 months after mating.

Estrus, the heat period itself, usually lasts five to seven days. During this time an egg (ova) develops within a follicle in one of the mare's ovaries. The developing follicle produces estrogen, and rising levels of this hormone account for the behavioral changes—"behavioral estrus"—you may see. Toward the end of estrus, she may squat to urinate frequently, raise her tail and "wink" her vulva. These signs are meant to let a stallion know that she's receptive to breeding.

Ovulation occurs a day or two before the end of behavioral estrus. Once the mature egg is released, what's left of the follicle develops into a structure called a corpus hemorrhagicum, which matures into a corpus luteum by six days after ovulation. It stops producing estrogen and begins to produce the hormone progesterone, which prepares the uterus for pregnancy. If the mare is bred and becomes pregnant, the corpus luteum and the placenta will continue to produce progesterone for the next 11 months. If not, the cycle repeats.

Diestrus is the 14- to 15-day period when the mare is not in heat. With progesterone dominant and estrogen levels low, signs of heat disappear. Near the end of diestrus, the mare's endometrium (the lining of the uterus) recognizes that she's not pregnant and begins to release other hormones. One of these is prostaglandin, which breaks down (lyses) the corpus luteum. An increase in follicle-stimulating hormone leads to follicular development with the start of the next estrus.

Anestrus is the winter dormant phase, brought about by short days and long nights. Your mare may start to show hormone-related behavior in February and March, when her reproductive system begins to wake up. During this transitional phase she may briefly come into heat, cycle in and out of heat or stay in heat for weeks.

Posted in Anatomy, Behavior, Breeding, Dressage, English, Eventing, Health, Horse Care, Hunter/Jumper, Illnesses & Injuries, Other Horse Sports, Riding & Training | Leave a comment

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