Managing Pregnant Mares

Managing Pregnant Mares

Mares are somewhat unique in the fact that even though their gestation period lasts the better part of a year, it's only in the final three months of pregnancy that owners must treat them as "pregnant mares."

Photo: Anne M. Eberhardt/The Horse

Three hundred and forty-five days, give or take about 25. That’s approximately how long your mare will be pregnant. Mares are somewhat unique in the fact that even though their gestation period lasts the better part of a year, it’s only in the final three months of pregnancy that owners must treat them as “pregnant mares.” 

“The foal really starts major growth in the last three months or so, the ‘last trimester,’ ” says Aime Johnson, DVM, Dipl. ACT, associate professor of theriogenology at Auburn University College of Veterinary Medicine, in Alabama.

Christine Aurich, DVM, PhD, Head of the Graf Lehndorff Institute for Equine Science, in Neustadt, Germany, agrees: “Up to eight months she’s mostly just another horse.” 

Of course, there are always exceptions to that rule—mares with complicated gestational histories such as repeat abortions, older mares, or those with particular health problems. But for typical healthy mares, here’s what you should be doing.

Riding and Other Exercise

Many owners believe riding a pregnant mare will harm her or even cause her to abort. Fear not, though, our sources say. If she’s generally healthy to start with and her pregnancy isn’t considered high-risk (for example, a mare with a history of pregnancy loss or abortion), saddle her up and enjoy a ride! And you don’t have to stick to a gentle walk/trot session. Up to about eight months of pregnancy you can continue a normal workout and even jump and compete, says Aurich. Some Thoroughbred mares continue racing until very late term, adds Johnson. 

Even so, Johnson likes to see mares on particularly light work during the first month of pregnancy as a special precaution. “Those first 30 days are really critical,” she says. “So she shouldn’t be doing any high-level exercise, especially in the hot summertime, until that pregnancy is more well-established.” 

In most cases regular riding should cause no problems past that first month. Johnson says that as the pregnancy progresses and the foal grows, there is a slight risk of the fetus damaging or even rupturing the abdominal wall during excessive exercise. In the rare event that this happens, the mare loses her ability to push the foal out and needs assistance with delivery. Mares with ruptured abdominal walls can no longer be used as broodmares or sport horses. Fortunately, though, abdominal wall ruptures are few and far between and should not dissuade owners from riding their pregnant mares, she says.

At the eight-month mark, however, it’s time to start reconsidering the workouts. The growing fetus will start taking a toll on the mare’s body—most of all her lungs, says Aurich. “The diaphragm is displaced in late pregnant mares, so they may have trouble getting enough air,” she says. Plus, horses have fairly inefficient placentas that don’t favor oxygen transfer to the foal. So while not likely to cause abortion, “if there’s any impairment, this can make both the mare and foal suffer.”

So in those last three months, stick to simple walk-trot hacking in the countryside. The mare might become so heavy in the final two or three weeks that riding is uncomfortable for her, Aurich says. But exercise continues to be important. “Late pregnant mares tend to stand still a lot in the paddock, and they can develop significant edema (fluid swelling) in the legs,” she says. You can keep that fetlock edema down and her spirits up with about 10 minutes of hand-walking twice a day. 

Housing and Social Life

You could be tempted to pamper your pregnant mare by keeping her in a clean, warm stall with soft, fresh bedding at all times. Keep in mind, though, that horses don’t exactly see stall life as a luxury. What she’ll want and need is to get out in the fresh air, move around, and spend time grazing with compatible pasturemates. Our sources agree that an open field with a shelter is ideal. But remember to check for tall fescue growing in the pasture. This grass often contains a fungus that can cause prolonged gestation, difficult birth, and low milk supply. Remove mares from fescue fields at around nine months of gestation, Johnson says. 

Ideally, house your broodmare in a large field with other pregnant mares or friendly geldings, and provide her access with some form of shelter.

Photo: Anne M. Eberhardt/The Horse

In the last few weeks of pregnancy, mares tend to separate out naturally from other horses. “They keep behind the herd; they don’t want to play or fight,” Aurich says. “They’re afraid of getting kicked.” So to keep her stress levels down, put your mare in a separate paddock with another pregnant mare or friendly companion. She can join a larger group again once the foal is about a week old.

You don’t need to worry too much about cold temperatures during pregnancy, Aurich adds, because horses are very cold-resistant. If she’s not clipped your mare most likely won’t need a blanket. In fact, blankets can be troublesome and get in the way during foaling and nursing. Just keep her dry and out of the wind. 

But you’ll want to make sure the mare doesn’t foal outside during winter, as newborn foals can suffer and even die from hypothermia (getting too cold). Aurich says it’s prudent to stall your mare at night near the end of her pregnancy.

Gestation-Friendly Food and Water

In the final eight to 10 weeks of pregnancy, a mare’s energy needs and nutritional requirements increase significantly. At a minimum, that means purchasing high-quality feed designed for pregnant mares and following the package directions. Better yet, ask your veterinarian or equine nutritionist to help you customize a diet for your particular mare.

“Each horse is different,” Johnson says. “A Quarter Horse mare might need a handful of grain a day to keep her in good body condition, while a Thoroughbred mare might need several pounds three times a day.”

While we want to be sure our mares have enough flesh on their bones to support themselves and their growing babies, we also need to make sure we don’t overfeed them. Johnson says too many owners allow their pregnant mares to become overweight, which can have serious consequences. 

“A lot of people think now the horse is eating for two, and as soon as they get that positive confirmation (of pregnancy) they start overfeeding,” she says. “But the extra weight she’ll have to carry plus the increased weight of the foal can lead to laminitis (a potentially life-threatening hoof disease). It can also cause fat deposits in the pelvis, which can narrow the birth canal, making foaling difficult.”

What overfeeding probably won’t do is result in a fat foal. In horses, the fetus takes what it needs, Johnson says, and the rest goes to the mare. Weigh your mare regularly if you have access to a scale, or use a weight tape (though it might be slightly inaccurate for pregnant mare bellies) to be sure she’s in line with your veterinarian’s recommendations for weight increase. Learn to judge body condition, and make sure your mare stays at a five or six (on a scale of one to nine) throughout the pregnancy. 

Pregnant mares should have access to abundant quantities of quality hay. They should also have unlimited access to clean fresh water—preferably from a running stream or the tap. Horses can contract the abortion-inducing bacterial disease leptospirosis from drinking stagnant water, Johnson says.

Supplements are almost never necessary, and our sources say a few can even harm fetuses. For example, devil’s claw, a common ingredient in many joint supplements, could cause uterine contractions, says Johnson. But most supplements have not been scientifically tested on pregnant mares. “It is better to avoid these herbal supplements we don’t know much about and be safe, rather than use them and realize too late that they are unsafe,” she says.

Preventive Care & Your Veterinarian

Generally speaking, pregnant mares suffer more serious consequences from disease than other horses, as many infections can lead to abortion, and some can be more difficult to treat because of medications that endanger the fetus. So keep your pregnant mares in a sort of quarantine, separated from horses such as young stock and competition animals that are more likely to pick up and carry illnesses, Johnson says. 

Also designate separate barn supplies, stall-cleaning equipment, and grooming tools for your pregnant mares—don’t use them with the rest of your herd, or just be careful to wash and disinfect them between horses, she adds.

Vaccines are important for both mare and foal immunity and won’t harm the fetus, so maintain your mare’s regular core vaccination program. Repeat most vaccines at about five weeks before foaling to help protect the foal. You’ll want to add rhinopneumonitis immunization at five, seven, and nine months of pregnancy to reduce the risk of abortion from that respiratory disease, Johnson adds. (Find the American Association of Equine Practitioners’ vaccination guidelines at aaep.org/info/vaccination-guidelines.)

Keeping your mare parasite-free is also essential to her health and welfare, so work with your veterinarian to design an appropriate deworming protocol. Avoid deworming in the last four weeks of pregnancy, though, as this can cause abortion, Aurich says. 

Owners sometimes neglect mares’ feet during pregnancy because they’re not riding and tending to them as frequently, says Johnson. While pregnant mares don’t usually have special hoof needs, they’ll still need regular trims on a normal schedule.

Finally, build and maintain a good relationship with your veterinarian. “Breeding mares is not like reading a cookbook,” Johnson says. “Each mare may have different needs, and your veterinarian will be an excellent resource. Plus, if you run into problems during the pregnancy or foaling, you’ll already have that relationship established.”

Take-Home Message

Your pregnant mare is a double treasure as the dam of an exciting new life in your barn. Although she might not need specialized prenatal care in the first eight months, the quality of those last three months of gestation are critical to both her health and that of her foal. It’s up to you to manage her in a way that will give them the best possible health and welfare.

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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