What are the signs of heat with a mare?

Here is an answer to your questions and some more info on heat cycles:

First of all, a mare will come into heat during the warmer months, when the days get longer, and that depends on where you live. In the northern states, the heat cycle is generally from May to September. In the southern states, they may come into heat all year round.

The mare's heat cycle is also called estrus cycle. During this time she will produce a fertile egg, and she will accept a stallion. The heat cycle usually lasts 18-31 days, but it is actually during the last 24 to 48 hours that she ovulates. During the beginning of her heat cycle, although she will probably accept a stallion, she may not be able to conceive. That is why breeders will generally breed the mare every other day until she goes out of heat to ensure that they catch her at the right time.

The signs of heat vary according to your individual horse, but it in most mares it is very easy to spot. Generally, the mare may act different to other horses, and may be more nasty to them, but act interested in stallions or geldings. When a mare begins her heat cycle, she may not accept a stallion at first; she may pin her ears, bite, kick, or chase any studdy-acting geldings away. Later, she will act interested in them, and will usually encourage them to mount her.

You will see mares in heat urinate in small amounts and usually hold their tails high for a longer time than normal after they urinate. They may stand and rest with their tails slightly up, or held to the side. Mares will also 'wink', a sign that tells a stallion she's ready to mate. Winking is when the mare opens and closes the lower part of her vulva.

She may also spread her legs and squat down, looking as though she is trying to urinate-- and she may.

The urine is yellowish or clear; there should never be blood or dark-colored mucus seen on a mare, this represents problems.

The best way to tell a whether a mare is in heat, if you are breeding, is to see how she acts toward a stallion; mares who are not in heat will be hostile, and may kick, bite, or whirl away from the stallion. This is why teasing (using a stud to determine whether a mare is in heat or not) is done very carefully, as it is unknown what either of the horses will do.
A mare IN heat will act interested in the stallion, and may nuzzle him, or will at least stand still for him.

You may be thinking, "How is it done in the wild if the mares are so hostile?" Well, stallions who are raised with mares learn when to leave them alone, and when it's OK to approach. An experienced stallion will act like any other horse around mares, and act studdy only when approaching a mare in heat. Mares are not always hostile to stallions, only when they are being bugged by a studdy stallion and they are not in heat. Mares and stallions get along well when left alone, as they both get to know one another. Mares are usually more hostile to stallions that they don't know-- all horses are often a bit nasty to the newcomers, until they get to know more about them.

Strictly used breeding stallions often do not live out with mares, and only see them when they breed. So, as you can imagine, they start snorting and prancing at any mare, which, if she is not in heat, will make her angry.

Some mares are very obvious when they come into heat-- they always hold their tails up, urinate a lot, and their behavior changes. Other mares may not be so flashy, but have quieter heat cycles. It just depend on your mare's temperament. Some mares will not show signs of heat unless brought near a stallion.

About 4-18 days after foaling (average is 9 days) a mare will come into 'foal heat'-- she may be bred during this time if she has had no foaling complications (tears, bleeding, infection, retained membranes, etc.). However, a mare like this may go out of heat if she is worried about her foal. It is best to haul her to the stallion's place when the foal is about 6 days old, so that she and her foal have time to adjust to the new premises.

If you want your mare to come into heat in winter or early spring so you can have an early foal, then you can cause her to come in by putting her in a stall at night leaving a light on in her stall for a few extra hours at night. This makes her body think it is breeding season, as the mare's body measures the length of the night, and causes her to cycle if it is short (it's not known how they do this, but they do). The mare, however, will shed her winter coat, too.

Some mares, if not kept around stallions, will come into heat if just taken around a stud.

Stressful situations can make a mare come out of heat; mares that have never been bred may be shy and not show heat signs even when around a stallion. 

Taken From ultimate horses

The Equine

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