By Breanne Hill

Sugar and spice and everything mean. That is what some mares seem to be made of when they are in heat.

As is the case with any animal, horses can be affected by the hormonal changes that occur during the heat—or estrus—stage of the reproductive cycle. This phase, in which a mare is most receptive to breeding, commonly manifests itself through such behaviorisms as tail swishing and squealing when around other horses and can be accompanied by a short attention span and a bad attitude.

In practically every equine discipline, there are trainers and owners who dislike working with female horses for this reason. They simply don’t want to put up with the monthly hassle created by a mare that is in heat.

To assume estrus is the only reason a mare gets moody, however, is a mistake. Veterinarians recognize that a horse that seems mentally imbalanced may be going through more than just the usual hormonal changes. She could be experiencing any number of health problems that have her behaving in a less-than-nice way.

Shaylyn Bliss, DVM, an equine reproductive specialist at Performance Equine Associates in Whitesboro, Texas, cares for hundreds of mares every breeding season. She says the key to determining whether your mare is being affected by a standard estrus cycle or is suffering from another medical condition is to know what is “normal” behavior for her.

“They’re usually having extreme problems with training or they’ve noticed a decline in performance, or the mare is being uncooperative and fractious and sometimes even aggressive,” Bliss said

According to Bliss, these clients have almost always diagnosed the horse themselves at home. And the gist of this diagnosis is usually that the mare’s problems are all related to her heat cycle.

“I’ve had several clients bring their mares in, and they claim a lot of things and try to associate that with the mare being in estrus,” Bliss said. “One of the things I hear most often is, ‘Her ovaries are sore and that’s preventing her from performing well.'”

In her experience, Bliss has found these home diagnoses are often incorrect.

“A lot of times, we’ll check the mare and she’s not even in heat,” Bliss said. “So I think for some reason people tend to want to blame the mare’s estrus cycle for everything that’s wrong. And I would say, most of the time, the mare’s problem has nothing to do with her estrus cycle.”

To avoid making this primary misdiagnosis, an owner should first be familiar with how his or her mare behaves on an everyday basis. Is she usually aggressive or submissive toward other horses—or toward people? Traditionally, has there been a difference in how she behaves when she’s in heat and when she isn’t? And, of course, do you really know when your mare is in heat?

If you admitted “no” to the last question, then it is there you should begin tracking down a diagnosis for your horse.

A mare’s reproductive cycle is relatively simple to pinpoint—if you’re willing to observe and mark the days off of a calendar. As a general rule, most mares only ovulate between March and October. During the winter, without artificial manipulation, such as special barn lighting, a mare’s reproductive organs go into “hibernation” and she will not be interested in breeding.

Throughout the spring and summer, however, a mare may ovulate every 21 days. Ovulation is prime breeding time, the window of the reproductive cycle when a mare is most likely to conceive. Estrus lasts approximately three to seven days prior to the onset of ovulation, and it is the point of the reproductive cycle when a mare is most likely to shows signs of being in heat.

Why is this? In female equines, heat-related behavior is prompted by low levels of the hormone progesterone. Progesterone levels are at their lowest right before ovulation.

The intensity of the resulting behavioral changes varies from mare to mare, but Bliss says it is perfectly normal for even the most reliable horses to act differently during this period.

“(Mares) are probably going to be a little more irritable than usual during estrus,” Bliss said. “They’ll show all the standard signs of posturing—tale lifting, squealing, backing up to other horses—and they’ll urinate frequently. I would consider this all to be normal behavior. They’ll just be a little bit crabby, as you would expect them to be.”

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