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.
Chronic toxicity is far more likely to be encountered.
The animals must consume the plants fresh or in hay for weeks to months before clinical signs appear. The toxin is cumulative, and continues to damage the liver until enough of the liver is affected to cause clinical signs. Approximately 50 to 150 pounds of ragwort needs to be consumed by a horse before signs appear. Cattle are more resistant to the toxic effects than horses and reports have indicated that cattle may be fed small amounts of ragwort hay without ill effects. There are reports that sheep are relatively resistant to this toxin.