Choosing the right breeding season

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You are here: Home / Articles / Horse care

Jonathan Pycock MRCVS

27 October, 2000

Mares do not naturally experience heat cycles (oestrus) during the winter and early spring months, although some mares will have a cycle all year round. In winter, a mare's reproductive system goes into a rest period, known as anoestrus.

After this winter inactivity, the cycle usually begins again in spring. The mare does not suddenly begin to cycle regularly, but has a "transitional phase" when the length is variable. Once the mare is truly in the breeding season, her oestrus cycles last about 22 days. When nature has its own way, the stimulus to "switch on" the mare to her cycle depends on the amount of daylight - the most important influence on mare reproductive performance. It is said that a mare is at her most fertile on Midsummer Day.

This effect of season on reproduction results in foals being born in the spring and early summer - when environmental conditions should be best for foal survival. Therefore, the natural breeding season of the horse is from May until August - and this is when highest pregnancy rates are likely to occur.

Birth dates

Unfortunatly, man has superimposed his ideas of when mares should foal on this natural pattern. Since the early 19th century, when 1 January was declaredthe official date of birth for Thoroughbred foals, irrespective of their actual birth dates, breeders have had problems attempting to put mares in-foal in the winter and early spring.

For some breeds, notably the Thoroughbred, an operational breeding season exists - 15 February until the first week of July. This "man-made" season means that many breeders try to get mares in-foal from the middle of February onwards.

This means that a foal born on 1 February and one born on

August would both be considered one year old on the following

January, despite a six months' difference in true age.

Cycle manipulation

Light can bring the mare to cycle earlier. Putting a mare "under lights" is popular among breeders and means providing extra light at the end of the day. It is also important that the mares are in good condition, brought in and well fed. Hormonal drugs are also prescribed.

Experience has shown that 16 hours of light stimulus (artificial and natural) is adequate. This means providing light from 7am until 11pm. The extra light is best added at the end of the natural daylight period and must be bright enough - a 200 watt clear bulb in the middle of a 4m x 4m box is best. A strip light should be 1.30m in length (40 watt). Take care to eliminate shadows, as they can prevent a good response.

Two important points are:

Extra lighting must be started early enough. Generally, a minimum of eight-10 weeks is needed and mares should be exposed to the lighting system by December. This should mean that they will begin to cycle normally by 15 February. You cannot decide to put a mare under lights and expect her to begin her cycle a week later. Forward planning is essential.
Mares should be in good body condition before they are given extra lighting, which may mean extra feeding.

Sometimes, vets may prescribe hormonal drugs (progesterone), which affect the mare's cycle. Mares coming to the end of a two-month artificial light programme may be given 10-15 days' treatment to give them an extra "boost" to begin their cycle. For it to work properly, it is important that the hormonal drugs are given in their feed every day. Most horseseat the drug with no problem.

Health checks

As well as making your mare cycle early, it is important to make sure that her reproductive tract is healthy. This is particularly important if she failed to become pregnant the previous year or lost her foal during pregnancy.

Ideally, such mares should be investigated in the autumn to allow time for any abnormalities detected to be treated.Even if you are putting your mare in-foal for the first time, it is a good idea to ask your vet to perform a thorough gynaecological check - known as the breeding soundness examination. The aims are to detect any cyclical abnormalities, physical defects or signs of inflammation and to assess the likelihood of a mare being able to conceive, carry a foetus through pregnancy and deliver a healthy foal.

Suitability for breeding is an assessment of the gynaecological health of that particular mare, not whether or not the horse is of sufficient quality or has a suitable temperament to breed.This involves an internal examination of the mare and it is important that you understand what your vet is doing. By a rectal (internal) examination, the ovaries, uterus and cervix are checked. Usually, the vet will use an ultrasound machine internally as well. The vagina and cervix will also be checked by looking at them through a speculum (a narrow tube down which the vet can shine a light) inserted into the vagina.

Avoiding infection

Infection is a risk in breedingand swabs taken as health precautions are an important part of the procedure. Two parts of the reproductive tract are commonly swabbed: the clitoris and the uterus. A swab is a small piece of sterile cotton wool twisted on to the end of a thin plastic rod. The cotton wool touches the part of the reproductive tract you want to check for bacteria, then the swab is sent to a laboratory for culture. The clitoris is swabbed to look for bacteria which are venereally transmitted during breeding.The Horserace Betting Levy Board publishes a code of conduct giving guidelines for the control of venereal diseases. It is updated annually and should be read by all owners planning to breed from their mare.

(Contact: HRBLB, 52 Grosvenor Gardens, London SW1W 0AU)The clitoral swab does not have to be taken when the mare is on heat, but it takes several days for the results to come through. A mare should not be covered until she is clear, in case she infects the stallion.

In many cases, a swab of the lining of the uterus will also be taken to see if there are any bacteria present or signs of inflammation, known as endometritis. Mares should be free from endometritis before they are covered or artificially inseminated.

Uterine biopsyA small piece of the lining of the mare's uterus may be taken for examination under a microscope - called a biopsy. The technique involves passing an instrument through the cervix and into the uterus. When the tissue is looked at under a microscope, certain features dictate the likelihood of the mare becoming regnant and giving birth.

This information helps owners decide whether to try to get the mare pregnant. Mares with a lot of scar tissue (fibrosis) in their uterus have a poor chance of getting in-foal.

Fertility expectations

Unlike other domestic animals, horses have never been selected for fertility. The criteria is whether they can gallop fast, jump well or look good. Unfortunately, these horses arenot necessarily the most fertile. How the mares are managed at stud has an enormous influence on pregnancy rates.

Pregnancy rates at any one heat vary from 35-70%. The overall pregnancy rates at the end of the season vary between 50-90%, with an average of about 70%.

Some mares lose the pregnancy through abortion and the live foal rate (the number of live foals born the following year) is less than the pregnancy rate. The current live foal rate in the UK is around 65%. This means that only two-thirds of mares which are covered will have a live foal the following year.