Sesquiterpene lactones appear to inactivate sulfhydryl groups of cellular enzymes. These bitter tasting compounds are present in all parts of the plant even when dry. As little as 0.5 - 1.5% body weight of green plant is toxic.
A upward branching, perennial 10-18 inches in height, arising from a clustered woody stem covered with old leaf bases. Leaves are clustered around the stem base. Multiple yellow flower heads with up to 12 ray florets. Similar species include H. odoata.
Animals that are compelled to eat bitterweeds tend to salivate. cough and vomit rumen contents. Rumen stasis and bloat are common chronic poisoning results in weak, debilitated animals which eventally become recumbent and die. Inhalation pneumonia may cause secondary vomiting.
Muscle weakness, ataxia
Remove animals from the source of the bitterweed and provide nutritious food and water.
Post-mortem examinations have found changes in the lung, heart, and kidney tissues.
Poisoning is infrequent when other forages are available.
1) Witzel DA, Jones LP, Ivie GW. Pathology of subacute bitterweed (Hymenoxys odorata) poisoning in sheep. Vet Pathol. 1977 Jan;14(1):73-8. doi: 10.1177/030098587701400109. PMID: 850997.