Senecio is very toxic both fresh and in hay, affected animals often die. All grazing animals may be affected, but horses and cattle are particularly susceptible. Young, growing animals are more susceptible than mature animals, and fetuses may be affected in utero. All above ground parts, with higher concentrations in the seeds. There are several species of ragworts.
Most ragworts grow along roadsides, in pastures, and in wet or waste areas.
Poisoning from ragwort may be chronic or acute, with the chronic form more commonly encountered. The toxic principle is a mixture of pyrrolizidine alkaloids which adversely affect the liver. The plant grows in late winter through the spring and tends to be incorporated in first cutting hays. Animals may not show signs until the fall or winter after eating this hay for some time. In horses especially, the clinical signs may develop months after ragwort hay is no longer being fed, making an accurate diagnosis difficult.