Breeding patterns in mares



As far as the stallion is concerned, he is ready to breed anytime or anywhere. The mare is different: she is classified as a seasonal breeder. She does not cycle all year long and does not accept the stallion when she is not in heat. In this article Robert N. Oglesby, DVM, explains how mares cycle.

Realize that what we see as breeding managers are the behavioral aspects of the reproductive cycle which vary tremendously from mare to mare. Underlying this behavior are physiological events that are much more consistent from mare to mare.

infologoNote that this article is written for the middle latitudes of the Northern Hemisphere. Reverse the seasons for the Southern Hemisphere. As you travel towards the equator, a mare’s first ovulation will have an earlier onset and as you go towards the poles, a later onset.


Winter Anestrus and the Transitional Phases

Most mares do not release eggs (ovulation) all year long. The average mare’s first ovulation of the year is in March and the last ovulation in October. Between October and March the mare’s reproductive organs shut down (anestrus) and most mares are not receptive to the stallion at this time.

It continues to be poorly understood as to the cause of the cessation of cycling in mares during the winter (Northern Hemisphere).

mare-foal-tb-stockThe study Absence of an association between melatonin and reproductive activity in mares during the non-breeding season (Fitzgerald and Schmidt, VI International Symposium on Equine Reproduction, Brazil, 1994 pp. 101-102) did not reveal a simple relationship between melatonin levels and estrous (cycling).

Interesting observations were:

  • that just because a mare continues to cycle one winter does not mean she will repeat the next winter.
  • a mare that foals in the winter is more likely to begin winter cycling than a mare that does not.

The beginning and end of the natural breeding season is capped by a transitional period of prolonged, mild receptivity to the stallion but with no ovulations. During this time eggs are developing on the ovaries but regress without being released.

Older (>18 yrs) mares usually require several extra weeks to the first ovulation, but recent research by S. Uni. of Ill. and Purina shows that when they are kept on a diet of 10 lbs Equine Senior(™ Purina Mills) they ovulate along with their younger companions.

Mares are noticably harder to settle early and late in the breeding season compared to the middle months. Increased fertility occurs about the third cycle of the season.


Spring and summer breeding patterns

A complete estral cycle is around 21 days long. It consists of

  • Estrus (receptivity to the stallion) lasting about 5 days
  • Ovulation (release of the egg from the ovary)
  • 24 more hours of receptivity following ovulation
  • Diestrus (not receptive to the stallion) lasting about 15 days

in-foal-mareIf the mare does not become pregnant she will cycle again. If she becomes pregnant she will not come back into heat until after giving birth.


Patterns of fertility and breeding strategy

Above it is stated that mares are receptive to the stallion for about 6 out of every 21 days. This is variable from mare to mare and even in the same mare during different times of the year. Towards the beginning and end of the breeding season mares are in heat longer.

Fairly constant is that mares go out of heat 24 to 48 hours after ovulation. This is very important to remember if you breed horses. The reason this is important is that fertility is markedly affected by the timing of stallion cover and ovulation. Sperm are not very long lived in the mare. They have a good chance of living 48 hours, but then their ability to fertilize an egg begins to drop off rapidly.

Look at the following pregnancy rates of mares covered at different times of the cycle:

  • Last covered 24 hours prior to ovulation: 67%
  • Last covered 48 hours prior to ovulation: 67%
  • Last covered 72 hours prior to ovulation: 50%
  • Last covered 96 hours prior to ovulation: 37%


Looking at the above numbers indicates that mare should be bred 48 hours before ovulation. But since the length of estrus varies from mare to mare this time can be hard to predict by just looking at her behavior. To get maximal conception rates you should start breeding a mare on the second or third day of receptivity to the stallion and then every other day until she goes out of heat.


Control of the estrus cycle

Day length is what drives a mare’s seasonal patterns of breeding. During periods of longer day length the mare cycles. You can keep a mare cycling by putting her under lights at night during the fall and winter. The amount of light recommended is 200 watts incandescent light per 12 ft sq. stall. Oddly, the light is more effective if added at the end of the day rather than the beginning. It is recommended you turn them on an hour before dark and leave them on till 11pm.

If a mare is allowed to enter anestrus it will take about 60 days of light to get her back ovulating.

This article reprinted with permission from, an internet information resource for the equestrian and horse industry since 1994. It has tens of thousands of documents on the web about horse care, diseases, and training.

First published on in 2005.

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2 thoughts on “Breeding patterns in mares

  • November 5, 2017 at 7:49 am

    I am trying to see if a mare can become pregnant a second time without any contact with a stallion .
    I was told it’s not possible but our mare looks pregnant again. She had a baby before we had know idea she was even pregnant because we have a lot of mares and able houses but all the mare hoers are fixed and we have stallions.

  • July 9, 2021 at 5:34 am

    will a mare go out of heat once bred


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