The Neotyphodium coenophialum fungal endophyte increases persistence and "ecological fitness" of the grass-endophyte combination when grown in mutualistic association with its tall fescue host (Bouton, 2000). However, ruminants consuming N. coenophialum infected tall fescue often suffer from the condition called fescue toxicosis (Stuedemann and Thompson, 1993), characterized mostly by poor weight gain and reproduction (see Chapter 16). Annual losses of US$600 million, earlier reported due to this problem, actually may underestimate the losses (Fribourg and Waller, 2004) to U.S. livestock producers (see Chapter 1). Ergot alkaloids derived from the endophyte association are considered to be the primary cause of fescue toxicosis (Hill et al., 1994) (see Chapter 12 and Chapter 13). With 80% of U.S. tall fescue area reported to be infested with N. coenophialum at a mean rate of 76% (Lacefield et al., 1993), most livestock grazing tall fescue pastures in the country probably suffer from some degree of fescue toxicity (Bouton, 2000).