ABSTRACT: This articte provides a brief overview of breeding from a mare. The first section details the oestrous cycle with some basic physiology. Considerations of the mare's health are discussed, with reference to general health (laminitis and other orthopaedic problems, teeth, vaccinations) and reproductive health (vulval conformation, conditions of the cervix, uterus and ovaries). Descriptions of different breeding strategies, in particular natural mating and artificial insemination, are discussed in the third section. Finally, there is a brief review of pregnancy diagnosis by ultrasonography.

Oestrous cycle of mare

The mare is regarded as a ‘long-day, seasonally polyoestrous breeder’. That means that during the spring, summer and early autumn months, normal non¬pregnant mares show repeated oestrous cycles lasting approximately 21 days. Oestrus (when the mare is receptive to the stallion) lasts for four to six days, and ovulation (release of the unfertilised egg from the ovary) usually occurs approximately 24 hours before the end of oestrus.

During this period, the predominant hormone produced by the ovary is oestrogen and this is responsible for the mare’s reproductive behaviour. Oestrus is followed by 16 to 17 days of dioestrus, when the hormone progesterone is produced by the ovaries, and the mare will reject the stallion.

During winter, most mares will pass through a period of anoestrus or sexual inactivity. The mare is rather non- responsive to stallion advances. Day length is the primary factor in controlling this seasonal ovarian activity. Long day length stimulates ovarian cyclicity, so during periods of increasing day length – the spring – the mare will start to undergo a normal oestrous cycle. This will continue until the mare becomes pregnant or the day length starts to decrease, towards autumn, when the mare is likely to return to anoestrus.

Health of mare

For a brood mare to conceive, the general health of the mare should be considered. Older mares have poorer fertility, and in particular older maiden mares (mares that have not been bred from previously) present a challenge. It is important that particular attention is paid to the mare’s feet, as laminitis is a major concern, particularly late in pregnancy when the she is heavier.

Many mares have had an athletic career before being bred, and some will show signs of arthritis. Pelvic fractures are of concern, and if the mare has had history of this, the pelvic canal should be assessed per rectum to ascertain the likelihood of problems during parturition (birth).

The mare’s teeth should be checked, as it is important that she is able to eat an increased diet late in pregnancy and during lactation. It is sensible too to ensure that a mare is up to date with her equine influenza and tetanus vaccinations.

In addition to general health, aspects of the mare’s reproductive tract health should be considered. Endometritis (infection of the uterus) is the most common cause of infertility. Swabs taken from the mare’s clitoris and/or cervix can help identify pathogenic bacteria in these areas and can be treated, if necessary.

The vulva is the first ‘barrier’ between the outside world and the uterus. If the ‘vulval seal’ is poor – usually as a result of poor vulval conformation – then air (and bacteria) are likely to enter the vagina and challenge the cervix (the opening into the uterus) making endometritis more likely. In cases of poor vulval conformation, it is common to perform a Caslick’s operation, in which the proximal portion of the vulva is sutured closed, to prevent entry of air into the vagina.

When considering breeding from a mare, it is sensible to check the reproductive tract. A speculum is placed in the vagina to look at the cervix and assess any damage or abnormalities. A poorly functioning cervix carries a poor prognosis for breeding.

The uterus and ovaries can be palpated and examined by ultrasonography per rectum. This will help detect the presence ol fluid or cysts in the uterus and, by assessing the ovaries, determine through what phase of the cycle the mare is passing.

Al versus natural breeding

A number of options exist for breeding. The simplest is paddock mating. This is when a stallion and mare are turned out in the paddock, so that when the mare is receptive, the stallion will ‘cover’ her. This has the lowest breeding efficiency and highest risk of injury to mare or stallion.

‘Teasing’ is the term used to assess the mare’s stage in her oestrous cycle by presenting her to a stallion or ‘teaser’ (an entire male horse) to determine whether she is receptive or aggressive to the stallion. When the mare is receptive (in oestrus), she can be bred from every second day, until she becomes unreceptive (following ovulation and entering dioestrus).

This is termed supervised mating and is more time-consuming than paddock breeding, but does reduce the risk of injury caused by an unreceptive mare.

In most breeding programmes, the two options above are considered inefficient, as only small numbers of mares can be bred using one stallion. Because of this, vets are often used to assess the optimum time for breeding by assessing cervical tone (via a speculum per vagina) and ovarian and uterine cyclicity (by palpation and ultrasound scan per rectum), with a view to using controlled mating or artificial insemination (Al).

The cervix tends to become relaxed and pink during oestrus. The uterus forms a distinct pattern of oedema on ultrasound scan, and the ovaries produce at least one large follicle that reaches approximately 40mm and softens prior to ovulation. Ovulation is the process where the follicle ruptures and releases an egg that passes into the oviducts.

Oviducts are the small lubes that connect the ovary to the uterus, and fertilisation takes place in the oviducts if semen is present when the egg is released. If fertilisation is successful, an embryo will be formed which will enter the uterus about five days after ovulation and will grow in the uterus to form a foetus.

Two options exist for insemination of mares. Some breeding associations, notably for Thoroughbreds, only allow natural insemination. In this situation, controlled mating is used. This means that the mare is examined by a vet as necessary, until she is showing signs of oestrus and has a follicle that is likely to ovulate.

She will then ‘visit’ the stallion and be covered (Figure 1). It is common to check the mare for ovulation two days later, and if this has occurred, she will then be scanned for pregnancy at least 14 days after ovulation. The aim is that each mare will require only one ‘cover’ to become pregnant, thereby allowing the stallion lo have the ability to cover a large number of mares in one season (often well over 100!).

Figure 1: A controlled mating

Most breeding associations accept the use of artificial insemination (Al). This involves collecting semen from a stallion, transporting the semen to the mare and inseminating her at the correct time. The semen can be transported fresh, chilled or frozen. If the mare and stallion are close – on the same premises, for instance – it is possible to collect from the stallion and inseminate the mare immediately.

If the mare can be inseminated within 12 hours, a preservative – called a semen extender – should be added to the semen which is then transported fresh. If t
he transport of the semen is likely to take up to 48 hours, then the semen is chilled and transported at 5°C. If longer is required for transport or the semen is to be stored for long periods then the semen is frozen and preserved at -196°C.

 Timing of insemination for AI becomes more crucial depending on the type of semen used. Ovulation should occur within 48 hours using fresh semen, 24 hours using chilled material and within 12 hours using frozen semen. As the timing of between insemination and ovulation decreases, the number of examinations performed by the vet increase, in an effort to forecast ovulation more accurately.

Pregnancy diagnosis

Pregnancy diagnosis can be performed by ultrasound examination in as little as 14 days after ovulation. At this stage, the foetus is too small to visualise, but the pregnancy sac is approximately 12 to 18mm in size. The foetus and its heart beat can be visualised at approx 22 to 25 days.

From then, the foetus continues to grow to about 10mm at day 32. Foetal sexing is possible at day 62 to 67, and again at around 120 days, but requires a good quality ultrasound scanner and a lot of practice to become accurate.


There are several decisions and considerations to be made when attempting to breed from a mare.

The health of the mare, type of cover – natural versus controlled versus AI -and veterinary involvement must all be considered. It is a process that is fraught with potential disappointments at every stage, but is also uniquely satisfying, especially when there is a successful outcome (Figure 2). 

Figure 2: Breeding from a mare is uniquely satisfying, especially when there is a successful outcome


Ian Cameron


Ian Cameron graduated from the Royal (Dick! Veterinary College in Edinburgh in 1999. His first job was in mixed practice in Malpas, Cheshire, before he moved to Rossdale and Partners in Newmarket in 2001. He completed his Certificate in Stud Medicine in 2007 and became a partner in the practice in 2008. His primary responsibilities are stud medicine, young stock management and sales.

To cite this article use either

DOI: 10.1111/j.2045-0648.2010.00014.x or Veterinary Nursing Journal Vol 26 pp 53-55


Veterinary Nursing Journal • Vol 26 • February 2011 •