Toxicity of Riddell's groundsel (Senecio riddellii) to cattle

The toxicity of Riddell's groundsel (Senecio riddellii) fed to calves in gelatin capsules, by gavage, or mixed in their hay ration was determined. Dosages were varied according to the pyrrolizidine alkaloid (PA) content of the plant, and the calves' responses were measured by clinical signs, serum enzyme changes, survival time, and histopathologic changes. Calves fed S riddellii to provide 10 mg of PA/kg of body weight/day in capsules or by gavage for 20 consecutive days did not develop clinical signs of seneciosis and did not have meaningful serum enzyme changes. However, feedings of the plant that provided 15 to 20 mg of PA/kg/day or more (gavaged or fed in capsules for the same time period) resulted in high mortality. The Senecio plant mixed in calves' hay ration was eaten slowly and reluctantly and was tolerated at dosages greater than 20 mg/kg/day, emphasizing that the toxicity was dependent on the rate at which the dosage was consumed and that mortality was not necessarily dependent on the cumulative dosage. Because of its high PA content, S riddellii presents a great hazard to cattle managed under conditions where they can consume quantities of the plant in short periods of time.