Tall fescue, Festuca arundinacea is one of the most abundant cool-season grasses in the southeastern United States, occupying over 35 million acres of land. 62% of pasture grasses are infected with the endophyte fungus, Neotyphodium coenophialum. N. coenophialum lives within the plants and seeds and produces varying amounts of ergot alkaloids. Horses are extremely sensitive to ergot alkaloids and become poisoned over time through consumption of the endophyte-infected grass or hay. The principal alkaloid responsible is ergovaline; the amount present in the plant varies considerably depending on the strain, plant part, stage of maturity, environmental conditions, and geographic region.

Fescue toxicosis is of particular importance to pregnant mares, as it can result in a number of different reproductive problems; some of which include abortions, stillborn foals, prolonged pregnancy, thickened or retained placentas, uterine infections, lack of or poor milk production, low immunoglobulin (IgG) levels in milk, higher rates of newborn foal deaths, premature placental separation (red-bag), foaling difficulty, lower conception or breeding rates, and altered serum hormone levels.

Growing horses and mature geldings and mares are also affected by endophytic-infected fescue, as studies have shown that it is associated with decreased weight gain and weight loss. Horses are also at a higher risk of developing chronic foot and leg disorders, an increased risk of laminitis, loose feces or diarrhea and more profuse sweating.