Whorled milkweed (Asclepias verticillata) has been showing up in more pastures and hayfields this year, raising concerns for both livestock safety and forage quality. 

While milkweeds are native plants that support pollinators, some species—like whorled milkweed—can be toxic to livestock if consumed. 

Knowing how to identify and manage this plant can help avoid losses and maintain safe forage systems. 

Identification 

Whorled milkweed is one of the smallest and most inconspicuous milkweed species, making it easy to overlook: 

• Growth habit: Slender, upright stems 1–3 feet tall, often forming small colonies. 
• Leaves: Narrow, linear leaves 2–4 inches long, arranged in whorls of 3–6 around the stem. 
• Flowers: Small white to greenish-white flowers in clusters at the top of the plant, blooming mid to late summer. 
• Latex sap: A milky white sap is exuded when leaves or stems are broken—a telltale sign of all milkweed species. 
• Roots: Deep, fibrous root system with rhizomes that help it spread. 

Toxicity and Risk to Livestock 

Whorled milkweed contains cardiac glycosides—compounds that interfere with heart function. It’s one of the most toxic milkweed species, especially late in the season or when dried in hay. Even small amounts can be dangerous. 

• Species affected: Cattle, sheep, goats, and horses are all susceptible. 
• Symptoms: Labored breathing, irregular heartbeat, muscle tremors, incoordination, and eventually death. 
• Toxic dose: As little as 0.1–0.5% of body weight in dried plant material can be lethal. 
• Treatment: Once clinical signs appear, treatment options are limited, and survival is often poor—making prevention critical. 

The biggest risk is contaminated hay, where animals may unknowingly consume toxic amounts. 

Management Strategies 

Early detection and prevention are key to reducing the risk: 

• Scout regularly: Especially in hayfields and lightly grazed areas. Pay attention mid to late summer when plants flower. 
• Watch hay fields closely: Seeds can begin to form while plants are still flowering, so early control is important to reduce spread. 
• Monitor purchased hay: Boughten hay can also be a source of contamination—visually inspect bales and ask about weed control history. 
• Avoid feeding contaminated hay: If milkweed is present in a hayfield, either avoid cutting or test bales visually before feeding. 
• Grazing management: Livestock typically avoid milkweed unless other forage is limited. Maintain adequate pasture availability. 

Control Options 

Controlling whorled milkweed can be difficult due to its deep roots and ability to spread by rhizomes. An integrated approach is best. 

Cultural 

• Competitive forage: Promote dense, vigorous pasture species that reduce bare ground and outcompete seedlings. 
• Avoid overgrazing: Weakens desirable species and opens the door for milkweed establishment. 

Mechanical 

• Mowing: Can suppress seed production if timed before flowering but will not control root systems. May require repeated cuts. 
• Follow-up opportunity: If mowed mid-summer, there is often enough time for regrowth to occur before fall, allowing for a fall herbicide application on the regrowth. 
• Hand pulling: Effective in small patches, but roots must be fully removed. 

Chemical 

Herbicides can be effective, especially on young, actively growing plants or regrowth after mowing. 

Note: Tordon 22K and Grazon PD are Restricted Use Pesticides. You must be a certified applicator to purchase and apply them. Be sure to check for and follow any grazing and/or haying restrictions. Always read and follow label directions carefully. 

Summary 

Whorled milkweed may be small and easy to overlook, but it poses a serious risk to livestock—especially when it ends up in hay. Knowing how to identify this plant, understand its toxicity, and apply the right combination of prevention and control practices is key to reducing risk. Whether you're managing native pasture or producing hay, staying vigilant and taking action early can help protect both forage quality and animal health. 

 Article by Ben Beckman, Nebraska Extension Livestock Educator.