Utah State University Extension
Tall larkspur reduces pasture use and can cause death in cattle. In the West, overingestion of tall larkspur causes average death losses of 4-5%, but can exceed 15% on some ranches. In addition, the presence of tall larkspur in some pastures may force ranchers to avoid those pastures during peak forage growth in late spring and early summer.
Extension, USU, "Reducing Losses Due to Tall Larkspur Poisoning" (2000). All Current Publications. Paper 1373.