Kentucky 31 (KY-31) tall fescue is not considered safe for horses, particularly pregnant mares. The grass commonly harbors a fungus that produces toxic compounds called ergot alkaloids, which can cause serious reproductive problems, reduced milk production, and in some cases, foal death. For non-breeding horses, the risk is lower but still worth managing carefully.
Why KY-31 Is Problematic for Horses
The issue isn’t the grass itself. It’s a fungus called Neotyphodium coenophialum that lives inside the plant. This endophyte fungus is invisible to the naked eye and doesn’t change the appearance or taste of the grass. It produces several types of alkaloids, the most harmful being ergovaline. When horses eat infected fescue, ergovaline interferes with normal hormone regulation, most critically by suppressing prolactin, the hormone responsible for milk production and reproductive function.
The fungus actually benefits the plant. Infected KY-31 is exceptionally tough, tolerating drought, heavy grazing, poor soil, and insect pressure better than uninfected fescue. That’s exactly why it became the dominant pasture grass across the southeastern and transition-zone states. An estimated 35 million acres of tall fescue grow in the U.S., and the vast majority of older KY-31 stands carry high endophyte infection rates.
The Danger for Pregnant Mares
Fescue toxicosis hits pregnant mares hardest. Mares grazing endophyte-infected KY-31 in late pregnancy can experience prolonged gestation, sometimes carrying well past their expected due date. They frequently develop agalactia, meaning they produce little to no milk after foaling. Their placentas become abnormally thick, heavy, and reddish in color, which creates a cascade of delivery problems.
One of the most dangerous complications is premature placental separation, sometimes called “red bagging.” Normally, the foal breaks through a thin, white membrane during birth. In red bag deliveries, the thickened placenta detaches from the uterine wall early and presents as a red, velvety mass at the vulva. This is an emergency because the foal loses its oxygen supply before it can breathe on its own. Without immediate intervention, the foal can suffocate.
Even when delivery goes relatively smoothly, foals born to mares affected by fescue toxicosis are often weak, have poor muscle mass, and face compromised immune systems. Stillbirths occur. Surviving foals may struggle to nurse because the mare has no milk, leaving them vulnerable to infection during the critical first hours of life when they need colostrum for immune protection.
Risk Levels Based on Pasture Composition
Not all fescue pastures pose the same threat. The risk depends on what percentage of the pasture is made up of infected tall fescue:
- Under 10% infected fescue: Very small risk to late-term mares
- 10 to 25%: Risk is small, but a safe pregnancy is not guaranteed
- 25 to 50%: Significant risk, especially during periods of grass stress like drought or heat
- 50 to 75%: High risk
- 75 to 100%: Very high risk
You can have your pasture tested for endophyte infection rates through your local agricultural extension office. This tells you the percentage of grass tillers that carry the fungus, which directly determines how dangerous the pasture is for your horses.
Conditions That Make Toxicity Worse
Ergovaline levels in infected fescue aren’t constant. They fluctuate with growing conditions, and two factors push them significantly higher: drought stress and heavy nitrogen fertilization. Research has shown that ergot alkaloid concentrations are highest in drought-stressed plants grown with moderate to high nitrogen application. The form of nitrogen matters too, with nitrate-based fertilizers increasing alkaloid content more readily than ammonium-based forms.
This means a pasture that seems manageable in a cool, wet spring could become considerably more toxic during a hot, dry summer, precisely when horses are spending more time grazing. If you’re fertilizing KY-31 pastures where horses graze, you’re potentially increasing the toxin load in the grass.
What About Non-Breeding Horses?
Geldings, stallions not actively breeding, and non-pregnant mares tolerate endophyte-infected fescue far better than pregnant mares. They can still experience some effects, including reduced weight gain, lower feed intake, elevated body temperature, increased respiration rates, and excessive salivation during hot weather. The restricted blood flow caused by ergovaline can also make it harder for horses to regulate body temperature in summer, since blood flow to the skin is part of how they cool down.
These effects are generally manageable and often go unnoticed in horses that aren’t under reproductive stress. Many horse owners graze non-breeding stock on KY-31 pastures without obvious problems, though performance and comfort during summer heat may quietly suffer.
Managing Pregnant Mares on Fescue Pastures
If your pastures contain endophyte-infected tall fescue, remove pregnant mares 60 to 90 days before their expected foaling date. Move them to a fescue-free pasture, a dry lot, or feed them hay that is not from infected fescue fields. This window gives the mare’s body time to clear the effects of ergovaline and restore normal prolactin levels before delivery.
If pasture removal alone isn’t enough or isn’t practical, a veterinarian can prescribe domperidone, an FDA-approved medication for horses that counteracts the prolactin-suppressing effects of ergovaline. It’s 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 isn’t producing adequate milk. This is a safety net, not a substitute for getting mares off infected pasture when possible.
Better Fescue Options for Horse Farms
If you’re establishing new pasture or renovating old KY-31 stands, you have two main alternatives: endophyte-free fescue and novel-endophyte fescue.
Endophyte-free varieties were the first solution developed in the 1980s and 1990s. They eliminated the toxic fungus entirely, but they also lost the toughness the endophyte provided. Without it, the grass has weaker seedling vigor, lower persistence under drought and heavy grazing, and shorter stand life. Endophyte-free fescue works best in multispecies forage mixtures, short crop rotations, or the most temperate parts of the fescue belt where environmental stress is lower.
Novel-endophyte varieties are the more practical long-term solution. These grasses contain a different strain of endophyte that still provides the plant with drought tolerance, insect resistance, and persistence, but does not produce ergot alkaloids. One example is the Protek endophyte, which was confirmed to be safe for animals while maintaining a stable relationship with its host plant. Novel-endophyte fescues retain the agronomic benefits of KY-31 without the toxicity. They cost more to establish, so it’s worth investing in good pasture management (rotational grazing, avoiding overgrazing, proper soil fertility) to protect your investment and keep the stand productive for years.
For horse farms where fescue makes agronomic sense, novel-endophyte varieties offer the closest thing to a best-of-both-worlds option: a durable, productive grass that won’t put your horses at risk.