Hemp dogbane, a poisonous herb, is found throughout the United States, including the western range states. The hemp portion of its common name comes from the fact that Indians used fiber from the bark for making rope. It also is called dogbane, Indian physic, American hemp, and rheumatism weed.
Hemp dogbane is a perennial that grows 1-2 meters tall. The leaves, that turn yellow in the fall, grow on opposite sides of the stems. The smooth-edged leaves are hairy on the lower surface. Flowers are greenish-white that develop into two long pods containing numerous seed with tufts of silky white hairs at their ends. The reddish stems contain a milky sap.
Normally, animals avoid hemp dogbane because of its bitter, sticky, milk-white juice. Sheep are more frequently affected than other animals, as they will eat large quantities of hemp dogbane leaves and tops if other forages are not available. This most often occurs when animals are turned onto harvested fields or on fall ranges when forage is scarce. Poisoning can also occur when livestock are trailed from summer to winter ranges and other forages are not available.
Where and When It Grows
Hemp dogbane grows on plains and foothills at elevations up to 2,000 meters. It commonly is found in gravelly or sandy fields, in meadows, and along creek beds, irrigation ditches, and fence lines in cultivated pastures. The plant begins growing in late spring or early summer.
How It Affects Livestock
The dogbanes contain cardiac glycosides that have physiologic actions similar to digitoxin. Dogbane's principle toxin has been identified as cymarin, which was once used as a heart stimulant for humans. Death from poisoning generally occurs 6 to 12 hours after animals eat the plant. Both dried and green plants are toxic. A lethal dose for most animals is reported to be about 0.5 gm/kg body weight, but as little as 15 gm of green leaves have been reported to cause death of some cows. Leaves are poisonous at all times, even when they are dry.
Signs and Lesions of Poisoning
Dilation of pupils
Blue coloration of mucous membranes
Progressive body weakness
Convulsions may occur
Mild myocardial degeneration
How to Reduce Losses
There are no reported treatments specific for hemp dogbane poisoning; so suggested treatment should be similar to those recommended for digitoxin toxicosis. This includes emetics in species that can safely vomit, atropine treatment if the animal has bradycardia, activated charcoal and possibly cathartics to minimize absorption, and electrolyte therapy to correct any electrolyte imbalances. Antidigitalis antibody fragments (FAB Digibind) may be useful, but binding affinities to Apocynum glycosides have not been documented.
Research results suggest that hemp dogbane may be controlled by repeated treatment of 2,4-D at 0.5 to 2 Kg per acre of acid equivalent. Follow recommendations and precautions when handling herbicides.