Rural Heritage Vet Clinic

Endophyte Fescue Toxicity

by Jim Brendemuehl

Much of the fescue grass grown in the United States and Canada is infected with an endophytic fungus, which produces several types of alkaloids that are toxic to animals. Fescue toxicity may occur in sheep and cattle, as well as in horses, although the syndromes are different. In sheep and cattle we see "fescue foot" and "fescue tail," in which the hooves and tail literally slough off due to avascular necrosis [death from lack of blood vessels]—the alkaloids cause constriction of the blood vessels to the feet and tail. Additionally, a "summer slump" syndrome in growing cattle is characterized by poor condition and lack of weight gains.

In horses the major abnormalities associated with consuming endophyte-infected fescue are reproductive, with the most common problems occurring in late gestation. The five classic signs of fescue toxicity in the pregnant mare are:

  1. Agalactia--failure to have normal mammary gland development and milk production.

  2. Prolonged gestation—the mare carries 30 to 60 days longer than the normal gestation length of 330 to 350 days, depending on breed.

  3. Dystocia—difficult delivery resulting from a combination of the foal being abnormally large (30 to 60 day's of extra growth) and failure of the mare's pelvic ligaments and muscles to relax, as they would in a normal delivery.

  4. Placental abnormalities—premature placental separation at birth, with the placenta being extremely thickened and edematous [swollen with fluids]. Horsemen call this condition "red bag," describing the presentation at delivery of the red velvety chorioallantois [outside] portion of the placenta instead of the normal glistening white amnion [inside portion of the placenta]. A result of the placenta's premature separation is the disruption of the foal's oxygen supply before delivery.

  5. Neonatal death and dysmaturity—A final piece of the syndrome is a high incidence of fetal or neonatal [newborn] death and dysmaturity, also known as "dummy" foal syndrome. Several factors contribute to this phenomenon, including disruption of the foal's oxygen supply due to the premature placental separation, as well as abnormalities in the foal's endocrine, or hormone, systems--the foal's hormone maturation is markedly delayed, resulting in the foal being maladjusted (dysmature). Such foals typically do not have normal suckle reflexes, are unable to stand, have poor thermoregulation [ability to control body heat], and are depressed and unresponsive. Among the foals that survive delivery the mortality rate is quite high, even with intensive management and care.

Although most of the toxicity seen in mares is associated with the last several months of gestation, problems are also seen earlier in gestation. Mares grazing infected fescue early in the season (January through April) typically have a delayed onset of cyclicity [coming into heat]. Mares grazing endophyte-infected fescue during the first 30 days of gestation have lower pregnancy rates and higher embryonic loss rates than mares grazing endophyte-free fescue. After the first 30 days of gestation and up until the last 30 to 60 days of gestation no greater incidence of abortions has been documented.

Management for pregnant mares and fescue should include the following practices:

  • Test all pastures annually for the presence of endophyte. While the fungus is spread only through seed and can't blow from one infected plant to another, infected seeds may be carried by birds and in the fecal material of other animals. Additionally, some so-called endophyte-free seeds are derived from infected seed that has been stored, which results in the fungus becoming dormant; after several years the fungus can reemerge, resulting in toxic alkaloid production.

  • If the fescue is determined to be endophyte infected, pregnant mares should not be allowed to graze the infected fields or eat infected fescue hay within two months of their due date. The level of toxic alkaloids in hay is not reduced with storage.

  • If for some reason pregnant mares cannot be removed from infected fescue pastures, they should be treated with the drug domperidone, available through your veterinarian. Although domperidone can be useful in cases where mares are not removed from infected fescue pastures, it is not a magic bullet. Make every effort to prevent pregnant mares from consuming infected fescue grass or hay during the last 60 days of gestation or while they are nursing.

Supplementing the diet of a pregnant mare grazing infected fescue with grain or legume (alfalfa) hay to dilute the toxins has not proven to be effective in preventing problems, as it has been in cattle. Mares show toxicity when consuming levels of less than 50 parts per billion (ppb), making them much more sensitive than cows to the alkaloids.


Jim Brendemuehl, DVM, PhD, is an equine Extension veterinarian at the University of Illinois Veterinary Teaching Hospital in Urbana, where one of his special interests is fescue toxicity in mares. He is one of this site's consulting virtual veterinarians available online to respond to your concerns about draft horse reproduction. This article appeared in issue of Spring 2000 issue of Rural Heritage.

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26 October 2011 last revision