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Larkspurs (Delphinium�spp.) are a major cause of cattle losses on western ranges in North America, especially on foothill and mountain rangelands.� Previous work in our laboratory has shown that there are substantial breed and individual animal variations in response to a standard dose of larkspur alkaloids.� Susceptible and resistant animals differ substantially in their response to larkspur alkaloids, and this response has a genetic basis as determined from DNA genotyping.� Within the Angus breed, we screened steers to provide groups of resistant and susceptible animals (n = 6/group). Steers were phenotyped by dosing with larkspur and measuring exercise tolerance. Phenotyped steers were grazed on a larkspur-infested rangeland (D. occidentale) in southeastern Idaho during summer for 2 years (2015 and 2016). Different steers were used each year.� Susceptible animals initially consumed about 2x more larkspur than did resistant animals both years, provoking serious symptoms of intoxication in susceptible animals. One susceptible steer died in year 2.� However, by the end of the year 1 trial, there were few differences in larkspur consumption between resistant and susceptible cattle.� During year 2, in the latter portion of the second grazing trial, resistant steers consumed about 2x more larkspur than did susceptible steers; resistant steers showed clinical signs of poisoning, but had no fatalities. �Serum alkaloid concentrations > 500 ng/mL always resulted in muscular paralysis and collapse.� Results to date suggest that selection of larkspur by steers with different responses to poisoning is dynamic and may not be predictable based on phenotype.� These results also indicate the increased risk of fatal intoxication in susceptible cattle compared to resistant animals while grazing larkspur-infested rangelands.