Fescue Toxicosis in Pregnant Mares: Risks and Prevention

Tall fescue covers roughly 35 million acres across the eastern United States. It's hardy, drought-tolerant, handles heavy traffic, and stays green when other grasses give up. Sounds perfect, right? Except for one massive problem: most of that fescue is infected with an endophyte fungus that can kill pregnant mares and their foals. If you keep broodmares and you're anywhere in the fescue belt, this is something you absolutely need to understand. Not eventually. Now.

What's Actually Going On Inside the Grass

The culprit is an endophytic fungus called Epichloë coenophiala (formerly classified as Neotyphodium coenophialum). "Endophyte" just means it lives inside the plant; you can't see it, smell it, or identify it by looking at the grass. The fungus and the fescue have a symbiotic relationship that benefits both organisms handsomely. The fungus produces alkaloids, primarily ergovaline, that make the plant more resistant to insects, drought, and overgrazing. The grass thrives. The fungus thrives. Everything's great unless you're a horse eating it.

Those ergot alkaloids are structurally similar to certain neurotransmitters, and they bind to dopamine receptors in the horse's body. This disrupts prolactin production, which is the hormone responsible for mammary development, milk production, and (critically in pregnant mares) the signaling cascade that leads to normal foaling. The alkaloids also cause vasoconstriction, squeezing peripheral blood vessels and reducing blood flow to extremities, the uterus, and the placenta.

Cornell's toxicology lab has documented ergovaline concentrations as high as 2,000 parts per billion in heavily infected seed heads during drought-stressed summers. For context, concentrations above 300 to 500 ppb are considered problematic for horses. That's a four-to-six-fold overshoot, and the mare grazing that pasture has zero ability to detect or avoid it.

Ergovaline isn't the only troublemaker. The fungus produces a cocktail of ergopeptine and ergoline alkaloids, with ergovaline just being the most abundant and best-studied. Concentrations spike during warm weather, seed head production, and periods of plant stress. A pasture that tests borderline in early spring can be genuinely dangerous by midsummer when seed heads are fully developed and alkaloid levels peak.

What Happens to Pregnant Mares

The effects on pregnant mares are devastating and well-documented. Here's what you're looking at when a mare in late gestation consumes endophyte-infected fescue:

• Prolonged gestation - Mares may carry foals 30 to 60 days beyond their due date. I've heard of cases going even longer. The foal keeps growing during this extended pregnancy, which creates serious problems at delivery. An extra month of fetal growth translates to a significantly larger foal trying to navigate a birth canal that hasn't received proper hormonal preparation.
• Agalactia - Little to no milk production. The mare's udder never develops properly because prolactin is suppressed. Even if the foal survives birth, it has no colostrum and no milk supply. Without that colostral transfer of immunoglobulins in the first 12 to 24 hours, the foal is immunologically naked.
• Thickened placenta - The placenta becomes abnormally thick and tough, sometimes described as "red bag" delivery. Instead of the normal white amnion presenting first, you see the cherry-red chorioallantois. This is an emergency because the foal is being separated from its oxygen supply. Every second counts during a red bag presentation; you must manually rupture the membrane immediately.
• Dystocia - Between the oversized foal and the thickened membranes, difficult deliveries are common. Some of these mares need veterinary assistance that would have been entirely unnecessary on clean pasture.
• Foal loss - Stillbirths and weak foals are frequent outcomes. Even foals that survive birth face the challenge of no colostrum, leaving them without passive immunity and vulnerable to every pathogen in the environment.

The mare herself can also be at risk. Retained placentas are common after fescue-affected deliveries, and retained placentas in horses are a veterinary emergency that can lead to laminitis, septicemia, and death if not addressed promptly. The vasoconstriction caused by ergot alkaloids also compromises uterine blood flow throughout late gestation, meaning the foal may be chronically oxygen-deprived even before delivery complications begin.

It's Not Just Pregnant Mares

While broodmares face the most dramatic consequences, endophyte-infected fescue affects other horses too. Reduced feed intake, poor weight gain, rough hair coats that don't shed normally, increased body temperature, and decreased tolerance for heat are all documented effects. Horses on infected fescue pastures in summer can struggle significantly with heat stress because the alkaloids cause vasoconstriction; the blood vessels near the skin constrict, reducing the horse's ability to dissipate heat through normal thermoregulatory mechanisms.

Geldings and non-pregnant mares won't show the reproductive effects, obviously, but they're still dealing with reduced performance and comfort. Weight loss despite adequate forage availability is a tipoff. Some horses develop a condition called "fescue foot" in severe cases, where vasoconstriction to the extremities becomes so pronounced that tissue in the hooves and tail begins to die. Rare in horses compared to cattle, but documented.

The Merck Veterinary Manual flags fescue toxicosis as one of the most economically significant mycotoxin-related conditions in North American equine practice. UC Davis researchers have pointed out that subclinical effects, the ones nobody notices because the horse isn't dramatically sick, probably affect far more animals than the obvious reproductive disasters do. A barn full of slightly underperforming horses on infected fescue is invisible until you move them to clean pasture and watch the transformation.

Growing horses deserve special attention too. Yearlings and weanlings on heavily infected pastures may show reduced growth rates and poor coat quality. The alkaloids interfere with growth hormone pathways in addition to prolactin, and young horses on infected fescue simply don't develop as well as their counterparts on clean pasture.

How to Know If Your Pasture Is Infected

You can't tell by looking. The endophyte lives inside the plant tissue and is invisible to the naked eye. The only reliable way to know your infection rate is to have your pastures tested. Your county extension office can help with this; they'll collect grass samples and send them to a lab that uses immunoassay or microscopic examination to determine the infection percentage.

In the fescue belt (roughly from Missouri to Virginia, and from Kentucky down to northern Georgia), infection rates of 80 to 90 percent are common in established stands. Some pastures test even higher. If your pasture has any tall fescue and hasn't been specifically replanted with endophyte-free or novel endophyte varieties, assume it's infected until proven otherwise.

Testing isn't a one-time event, either. Infection rates can shift over time as management practices change. A pasture renovated five years ago may be creeping back toward high infection if endophyte-infected fescue from neighboring properties seeds in. Test every few years to stay current.

Prevention and Management

So what do you actually do about it? You've got several options, and most good broodmare operations use a combination:

Remove mares from fescue pastures. The most straightforward approach. Get pregnant mares off infected fescue at least 60 to 90 days before their expected foaling date. Some researchers recommend removing them even earlier, by day 300 of gestation at the latest. Move them to fescue-free pastures, a drylot, or pastures that have been overseeded with other grass species through good pasture management. This single management step prevents the majority of fescue toxicosis cases.

Domperidone treatment. This is the pharmaceutical option. Domperidone is a dopamine antagonist that blocks the effects of ergot alkaloids on prolactin production. Administered daily starting 10 to 15 days before the expected due date, it can help restore mammary development and milk production. Your vet needs to prescribe and monitor this. It's not a substitute for removing the mare from fescue; think of it as a safety net, not a primary strategy. Research from the University of Kentucky has shown domperidone to be highly effective when used correctly, but timing is critical.

Pasture renovation. The long-term solution is replacing infected fescue with novel endophyte varieties. These cultivars contain endophyte strains that still benefit the plant but produce alkaloids that don't harm livestock. MaxQ, Texoma MaxQ II, and other branded varieties fall into this category. Renovation is expensive and time-consuming: you have to completely kill the existing stand, wait for the seed bank to deplete (often a full growing season), and then establish the new variety. But it solves the problem permanently for that pasture.

Dilution. Overseeding with clover, orchardgrass, or other species reduces the percentage of fescue in the mare's diet. It doesn't eliminate the risk, but it reduces exposure. This works best as part of a broader management plan rather than a standalone strategy.

Hay considerations. Infected fescue hay retains its alkaloid content. Drying and baling don't destroy ergovaline. A mare removed from infected pasture but fed infected fescue hay is still at risk. Check your hay source. Ask questions. If you're buying mixed-grass hay from the fescue belt, know what you're getting.

Monitoring and Blood Work

Your veterinarian can monitor prolactin levels in late-gestation mares to assess whether ergot alkaloid exposure is suppressing the hormone. AAEP guidelines suggest checking prolactin starting around day 300 of gestation, particularly in mares with any fescue exposure history. Normal prolactin should be climbing steadily in the last trimester; flat or declining levels scream fescue involvement even when the mare looks fine outwardly. Texas A&M's theriogenology department recommends serial monitoring every two weeks through foaling for mares in suspect environments. Cheap bloodwork. Priceless information.

The Economic Reality

Fescue toxicosis costs the U.S. livestock industry hundreds of millions annually, with horse losses representing a significant portion. A single lost foal represents not just the monetary value of that animal but the mare's entire breeding season, the stud fee, veterinary costs for the complicated delivery, and potentially the mare's future reproductive capacity if complications are severe enough. Prevention, even when it requires pasture renovation or supplemental feed purchases, is overwhelmingly cheaper than treatment.

For anyone managing broodmares, fescue toxicosis awareness isn't optional. It's survival knowledge. The alkaloids don't have a taste or smell that horses avoid. They'll graze infected fescue happily right up until things go terribly wrong. Testing your pastures, knowing your foaling dates, and having a plan are the difference between healthy foals hitting the ground and emergency vet calls at 2 AM. Talk to your veterinarian. Talk to your extension agent. Get the tests done. This is one of those problems that's entirely preventable if you know it exists.

Frequently Asked Questions

Sources

• Texas A&M AgriLife Extension - Tall Fescue Endophyte Toxicosis in Horses tamu.edu
• University of Kentucky College of Agriculture - Fescue Toxicosis uky.edu
• Cornell University College of Veterinary Medicine - Ergot Alkaloid Toxicosis cornell.edu
• AAEP Proceedings - Management of the Fescue-Affected Mare aaep.org
• Merck Veterinary Manual - Fescue Poisoning merckvetmanual.com
• UC Davis Center for Equine Health - Toxicology and Plant Poisoning ceh.vetmed.ucdavis.edu