Anderson larkspur (Delphinium andersonii A. Gray) is a toxic plant responsible for cattle death losses in the western United States. The objectives of the present study were to determine when cattle consumed Anderson larkspur in relation to plant phenology and toxicity, and to determine if animal age influenced selection of Anderson larkspur. These grazing studies were conducted on semiarid sagebrush rangeland near Picabo, Idaho. Eight 6–7-yr-old mature Angus cows were used in 2008, whereas during 2009, 12 Angus cattle were used, including six yearling heifers and six 4-yr-old cows. The overall density of Anderson larkspur was 2.8 plants · m⁻² during 2008, and 4.9 plants · m⁻² during 2009. Total toxic alkaloid concentrations in Anderson larkspur plants were near or above 5 mg · g⁻¹ during both studies. During 2008 consumption peaked during the late flower and pod stage of growth. Overall in 2008 cows ate 3% of their bites as larkspur. During 2009 heifers ate about twice as much Anderson larkspur as did mature cows (5.1% of bites vs. 2.9%, respectively). Heifers repeatedly consumed sufficient larkspur that they collapsed; however, no animals were fatally intoxicated. Heifers appeared to become transiently averted to larkspur; however, heifers resumed consumption of D. andersonii after a period of one to several days of low or no consumption. Livestock management to reduce losses to Anderson larkspur should include timed grazing to avoid infested pastures during full flower to pod phenological stages, and grazing with older animals rather than yearling heifers.