Tall fescue (Festuca arundinacea) is a nutritious cool-season perennial grass that grows on more than 35 million acres in the United States. Introduced from Europe more than 100 years ago, it didn’t become popular until the mid-1940s after it was found to be productive and hardy in a wide variety of climates. It now provides hay and pasture for more than 700,000 horses. But all is not good news.

Stockmen noticed problems when cattle grazing this grass became unthrifty and intolerant of hot weather, with reduced milk production and conception rates. Mares had reduced fertility, prolonged gestation, oversized foals, foaling problems, thick placentas, and lack of milk. The cause of these problems was discovered to be an endophyte fungus in the plant.

The Fungus

An endophyte is a plant that lives within another plant. The fescue fungus Neotyphodium coenophialum produces a group of alkaloids (a type of chemical) that are toxic to grazing animals. The fungus isn’t visible; it grows within cells and can only be detected by lab analysis. It has a symbiotic relationship with the grass, making the plant more resistant to parasites, more tolerant of overgrazing and poor soils, and more hardy and long-lived (especially in drought and in warm southern areas). Also, insects and rootworms prefer eating fungus-free fescue.

“The alkaloids in the fungus serve as natural pesticides,” says D.L. Cross, PhD, MS, professor emeritus of animal and veterinary sciences at Clemson University, specializing in veterinary pharmacology and toxicology.

The fungus and toxins are found throughout the plant; however, the toxin and spores are in higher concentrations in the seeds. It can only be spread through the seed; it is not transmitted from one plant to another. There is endophyte-free seed available, but it’s not easy to establish fungus-free pasture, especially in the South, because the plants are less hardy and not as productive as infected ones. Stand persistence is a problem for the fungus-free grasses in most parts of the United States. Additionally, it is difficult to eradicate infected grass. If any infected plants or seeds survive, they eventually take over the pasture again because they are more hardy, crowding out the endophyte-free plants. Infected seed can also come in from surrounding areas or from hay being fed.

How the Toxins Affect Horses

“There may not be uniform production of toxins from year to year in a pasture,” says Susan White, DVM, MS, Dipl. ACVIM, professor of large animal medicine at the University of Georgia. “Horses may have no clinical signs one year, and the next year different weather conditions or degree of fertilization may enhance production of endophyte and toxin.”

When infected fescue pastures are heavily fertilized, the increased grass growth results in more endophyte and more toxin, she says. Drought-stressed grasses also produce more toxin.

“In the Southeast in 2005, we had rain from March until mid-August and grass grew abundantly,” says White. “Then it got hot and dry, stressing the fescue (which went to seed) and producing high concentrations of toxin.”

The weather, time of year, how fast the grass heads out, whether or not horses eat the seeds, and whether or not other feed is used as part of the diet factors into whether clinical problems occur, she says.

Horse owners should keep pastures mowed, not letting the grass go to seed. Some try to dilute the effects of the fescue by planting a legume such as red clover in the pasture to increase the nutrient quality and reduce the amount of fescue eaten, but it usually must be reseeded every three years since fescue out-competes other grasses.

“Pregnant mares may show toxicity signs even on pastures with very low levels of toxic grass present, and even when mowing is utilized to reduce intake of toxins,” says Cross. “Since horses don’t have a rumen (with bacteria that can detoxify much of the toxin), they are more sensitive to endophyte-infected grass than are cattle.”

Cattle, or horses other than broodmares, can also suffer from eating the endophyte-infected fescue, but those problems can be minimized if grazing or hay harvest is done before seed heads form.

“The ergo-peptides in the toxin constrict blood vessels and act on the central nervous system by activating dopamine receptors,” says White. Dopamine is a chemical messenger that affects secretion of hormones that play important roles in reproduction.

Cattle eating infected fescue look unthrifty, and this same unthriftiness occurs in horses as well. Young horses might not grow well, and adults don’t perform well. In a hot environment, the constriction of blood vessels near the body’s surface hinders an animal’s ability to dissipate heat. In cattle, fescue toxicosis causes impaired blood circulation to the extremities and occasional loss of ears, tails, or feet. This doesn’t happen in horses, but the constriction of blood vessels might be a factor in laminitis.

“This has not been proven, but has been hypothesized because of the effect on blood vessels in the hoof,” says White. “Clinicians in the Southeast often observe more laminitis in August and September (with an unknown etiology) in horses that are on fescue during our dry time of year.”

Problems for Mares and Foals

Broodmares should not be grazed on high-risk pastures during breeding season and in late pregnancy. Once past 30 days’ gestation, they seem to be able to safely graze infected fescue until about 300 days of pregnancy. After that, the alkaloids might result in abnormalities that include increased gestation length, lack of milk, foal and mare mortality, weak and dysmature foals, thickened placenta, elevated estradiol (an estrogen hormone), and reduced serum prolactin and progestogen levels in the mare, says Cross.

Signs of approaching parturition are often lacking. The mare might have no udder development prior to foaling and does not show normal signs of readiness (relaxation of vulva, etc.). Although the mare goes over her due date, the owner might not be expecting her to foal since she exhibits no outward signs. Thus, the birth might be unattended, with more risk for loss of the foal or mare, says Cross.

Changes that normally occur in the mare (to allow the foal to be born easily) are interrupted by the toxin. The cervix might not dilate, ligaments might not be relaxed enough to allow easy passage of the foal, and the uterus might have poor contractions. The reproductive tract is not ready for birth when hormones are hindered by the toxin.

In controlled studies at Clemson University, Cross observed that the foal could be rotated 90 to 180 degrees apparently due to a lack of preparation in the birth canal and the large, dysmature foal making birth even more difficult. The placenta is often thick and tough, making it heavier and less elastic than normal. Even if a foal is presented normally, it might not be able to break through the thick placenta and will suffocate unless someone is there to assist. The placenta also can detach too soon, preceding the foal through the birth canal (a condition commonly called red bagging). Retained placenta after foaling is more common in mares that have grazed endophyte-infected fescue.

Partly due to the increased gestation length and altered endocrine status of affected mares, the foal might be abnormally large-framed, emaciated (with poor muscle mass), weak, and dysmature (organs are not ready to make the adjustment from the uterus to the outside environment).

“Organs have not gone through the ‘readiness for birth’ procedures that happen in normal foals during the last few days of gestation,” says White.

Foals born alive might be hypothyroid (with inadequate thyroid hormones, which are important in body metabolism and in getting the foal ready for birth). “The foal has decreased ACTH (adrenocorticotropic hormone), decreased cortisol, decreased functional thyroid hormone, progestin, etc.,” she says. “The effect of all this adds up to lack of readiness for birth. The toxin is probably affecting the foal’s pituitary gland.”

These foals require a lot of care and can be hard to save. The mare often has no milk, and even if the foal is given colostrum from a mare other than its dam, it might have some reduced ability to absorb antibodies and is thus more vulnerable to infection. Some foals are stillborn due to premature placental separation or lack of lung maturation.

A high percentage of mares on infected fescue do not bag up before foaling and fail to produce milk. Instead of milk, mares may produce a brown or straw-colored fluid that is dark, and oily, with little nutritional value or antibodies, says Cross. “Studies at Auburn University in Alabama showed that mares removed from infected fescue too late (after 330 days gestation) often failed to produce normal concentrations of antibodies in their milk, even if they had udder development and adequate milk production,” he says.

Prevention and Treatment

The best prevention involves careful management of mares. “The grass can be tested for presence of the endophyte,” says White. “If you have infected grass, the safest procedure is to remove mares. Hay made from infected fescue also contains toxic alkaloids. Keep mares off infected pasture or hay for the last 30 to 60 days (before foaling).”

If the mare must be on infected fescue, she can be treated with domperidone (trade name Equidone), a dopamine antagonist that fills the receptor sites where the toxin binds, blocking the receptor sites so the toxin can’t exert its effects. Cross says, “The drug blocks the effects of the alkaloid at the cellular level, preventing the ergot alkaloids from interacting with D-2 dopamine and alpha-1 receptors throughout the peripheral tissues of the mare’s body.”

Domperidone can be given to mares in late gestation. The research and development of domperidone used rats and was done by Cross and his graduate students at Clemson University. In one field study, domperidone was given orally to 1,423 mares on infected fescue. The drug increased serum prolactin and progestogens; mares began to produce adequate milk when the drug was given 10 to 15 days prior to their due dates. Treated mares had live, healthy foals and normal gestation lengths, normal placentas, and foaled normally.

The drug has proven to be safe, with no adverse effects on the foals’ central nervous systems. Unlike other dopamine antagonists, domperidone does not cross the blood-brain barrier, and it does not cause nervousness nor lethargy. Equidone is an oral gel produced by Equi-Tox Inc., and is only available through veterinarians by prescription. Dosage depends on severity of the problem.

“You can’t treat every mare the same,” comments Cross. “In some instances a pasture contains other grasses, while on some farms it’s pure fescue. Dosage depends on the amount of fescue being eaten. The main thing is to get mares on the medication prior to foaling. Many mares are removed from fescue 15 to 30 days prior to expected foaling and started on the drug 15 days prior to foaling.” Cross holds four U.S. and several international patents on the domperidone technology, and he says Equi-Tox Inc. has funded the expensive regulatory approval process to make the prescription drug available to veterinarians.

The worst problems veterinarians run into are instances where owners are pasture breeding and don’t have breeding dates, says Cross. Veterinarians also see cases in which the mare has foaled, but is not lactating. “We double-dose those mares for at least three days (a dose in the morning and in the afternoon, then a single daily dose for five more days),” says Cross. Treatment for agalactia (no milk) is 1.1 mg per kg of body weight, twice a day. Most mares respond well to this and come into lactation in 24 to 48 hours. The sooner the medication is given after foaling, the quicker the mare will come into milk. However, by giving the drug to the mare a few days prior to calculated foaling, the 24-48 hours of lactation problems can be avoided. If the mare is agalactic (no milk), the foal must be fed substitute colostrum and milk replacer until full lactation occurs. In some cases, IV serum with antibodies must be administered.

“Many people think the drug only stimulates lactation, but it also brings mares into parturition close to expected foaling date,” he says. “The mares have more normal placentas, normal vulval swelling, full udders, and appear prepared for foaling. In our studies, mares foaled very close to their calculated foaling dates, whereas mares not on the drug averaged going 27 days past the expected due date.” Foals from treated mares are much more vigorous and appear normal, he adds.

Peter Sheerin, DVM, Dipl. ACT, of Rood and Riddle Equine Hospital in Lexington, Ky., says mares tend to come into milk when given the drug and occasionally might stream milk for a number of days before foaling and lose the colostrum. “If a mare starts to bag up and drip milk and doesn’t foal shortly thereafter, you must have another source of colostrum to feed the foal when it arrives,” he says. Cross says this happens most often for mares on pastures with low levels of fescue and veterinarians are advised to halve the dosing level until foaling in these instances.

Take-Home Message

There are many horse farms with endophyte-infected fescue in the fields, and many farms feeding hay that contains infected fescue. While this infected grass can cause many problems in mares and other horses, there are ways to manage the mares and fields to minimize its consequences to horse health.

 SAFE INFECTED FESCUE? Grazer-Friendly Endophyte

There is a new endophyte-infected seed on the market, with a genetically altered form of endophyte that is less harmful to grazing animals. “Two brands of this fescue are MaxQ (developed in Georgia) and ArkPlus (developed in Arkansas and Missouri),” says Susan White, DVM, MS, Dipl. ACVIM, professor of large animal medicine at the University of Georgia. “Fescue is a nutritious grass that fills a need; animals in many regions depend on it, and if the new fescue proves durable, many animals will benefit.”

James P. Brendemuehl, DVM, PhD, Dipl. ACT, visiting assistant professor of veterinary clinical medicine at the University of Illinois, adds, “I don’t think there’s enough data yet (number of acres and number of animals exposed) to know for sure. My gut impression is that it will have some of the same issues as endophyte-free fescue; it probably won’t be as hardy