Provided mares have adequate feed and are in reasonable bodily condition, they have a chance to commence their breeding cycles with the onset of spring weather.

The breeding season

In the winter, many mares have a period of very low sexual activity, usually known as winter anoestrous. In some mares this is quite short and can even be non-existent in the mare that displays oestrous cycles all the winter.

However in the late winter and early spring, the lengthening of daylight helps portions of the brain to stimulate the production of hormones necessary for the functioning of the breeding cycle. The first or initial cycle each year can be quite irregular, may not produce fertile follicles, and may extend for up to 5-6 weeks before the mare finally goes off. The following or next heat period tends to be more regular and often proves more fertile as far as the production of follicles is concerned.

The normal breeding cycle

In order to understand abnormal or problem mares, it is first necessary to understand the normal mare’s behaviour in a breeding cycle.

A normal breeding cycle is comprised of segments of different responses, which normally cover an 18-23 day cycle. At the commencement of the season, mares are "teased" by means of a male animal, usually a pony stallion. Early in the season, August and September, mares may be slow to display signs of being in season.

Signs of heat

When being approached by the teaser, mares in season usually display one or more of the following signs:

  • They may squeal and half offer to kick, but don’t vigorously carry through
  • They may pass small quantities of urine as they adopt a straddle leg position with their hind legs, at the same time carrying their tail higher than normal
  • They may be observed to “wink” with the lower folds of the vulva, which usually covers the clitoris
  • The colour of the mucous membrane of the lip of the breeding tract, the ‘vulval lip’, is usually pale pink in early oestrous and gradually becomes more puce coloured as the heat period intensifies.

As the mare reaches the peak of the heat display period, she will often “show” continuously and even be difficult to drive away from the location of the teaser or stallion. Unfortunately, there is not a good relationship between the intensity of display and the likelihood of the mare producing a suitable fertile follicle for conception. This is particularly true in September and October when mares often have long heat cycles and may not produce suitable follicles for mating.


Mares at some stage of the “display” or heat” may ovulate. This can be quite irregular early in the season but once it occurs, the mares cease to display oestrus within 24-48 hours of ovulation and become quite sour. When approached by the teaser, they will squeal, lay their ears back and kick vigorously. Both stallion and teaser can be injured if they incautiously approach the mare in this stage. This period is known as ‘dioestrus’ and usually lasts 10-16 days, when the normal mare, if not pregnant, will again start to display oestrus behaviour.

The normal mare usually has a follicle or follicles developing when the first signs of being in season occur. In many mares, this reaches a peak in 3 to 7 days with the follicle becoming mature. The maturation of the follicle leads to ovulation. Ovulation is the final step in which the fluid filled structure (the follicle) containing the mature egg, gradually discharges the fluid and the egg from the ovary down into the fallopian tubes. It is here that fertilisation occurs.

Service and conception

It is preferable that service has been given just prior to this release or ovulation. Sperm from highly fertile stallions can last up to 4 –5 days in the breeding tract of the mare, but the released egg must be fertilised in 3 - 4 hours after release otherwise conception does not occur. The mare may show an increased heat display at this period, which often extends 12 hours after ovulation has occurred. This can be confusing to inexperienced owners who see a mare extremely well in season, but which, on veterinary examination, has already ovulated. The mare readily accepts service by the stallion even though ovulation has occurred; this of course results in a much lower, or even zero conception rate, once ovulation has occurred.

The mare then goes ‘off’, or out of season. This is reasonably typical of the behaviour pattern of normal cycling mares.

Factors that upset normal breeding cycles

Factors that upset these normal cycles are poor nutrition, long hair coat, adverse weather conditions, and injudicious use of hormones while racing followed by too short a time for let-down before the breeding season commences.

Long hair coat indicates the existence of the winter anoestrus-type pattern. These mares will often cycle without follicles or with very poor follicles. Cold, overcast weather usually decreases oestrus display.

Early in the season, the two principal hormones produced by the mare to control the oestrus cycle are often unbalanced with the hormone responsible for follicle production being produced in excess, while the second hormone that is required for normal development and ovulation of the follicle is under produced. It is this factor that prolongs the initial heat periods and also causes the low ovulation percentage, frequently as low as 40% in September and October mares. Basically, this means that only four mares in every ten actually ovulate naturally, hence a high number of returns in these mares if they are not treated with additional hormone to assist ovulation in these months.

Treatment of low ovulation rates

This low ovulation rate, particularly in dry mares, has to be treated by the use of lutenising hormone, which assists the normal process of maturing of the follicle. This hormone is given only when a follicle larger than 35mm is present in the ovary. If the hormone is given for smaller follicles, its action is irregular or it may work very poorly and give undesirable results. This hormone is quite powerful and must be used carefully, and only when necessary. Overuse and indiscriminate use can lead to lower ovulation — the exact opposite to that for which it is intended.