Caring for a Pregnant Mare

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PHOTO: PETER ARNOLD
It takes a pregnant mare about eleven months to gestate and deliver a foal.

If Tennyson was right — that “In the spring a young man’s
fancy lightly turns to thoughts of love” — well, then,
horses and humans aren’t so far apart. Horses are what are
known as long-day breeders, which means that, with longer
days just around the corner, now’s the time to start
getting your broodmares ready. Good preventive medicine
will help to ensure the health of a pregnant mare and increase the
likelihood of a live foal.

Good Beginnings

Broodmares should be in good overall health and be up to
date on vaccinations, Coggins testing, and deworming. Good
dental care will enable your mare to chew her feed properly
and maintain her weight throughout pregnancy. For optimal
conception rates, broodmares should be neither too thin nor
too fat. What’s the right weight? You shouldn’t see but
should easily feel your mare’s ribs, and there should be
some fat around its tail head. Also, the bones of its
pelvis, neck, and shoulders should blend smoothly with the
rest of the body. The spring transitional period (time to
come into first heat in which ovulation occurs) is longer
in thin mares than in mares in good body condition.

Many
broodmares have some musculoskeletal or lameness problem
that makes them unsuitable for athletic use. Most of these
conditions will not affect their ability to carry a foal.
However, serious conditions that cause chronic pain will
decrease conception rates. And, as the foal grows and the
mare’s weight increases, some lameness problems may become
more severe.

It’s important that prospective broodmares
have excellent conformation of their external reproductive
tract to minimize the possibility of uterine infections.
Ideally, the lips of the vulva should be perpendicular to
the ground and not slope toward the mare’s anus.. A
veterinarian can use ultrasound to examine the internal
reproductive tract for uterine cysts, which may hinder the
mare’s ability to conceive and maintain pregnancy.
Ultrasound can also be used to image the ovaries and detect
follicles and ovarian tumors.

Increased amounts of fluid in
the uterus may sometimes be found with uterine infections.
If a uterine infection is suspected, a sterile swab can be
passed into the uterus to check for telltale white blood
cells. A culture can then be performed to identify the
infective bacteria and determine which antibiotics will be
most effective.

A speculum passed into the uterus can be
used to check for urine pooling in the vagina and to assess
the cervix. Injuries to the cervix can occur during
dystocia (difficult or abnormally painful births) and may
not allow the cervix to close properly during subsequent
pregnancies, increasing the risk of uterine infections.
Biopsies taken from the inside lining of the uterus are
often very useful in determining if there is scarring or
inflammation of the uterus. Uterine biopsies are graded,
enabling your veterinarian to predict your mare’s chances
of carrying a foal to term.

Eating for Two

For the first two trimesters, the pregnant mare’s
nutritional requirements are the same as before pregnancy.
Mares in early- and mid-gestation can easily maintain their
weight on good quality legume (clover or alfalfa) pasture or
hay and a trace mineral salt block.

The period of greatest
development is during the last trimester, when 60% to 65% of
fetal growth occurs. The mare’s nutritional needs increase
during this time to meet her own requirements, as well as
those of the rapidly growing fetus. In the last three months
of pregnancy, the mare’s protein and energy requirements
increase 32% and 20%, respectively. Unless you are feeding
your mare a high-quality forage, she may need 1/4 to 3/4 pounds
of a high quality grain mix per 100 pounds of body weight
daily to meet her increased requirements. She’ll also need
more calcium and phosphorus in order to mineralize the
developing fetal skeleton. Calcium requirements increase by
85% in the last trimester — an amount that can’t be met simply
by increasing your mare’s feed. At this stage, you’ll want to
supplement a grass hay diet with a 50:50 mixture of trace
mineralized salt and dicalcium phosphate, fed free-choice. To
avoid possible bone disease in foals, it’s important that
pregnant broodmares be fed nutrients to meet but not exceed
recommended requirements .

Keep off the (Fescue) Grass

Never feed pregnant mares tall fescue grass during the
last trimester of pregnancy, as this may lead to prolonged
gestation, oversized foals, dystocia, decreased milk
production, and thickened placentas, or afterbirth. Tall
fescue grass is the most widely grown forage in the U.S. and
is found mainly in the Southeast and Midwest, where there’s
an estimated 35 million acres of the stuff. The toxicity of
fescue is linked to a fungus that grows between the cells of
the plant and produces toxins. While the fungal toxins are
generally bad news for animals, they’re a boon for fescue
grass, boosting the plant’s resistance to drought and insects
and enhancing its growth. It is estimated that 80% of fescue
pastures are infected by the fungus.

Horses are more
susceptible to infected fescue than are cattle, and as little
as 5% to 10% infected tall fescue plants in a pasture can
lead to fescue toxicity in mares. Gestation can be prolonged
for several weeks, resulting in a foal that is much larger
than normal, yet at the same time developmentally premature,
with overgrown hooves, fine hair, lax leg tendons, and,
perhaps, missing incisors.

Many foals born to mares fed
infected fescue will not survive. Only one foal in 11 lived
past the first few days of age in one study. In some cases,
the placenta becomes so thick that the foal, unable to break
out, suffocates. In others, udder development and milk
production are inhibited, leaving the mare with dangerously
little or no colostrum, a necessary source of protective
antibodies for the foal.

It is possible to test tall fescue
grass or seed to determine if the infective fungus is
present. And noninfected varieties of fescue have been
developed, although they are not as hardy. Also, drugs have
recently become available that help to offset some of the
toxic effects of infected fescue. But the absolute best
prevention remains to avoid feeding tall fescue grass, and
that means pasture, hay, and haylage during the last
trimester.

Vaccination and
Deworming

Pregnant mares should be vaccinated for equine herpes virus
1 at five, seven, and nine months of gestation. Equine
herpes virus 1 is a common cause of late term spontaneous
abortion in horses and may also cause respiratory and
central nervous system disease. To reduce the risk of
exposure to equine herpes virus, you’ll also want to
isolate your pregnant mares from other horses that have a
history of respiratory tract disease or spontaneous
abortion.

Vaccination against common equine diseases
such as equine encephalomyelitis, tetanus,
rhinopneumonitis, and influenza should be performed one
month before the expected foaling date to promote high
immunoglobulin concentrations in the colostrum. The foal is
completely dependent on colostrurm for its ability to fight
infections. Vaccinating the mare before foaling will ensure
that the foal will be able to mount an effective immune
response against many of the diseases it is likely to
encounter after birth.

If the mare is to be moved to a
different farm to foal, this should be done at least one
month beforehand to allow her to produce antibodies against
any diseases present in the new environment.

Deworm your
broodmare during the last month of gestation to reduce
parasite burdens and decrease the number of eggs to which
the foal may be exposed. Most currently available dewormers
are safe for pregnant mares. Some veterinarians recommend
deworming one or two days after foaling to reduce the
foal’s risk of contracting roundworms or small stomach
worms.

Gestation and Birthing

Normal gestational length — calculated from the last day the mare
stood for breeding — ranges from 320 to 350 days. You’ll find
accurate breeding records very useful for predicting
foaling times, allowing you to keep an eye on your mare
during the days that foaling is expected.

Prolonged gestation (greater than 350 days) doesn’t
necessarily indicate an abnormality, especially if the mare
has no history of exposure to infected tall fescue grass.
Prolonged gestation may occur normally in mares bred early
in the year, in those carrying colts, and in those that are
undernourished during the second half of gestation.

Late-term foals may be examined using ultrasound. Your
veterinarian may want to examine the placenta and determine
the fetal heart rate, which can be an indicator of fetal
stress.

It is difficult to predict foaling times based on udder
development, appearance of udder secretions, and relaxation
of the tail head and other pelvic ligaments. However,
frequent examination may allow a reasonable guess as to
when foaling is imminent. Usually, the mare’s udder will
not appear to enlarge until three to 30 days before
foaling. Maiden mares may not develop an enlarged udder
until one to two days before foaling.

Normal colostrum
ranges from honey-like to thick and yellowish. As foaling
approaches, some mares may leak colostrum and it may
accumulate on the teat ends (waxing). If the mare leaks
enough, you can milk her periodically before foaling,
freeze the colostrum, and give it to the foal after birth.

Water hardness tests and some tests specifically made for
use in mares may be used to predict when mares will foal.
These tests measure colostral concentration of calcium,
which increases as the mare approaches her foaling date.
However, these tests are more useful to predict when a mare
will not foal, rather than when she definitely will.
Several electronic devices are also available that set off
a remote alarm when the foal passes through the birth
canal.

Be sure foaling stalls are disinfected, clean, warm,
and dry. The stall should be large enough to easily
accommodate the mare, foal, and anyone who assists during
foaling. Mares are very likely to lie down during foaling
and so there must be enough room for the mare to stand up
easily. If foaling appears imminent the mare’s vulva may be
washed with an antibacterial soap and her tail wrapped to
minimize contamination.

Most mares will foal at night or
very early in the morning. Labor progresses through three
stages. During the first stage, the foal positions itself
for delivery, a process that can take anywhere from 30
minutes to four hours. During this stage, the mare may
appear uncomfortable and show clinical signs resembling
colic, such as sweating and pawing at the ground or at its
abdomen. Second-stage labor begins with rupture of the
fetal membranes (water breaks) and ends with delivery of
the foal. This stage is normally very rapid and should not
exceed 40 minutes. If the foal is not delivered within this
period of time, a true emergency exists and you should call
your veterinarian. Third-stage labor involves passage of
the placenta and should be complete within four hours.
After the placenta is passed, carefully examine it for
missing pieces. It’s important that no part of the
afterbirth remain in the uterus. Retained placenta in mares
can cause serious complications, including founder
(laminitis) and severe uterine infections, that require
veterinary attention.

Breeding mares and following them
through gestation to foaling can be a very rewarding
experience. Good preventive health care along the way will
increase the chances of a healthy mare and foal. Your
veterinarian is the best source of advice for health cafe
and any problems that may be a concern in your area.