Introduction
With some minor breed differences, most mares have similar reproductive physiology and, from that respect, may be expected
to respond similarly to a canned breeding management protocol. When designing breeding management protocols for a particular
mare or stud farm, you must take into account many external factors. These should include the type of semen or breeding used
(live cover, fresh AI, chilled/shipped AI, or frozen), when the stallion (or semen) is available, if the mare is to hold the
pregnancy or be an embryo donor, if the mare is to be bred on foal heat, your availability to that farm (Do you visit every
day? Do you work weekends?), and perhaps other factors unique to that particular farm. All of these variables will influence
your decisions about when to schedule follow-up examinations, when to give certain drugs, when to inseminate, etc. This lecture
will focus mostly on the normal physiological changes in the mare reproductive cycle and how we can manipulate the cycle.
It is up to you to figure out how to effectively use those tools to manipulate the cycle around whatever unique parameters
surround your particular farms. No matter what protocols you develop, the end goal is the same: foals on the ground the
following year.
Estrous behavior
The earliest form of breeding management in horses was simply to tease the mare to the stallion and allow them to breed periodically
while the mare showed signs of being in heat. Even with the technical advances of our day, including ultrasound and hormonal
manipulation, the mare's natural reproductive behaviors still play an important role in determining if and when to breed her.
Ultrasound allows us to monitor uterine edema, a very useful tool in monitoring the mare cycle, but inflammation can also
cause edema. Follicles generally reach a certain size before they ovulate, but diestral follicles can reach those large sizes
and then regress instead of ovulating. The cervix generally softens during estrus, but maiden mares will often have a tight
cervix throughout their cycle. With all of these variables, it is often useful to be able to take a mare sending mixed signals
through our routine reproductive examination and expose her to a stallion. Whether she is showing behavioral signs of estrus
("teasing in") or not ("teasing out") can be the deciding factor as to how you should proceed with her protocol.
Behavioral signs of estrus in the mare classically include raising the tail, squatting, everting the clitoris ("winking"),
and urinating. The ears are alert and generally forward. The mare may squeal, playfully kick out (not to strike), or turn
to push her hind end towards the stallion. A mare in diestrus will generally clamp her tail down tight over her vulva, keep
her ears pinned back, squeal, and kick out to strike. The mare will be reluctant to approach the stallion.
The most important hormone regulating estrous behavior in the mare is progesterone. Progesterone is produced by the corpus
luteum and exerts a strong inhibitory effect on estrous behavior, making diestrous mares strongly averse to the advances of
the stallion. Estrogens (high during estrus, baseline during diestrus) do enhance estrous behavior, but are often not necessary
for the mare to show signs of estrus. When the corpus luteum regresses at the end of diestrus and becomes inactive, no longer
secreting progesterone, the mere absence of the inhibitory effects of that hormone are often enough to cause the mare to become
receptive to the stallion. This is evidenced by the behavior of some ovariectomized mares, who will continually show signs
of estrus, despite the absence of both progesterone and estrogens. Not all mares are this sensitive to the effects of progesterone,
and some do require supplementary estrogens to show signs of being in heat. Another clinical application of these hormonal
interactions is that mares will usually tease to the stallion for at least 24 hours after ovulation. Estrogen drops precipitously
up to 24 hours prior to ovulation, but progesterone does not climb to levels significant enough to suppress estrous behavior
for 1-2 days post ovulation. Just because a mare is still teasing to the stallion does not, therefore, mean that she still
has a dominant follicle, and since the oocyte is only fertile for about 6-8 hours post-ovulation, it does not mean the mare
is still in the fertile part of her cycle.
In a program where the mares are teased regularly to a stallion, the number of days that a mare has been teasing can be helpful
information. Early in the breeding season, estrus may last up to 7 days. At the height of the breeding season, the estrus
period shortens considerably and may only last a few days.
