The Expectant Mare We often think of pregnancy as a delicate and fragile
condition. When it comes to horses, this perception is perhaps due to the mare's
relatively poor reproductive performance in comparison to other domestic animals.
However, in a natural setting, the mare does comparatively well reproductively.
Therefore, this seemingly poor performance is due as much to improper management
as to any reproductive deficiency. Fortunately, management is something we can
control.
As a conscientious owner, you probably have many
questions about caring for your expectant mare. In truth, you may be a little
worried. Relax. With a little TLC, your mare should progress through her pregnancy
without mishap. Proper nutrition, deworming, exercise, and vaccinations will
help ensure a healthy pregnancy, and you can look forward to the birth of your
foal with greater confidence.
PRECARIOUS BEGINNINGS
The earliest days of an embryo's existence are perhaps
the most precarious. During the first 30 days, there is a 10-15% chance that
the embryo will be resorbed. Stress, illness, uterine infection, hormonal abnormalities,
the presence of twins, and other factors have been implicated in early embryonic
loss. Often, the cause remains undetermined.
When the mare conceives, the fertilized egg (zygote)
travels down the fallopian tubes and enters the uterus around day 6-7. It migrates
throughout the uterus until about day 16 and typically implants into the uterine
wall at 6-8 weeks. By day 12-13, the embryonic vesicle is usually large enough
to be detected by ultrasonic examinations, during which an image is made by bouncing
sound waves off tissues. For practical reasons, some breeding farms simply tease
the mare 14-20 days after her last breeding date to see if she comes back into
estrus (heat). If she does not, the pregnancy may then be confirmed by ultrasound
or trans-rectal palpation at approximately 30-35 days postbreeding.
Neither teasing, palpation, nor ultrasound has
been shown to harm the developing embryo or endanger the pregnancy. However,
because of the embryo's uncertain beginning, it may be wise to have the pregnancy
reconfirmed at 45, 60 or 90 days.
TROUBLESOME TWINS
Some reproductive specialists recommend an ultrasound
exam at 14-16 days post-ovulation to detect twins. Early detection of twins provides
an opportunity to eliminate one embryo, thus allowing the other to develop normally.
This is commonly done because twins pose a number of risks:
- In 95 percent of mares with twin embryos, one
or both embryos are resorbed or aborted during the first 60 days. However, waiting
to see if this occurs naturally could delay or interfere with a subsequent successful
pregnancy.
- Of the small percentage of twins that survive
in utero past 50 days, it is highly unlikely that two healthy foals will be born.
If either survives, it may be small and weak.
- Most twins surviving past 50 days will spontaneously
abort at 6-8 months.
- Mares carrying twins are more likely to give
birth prematurely (before 300-320 days). Premature foals may have serious medical
problems and are less likely to survive.
HELPING NATURE TAKE ITS COURSE
Good broodmare management is the best aid for helping
the mare make it through the critical first 30-60 days of pregnancy. The mare
should go into the breeding season fit and perhaps gaining weight. Severely underweight
mares will have more trouble conceiving than will mares of appropriate weight.
Avoid stressing the mare as much as possible. Stress can cause a drop in progesterone,
a hormone which helps maintain pregnancy. Illness and/or fever can cause the
mare's system to secrete prostaglandins, which may cause abortion.
USE GOOD JUDGMENT
- Transport your mare only if necessary.
- Use caution when exposing your mare to other
horses. You should avoid any undue risk of injury or disease transmission.
- Provide nutritious forage, but don't overfeed.
Supplementing with vitamins and minerals is unnecessary in mares being fed a
balanced diet.
- Make sure the mare is current on vaccines and
deworming. Consult your veterinarian for recommendations regarding specific vaccinations
and deworming interval during pregnancy.
- Do not administer hormones or other drugs unless
specifically prescribed by your equine practitioner.
- Carefully evaluate the mare before deciding whether
to breed on foal heat. Consult your veterinarian.
COASTING THROUGH MIDDLE PREGNANCY
Unless there are special circumstances, during the
first 7 months of pregnancy, treat your mare as you would a non-pregnant one.
She will benefit from moderate riding or exercise. The ration should be composed
primarily of high-quality forage in approximately the same as pre-pregnancy amounts.
Extremes in weather can alter her nutritional
requirements and should be taken into account when formulating the ration. She
should always have plenty of clean, fresh water. The mare will also benefit from
routine hoof and dental care, standard vaccinations, and regular deworming.
VACCINES
Vaccinations should be current, since infectious
diseases can trigger abortions. A four-way inoculation for Eastern and Western
encephalomyelitis, influenza, and tetanus is recommended at the beginning of
pregnancy. A booster should be given one month prior to foaling to increase the
antibody level in the mare's colostrum (first milk) and help protect the newborn
foal from disease. Also, the mare should be vaccinated for equine rhinopneumonitis
(commonly called virus abortion or rhino) at five, seven, and nine months' gestation.
Consult with your local veterinarian regarding other vaccines that may be advisable
in your area, such as rabies and botulism.
DEWORMING
Most deworming agents available today are relatively
safe for pregnant mares. Consult your veterinarian to establish an effective
and safe deworming schedule for your mare. It is especially important to deworm
the mare within several weeks of foaling, because the mare will be the primary
source for infecting her foal with parasites. Of course, manure should always
be properly disposed of.
CHANGING NEEDS
During the last four months of pregnancy, the foal
will grow rapidly. To accommodate this growth, the mare's energy needs will increase.
Even so, special nutritional supplements are probably unnecessary. Good-quality
hay and forage should remain the bulk of the expectant mare's diet. Concentrated
feeds, such as grains, may be added to the ration to bolster energy intake without
adding excess bulk.
Use the mare's body condition as your guide to
how she's faring. Adjust the ration accordingly. The mare should not become obese.
Your veterinarian can advise you regarding a proper nutritional program for your
mare.
Exercise during the last four months of the mare's
pregnancy should be light to moderate. In fact, a pastured mare will get as much
exercise as she needs just grazing. Vigorous exercise is not recommended.
HOME STRETCH
 The
average length of pregnancy in the mare is 338-343 days. However, normal gestation
can range from 320-380 days. You needn't become overly concerned if your mare
is past due. Prolonged gestation is not generally associated with problems or
extra large foals. If your mare's pregnancy extends much past 340 days or you're
concerned, ask your veterinarian to examine her to determine if the mare is still
pregnant and confirm that all is well.
SUSPECTED ABORTION
Mares do occasionally abort. If you notice a vaginal
discharge or dripping milk during pregnancy, contact your veterinarian. If you
find the remains of a placenta or fetus, save it for your veterinarian to examine.
It may be possible to ascertain the cause of abortion and treat the mare accordingly.
Mares can and do abort without ill effects. However, it's always a good idea
to have her checked by your veterinarian, because some complications of abortion,
such as a retained placenta, can be life-threatening to your horse.
IMPENDING BIRTH
There are obvious as well as subtle signs of impending
birth. The time frame during which they occur varies from mare to mare. The most
obvious and reliable are:
- Filling of the udder (two to four weeks pre-foaling)
- Distension of the teats (four to six days pre-foaling)
- Waxing of the teats (one to four days pre-foaling)
- Obvious dripping of milk
More subtle signs include:
- Softening and flattening of the muscles in the
croup
- Relaxation of the vulva
- Visible changes in the position of the foal
PREPARING FOR BIRTH
Your eleven-month waiting game will be over before
you know it. To prepare, brush up on your foaling knowledge with the companion
AAEP educational brochure, Foaling Mare and Newborn. Your veterinarian will be
happy to supply it and will also be able to answer any further questions you
may have about caring for your expectant mare.
This brochure was developed by the American Association
of Equine Practitioners through a grant from Bayer Corporation.
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