Scientific Name: Apocynum cannabinum
Distribution: Throughout United States and Canada
Type of Poison: Cymarin (a glycoside)
Signs of Poisoning:
Growth Characteristics: Hemp dogbane grows up to 3 feet tall. Its stems are purplish to red, glabrous at the bottom, and hairy at the top. The stems exude a milky sap when broken or damaged. It is rhizomatous.
Flower: Flowers grow in dense cymes found at the top of the plant and in the leaf axils. They are white to greenish-white, are very small, and have five lobes.
Leaves: Leaves are opposite and oblong. The margin is entire, and the leaf surface can either be glabrous or somewhat hairy. The leaves are poisonous at all times, even when they are dry.
Hemp dogbane grows on plains and foothills at elevations up to 7,000 feet. 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.
Normally, animals avoid hemp dogbane because of its bitter, sticky, milk-white juice. Sheep are more frequently affected than other animals; they will eat large quantities of hemp dogbane leaves and tops if other forb-type plants are not available. Hungry animals may eat hemp dogbane when they are turned onto harvested fields or new range in the fall or when other forage is scarce. Poisoning may also occur when sheep, cattle, and horses are trailed from summer to winter ranges.
Death from hemp dogbane poisoning may occur six to 12 hours after animals eat the plant. A lethal dose for sheep is about one-half to 1 ounce per 100 pounds of body weight. A lethal dose for cattle and horses is about one-half to three-fourths ounces per 100 pounds of body weight.
There is no known treatment for hemp dogbane poisoning. Research results show that hemp dogbane may be controlled by repeated treatment of herbicides.
L.F. James, R.F. Keeler, A.E. Johnson, M.C. Williams, E.H. Cronin, and J.D. Olsen. Plants Poisonous to Livestock in the Western States. U.S. Department of Agriculture, Agriculture Information Bulletin 415, 90 pp. 1980.
Tenaglia, Dan. Apocynum cannabinum. Missouriplants.com. 2008. Accessed 28 July 2009. http://www.missouriplants.com/Whiteopp/Apocynum_cannabinum_page.html