Breeding From Your Mare 

Understanding the basic physiology and anatomy of your mare will help you understand what is involved in successfully breeding healthy foals.

Reproductive Anatomy:

Mares have two ovaries. The ovaries are situated in the dorsal (upper) part of the abdomen, just behind the kidneys. The ovaries are the female gonads and they produce and release eggs. They are also the site of production of the female sex hormones oestrogen and progesterone. The uterus (womb) is comprised of two horns and a body, all being suspended from the roof of the abdomen by the broad ligament. The cervix protects the entrance into the uterus from the vagina.

Figure 1: The mare’s reproductive system           Figure 2: The mare’s reproductive system in situ

The mare's reproduction system              

The oestrus cycle:

In the Northern Hemisphere mares are most sexually receptive and active from April through to September. During winter, mares become sexually inactive and are said to be in a state of anoestrus. As spring approaches and day light length increases, the pineal gland within the brain produces less of the hormone melatonin and the mare enters a transitional period. (Sometimes mares can exhibit intermittent grumpy behavior during the transitional period.) With rising hormone levels, follicles within the ovaries are stimulated to grow. Initially lots of follicles grow simultaneously until one becomes dominant and ovulates releasing the egg from the follicle. Following this, regular oestrus cycles will occur throughout the summer months.

Each oestrus cycle lasts around 21 days with the mare being in season (sexually receptive) for 3-8 days. During this time the mare will show variable behavioural changes such as frequent posturing (see below) and she will also be receptive to the stallion, allowing him to nuzzle her.

Figure 3: Mare in season

These behavioural changes occur due to increasing levels of the hormone oestrogen, which is released from the follicles. Ovulation usually occurs 24-48 hours before the mare goes out of season and what remains of the follicle after ovulation forms a structure within the ovary called the corpus luteum (C.L). The C.L. produces another hormone called progesterone. Simply speaking progesterone has the opposite effect to oestrogen and the mare will now be out of season and no longer receptive to the advances of the stallion. At around 16-17 days, if a pregnancy is not detected, the lining of the uterus produces a hormone called prostaglandin, which destroys the C.L. and consequently the levels of circulating progesterone decrease. This reduction in progesterone allows follicles to develop again and then the mare returns to season. However if the mare is pregnant the C.L. will remain and with the help of other hormones, the pregnancy will be supported and there will be no return to osetrous.

Preparing your mare for stud

Prior to sending your mare to stud there are a number of things that need to be considered, such as her having a veterinary examination to ensure that she is fit to breed.

In order to perform this safely your veterinary surgeon may require stocks, although quiet, well-behaved mares can be assessed over a stable door.
A typical breeding soundness examination includes:

•    A physical examination
•    Rectal examination
•    Ultrasound scan
•    Reproductive examination

At the same time as the breeding soundness examination the veterinary surgeon will also perform clitoral swabs in order to make sure that the mare is free from the bacteria (Taylorella equigenitalis), which causes the venereal disease Contagious Equine Metritis (CEM). Blood tests are also performed to rule out viral infections, namely Equine Viral Arteritis (EVA) and Equine Infectious Anaemia (EIA) prior to entering the stud.

Pre-stud checklist

Before sending your mare to stud you will need to consider whether the following are required and up to date:

•    Vaccinations (tetanus, influenza and herpes virus (many studs stipulate that horses are vaccinated against equine herpes virus in order to minimize the risk of abortion storms)
•    Worming
•    Dental health
•    Routine hoof care
•    Overall physical condition of the mare

Most stud farms will impose a 2-week period of isolation for any new arrivals. This allows new arrivals to be monitored for any infections, which they may be incubating, which they may not have been tested for, thus safe guarding the stud farm from potentially serious infections. If your mare is pregnant and you are sending her away to stud to foal, then you should contact the stud for further information, as the mare may well need to arrive at the stud at least 3-4 weeks prior to her foaling date.
Central Equine Vets can provide you with further information on both what is involved with a breeding soundness examination as well as providing advice on appropriate considerations for breeding your mare.


Mares can be mated or “covered” in one of two ways:

1.    Natural mating with a stallion

2.    Artificial insemination (A.I.)

With artificial insemination semen is collected from a stallion, which maybe from a different part of the country or a different country all together, thus giving you, the breeder, a wider range of choice of sire. The collected semen is then stored, and at a later date, placed into the uterus of the mare.  
Mares are often covered naturally, especially in the thoroughbred industry.

Advances in equine reproductive knowledge and techniques mean that it is now possible for a veterinary surgeon to examine the mare to see what stage of her oestrous cycle she is in. Using modern hormone drugs it is then possible to manipulate the mare’s oestrous cycle in order to induce ovulation and thus mate the mare as close to the ovulation as possible. This means that fewer matings are required.

Artificial Insemination (A.I.)

A.I. involves collecting semen from the stallion and then implanting it into the mare. The semen is collected by having the stallion either mount a teaser mare or dummy. Once mounted, the stallion ejaculates into an artificial vagina (A.V.), and the semen is collected. From here it can be used immediately or it maybe chilled, or frozen, and then transported to the stud farm where the mare is stabled.

The mare is then inseminated once she is in season. The semen is thawed (if frozen) and inserted into the mare’s uterus through the vagina and cervix via an A.I. gun (pipette).

Advantages of A.I.?

•    Reduced risk of spreading venereal diseases as there is no direct stallion-mare contact
•    Reduced risk of injury to the horses and handlers compared to natural mating
•    Achieves mating whilst the sire and dam are on different stud farms, or even in different countries

If you are considering breeding from either a mare or stallion Central Equine Vets will be able to provide you with further advice as to which method is most appropriate for you.

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