Broom snakeweed and threadleaf snakeweed are perennial weeds common on rangelands from Canada to Mexico. Sheep and cattle have been poisoned in large numbers from eating snakeweed; however, most losses occur in cattle. Snakeweed should be considered for its toxic properties and for its abortifacient effects. Death may occur but the principal effect of poisoning is abortion. The plant is most toxic during leaf formation and the early rapid growing period. Cattle eat snakeweed only when other feed is scarce such as during winter.
Where and When It Grows
Broom snakeweed and threadleaf snakeweed are very similar in appearance. They are distinguished by the number of ray flowers per head: three or more for broom snakeweed and two or less for threadleaf snakeweed. Snakeweed grows on the drier range areas. Its presence in large amounts is indicative of abused rangeland from overgrazing, fire, or other disturbance. It may grow during the fall if moisture is adequate; however, growth usually occurs in early spring. Snakeweed is said to be more toxic when growing on sandy soils than on range limestone soils (referred to as hard soils).
How It Affects Livestock
The amount of snakeweed needed to cause death or abortion in cattle varies greatly. Cows near term may give birth to small, weak calves that may survive if properly cared for. Cows that abort have a persistently retained placenta. Aborting cows may develop lesions of endometritis and septicemia accompanied by an increase in body temperature. These cows are at risk unless properly treated by a veterinarian. Factors affecting dosage include site of growth and stage of growth.
Signs and Lesions of Poisoning
Periodic mucopurulent nasal discharge with crusting and sloughing of nasal mucosa
Anorexia and weight loss
Rough hair coat
Diarrhea followed by constipation
Hematuria may occur
Hydropic degeneration and necrosis of the liver, mild icterus
Uterine edema and possible hydrops of fetal membranes
Pregnant cows may have periodic vulvar swelling and earlier-than-normal udder
development. They may abort; there is usually a retained placenta
How to Reduce Losses
Improve range condition to reduce snakeweed density. Cattle should not be grazed where there is an abundance of snakeweed growing on sandy soil during the time rapid growth is taking place, especially if there is a scarcity of other feed. Poisoned cows should be removed from snakeweed areas and given nutritious feed until they recover.
In the Southwest, snakeweeds can be reliably controlled by spraying in the fall using picloram (100 to 200 gm ae/Ac) or metsulfuron (50 gm ai/Ac). Further north in the Plains and Intermountain regions, these same herbicides and rates must be applied in early summer while snakeweeds are in the vegetative stage and growing rapidly. Follow all precautions for handling herbicides.