Many Senecio species contain pyrrolizidine alkaloids toxic to humans and livestock when ingested in a single large quantity or in small amounts over time. Cattle, horses, goats, and young animals are more susceptible to poisoning than sheep. Introduced from Eurasia. Plants are toxic and have historically caused heavy livestock losses on pastures and rangelands in the Pacific Northwest. Animals seldom eat mature plants, but poisoning can occur when seedlings are grazed accidentally along with other forage or when contaminated hay is ingested .
Bees foraging on tansy ragwort produce bitter honey tainted with alkaloids.
Infestations are most troublesome during cool, moist periods. Common groundsel also contains toxic alkaloids, but livestock losses are uncommon. Most poisonings are also due to ingestion of contaminated hay over a period of time. Toxicity to livestock is undocumented. In England, Oxford ragwort was introduced as an ornamental to the Oxford Botanic Garden in the late 1700's.