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Feeling the Heat: Estrous in Mares

Squealing. Kicking. Squatting. Urinating. Stopping to gawk at a passing horse. Distracted and not paying attention to you. Anyone with a mare knows what this means!

Why is this happening?

There are two major ovarian hormones that rule a mare's behavior during her reproductive cycle: estrogen and progesterone. Estrogen is high and progesterone is low when a mare is in estrus, or "in heat" and the converse is true when she is in diestrus, or the time between periods of heat. When estrogen is high, this cues the mare to show receptive behaviors to a stallion. This can often be anywhere from annoying to downright unmanageable when trying to handle or ride your mare. When progesterone is high, mares tend to be calmer, although they can sometimes display aggression and agitation when around other horses.

What can we do?

The first step in deciding how to manage a mare's behavior during her heat cycles is to determine if it is in fact related to increased estrogen levels (is she only showing undesirable behavior when she is "in heat") or is it a more constant issue, including when she isn't actively in heat. If the latter is the case, then hormonal management may not be effective in affecting her behaviors and another course of action may be required. Your veterinarian can assist you in determining which applies to your mare via reproductive exam and ultrasound to determine where she is in her estrous cycle.

Estrus Suppression Strategies

Following are some of the options available to help control and/or minimize the effects of your mare's cycles and make life a little more pleasant for everyone involved!

Altrenogest (aka Regu-Mate, OvaMed, or Altren):

This is the gold standard for estrus suppression. It utilizes the prescription drug Altrenogest which is progesterone-like. It is given orally once daily. There are a couple options regarding the frequency of administration. First, you can give it 4-5 days before and then through an event to keep the mare from “coming in” during the show. A few days after stopping the drug she will come into estrus, but at least it wasn’t at an inopportune time. Secondly, you can give it daily through the whole show or riding season. This is very effective, but also very expensive.

Extreme caution has to be used when handling and administering this medication. It absorbs through skin easily and shouldn't be handled by people with certain medical conditions or women who are or may be pregnant. Wearing protective gloves is a must.

If these downsides aren’t ideal, there are a few other options, but one major caveat is that these are non-FDA approved products. However, anecdotally there have been positive reports of estrus suppression or control of undesirable behaviors in mares.

Progesterone Implant:

These pellets are made of progesterone and estradiol benzoate and are injected under the skin just below the crest of the neck. They are usually placed under the mane so the small lump that occurs can be hidden. These pellets are meant to be used in steers at the feedlot to enhance weight gain, but can also work to suppress estrus in horses. The duration of effect is variable depending on the mare, but is slow release and in general last anywhere from 2-6 months. Infection at the injection site is a possible side effect, but is very rare.

Medroxyprogesterone/Depo-Provera Injection:

This is a compounded progesterone product that is injected in the muscle at specific time intervals. The oil-based product is given monthly, is expensive and can have local reactions due to the oil base. The aqueous product is given weekly, has fewer reactions and is less expensive. The latter is the drug we use and have had great success.

Herbal Supplements:

Herbal oral supplements contain proprietary blends of herbs believed to have calming properties. A common herb in mare behavioral supplements is raspberry leaves, which are thought to regulate the smooth muscle tone of the reproductive tract, which can help to relieve cramping and thereby make the mare less uncomfortable.

And how about this for thinking outside the box...

Marble Implant:

Placing a sterile 35mm marble, or acrylic sphere, in the mare’s uterus soon after ovulation sends the body a false signal of pregnancy. The marble mimics the size of a conceptus and the pressure in the uterus then signals the mare's body to go into pregnancy mode. Studies have shown that this effect will last for about 3 months. This procedure doesn’t work on all mares and some mares push them out, unbeknownst to their owners. Side effects can include chronic uterine inflammation or infection, scarring, and infertility, especially if the marble is left in long-term.

There is no one-size-fits-all treatment strategy for managing estrus in mares so it is important to talk with your veterinarian to determine which is the best option for your mare.

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