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.

Management Strategies

Both species of conyza are unpalatable, and livestock eat them only when forced. Coulter conyza is normally a problem in drought when the plants are young and forage is limited. Horsetail is usually consumed only after it is treated with the herbicide 2,4-D; do not allow animals access to treated plants until the plants are dead and dry. These plants readily invade disturbed areas and often increase after mechanical brush control has disturbed the soil. Coulter conyza has increased where Spike 20P® was used to control woody plants.