Horse owners and breeders should take into account that when it comes to breeding, stallions and mares are different. This article explains that while stallions are ready to breed on command, mares have breeding seasons and will not be interested in the stallion at all if she is not in heat. The reproductive cycle of each mare is different, but there are physiological events that allow the breeders to determine if the mare is ready to breed. Further experiments can be conducted to study this more using  stereoscopic microscopes.
Expert breeders have identified through studies that mares ovulate at least twice a year, in March and in October. The mare’s reproductive organs tend to be inactive in between these months, or a period that can be described as anestrus. Breeding should not be attempted during between October and March.
Horse breeders in the Northern Hemisphere are at a loss to explain the possible cause of the cessation of breeding cycles in mares especially during the winter season. This article states that their study aided by various microscopes like stereoscopic microscopes did not find a correlation between melatonin levels and estrus or the regular period of sexual excitement in many female mammals during which the animal seeks to mate. Although there have been significant observations that may explain these occurrences.
There is a transitional period that marks the beginning and the end of the breeding cycle. Along this time, eggs are being developed for ovulation and may regress as it is failed to be released.
According to the article, a full estral cycle is 21 days in duration. There are phases that culminate this cycle. First, the estrus or the mare’s interest in the stallion, which lasts for about 5 days. Secondly, the ovulation, when the egg is released from the ovary. There are 24 more hours of receptivity (estrus) after ovulation, and then the diestrus phase, where the mare is not receptive to the stallion. This period lasts about 15 days. Bear in mind that the phases are variable depending on the mare. Although towards the beginning and end of the cycle, mares generally are in heat for a longer time. Also, constant factors that breeders should take note of is that mares go out of heat 24 to 48 hours following ovulation. Therefore, the timing of fertilization is significant. Sperm do not live very long in the mare and may last only up to 48 hours, after which their ability to fertilize the egg declines. It is then recommended that the mare be bred 48 hours before ovulation for the best chances of successful foaling. For maximum chances of conception, it is suggestes that the mare is bred on her third day of receptivity to the stallion and until she goes out of heat, every other day after that. View how their sperm and egg cells look like using microscopes such as stereoscopic microscopes.
If horse breeders are successful in making the mare pregnant, she will not cycle again until after she gives birth. If she is not with foal, then the cycle will repeat itself.
The estrus cycle can be controlled by placing lights in the mares stable. The amount of light or day length is what encourages seasonal cycles of breeding. It is recommended that breeders should read up on more text regarding the prolongation of estrus and to consult breeding experts.

ORIGINAL TEXT:
Introduction
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 we will explain 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.
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). This study 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.
[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.]
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 (TM 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
1. Estrus (receptivity to the stallion) lasting approximately 5 days
2. Ovulation (release of the egg from the ovary)
3. 24 more hours of receptivity following ovulation
4. Diestrus (not receptive to the stallion) lasting approximately 15 days.
If 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 approximately 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.
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