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Reproductive Health

Understanding the basic physiology and anatomy of the brood mare will provide owners with information that is vital for successfully breeding healthy foals.


The mare has two ovaries that lie in the dorsal (upper) part of the abdomen, behind the kidneys. The ovaries are the female gonads and produce and release eggs. They are also the site of production of the female sex hormones oestrogen and progesterone. The uterus is made up of two horns and a body, all being suspended from the roof of the abdomen by the broad ligament. The cervix guards the entrance into the uterus from the vagina.

The Oestrus Cycle

Mares are ‘long day’ breeders and in the northern hemisphere generally, are most sexually receptive from April through to September. During winter the mares become sexually inactive and are in a state of anoestrus. As spring approaches and day length increases a gland within the brain, the pineal gland, produces less of a substance called melatonin and the mare enters a transitional oestrus period. In response to rising hormone levels follicles are stimulated to grow simultaneously within the ovaries. Initially lots of follicles grow simultaneously until one becomes dominant and ovulates. After this, regular oestrus cycles will occur through 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. The mare will also be receptive to the stallion, allowing him to nuzzle her, while she squats and urinates and winks her clitoris.

These behavioural changes are 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. The follicle that has ovulated forms a structure called the corpus luteum within the ovary,which produces another hormone called progesterone. Progesterone, in simple terms, has the opposite effect to oestrogen.The mare will now be out of season and aggressive towards the stallion’s advances. At around 16-17 days the lining of the uterus produces a hormone called prostaglandin destroying the corpus luteum. This reduction in progesterone allows follicles to develop again and the mare to return to season. If the mare is pregnant the corpus luteum remains allowing pregnancy to continue suppressing further returns to cycle.

Preparing your mare for stud

There are a number of things that need to be considered before sending your mare away to stud. For more detailed information please contact the surgery.

Breeding Soundness Examination of the mare

Prior to sending your mare to stud it is adviseable to have her examined by your vet to ensure that she is fit to breed. To perform this safely stocks may be required ( we have stocks at the practice and many clients bring the mare to us) although QUIET,WELL BEHAVED mares can be assessed over a stable door.

The breeding soundness examination will include:

  • a physical examination
  • a rectal examination
  • ultrasound scan
  • reproductive examination

At this time we will also be able to perform clitoral swabs and blood tests required by the stud/stallion owner to confirm that your mare is free of venereal disease.


please ensure you allow plenty of time for this before arranging to take your mare to stud.

Venereal Disease

  • the control of venereal disease is a very important aspect of stud/breeding management. ALL visiting mares should be swabbed (preferably before entering the stud premises but in any case before mating is allowed).
  • a stud/stallion owner that does not insist on the routine precautions of swabbing carries a major risk for the mares that visit.
  • all stallions should be swabbed prior to the start of the breeding season

CEM – contagious equine metritis

CEM is one of the major venereal pathogens that we swab breeding equines for. It is a highly contagious venereal disorder that is transmitted between mares by the teaser or stallion.

  • Transmission can also occur through the use of infected semen for artificial insemination.
  • Carrier mares and stallions may show no signs but remain infectious.
  • Infected stallions and teasers are most often passive carriers with no clinical symptoms of the disease.
  • In mares infertility is the most obvious sign.
  • In mares severity varies from mild/unapparent to severe/obvious and signs include purulent vaginal discharges appearing 2-4 days after mating and an early return to season 7-10 days after mating.
  • Treatment of mares affected by CEM does not appear to be effective;usually the acute symptoms are self limiting however there remains the risk of symptomless carriers.
  • Treatment of infected stallions is difficult involving antibacterial washes and antibiotic therapy.

EVA – equine viral arteritis

This disease causes abortion,infertility,upper respiratory tract disease and arteritis.

  • most studs/stallion owners require that your mare be blood tested for EVA prior to arrival
  • a rectal examination
  • ensure that your chosen stallion has been tested negative for EVA at prior to the start of the breeding season
  • vaccines are available and are effective in preventing the disease but they do not result in the cure of an infected carrier stallion. Care must also be taken when interpreting blood results if the animals have been vaccinated.

Please contact our equine team if you have any queries regarding swabs and blood tests and we will be happy to advise you further.



HERPES VACCINATION – herpes is the commonest cause of contagious abortion in the UK and a significant cause of late abortions and stillbirths. We strongly advise that all mares are vaccinated against herpes virus at 5, 7 and 9 months of pregnancy.

TETANUS – as foals gain initial immunity from the mare’s colostrum it is adviseable that this immunity be boosted by vaccinating the mare in the last few months of pregnancy.

INFLUENZA – recommended that pregnant mares are given a booster 3-5 weeks before foaling. This can be combined with the tetanus booster.

ROTAVIRUS – again the pregnant mare is vaccinated in order to provide natural immunity to her foal. Rotavirus is a cause of contagious viral diarrhoea in foals. The vaccine is mainly used on yards where there has been previous problems with rotavirus.




If you require any further information on either a breeding soundness examination or advice on appropriate considerations for your mare please contact our Equine team.



Mares can be “covered” in one of two ways. The first and obvious being when they are naturally covered. The second is where semen is collected from a stallion and at some later date placed into the uterus of the mare – AI or artificial insemination.

Natural covering – mares are often covered naturally especially in the thoroughbred industry. The stallion is either free to run with the mares or he is led out to cover the mare every day/every other day whilst she is in season.

AI – the use of AI is becoming more and more popular,allowing the use of stallions from abroad as well as at home. Artificial insemination includes the use of:

  • fresh semen
  • chilled semen
  • frozen semen

Most breeding programmes that use AI incorporate the use of certain drugs to facilitate ovulation and repeated ultrasound examinations of the reproductive tract to time ovulation. Timing of ovulation and insertion of semen is crucial when using AI.

To breed mares successfully with shipped, cooled semen, and to minimize disappointment and financial losses , all parties involved need to cooperate in order to to coordinate the semen shipping with the timing of the mare’s ovulation.

Management of mares to be bred by AI is intensive and if you are considering this contact us to discuss this in more detail and to advise on the method most appropriate for your mare.

Animed Veterinary Group is part of CVS (UK) Limited, a company which owns over 300 veterinary practices in the UK. Company Registration Number 03777473. Registered Office CVS House, Owen Road, Diss, Norfolk, IP22 4ER