from the Vet Corner Archives

Part of Horse Previews Magazine website. Posted on 01/07/2001; 2:00:00PM.

Veterinary Corner 01/01: Managing Heat Cycles in Breeding & Performance Mares

by Katherine Burnett, DVM
Edgecliff Equine Hospital
S. 1322 Park Road, Spokane, WA 99212 * 509/924-6069

Mares that readily show heat during the breeding season are a breeder's dream - easy to "catch" in foal. Failure to show heat is common and frustrating, however. This problem is almost always due to the fact that the mares are having a normal heat cycle, but that we fail to detect it.

There are many ways to tease mares. The most foolproof way is to build a small corral (stall-sized) for the stallion and place him in it. The mares are then turned out into a larger paddock that surrounds the stallion pen. The pen should be sufficiently large that the mares can back away enough not to be intimidated by the stallion. Even the most timid mares will usually eventually show heat in this instance. The first sign is that they leave the corner of their paddock and start to move closer to the stallion.

This penning method works especially well for the mare that is to be bred for the very first time. Often, because of their fear of the unfamiliar, maidens will fail to show. Mares with a foal by their side are also hard to catch in heat, may also respond more favorably if allowed freedom to approach the stallion on their own. Allowing the mare to approach the stallion on her own while loose in a pen, rather than forcing her to get too close (such as teasing over a wall) is far less threatening to all mares. Excessive restraint, such as twitching, may also keep mares from showing heat.

Many breeders are concerned about the mare kicking and injuring the stallion. While this is a valid concern, careful attention to the behavior of the mare will usually tell you whether or not she will stand to be bred. True estrus (heat) is characterized by lifting the tail, "winking" the vulva repeatedly, and a characteristic breeding posture ("squatting with the hind legs). Most mares will not stand for breeding during the entire heat cycle, however, and unless careful attention is paid to the mare's behavior, she may kick the stallion if breeding is attempted at an inappropriate time. Ideally, the following signs will be evident as well:

Mares usually show willingness to be bred by posturing (squatting), buckling one leg forward, and showing a "mating ear" - an ear position that is neutral - neither forward nor back. The angle of the neck is in a neutral position (twitching makes it hard to see this), and the lip may droop. Wild mares may even look back at the stallion before he mounts. It is important to give the mare some room to move forward for at least a couple of feet as she is being mounted. Otherwise she may "explode" or suffer a vaginal injury.

I am often called out to tranquilize mares before breeding. Careful attention to the above procedures, however, may make this procedure, as well as twitching, hobbling, or other such forms of restraint, unnecessary in the vast majority of cases. At the University of Pennsylvania, which has a long-standing and highly successful breeding facility, hobbles and twitches are not usually used. Instead, careful attention is paid to teasing and readiness to breed, and felt breeding boots are placed on the hind legs of the mare for extra assurance against injury.

Of course, there are other reasons that mares will not show heat. Undetected pregnancy should always be considered. Some mares will prefer not to breed to a particular tallion. Other problems include exposure to steroids, or ovarian tumors (rare).

Performance problems in mares are often attributed to the heat cycle. It's important to distinguish problems that are and aren't associated with heats, however. Some mares are very submissive, and will cower and squirt urine due to stimuli other than a stallion. A raised tail and winking of the clitoris are usually absent. This is a training problem, and the horse's fear must be worked through.

A mare's performance can suffer during heat. Some will respond in heat to other mares or even people. Some will even do this under saddle. In this case, the most effective drugs are daily injectable or oral forms (Regumate) of progesterone. At currently used dosages, Cattle steroid implants and Depo Provera (a human birth control drug) have been shown to have erratic and often limited effectiveness in moderating a mare's response to a stallion.

Some mares show performance problems during the phase of their cycle when they are not in heat. This phase is called diestrus. In this case, other hormones such as prostaglandins or HCG (human chorionic gonadotropin) may be given by injection throughout the show and training season. None of the above drugs (including progesterone) have been shown to interfere with future fertility.

Long-acting tranquilizers have had some positive effects on these problem mares in recent studies.

If the mare's behavior is not a problem during the middle of the winter, when she is not cycling, and she responds favorably to hormone therapy as a treatment for performance problems, than surgical removal of the ovaries is a possible solution. This surgery can be performed with an endoscope, avoiding the large incisions and many of the complications of surgeries in the past.

Dr. Katherine Burnett, DVM

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