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Low larkspurs (Delphinium nuttalliunum Pritz., Delphinium andersonii Gray) and plains larkspur (Delphinium geyeri Greene) often poison cattle grazing on western North American rangelands. The dominant toxic alkaloid in larkspur is methyllycaconitine (MLA); other very toxic alkaloids in low and plains larkspurs are nudicauline and geyerline. Toxic alkaloid concentrations in larkspur near or above 3 mg g-1 present significant risk to grazing cattle. D. nuttallianum from Utah and Colorado, D. andersonii from northern Arizona, and plains larkspur (D. geyeri) from Wyoming were collected for analysis. Concentrations of MLA in D. nuttallianum ranged from 0.8 to 4.5 mg g-1 in Utah and Colorado; total toxic alkaloid concentrations were often above 3 mg g-1. D. nuttallianum differed (P = 0.09) in MLA concentration between locations but not phenological stages (P = 0.41). Concentrations of nudicauline ranged from 0.7 to 4 mg g-1 in D. nuttallianum and were different (P = 0.01) between locations and phenological stages (P = 0.004). D. andersonii was consistently toxic because the total toxic alkaloid concentration fluctuated from 3 to 6 mg g-1 over the growing season. The concentration of geyerline in D. andersonii was equal to or greater than MLA at each phenological stage, thus adding to the toxicity. The concentration of toxic alkaloids in D. geyeri was typically highest (2-4 mg g-1) in immature plants, although toxic alkaloid concentrations in plants during the pod stage of growth sometimes increased. Only trace amounts of nudicauline were found in D. geyeri, as most of the alkaloid fraction was other N-(methylsuccinimido) anthranoyllycoctonine (MSAL) alkaloids. Total alkaloid concentration (MSAL and non-MSAL alkaloids) in D. geyeri sometimes exceeded 15 mg g-1. Concentrations of toxic alkaloids in D. nuttalliunum, D. andersonii, and D. geyeri often did not significantly decline during the growing season as typically found in tall larkspurs. Thus, risk to grazing cattle may remain high until these plants are dormant.  The Rangeland Ecology & Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact lbry-journals@email.arizona.edu for further information. Migrated from OJS platform August 2020