The Neotyphodium endophyte (see Chapter 1) is not uniformly distributed throughout tillers and plants, as can be seen in the variable intensities of stain in Fig. 15-1. Some researchers argue that the total amount (mass) of endophyte present in a field should be measured; unfortunately, quantitative measurement of endophyte hyphae biomass is complex and expensive. For most practical problems, measuring the proportion of infected tillers is adequate to describe the potential severity of an endophyte field "problem." A "rule of thumb" proposed by some is that beef stockers will lose about 45 g/d (0.1 lb/d) of potential gain for each 10% of the plant population that is endophyte infected (E+). Appreciable losses of livestock production can occur for endophyte levels greater than 20 to 30% (Stuedemann et al., 1985; Crawford et al., 1989; Fribourg et al., 1991; Thompson et al., 1993).
Adding to the question of "what do we measure?" is the fact that some of the alkaloids (see Chapter 13) produced by the plant-endophyte association are toxic to livestock, not the endophyte per se (see Chapters 12 and Chapter 20). Although there are 30 to 40 alkaloids produced by endophyte-fescue associations, most interest has focused on ergovaline and other lysergyl alkaloids. For some purposes, measurement of lysergyl alkaloid concentrations may be more important than estimates of the presence of the endophyte. This is especially true in the marketing of tall fescue cultivars infested with nontoxic endophytes. Now we are interested in whether those endophytes are producing toxic alkaloids, as well as their actual concentrations, expressed as micrograms per kilogram (parts per billion, ppb).
While past research has implicated ergovaline loosely as the compound responsible for tall fescue toxicosis, research since 2001 indicates that ergovaline may be only one part of this syndrome. Hill et al. (2001) found that ergopeptine alkaloid transport across gastric tissues was lower than transport of lysergic acid and lysergol, with the implication that the latter alkaloids were the cause of observed physiological responses in livestock. In addition, recent work from Arkansas and Missouri in which synthetic ergovaline was fed to lambs indicated that ergovaline elicited signs of tall fescue toxicosis, but signs in animals fed equivalent levels of ergovaline with other alkaloids were more severe (Gadberry et al., 2003). Others suggest that ergovaline is degraded in the rumen to lysergic acid and ergoline and that these compounds are absorbed (De Lorme et al., 2007). At this stage, evidence is not strong enough to recommend measurement of lysergyl alkaloids in preference to ergovaline for commercial purposes; however, further research on this issue is required.