Toxicity of threadleaf groundsel (Senecio douglasii var longilobus) to cattle

Threadleaf groundsel (Senecio douglasii var longilobus), a common plant on much of the rangeland of southwestern United States, was experimentally fed to 55 cattle by gastric gavage, mixed in alfalfa hay, or pelleted in feed. Since the pyrrolizidine alkaloid (PA) content of threadleaf groundsel fluctuates greatly, dosage was determined by the plant PA content. Animals that died survived up to 506 days. Whether they were acutely or chronically affected they had typical signs of seneciosis. Major gross and histopathologic changes are reported. The estimated minimum lethal plant dose by gavage was sufficient plant to provide 13 mg of PA/kg of calf body weight/day for 15 days or a total of approximately 200 mg of PA/kg in a 15-day period. Cattle that consumed up to 600 mg of PA/kg total in 20- to 100-day periods in hay or pellets were unaffected or minimally affected. These findings emphasize the importance of the time-dose relationship in PA toxicosis in cattle and indicate that there is a threshold level of PA that must be exceeded for the toxicosis to develop.