Tall fescue (Lolium arundinaceum) is a perennial grass adapted to much of the eastern half of the United States and widespread in the Central Ohio Valley, including Kentucky. Much of the tall fescue contains a fungus, called an endophyte, which, together with the fescue plant, produces chemicals called ergot alkaloids that can be harmful to grazing animals. The best known of these ergot alkaloids is ergovaline.

Fescue toxicosis in horses is most frequently associated with hormonal changes in late gestation mares, causing gestation lengths to extend beyond expected due dates. It is also associated with thickened placentas and abnormal placental separation when birth occurs, as well as with dystocia (difficult birth) and agalactia (lack of milk production by the dam).

In cattle, the most frequently reported signs of fescue toxicosis are associated with vasoconstriction (blood vessel constriction). This can result in cattle overheating in the summer because they cannot dissipate heat effectively and sometimes causes signs of gangrene in the winter due to insufficient blood flow to the hooves or tail switch. Fescue toxicosis in cattle is also associated with low average daily gain, poor growth rates, and lowered fertility. Reports in the literature of whether grazing endophyte-infected fescue can cause poor growth rates or reduced performance or fertility rates in horses are inconsistent, and researchers have not investigated the mechanisms that might contribute to these effects. Therefore, several years ago, the laboratory of Karen McDowell, PhD, associate professor at the University of Kentucky Gluck Equine Research Center, performed some experiments to determine if endophyte-infected fescue caused vasoconstriction in horses, similar to cattle.