Tall Fescue Endophyte Toxicosis in Beef Cattle: Clinical Mode of Action and Potential Mitigation through Cattle Genetics

Tall fescue (Festuca arundinacea Schreb.) is the most commonly used cultivated grass in the United States to feed beef cattle. Tall fescue is a cool-season perennial grass that many cattle producers ‘can’t live with, but can’t live without’ because of its hardiness and good forage yields, but adverse effects on cattle well-being and yields. The history of this forage and its effects on animal performance have been extensively reviewed (Hemken et al., 1984; Bacon et al., 1986; Stuedemann and Hoveland, 1988; Porter and Thompson, 1992; Stuedemann and Thompson, 1993; Porter, 1994; Bacon, 1995; Paterson et al., 1995). Tall fescue was unintentionally introduced from Europe sometime in the 1800s. Early university research on growing tall fescue in the U.S. began between 1907 and 1918 in Oregon and in Kentucky in 1931 (Alderson and Sharp, 1993). Tall fescue, primarily the Kentucky-31 variety, was planted across the U.S. throughout the 1940s and 1950s because of its excellent growth under various environmental stressors. Tall fescue may be found across the eastern half of the U.S. and the Pacific Northwest covering an estimated 25 to 40 million acres of pasture and hayland. It has been estimated that over 90% of tall fescue pastures in the U.S. are infected with the fungal endophyte Neotyphodium coenophialum (Bacon and Siegel, 1988; Glenn et al., 1996). Tall fescue and the endophyte share a natural, symbiotic relationship. The endophyte protects the host plant from environmental stressors such as drought, insects, nematodes, disease pathogens, and grazing by herbivores such as cattle.