Spring is a key time in controlling the spread of weeds, and control is most effective on young, growing, immature plants. Some of the most prominent poisonous weeds are leafy spurge, larkspur and lupine.
Leafy spurge usually reaches 3 feet tall with narrow leaves that have a “milky” juice when broken off and yellow or green flowers, and is typically seen along roadsides and in dry, unused areas. Although cattle do not typically seek out and consume leafy spurge, most poisoning cases occur when it is baled into hay. The tough thing about controlling leafy spurge is: It is resistant to many control methods and has high seed production, making it extremely difficult to eradicate.
Larkspur and lupine are two other poisonous plants best controlled early in the spring. Larkspur is easily identified by the “spur” pointing backward off the flowers, which range in color from white or yellow to purple or pink and are found on open hillsides and meadows. Poisonings typically occur in the spring in areas that are overgrazed when few other forages are available.
Finally, lupine – identified by its “wheel spoke” leaves – are usually found along foothills and mountains in areas with sagebrush or aspen trees. In the spring, cattle do not actively seek out lupine when other forages and grasses are available but will look for the green plants higher in protein later in the grazing period. Pregnant, early gestation cows who consume lupine can give birth to calves with serious defects, including cleft palates, bone and muscle deformities commonly called “crooked calf” disease. To avoid poisonings, keep cattle out of affected pastures in their first trimester of gestation. The best time to control larkspur is before flowers up, and lupine is best controlled at the bud stage with herbicides.
Spring is an exciting time as cows and calves head out to pasture but, while you’re out there checking on your herd, keep an eye out for these problem weeds and take action before it’s too late. As the saying goes, “An ounce of prevention is worth a pound of cure.” 
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Carmen Willmore
- Extension Educator
- University of Idaho Extension – Lincoln County
- Email Carmen Willmore
