Mare Reproductive Information
The Mare Reproductive Cycle
Most mares will not cycle all year round, but instead cycle mainly in spring/ summer. As day length begins to increase in late winter/ early spring this begins to stimulate the production of hormones which allows them to cycle. Therefore for most horses in the southern hemisphere, the breeding season runs from the beginning of September until the end of February.
Mares will cycle approximately every 21 days during the breeding season. Of these 21 days there are 5 days when the mare is “in season”. A mare is in season when they are receptive to be bred. Ovulation occurs towards the end of this period, usually on day 5. Most mares will display certain behavioural signs when they are in season – these include showing interest in the stallion (or other geldings!), “winking” of the clitoris, passing small amounts of urine etc. It is important to remember that some mares show no behavioural signs of being “in season”. The veterinarian can also tell if a mare is in season by the appearance of the uterus and cervix. When inseminating a mare it is important that insemination occurs as close to ovulation as possible.
When examining your mare to determine the best time for insemination the veterinarian will use ultrasound to examine each ovary, as well as the uterus. The ovaries are examined for follicles, which are fluid filled structures that each contain an ovum (egg). These usually grow as the mare is in season, finally ovulating at the end of five days, by which time they are usually 40-45mm in diameter. Most cycles only result in one, or occasionally two follicles ovulating. The uterus will typically show characteristic changes as the cycle progresses.
Pregnancy
A horse’s gestation period is approximately 341 days (range 305 – 365+ days). The first pregnancy test is routinely carried out at 14-16 days, with follow up scans at 26 days and 42-45 days.
It is important during the first pregnancy test that the mare is checked for twins, particularly if we know that there was more than one ovulation. Mares are not designed to carry twins. Even if they do survive until birth (most do not), they will be weak and underdeveloped. The reason for this is thought to be that the equine placenta cannot adequately provide for more than one foetus. It is much easier to deal with twins if the first pregnancy test is performed early, at the recommended time. Usually one can be pinched off, or sometimes left to resorb depending on the circumstances.
Udder development usually begins to occur approximately four weeks prior to foaling. In the last few days prior to foaling the mare will usually “wax up”. Milk test kits are available to help predict the time of foaling more accurately.