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Equine Reproductive Service

‘Stud medicine’ is a veterinary subject area concerning the reproductive and medical management of mares, stallions and foals. In the U.K. the equine breeding season is in the spring and summer months as mares are ‘long-day breeders’ – this being when they have regular oestrous cycles or ‘seasons’ - and are able to be bred.  The thoroughbred breeding season is artificially brought forward to earlier in the year through managemental, hormonal and environmental manipulation.

A mare comes into season roughly every three weeks, with each oestrous period lasting about five to seven days. Ovulation occurs about 24 hours before the end of behavioural oestrous.  The mare will continue cycling until around October, when the ovaries become progressively less active, and eventually stop cycling altogether over the winter months – so called ‘anoestrous’.  As day length increases in the spring mares start to show erratic oestrous behaviour with prolonged periods of apparently being in season but without any ovulation occurring.  This ‘transitional’ stage continues until the hormones involved reach sufficient levels that ovulation eventually occurs, and the mare goes on to cycle on a regular basis.

Stallions are also affected by the seasons, for instance, sperm production reaches a maximum between May and July.

Artificial insemination involves the use of fresh, chilled or frozen semen. There are advantages and disadvantages to breeding mares using artificial insemination compared with using a ‘natural cover’.  From a veterinary perspective the main difference is that the timing of insemination must be a lot closer to the point of ovulation when using artificial techniques – particularly so for chilled and even more so for frozen semen.  This invariably requires the use of repeated physical and ultrasonographic examinations of the mare’s reproductive organs to get the timing ‘right’.

From an owner’s perspective there are some general issues such as the mare’s age and fertility, mare safety and the higher costs of A.I. which need to be considered. With reference to chilled semen, this technique generally produces higher conception rates and requires less intensive gynaecological work compared with using frozen semen Even so it must be inseminated within 48 hours of collection.  It is essential to find out what you are paying for when buying semen – e.g., how many doses of semen will you get and are there any additional transport costs?  Semen coming from abroad must come with the relevant paperwork. Semen transported without an official export health certificate from the country of origin cannot be legally inseminated into U.K. mares.  It is important to check that the stallion’s semen chills and travels well (as not all do) and find out which days of the week semen can be delivered and how much notice you will have to give in order for the semen to be collected and sent.

Frozen semen can be collected from stallions anywhere in the world and sent to the mare owner well in advance, avoiding potential late delivery. It is important that it is ordered from a reputable stud,  so ask for details of semen quality and post-thaw motility (some stallion’s semen does not freeze well).  Again, it is important to know what you are paying for and how many doses.  Semen handled badly during transport may not be viable. Appropriate export health certification accompanying the semen is also vital.

It is recommended all mares which are to be bred have a pre-breeding examination beforehand.  This involves a physical and ultrasonographic examination of the ovaries, uterus and cervix and testing the mare for venereally transmitted infections.  These tests may involve a blood sample for Equine Viral Arteritis (EVA) and Equine Infectious Anaemia (EIA) and always involve a clitoral swab for Contagious Equine Metritis (CEM) and two other bacteria causing venereal disease.  If any problems are identified at this stage they can potentially be dealt with before the breeding season starts.  It can take 7 -10 days to get the results so please allow enough time for this.  This examination is especially important for maiden mares, barren mares and older mares (13 years plus).  The autumn is a good time for investigating potential fertility problems in ‘barren’ mares, that is, those that have failed to conceive in the previous stud season.  Techniques used include uterine swabs and biopsies, ultrasonography, hormone testing and endoscopic examination of the uterus.

Pre-breeding assessment of stallions is also possible and can include a physical examination, observation of libido and mating ability and swabbing for venereal disease.

Catley Cross Veterinary Clinic offers packages for mares which are to be bred with artificial inseminationPackages are only available for mares boarding at the Clinic for their reproductive work. Catley Cross Veterinary Clinic is a BEVA Approved Centre for fresh, chilled, and frozen artificial insemination and our vets and nurses are familiar with the latest techniques and ideas in the field of equine reproduction.

Before entry into a packaged AI program a mare has to have a negative CEM (clitoral) swab and has to be confirmed she is cycling normally within a month of the planned breeding. This can be done at the same time as a pre-breeding examination and may require the use of a blood sample to measure progesterone levels.


All work is to be carried out at Catley Cross. Packages include all routine work, livery (stabled with turnout) and routine drugs relevant to breeding. Some mares are not suitable for a package such as those with a poor breeding record.  Treatment of ovarian or uterine problems or non-cycling mares is not included. 


  • Scans prior to insemination
  • Prostaglandin injections (if required) and ovulation hormone
  • Livery at Catley Cross (for no longer than 5 days)
  • Insemination
  • Liaison with stud
  • Semen evaluation
  • Post-insemination oxytocin injections
  • One routine post-insemination uterine flush (if required)
  • Post-insemination scans
  • Pregnancy scans at 15 days and 25 days after ovulation if carried out at the clinic


All prices are inclusive of VAT and correct of February 2023 AND INCLUDE UP TO FIVE DAYS STABLED LIVERY WITH TURNOUT

  Standard Price
Chilled price for first cycle £593.79
Chilled price for subsequent cycles £493.79
Frozen price for first cycle £730.92
Frozen price for subsequent cylcles £630.92

Pre-breeding exam

  Standard Price
Exam (including taking samples) £71.25
Lab fees per swab (approx.) £31.14
Lab fees per progesterone test (approx.) £22.20
Postage for samples (approx.) £13.08


Breeding work carried out at home premises

We are very happy to carry out stud work at the home premises, where the work will be charged out on an individual basis, as per our standard charges.

Pregnancy diagnosis and twin management

It is usual for a mare to have her first scan for pregnancy 15 or 16 days post-covering or insemination. This allows early detection of twins, which if left untreated, usually leads to the abortion of both foetuses. Treatment of twins at this early stage is possible and involves crushing one of the early embryos, leaving the other to go on and develop normally. This type of treatment is risky beyond day 18 after which both embryos are often fatally damaged during any attempted crushing.

In the normal situation a second ultrasound scan for pregnancy is essential at 25-30 days post-ovulation. This allows confirmation of a foetal heartbeat and re-checks the mare for the presence of twin embryos.

Other useful times to scan during pregnancy are at around 42 days to check general development and between 60 – 65 days to check the sex of the foetus. This is a specialist procedure and Craig is still gaining experience in this technique. The foetal sex scan is therefore being offered free of charge if done at the same time as other work on the premises. Scans to assess the well-being of the late-term foetus can also be carried out in the last month or two of pregnancy.