The discovery in New Zealand of N. lolii strains lacking lolitrem B, an alkaloid causing ryegrass staggers, but containing peramine, an insect deterrent (Latch, 1997) made it possible to design beneficial associations in perennial ryegrass. Strains of N. coenophialum lacking ergot alkaloids also were found (Bouton et al., 2000), leading to commercial seed production of tall fescue cultivars with beneficial endophytes (see Chapter 20), resulting in symbiotic combinations in which the host plant benefits from the endophyte (Bouton et al., 2002). Tall fescue with novel endophytes has furnished excellent performance of lambs (Parish et al., 2003a), steers (Parish et al., 2003b), and beef cows and calves (Watson et al., 2004), while capitalizing on the endophyte infection by maintaining good stand survival. Planting novel endophyte tall fescue is the most dependable method of eliminating animal losses from tall fescue toxicosis. Replacement of toxic tall fescue pastures involves substantial cost and will take time; however, the improvement in animal production will repay amply the investment.