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. It spread rapidly throughout the British Isles where it is hybridizing with common groundsel . In California, Oxford ragwort was discovered growing at the Berkeley Botanic Garden, but has since been considered eradicated. It is native to the mountainous regions of Central and Southeastern Europe where it is not weedy.