The biggest dietary threat to a pregnant mare is tall fescue grass infected with an endophyte fungus, but the full list of what to avoid includes specific pasture plants, moldy feeds, excess minerals, and certain medications or supplements. Getting any of these wrong can lead to prolonged gestation, weak foals, abortion, or life-threatening complications at birth.
Tall Fescue: The Most Common Threat
Tall fescue is widespread across the eastern United States, and most of it harbors an endophyte fungus that produces ergot alkaloids. These compounds suppress prolactin (the hormone responsible for milk production), constrict blood vessels, and raise body temperature. In pregnant mares, the result is a cascade of problems: prolonged gestation, thickened placentas that don’t separate properly, large and weak foals, difficult deliveries, and a complete failure to produce milk.
The standard recommendation from university extension programs is to remove pregnant mares from infected fescue pastures at least 90 days before their expected foaling date. That buffer gives the mare’s hormone levels time to normalize. If your pasture contains tall fescue and you can’t confirm it’s endophyte-free, treat it as contaminated. Replacing fescue with other grasses in broodmare pastures is the most reliable long-term solution.
Toxic Pasture Plants
Several common pasture weeds can cause abortion, birth defects, or fetal death in mares. Buttercup (Ranunculus bulbosus) is one that’s often overlooked. In one documented case on a Thoroughbred farm, seven mares confirmed pregnant between 30 and 45 days of gestation all aborted after grazing in buttercup-infested pastures. Mares in early pregnancy appear especially vulnerable.
Locoweed (Astragalus and Oxytropis species) causes congenital abnormalities and abortions in horses and other herbivores. Some pasture plants also accumulate nitrate, a known cause of abortion. Wild cherry, red maple, and plants in the sorghum family (including sudan grass) pose additional risks ranging from cyanide poisoning to oxygen deprivation in the fetus.
Walk your broodmare pastures regularly and remove or fence off any suspicious plants. If you’re unsure what’s growing, your local agricultural extension office can help with identification. Pregnant mares that are short on forage are more likely to eat weeds they’d normally ignore, so keeping pasture quality high and providing adequate hay reduces the chance they’ll graze on something dangerous.
Moldy Hay and Feed
Mold in hay, grain, or bedding produces mycotoxins that can disrupt reproductive function. Zearalenone, a mycotoxin produced by Fusarium molds commonly found in corn and small grains, mimics estrogen in the body and impairs normal reproductive function in farm animals. Other mycotoxins like aflatoxins stress the liver and compromise overall health during a time when the mare’s body is already working hard.
Inspect hay before feeding it. Dusty, discolored, or musty-smelling bales should be discarded entirely, not just pulled apart. Grain that has been stored in humid conditions or has visible mold growth is equally risky. Proper storage in dry, ventilated areas prevents most mycotoxin problems before they start.
Excess Iodine
Iodine is essential in small amounts, but too much causes serious thyroid problems in foals. In one well-documented case on a Thoroughbred stud, four foals were born with massively enlarged thyroid glands and severe leg weakness. The cause was traced to a commercial feed pellet with unusually high iodine content. The mares had been consuming roughly 83 mg of iodine per day, far above normal requirements.
The foals’ thyroid glands were hyperplastic, meaning the tissue had grown abnormally in response to the excess iodine. Treatment with supplemental iodide didn’t help because the problem wasn’t a deficiency. Check the iodine levels in any commercial feed or mineral supplement you’re using, and avoid stacking multiple iodine sources (kelp meal, iodized salt, and fortified grain together, for instance).
Excess Selenium
Selenium is another trace mineral that’s vital in small doses but toxic when overfed. The U.S. FDA has set a maximum concentration of 0.3 parts per million in animal feeds for this reason. Chronic selenium toxicity, sometimes called alkali disease, causes loss of mane and tail hair, hoof deformities, and decreased reproductive performance. A single dose at or above 1 mg per kilogram of body weight can cause acute cardiovascular collapse.
Selenium levels in soil vary dramatically by region. Parts of the western and central United States have naturally high selenium in forage, while the Southeast tends to be deficient. If you’re in a high-selenium area, adding a selenium supplement on top of fortified grain can push your mare into toxic territory. Test your hay and know what’s in your commercial feed before adding any selenium supplement.
Too Much Vitamin A
Vitamin A from preformed sources (retinol, retinyl palmitate) is teratogenic at high doses, meaning it causes birth defects during fetal development. While the specific thresholds studied most extensively apply to humans and laboratory animals, the biological mechanism is consistent across mammals: excess retinol disrupts normal development of the heart, neural crest tissues, and other structures during organogenesis. Fresh green pasture and quality hay typically provide ample vitamin A through beta-carotene, which the body converts as needed and doesn’t accumulate to toxic levels the way preformed vitamin A does.
The practical risk comes from over-supplementation. If your mare is on good pasture and also getting a fortified grain mix plus a vitamin supplement, you could be tripling her vitamin A intake without realizing it. Review all sources and total them up rather than assuming more is better.
Medications and Supplements to Avoid
No deworming products should be given during the first 60 days of gestation, when the foal’s organs are forming. After that window, most standard dewormers are considered safe, but always check the label for pregnancy-specific warnings.
Steroids are a significant concern. This includes joint injections, treatments for allergic reactions, and anabolic steroids. Both the mare’s own hormone balance and the developing foal can be affected. If a pregnant mare has a condition similar to gestational diabetes, even a low steroid dose that would normally be harmless could trigger laminitis. Routine joint injections should be postponed until after foaling.
Common anti-inflammatory drugs like phenylbutazone and flunixin meglumine aren’t labeled for use in pregnant mares. A single conservative dose for an acute problem like colic is generally considered low risk by veterinarians, but repeated or chronic use can cause kidney problems or harm the fetus. Anti-spasmodic drugs used for gas colic carry their own risk: they may relax the cervix, which is dangerous during pregnancy.
Ulcer medications like omeprazole (sold as GastroGard) lack safety data for pregnant and lactating mares, so veterinarians typically advise against using them during pregnancy. The progestin altrenogest is sometimes given to mares with a history of pregnancy loss, but it’s not recommended for healthy mares with normal pregnancies.
General Feeding Principles
The safest approach is to keep the diet simple and based on quality forage. A pregnant mare’s caloric needs don’t increase significantly until the last trimester, when the foal is growing rapidly. During those final three months, gradually increasing grain or a commercial broodmare feed covers the extra energy and protein demands without requiring a complicated supplement regimen.
Audit everything your mare consumes: pasture, hay, grain, mineral blocks, loose minerals, and any added supplements. Overlap between fortified feeds and standalone supplements is the most common route to mineral toxicity. When in doubt, a hay analysis from a forage testing lab costs relatively little and tells you exactly what nutrients your mare is already getting from her base diet, so you only fill the gaps that actually exist.