Coulter Conyza, Horsetail Conyza

Conyza coulteri

Asteraceae (Sunflower Family)


Conyza coulteri and C. Canadensis are herbaceous, 3- to 6-foot-tall annuals of the sunflower family. The leaves of C. canadensis are long and narrow, while those of C. coulteri are lobed and rounded on the ends. The many-flowered heads grow in leafy spreading branches or clusters, becoming feathery white after flowering.


Coulter conyza is confined to the western third of the state, while horsetail may be found in all areas of Texas. Both species are commonly found in disturbed sites.

Toxic Agent

The toxic agent of these plants is unknown. Coulter conyza has poisoned sheep, goats and cattle in experimental trials. During drought, this plant has been responsible for serious losses of cattle in the Trans-Pecos. Horsetail conyza has not been fed experimentally, but cattle and goats have been lost after consuming the plant. Both result in polioencephalomalacia.

Signs of Livestock Ingestion

Clinical signs associated with poisoning by these plants are related to brain damage and may include: Walking in circles; Hyperexcitability; Muscle tremors; Apparent blindness; Coma; Death.

If begun early enough, treatment with thiamine may reverse the condition.