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When Do Horses Go Into Heat & What Age Do They Stop Cycling?

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We decided to replace an older broodmare with a young prospect. The first question we need to answer is when will our new horse go into heat and be old enough to breed, and how much longer will our older mare continue to cycle?

Most horses have their first heat cycle before turning two years old and stop cycling at twenty years old. Typically a horse’s estrus cycle lasts three weeks and is affected by age, location, and time of the year. But, most importantly, horses are individuals, and their cycles may vary from the standards.

Most horse owners have limited knowledge about mares’ heat cycles. But if you intend to breed horses, there is a lot you need to know about horses’ estrus cycles.


Horses heat cycles

A mare in heat can be difficult to handle; this is especially true if there are one or more stallions nearby. There are several signs that the mare feels this way, and knowing what they are can help you cope with this natural cycle.

Fillies first heat

A filly usually comes into heat the first time in the yearling year. This does depend on a few factors, including the season. Unlike humans, horses usually don’t go into season during the winter months. Their breeding season typically starts in April.

It is also dependent on whether or not the filly is around other horses, particularly mares that are in or getting ready to be in heat. Stallions are another possible cause of earlier than normal heat, while the season can make a filly go into a first heat later.

Length of estrus cycles

Mares typically have a three-week estrus cycle. They can be in heat for as little as two days or as many as ten days. This will last until the days begin to grow shorter again, usually around September.

This cycle may be different depending on location. In the farther northern part of the northern hemisphere and the south part of the southern, the cycling time will be shorter. In the middle parts of the globe, it may be longer.

Why the length of heat cycles is important to breeders.

Knowing your mares’ breeding cycle is critical for most breeders because of how birthdays are defined in many horse registries. Mares have an eleven-month gestation cycle, and all foals in the Northern Hemisphere are considered born on January 1 of the year of their birth.

So if your foal is born in May 2020, on January 1, 2021, it is deemed to be one year old. Ideally, you want your mare to have her foal as early in the year as possible. To ensure your foal is born early in the year, it’s critical to understand a mares estrus cycle.

For horse racing, a couple of months can make a huge difference. For example, most tracks have races for two-year-olds in the fall season. For horses born late in the year, they could be matched against horses six or seven months older or miss their entire two-year-old season.

Age is also important in many other equine disciplines that horses compete by age, such as dressage, jumping, and barrel racing futurities.

Coping with mares in heat.

Mares in heat can be a problem, especially for those new to horse ownership. There are a number of things that they may do (or not do). Many are relatively easy to handle, once you know why they are doing it.

Mares in heat will urinate more and may stand differently. They will likely lift their tail higher and you may see the vulva “winking.” They will open and close it a lot. This can be worse if there is a stallion nearby, as mentioned. Other sexual behavior may be exhibited as well.

What you need to know.

There is a list of things to know about how to handle the mare. First of all, never approach from behind. This is unwise in any case, but especially around a mare in heat. When grooming, start at the head and neck, rather than the flanks. They will be more sensitive now.

Mares in heat may also feel some pain; not unlike the cramping women may feel during their menses. If that happens, the vet may suggest you give her a pain reliever. There are some that are especially for horses and gentle enough to use.


When does a mare stop having heat cycles?

The last question is when does a mare stop having heat cycles. Like older women, the exact time is not going to be uniform. The official age given is usually about the age of twenty. Horses can live up to thirty years, so it is not unlike menopause.

Again, this will depend. Some may stop at the age of eighteen, and a few maybe younger. Some may also be older, even up to about twenty-two years old. However, as a mare gets older, there is much less likelihood of pregnancy resulting from heat.

If you are new to horse ownership or this is your first mare, it is wise to talk to the previous owner and to your vet. They can give you an idea as to what to expect and how to deal with it. The vet can go over options if you wish to avoid heat and which is the best way to handle the situation.

Can you stop a horse from going into heat?

Why would someone want to mess with the cycle of a mare? Just as there are for human women, there are many reasons. As mentioned below, showing a mare in heat may be somewhat problematic. There are bound to be plenty of other horses and some may be stallions.

On top of that, having a mare exhibit some of the symptoms of being in heat while attempting to show could be disastrous. Not just for the show, but for both horse and rider. The mare may not be paying as much attention as she should and some of her behavior may not be circumspect.

If the mare is going to be ridden by someone who isn’t familiar with horses it could be equally disastrous. A lot of care has to be taken around a mare in heat, although not all horses show these problems to an extreme.

If you don’t want the mare to go into heat for some reason, there are ways that may help you avoid it. However, not all of them work and some may cause reproductive problems in the future. Knowing these can help, especially if the mare is going to do show work.

Picture of a chesnut baby horse.


The easiest methods tend to be the use of medications, as they are likely to be more reliable. There are two drugs that can be used. One is more of a long-term birth control medication. The other has to be used during the entire period the mare is in heat.

The first choice is often used for show mares that the owner may prefer not to breed down the road. Regumate can cause a mare to not be fertile temporarily. Unfortunately, it doesn’t stop when the drug is stopped. It will take time to wear off.

Oxytocin is another option. Most women who’ve had children are familiar with this hormone; it is what brings on birth. However, the right dosage may also fool the mare’s uterus into thinking it’s already with foal.

Marble trick

The last trick is one that works about forty percent of the time. A sterile marble is inserted into the mare’s cervix, just as her heat is ending. Sometimes the mare will “slip” the marble; i.e. it will come out similar to a miscarriage.

When it works, the effect lasts about three months. Once removed, the mare can be bred easily. However, if it is left in too long it can cause some severe problems, including infertility. This is usually due to infections.

Below is a helpful YouTube video that explains when horses first go into heat.

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