Breeding Information

This page gives you some information and advice about breeding from your mare including physiology, anatomy and details about artificial insemination.

For further information about equine artificial insemination at Paragon Veterinary Group please follow this link.

Reproductive Physiology

Mares are seasonal breeders. In the Northern hemisphere foals are born in Spring and in the Southern hemisphere foals are born in the Autumn.  Ovulation activity is affected by day length, so most mares stop ovulating in the winter months with the shortening day length


The mare’s oestrus cycle is on average 21 days long but can vary between 18 and 24 days in individuals. Oestrus lasts between 3 and 9 days (average 5 days) and ovulation takes place 24-48hours before the end of oestrus. The signs of oestrus in the mare are changes in the behaviour, frequent urination, raising of the tail and displaying the clitoris. Some mares show very obvious behavioural changes associated with oestrus (being in season) and other show very few signs especially if they have no stallion or gelding contact.




The duration of pregnancy in the mare is very variable, in most mares being between 315 and 360 days long. The average is 340 days in the thoroughbred mare (approx 11 months).

The foal heat is the first oestrus and usually occurs 5-12 days after foaling . It is usually associated with diarrhoea in the foal due to the hormonally mediated change in the milk .

Reproductive Anatomy of the horse


Pre-breeding Assessment of Mares

A general assessment of body condition and health is made. Then a more in-depth examination of the vulva is made for any conformational abnormalities. A speculum exam of the cervix is conducted and ultrasound examination of the uterus and ovaries, to assess reproductive health and the stage of the oestrus cycle the mare is in.

The results of these examinations allow an assessment of the mare’s potential to breed and any recommendations for reproductive treatment can be made.


Choosing a Stallion

The choice of stallion used should be based on the type and future use of the foal you are looking to produce. The temperament of the stallion is important as well as his performance, conformation, fertility and the quality of his offspring. Most large studs will be able to give this information and if the stud is local, it is a good idea to make an appointment to visit.

When you've decided which stallion to use, you then have to consider the options of natural service, chilled semen or frozen artificial insemination. If the stallion is abroad or has died, frozen semen may be the only option. At UK based large studs most offer natural service or chilled semen options. Individual stallion owners often only offer natural service. It is important to establish the basis of the package you are paying for; if it is a set number of inseminations or straws or if you are buying a pregnancy or live foal.

Disease Prevention

Most studs insist mares are swabbed and certified clear of contagious equine metritis (CEM) prior to being admitted to stud. This involves a visit from your Vet who will collect a clitoral swab from the mare. This must reach the laboratory within 48 hours so appointments are made Mondays to Thursdays only. The culture takes a week before the certificate can be issued so forward planning is required. Large studs also insist on a blood sample for Equine Viral Arteritis (EVA), this can be collected at the same time that the swab is carried out.

Preparing a mare for breeding

Mares should be in lean to good body condition. Overweight mares have reduced fertility. They should be fed to maintain body condition with a complete feed or with a mineral supplement. Make sure the mare's tetanus vaccination status is up to date and arrange a booster vaccination if she has not recently been done. Make sure the mares feet are trimmed, as foot soreness can reduce fertility. Note when your mare was last in season so she can be delivered to the stud or the AI centre at the optimum time. If your mare has not been wormed in the last month administer a wormer one week before delivery. 

Artificial Insemination

Artificial insemination is the injection of semen into the uterus other than by natural service by a stallion.


Processing and lifespan

When to inseminate


Raw 10 minutes

24 - 48 hours before ovulation


Extended 22C 

4 hours

24 - 36 hours before ovulation


Extended 5-10C

36 hours

12 - 18 hours before ovulation


Processed -196C

Long periods

6 - 8 hours before or up to 6 hours after ovulation


The use of both raw and fresh semen require the stallion to be kept at the same premises and insemination carried out immediately.

At Paragon we use both chilled and frozen semen. With chilled semen the semen is collected from the stallion at the time judged by the Vet to be optimum by repeated ultrasound examinations and the use of an ovulation induction agent. The semen is collected, extended and chilled and sent on an overnight carrier. It is inseminated through the cervix on arrival.

With frozen semen the semen is ordered and delivered in advance of the mare arriving. Using repeated ultrasound scans and an ovulation induction agent the optimum time for insemination is judged this must be within 8 hours of ovulation due to the short survival time of frozen semen. We use deep uterine insemination for frozen semen. Which means it is delivered close to the correct oviduct done into which the egg will pass.


The Stallion Stud’s Role in AI

The Stallion stud is responsible for disease surveillance in the stallions which includes swabbing and blood tests. Any imported semen should be accompanied by health certificates to show these tests have been carried out.

The stallion stud carries out the collection, storage, quality control checks and transport of semen. They also issue covering certificates once a mare is scanned in foal.


The Vet’s Role in AI

The Vet is responsible for assessing the suitability of the mare for A.I. Regular ultrasound examinations of the uterus and ovaries need to be conducted to accurately predict when ovulation will occur and the optimum time for insemination. The Vet carries out the insemination procedure although licensed technicians can perform routine cervical insemination, but are not permitted to carry out the ultrasound examinations or deep uterine insemination. The Vet will also examine the mare with ultrasound after insemination to ensure ovulation has occurred and to examine for the build up of fluid.

Pregnancy diagnosis is normally carried out at 15-16 days by ultrasound examination. This allows the identification and treatment of twinning.


18 day pregnancy on ultrasound scan


Advantages of AI

  • Increased stallion choice for mare owners
  • Decreased risks of injury to mare, stallion and handlers
  • Less travelling time for mares and foals
  • Stallion can cover more mares
  • Improved hygiene and reduced spread of infection?
  • Better conception rate in some problem mares
  • Semen quality control

Disadvantages of AI

  • Increased Veterinary intervention and increased cost
  • Reduced sperm survival time in mare
  • Disease risk in imported semen
  • More risk of error or fraud
  • Not permitted by some breed societies e.g. T/Bred
  • Distorts genetic base and financial structure of breeding industry?
  • Costs