Tall fescue is the predominant cool-season perennial forage that supports approximately 20% of beef cattle in the United States (West and Waller, 2007). Tall fescue is one of the most abundant introduced forages in the United States (see Chapter 3). Favorable agronomic attributes of KY-31 tall fescue encouraged widespread planting throughout the mid-southern United States (Stuedemann and Hoveland, 1988) starting in the mid 1940s (see Chapter 1). Over 90% of the U.S. tall fescue pastures are infested with the endophytic fungus Neotyphodium coenophialum (Sleper and West, 1996). The negative effects of toxins produced by N. coenophialum on cattle were first described in the United States as field observations of cattle grazing KY-31 pastures in the early 1950s (McLaren and Fribourg, 1991). These early researchers referred to the signs observed (unthriftiness, unshed haircoat, increased respiratory rate, and decreased heat tolerance) as "fescue poisoning" and investigated the possibility that ergot might be involved in the poor performance of the animals. Several ergot alkaloids (see Chapter 13) have been isolated from E+ tall fescue. The ergopeptine alkaloid ergovaline was the primary alkaloid isolated from E+ tall fescue plants and seed that exhibited toxicity to cattle (Porter, 1994). When animals grazed tall fescue infected with endophytes not producing ergovaline, animal performance was similar to that of animals consuming E- tall fescue (Nihsen et al., 2004; Parish et al., 2003b; Watson et al., 2004) (Fig. 16-1 and 16-2). These findings support the claim that ergovaline is the alkaloid responsible for tall fescue toxicosis.
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Fig. 16-1. Angus steer in July after having grazed E+ tall fescue since March at Knoxville, TN, in 2003. Note the retained winter haircoat and unthrifty appearance, as compared to the sleek and shiny haircoat of steer in Fig. 16-2.
Fig. 16-2. Angus steer in July after having grazed E- tall fescue since March at Knoxville, TN, in 2003.
During the last several decades, production problems in the beef cattle industry associated with cattle grazing E+ tall fescue have been identified as three disorders: fescue foot (Garner and Cornell, 1978; Jacobson et al., 1970; Merriman, 1955), bovine fat necrosis (Stuedemann et al., 1985; Williams et al., 1969), and summer slump or summer syndrome. The latter is the most costly and is commonly referred to as fescue toxicosis (Schmidt and Osborn, 1993). The economic losses in the United States associated with fescue toxicosis have been estimated at more than $600 million/yr for beef cattle alone (Hoveland, 1993). Allen and Segarra (2001) concluded that these estimates were low when both growth and reproduction losses were considered. Forage and animal management practices that alleviate or reduce the maladies (see Chapter 12) associated with ingesting E+ tall fescue would promote animal health and well-being.
Several reviews have been published on the effects of consuming E+ tall fescue on grazing cattle (Bush et al., 1979; Fribourg and Waller, 2004; Gunter and Beck, 2004; Hemken et al., 1984; Paterson et al., 1995; Stuedemann and Hoveland, 1988; Thompson et al., 2001; Thompson and Stuedemann, 1993). These reviews provide more detail on the research studies that have been conducted on the effects of N. coenophialum on cattle and serve as excellent resources.
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