The extrinsic factors in mare reproduction are those that can be manipulated.
They can be influenced from a management perspective. "Sorting out how important each of those factors is in an individual
mare and her case of embryonic loss is not that easy," Barry Ball, DVM, Ph.D, University of California, Davis, suggests.
Foal heat and lactationA number of studies indicate a significantly reduced fertility, increased embryonic and early fetal loss of foaling mares
bred on the first postpartum estrus (foal heat) compared with foaling mares bred at later estrous periods.
Feces in vagina
Higher incidence of embryonic and early fetal loss has also been seen in lactating versus non-lactating mares.
"There have been suggestions for a long time that mares bred in foal heat were a lot less fertile and also had a higher incidence
of embryo loss," Ball explains. "That's not clear from every study. Some say that it is, and others suggest that there is
no effect on embryo loss. To me, there is consensus that there is higher embryo loss associated with foal-heat breeding.
It might be related to the health of the uterine environment in the immediate post-partum mare. The possibility is that things
are not clear in the uterus prior to breeding."
StressMaternal stress has been proposed as a factor in embryonic or early fetal loss in mares.
The role of maternal stress and its possible relationship to progesterone concentrations and embryonic loss in mares requires
Stress is thought to decrease progesterone secretion, which terminates pregnancy. Stressors include excessive heat, van transport,
pain, social situations and violent weather. In regard to van transport, work done at Colorado State University has not shown
any detrimental effect of routine transportation of mares during early pregnancy.
NutritionA poor plane of nutrition, especially lower protein and energy levels, has been shown to be linked to embryonic loss in mares.
It was shown that mares on poor quality pasture had more embryonic losses between days 25 and 31 than did mares that received
a supplement or better quality grazing.
There are several nutrients, including vitamins A and E that are lower under such circumstances. Mares with poor body condition
due to poor nutrition had lower pregnancy rates at 30 days, and had more pregnancy losses noted between days 30 and 90.
In another study, 90-day pregnancy rates were lower in mares that were in thin body condition at the start of the breeding
season than in mares in better body condition. Mares that were not in good body condition at the time of foaling had reduced
fertility, and suggested a higher embryo loss rate.
After Caslick's procedure
Prevention, management of early pregnancy loss
Some management practices can help prevent early pregnancy loss.
Ensuring good nutrition, reducing stress, selection of good sires and testing and careful breeding practices can limit or
alleviate extrinsic factors.
Good quality pasture, hay and supplemental nutrition will help keep breeding mares in good body condition.
Moderation of heat stress and reduction of physical stress during mare transport may assist to reduce embryonic loss from
stress factors. Cleanliness of the mare during and after breeding may help to reduce post-breeding endometritis.
Other stepsOther steps can be used to alleviate embryonic loss.
Oocyte transfer to bypass the oviduct may assist mares when primary embryonic defects are not the cause.
Artificial insemination may reduce the problems of uterine contamination of natural breeding and resultant endometritis.
Antibiotics may also assist to reduce possible uterine infection. Exogenous progesterone, though commonly used, may not be
that effective in preventing pregnancy loss in every case, but may be beneficial in certain situations. Careful monitoring
of early pregnancy may be especially helpful, so that if early pregnancy loss occurs, it is possible to rebreed the mare.
Mare managementFertility will be better if the mare population is managed carefully.
"I think if you take a random group of mares and don't carefully manage them, that's when you come up with these 70 percent
conception rates, and 55 percent foaling rates," suggests Bud Hallowell, DVM, of Auburn, Wash.
Good breeding practices, good husbandry, and good management can lend some control of the mares. It is good to keep recently
bred mares together in smaller groups, so they are more compatible; manage the mares, so that they are happy and well nourished.
With good management there is not this 'war' going on so that the mares are not stressed socially. "When management begins
to take care of the situation, you generally see the conception rate and foaling rate improve," Hallowell explains. "I would
see 90-95 percent conception rates and hopefully 80-85 percent foaling rates."
In addition to mare socialization and stress reduction, "We did begin to look askance a little more at mares that had poor
breeding histories, but basically what we did was just change the environment that a given group of mares were living under.
They became happier, more content, less stressed, and had better nutrition - things we know are important things in regard
to better management practices."
On a big farm where there are a lot of mares coming and going, where they are mixing the horses together, taking some out,
putting new ones in, those situations can produce mare stress. If that situation is eliminated, the fertility of the mares
may change. "That's basically why I say that normal fertility of mares isn't that bad, it's just that sometimes we just manage
them poorly," Hallowell concludes. "Mares are pretty fertile as long as they've got plenty of feed, the pressure is not too
great on them, or they are not a very old group of mares."
Mare veterinary care proceduresFrom the practitioner's perspective, much of the occurrence of early embryonic loss prior to 40 days depends on the mare.
The primary candidates are older mares and mares that have had problems, whether foaling problems, or those with endometritis
early in their season. This also is probable cause for increased EEL losses seen in lactating mares and those bred during
"The older mares have several things going against them," says Pete Sheerin, DVM, Rood & Riddle, of Lexington, Ky.
"I don't think there's a lot of EEL," says Sheerin, "though there is a certain percentage of it in the field, probably 5-10
pecent of mares bred, which is obviously more than we want to see."
With mares bred in the natural setting, with actual stallion cover, the contamination of the mare's caudal reproductive tract
from natural service is potentially greater than the controlled environment of an artificial insemination program with use
of sterile sleeves, sterile pipettes, etc. With this potential for increased contamination, "We use a lot of post-breeding
infusions," says Stuart Brown, DVM, of Hagyard-Davidson-McGee in Lexington Ky., "and then maximize our efforts with the Caslick's
Though every mare does not have to be treated in this way, there is a certain population of mares, approximately 15 to 20
percent Brown estimates, that experiences delayed uterine clearance, being unable to clear their uterus of normal levels of
contamination post-foaling and post-breeding. It's hard to predict which mares are likely to be susceptible. Of this group
there are a certain number of mares bred that previously had a difficult foaling or had some type of contamination post-foaling,
leading to a less than ideal uterine environment. There is then inflammation in the reproductive tract that detracts from
their ability to maintain normal uterine defense mechanisms.
They may easily pick up infections due to streptococcus or coliform bacteria.
Post-breeding infusions help to keep that contamination to a lower level, preventing the bacteria from establishing an infection.
Post-breeding infusion is often done with antibiotics, such as Timentin® (ticarcillin disodium/clavulanate potassium) or a
pH-balanced solution of penicillin and genticin.
This combination is broad spectrum, economical and effective to combat the two most common organisms of concern. Timentin®
and some of the other stronger antibiotics are used for more persistent problems and for mares that have known histories related
to endometritis. It may be debatable, according to academicians, as to the necessity of using post-breeding infusions, but
in practice they do appear to be beneficial in improving fertility rates.