By Robert N. Oglesby, D.V.M.


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 11 pm.

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

Note 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.