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Moody Mare - Why?
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Moody Mare - Why?

Just like people, some mares can suffer a hard time during their reproductive cycle. Many show no changes in their demeanour whereas others can transform overnight from placid, loving creatures into "mareish", squealing lunatics!

Mares affected by the hormonal changes that occur during oestrous can become aggressive, oversensitive, nappy or even suffer from pain and show the symptoms of colic. All of this can be most frustrating for an owner particularly if the mare concerned is used for competition, but understanding how your horse's cycle works can help you to understand why she behaves as she does at this time.

The reproductive cycle explained

Mares are referred to as seasonal polyoestrus breeders. Put simply, this means that they have many breeding cycles during the annual breeding season. The season coincides with the lighter, warmer months of the year; in the UK from March through until October. This is why stud farms use artificial lighting to induce oestrus. Hormonal activity and seasons cease during the cold, dark months of the winter and the system is not active. Mares are intrinsically programmed in this way so that foals are not born during the harshest time of the year when the weather is at its coldest and grazing is scarce. During the spring and summer months there is plenty of lush, nutritious herbage available so the mare is able to provide rich, plentiful mioestruslk for the foal ensuring he gets the best possible start in life.

The oestrus cycle lasts about 22 days. The first five or six days is called the pro-oestrous and oestrus stage and the remaining 21 to 22 days is referred to as the dioestrus. As spring approaches, a transitional period occurs during which follicular development commences within the mare's ovaries. Each ovary will harbour a number of follicles at different stages of development. A follicle-stimulating hormone encourages the follicles to grow. At ovulation, the follicle will measure about 35mm to 40mm in length. During pro-oestrus at around day five or six, one of the follicles will ovulate, producing an egg. A second follicle matures and releases its egg a few days after the first has ovulated.

As ovulation approaches and the follicles grow larger, follicular-stimulating hormone levels increase. About half way through this period the ovary releases oestrogens and it is these hormones which cause the mareish behaviour. When ovulation occurs, the follicle ruptures, releases the egg and leaves behind a fluid-filled sac.

At the end of the fallopian tube (oviduct) are tiny structures like fingers (infundibulum) which form a cup beneath the ovary. As the egg is released from the follicle at the base of the ovary the infundibulum catch the egg and direct it into the oviduct. If the mare is mated, this is where fertilisation will occur. The remaining maturing follicle continues to develop and ovulates about five days later. The consequent peaks in hormones cause prolonged behavioural changes throughout oestrus.

The fluid-filled sacs left behind after ovulation degenerate forming the corpus luteum. This produces the hormone progesterone which takes the mare into the dioestrus stage of her cycle. During this phase, the progesterone relieves the mareish behaviour and she is not receptive to mating.

Physiological effects

In the presence of oestrogens the mare is much more aware of what is going on around her. While she is receptive there will be a lot of swelling throughout her reproductive tract and it is this which can cause discomfort, pain and colic in some mares. The uterus contains folds of tissue which swell when the mare is in oestrus and there is also much more mucous and general secretions present. The mare will urinate more frequently to spread pheromones, the chemical messengers which tell a stallion that she is ready to mate.

All these changes can cause pain and this is why your mare may become reluctant and uncooperative during ridden work or even when being groomed. The ovaries are situated just behind where the saddle rests so it shouldn't actually interfere directly with them but the swelling and inflammation within the reproductive tract will make the whole area behind the saddle tender and sensitive. Pressure from the rider's leg and driving aids from the seat and back can also cause discomfort at this time. Your mare is also very interested in sex and boys at this time too; little wonder then that she's not that keen to perform a perfect half-pass for you during a dressage test when there's a handsome, strapping stallion showing off his flashy extended trot in the arena next door!

As if all this wasn't bad enough, it's also thought that changes during oestrus can also affect other body systems too. Subtle lameness can become more acute and a minor back problem may become ten times worse.

How can you help?

Progesterone injections

Progesterone is released naturally during the last part of the mare's cycle and serves to curb extreme, unwanted behaviour. Injections of this hormone can be given to help your mare.


Regumate is a prohibited substance for competition horses so you can't use it if your mare is a performance horse. It's a synthetic progesterone supplement which you add to your mare's feed. Regumate works by mimicking pregnancy and preventing oestrus thus removing season-related behaviour.

Herbal feed supplements

There are a number of different herbal feed supplements available which mimic the effects of progesterone. Because the ingredients are all natural, they are permissible under competition rules.

PRIDs (Progesterone Release Inrauterine Devices)

These are implants more commonly used in cattle although there are equine versions. These work in the same way as progesterone supplements.

Melatonin supplement

The production of GnRH (gonadotrophin-releasing hormone) drives the production of the follicle-stimulating hormone. Reducing the production of GnRH therefore can prevent oestrus at an early stage. Feeding a melatonin supplement will stop the production of GnRH. Melatonin is produced by the mare's body at night. During the winter time when daylight hours are shorter, large quantities of melatonin are produced, preventing the mare from coming into season even during the summertime.


This involves surgical removal of the ovaries. Obviously, this is an expensive option and should not be undertaken lightly. Your vet will advise that you try all other options available to you before going down this route.

Disclaimer: Of Horse! and sponsors do not endorse nor validate the accuracy of a blog post. Each article is the opinion of the blogger.

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  1. PonyGirl
    Very interesting article. At one time, many mares were given Regumate on the track. A small percentage of them became very study and would try to mount my pony when they were on it. One mare in particular was so bad, I couldn't even get near her without her going up. I don't know if these mares had underlying problems or if this was a side effect of the Regumate. I have also seen several mares who have cysts on their ovaries which makes them extremely temperamental and touchy when they're in season. I'm not sure what the current treatment is for this, but a vet will be able to diagnose and treat this condition. The cysts definitely affect performance even when the mares are out of season.
    1. autumnap
      Thank you. Interesting comment on Regumate. I wonder if it contains testosterone? That might explain the mounting behaviour. Someone I knew had a mare with an ovarian cyst and she was a complete nightmare. In the end a scope confirmed the problem and if memory serves she had surgery to remove it. I had a mare once who was fine except that she made a horrible sticky mess of her tail and insisted in squirting all over the place (including all over me on one occasion) when she was groomed - nice! x
      1. PonyGirl
        I think perhaps the Regumate does have testosterone in it. Before Regumate came out, they gave mares Equipoise, which was testosterone, resulting in the same behavior. The mare I'm currently ponying with cysts was terrible to gallop last year, so the trainer started ponying her. The rider's weight seemed to be bothering her, and I mentioned it might be a cyst. (She would always wring her tail with the rider,which is one sign of potential hormonal problems). The vet checked her, and she did have a cyst. He treated her non-surgically. Lately, she started not breaking and running poorly. I watched her last race and noticed she was not striding out at all behind. I mentioned to the trainer, that the spring weather and her returning hormones might have caused the cyst to flare back up, and sure enough, it had returned. They treated her, and I took her to the gate to work and see if that helped. She broke so hard, she nearly stumbled, and turned in a beautiful work. So apparently she is feeling much better now. When the trainer got her she was a miserable, nervous wreck who hated to go to the track. Now she squeals and plays as soon as she sees my pony, and coming home is completely relaxed and happy. It's horses like her that make my job worthwhile.
        1. autumnap
          That's a really good example of how "listening" to horses through observation (and common sense) can help to overcome problems and achieve the desired result without having to resort to force. Too many people give up without persevering to get to the root of the horse's problem. x
          1. Lori L
            You could not have said it better!!
  2. Rene Wright
    Rene Wright
    Voted. I haven't witnessed Cookie's heat cycles in a manner that seems different to her off season attitude. It could very well be because she's the only horse around so there's no gelding or stud to wake up those responses. I have heard that raspberry leaves are great for both mare and geldings should either get "hot" during competitions and even trail rides. Great blog! x
    1. autumnap
      Thank you kindly! It could just be that Cookie is a chilled out soul who doesn't suffer from PMT - lucky you! x
  3. Lori L
    Great article, Atumn!! I do not own any more horses, but I did take into consideration of my sweet Arab/Welsh mare when she seemed a bit grumpy as I suffered horribly with my own 'seasons'. Horses are so wonderful. They deserve our consideration and love!

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