Tall fescue grass is one of the most common pasture grasses in the United States, but it poses a serious risk to horses, particularly pregnant mares. The danger comes not from the grass itself but from a fungus living inside it. About 95% of tall fescue pastures in the U.S. carry some level of this fungal infection, which produces toxic compounds that can cause everything from poor milk production to foal death. For non-breeding horses, the risks are lower but still worth understanding.
Why Most Tall Fescue Is Toxic
Tall fescue commonly harbors an internal fungus called an endophyte. This fungus lives between the plant’s cells, invisible to the naked eye, and produces a group of toxic compounds known as ergot alkaloids. The most problematic one is ergovaline. The endophyte actually benefits the plant by making it more drought-tolerant, pest-resistant, and hardy, which is exactly why infected fescue has become so dominant across pastures from the Southeast to the Midwest.
When horses eat infected fescue, ergovaline enters the bloodstream and mimics several natural signaling chemicals in the body, including dopamine and serotonin. This molecular mimicry triggers two major problems. First, it causes blood vessels to constrict, reducing circulation to extremities and the uterus. Second, it shuts down prolactin, the hormone responsible for milk production. In horses, this effect is especially severe. While cattle experience reduced milk output, horses become completely unable to produce milk, a condition called agalactia.
The Danger to Pregnant Mares
Fescue toxicosis is most devastating in broodmares during late pregnancy. The list of complications is long and serious: prolonged gestation, thickened and toughened placentas that make delivery difficult, complete failure to produce milk, weak or premature-looking foals, and increased mortality for both the mare and foal. Hormonal disruption lowers progesterone levels while raising estrogen, throwing the normal signals of late pregnancy into chaos.
The thickened placenta is particularly dangerous because it can prevent the foal from separating properly at birth, creating an emergency situation. And even when delivery goes smoothly, a mare with no milk cannot feed her foal. Without intervention, that foal will not survive. Supplementing with selenium or extra feed energy does not reduce these effects. The toxicity is chemical, not nutritional.
The standard recommendation from Penn State Extension and other university programs is to remove pregnant mares from infected fescue pastures 30 to 90 days before their expected foaling date. This window is when ergovaline causes the most harm to late-stage pregnancy. For horse operations that cannot avoid fescue entirely, a veterinary medication called domperidone can block the effects of ergovaline on prolactin production. It is given orally once daily starting 10 to 15 days before the expected foaling date and can continue for up to 5 days after foaling if the mare still isn’t producing enough milk.
Effects on Non-Breeding Horses
Geldings, stallions, and non-pregnant mares face far less risk from infected fescue, but they are not entirely immune. Horses grazing toxic fescue during hot weather can experience increased sweating and difficulty regulating body temperature because of the blood vessel constriction caused by ergot alkaloids. Reduced blood flow to the skin makes it harder for the body to cool itself. Some horse owners also report rough, slow-shedding coats in horses kept on fescue pastures year-round, likely related to the suppression of prolactin, which plays a role in seasonal coat cycles.
For a healthy adult horse that is not pregnant or lactating, grazing moderate amounts of infected fescue is unlikely to cause a crisis. But it is still not ideal, and the risks increase during summer heat and when horses are consuming fescue seed heads, which concentrate the highest levels of ergovaline.
Endophyte-Free and Novel Endophyte Varieties
Not all tall fescue is dangerous. Plant breeders have developed two alternatives. Endophyte-free fescue has had the fungus removed entirely, making it completely safe for horses. The tradeoff is that without the endophyte, the grass loses much of its toughness. It is less drought-tolerant, less pest-resistant, and tends to thin out over time, especially in the hot, humid climates where fescue is most commonly grown.
Novel endophyte fescue varieties contain a different strain of fungus that still helps the plant survive but does not produce the toxic ergot alkaloids. These varieties offer the best of both worlds: a durable, persistent pasture grass that is safe for horses. If you are establishing new pastures or renovating old ones, novel endophyte fescue is the strongest long-term option for horse operations.
Managing Existing Fescue Pastures
Replacing an entire pasture of infected fescue is a multi-step process. The University of Georgia Extension recommends a “spray-smother-spray” approach: mow at least twice to prevent seed heads from forming, then use herbicide to kill the existing stand, plant a smothering crop, and spray again before reseeding with a safe variety. A simpler alternative for smaller operations is the “spray-spray-plant” method, where you prevent seed head production in spring, spray herbicide in late summer, spray again four to six weeks later, and plant the new fescue within one day of that second application. The urgency of planting quickly matters because infected fescue is aggressive and will recolonize bare ground fast.
If full pasture replacement is not practical, dilution is the next best strategy. Interseeding legumes, especially white clover, into existing fescue stands reduces the proportion of toxic forage horses consume at any given time. Clover also adds nitrogen to the soil and improves overall forage quality.
Grazing management also helps. Ergovaline concentrations are highest in seed heads and during the plant’s reproductive growth stages, so timing grazing to avoid those periods reduces exposure. Avoiding fescue grazing during summer months is also recommended, both because heat amplifies the toxicity symptoms and because the plant is under stress and concentrating alkaloids. Some producers use targeted herbicide applications to suppress seed head development without killing the entire stand, which can meaningfully lower toxin levels in the pasture.
The Bottom Line on Fescue and Horses
Tall fescue is safe for horses only when it is free of the toxic endophyte or contains a novel endophyte strain. For pregnant and lactating mares, infected fescue is genuinely dangerous and can be fatal to foals. For other horses, infected fescue is a manageable but not ideal forage. If your pastures contain tall fescue and you are unsure of its endophyte status, your local agricultural extension office can test a sample. Knowing what is growing in your fields is the first step toward keeping your horses safe on them.