Breeding for early foals

Most mares don’t naturally experience heat cycles (oestrus) during the winter and early spring months, although some mares will cycle all year round. In winter, a mare’s reproductive system normally goes into a rest period, known as anoestrus.

After this winter inactivity, the cycle usually begins again in spring, in response to the increase in daylight hours. The mare has a “transitional phase” when the length is variable before she starts to cycle regularly. Once the mare is in the breeding season, her oestrus cycles last about 22 days.

The natural breeding season of the horse is from May until August, which is when highest pregnancy rates are likely to occur. However breeders have been trying to put mares in-foal in the winter and early spring man since the early 19th century, when 1 January was declared the official date of birth for Thoroughbred foals. This means a foal born on 1 February and one born on 1 August are both considered one year old on the following 1 January, despite a six months’ difference in true age.

Experience has shown that 16 hours of light stimulus (artificial and natural) is adequate to encourage early oestrus. 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 200watt 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.

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 also 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 every day

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.

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