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The free base and IV-oxide pyrrolizidine alkaloidcontents of a number of Senecio, Amsinckia, and Crotalaria species, which occur as range plants in the United States, have been measured in order to study variations with season, location, and species. Sampling on a monthly basis during the growing season, carried out for a 3-year period, showed considerable inter-and intraspecies variation in alkaloid content. In most species the total alkaloidcontent reached a maximum at the preflower or early bud stage.

Range plants that contain hepatotoxic pyrrolizidine alkaloids (PAs) cause considerable economic loss to the livestock industry (Mathews, 1933; Snyder, 1972). Cattle and horses are most often affected but otheranimals are susceptible (Bull et al., 1968). Sheep, however, appear to be quite resistant to the toxic effects, and their use as a control for these plants by heavily grazing infested ranges has been proposed …