How Long Are Mares In Heat?

Last Updated on November 24, 2020 by admin

Anyone who owns a mare knows that managing her heat cycle can be a delicate and difficult skill to master!  Mares, like many other mammals, can completely change in temperament when going through their regular cycles. What happens to mares in heat?

As always, being informed and proactive about dealing with a mare in heat will make the actual process easier and less stressful on both horse and owner. 

 Knowing when to expect your mare’s cycles, what symptoms to expect from her, how long her cycles last, and what you can do to keep her as comfortable as possible will help you prepare for dealing with her cycles.

These things and more are what will be discussed in this article.  Knowledge and experience are your best defenses against dealing with health-related issues and situations as a horse owner.  

So, look no further than this article to learn more about when mares go into heat, how to know when a mare is in heat, how long a mare is in heat for, and what you can do to help a mare in heat.

Mares in Heat: How Often Do They Come?

Mares have a complicated cycle.  Unlike many other mammals, mares do not go into heat year-round.  Instead, mares become anoestrus, which means their cycle pretty much doesn’t exist.  But, as the weather warms up and the sun is out for longer, this changes.

In early spring ( for the US, probably March or April), mares will begin to experience a regular cycle.  Sometimes it takes a while for their cycles to become normal and predictable, but they will begin at roughly this time of year.

Mares in Heat: How do You Know?

Every horse is different, and every mare is different.  Therefore, the signs that a mare is going into heat, or is already in heat vary from horse to horse.  Though, thankfully, there are some shared symptoms. 

As people in the horse world would say, mares in heat become extra “mare-ish.”  Mares typically become extra irritable and grumpy.  They do a lot of tail raising and urinate more frequently than normal.

Mares in Heat: How Often Do They Come?

In extreme cases, they may even lash out by biting or kicking at you or other horses.  Sometimes they become more vocal, calling to their friends if they are out of sight. They can also become more sensitive around their stomach and flanks.

Mares in Heat: How Long Will They Be in Heat?

Typically, a mare’s cycle lasts only three weeks or twenty-one days.  Out of these twenty-one days, a mare will be in heat for five of them. So, that’s a little under a fourth of the time.  

Every horse is unique, and some mares will deviate from this average, but it’s a good basis to plan off of. Remember, mares do not go through cycles year-round.  They will typically be on this three-week cycle from mid-spring through the end of fall.  

The actual months of a mare’s cycle will vary depending on where you live and what the climate is like where you live.  But, during the months your mare experiences a cycle, it’s safe to say she will be in heat for about five days every three weeks.

What You Can Do to Help

Heat Cycles are a natural part of a healthy mare’s life.  Many horse owners choose to simply work around their mare’s cycles and choose to let them run their course.  

But, if circumstances do not allow for the behavior a mare presents during her cycles, there are several things that can be done to help her symptoms change.

Regular Vet Checks

First and foremost, if your mare is showing extreme signs of aggression or irritability, make sure she gets looked over by the vet.  It can be easy to write a mare’s negative behavior off as her “being in heat.”

But, this is not always the case.  Sometimes her behavior may be due to an actual health impairment, injury, or illness not at all related to her cycle.  It never hurts to have a vet look her over before assuming her behavior is due to her cycle. 


Second is supplements.  There are many supplements that help regulate mare’s hormones.  Be sure to do your research on which one will be best for your mare, in conjunction with her behavior, her regular feed schedule, and other supplements she may be getting.

These hormone-regulating supplements may help her calm down, and they may help her pain and discomfort decrease.

Sterile Marbles

Third are sterile marbles.  Sterile marbles are inserted only by veterinarians, and they trick the mare’s body into believing that she is pregnant.  This then stops her regular cycles altogether.  

Some horse owners have had positive experiences with sterile marbles, and some have watched their horses suffer unpleasant side effects from them. Be sure to consult your vet before deciding this is the best course of action for your mare.

Mares in Heat: How Long Will They Be in Heat?


Mares can be the best partner you’ve ever had if you know how to maintain them properly.  Knowing how to manage their cycles is a significant part of that maintenance. Your mare will thank you, and you will thank yourself if you prepare for her cycles beforehand.

There are many options for managing your mare’s cycles, and the decisions are all up to you as a horse owner.  What is best for your horse is exactly that; what is best for your horse; not your neighbor’s, not your trainer’s, but your individual horse.

Every horse is different, and every mare is different.  Every mare’s cycle is going to be different, and every mare is going to need different care during the time she is in heat.

I hope this article helped you better understand when mare’s go into heat, how to tell if a mare is in heat, how long a mare is in heat, and what you can do to help a mare in heat.  If so, please share this article, and share with us your experiences dealing with mares in heat!


How Do You Know If A Horse Is Pregnant? 

A mare will likely have multiple signs she is pregnant, but they might not be obvious if the pregnancy is unexpected. Those actively breeding are going to be monitoring these signs and taking the mare in for ultrasounds. However, there are cases of accidental exposure or purchasing a mare that has previously been exposed. Signs may include no estrus cycle (she doesn’t go into heat), larger abdomens, visible changes to the udder, foal movement, or elevated progesterone. If you suspect a mare is pregnant, your veterinarian can perform an ultrasound as soon as two weeks after possible conception. 

How Long Can A Horse Go Past Her Due Date? 

Horses typically have a gestation period of 11-12 months. The most commonly reported pregnancy lasts 330 days (11-months), but veterinarians usually use a 340-day period for due date estimates. Gestation typically maxes out at 380 days, well into the 12-month mark. However, overdue mares may be at risk for birthing foals small in size due to delayed development. Contrary to popular belief, these late foals are rarely “too large” to cause any delivery issues. Horses typically follow the same gestation pattern, so late foalers are likely to continuously have longer pregnancies.

Will A Pregnant Mare Accept A Stallion? 

In some cases, a pregnant mare may still accept a stallion. Although not common, some mares (particularly in later stages of gestation) will show signs of heat and allow stallions to mount. This can be due to the gonad development of the fetus, which may produce additional hormones. When this occurs, it is not a true estrus cycle and the result is only behavioral. “Heat-like” behavior is more common when mares are in herd settings. On the contrary, some stallions may even mount unwilling mares. If this occurs and a stallion somehow succeeds, this can cause damage or even death to the developing fetus. 

Can You Ride A Horse While She Is Pregnant? 

Much of a mare’s pregnancy and foaling process is determined by her conditioning prior to conception. Most healthy pregnant mares that were routinely worked or ridden prior to conception can be ridden during the majority of the pregnancy. However, mares should not be ridden in the last few final months before the due date, or immediately after foaling. Like any other time, riders should be careful to avoid overexertion of their horse. Humans and horses both benefit from moderate healthy exercise during pregnancy. If you have questions regarding your mare’s health or pregnancy complications, speak to your equine veterinarian for guidance. 

How Many Babies Can A Horse Have In A Lifetime?

Some horses will be used as broodmares and foal multiple times in their lifespan, while other mares may only be bred once. Optimal foaling age for a mare ranges from 4-15 years old, with decreased fertility and increased risk after age 15. Using this range, annual breeding would result in 11-12 foals in a lifetime. However, some broodmares are reported as having upwards of 15-20 foals throughout their lives! Many experienced and healthy broodmares are bred beyond the age of 15, so long as estrus cycles remain functional and normal.