Species specifics: Milkweed

Fast facts about milkweed

Milkweed contains toxic cardiac glycosides that can poison cattle and horses even though the plant is generally considered unpalatable under normal grazing conditions.
The plant is best known for its milky white sap, distinctive seed pods and clustered flowers that range from pink and purple to greenish-white depending on species.
Over 100 species of milkweed in the Asclepias genus family can be found across North America, all with similar characteristics including varing degrees of toxicity to livestock including cattle and horses.

What to look for

Milkweeds (Asclepias spp.) is a perennial broadleaf weed found throughout much of the United States, with more than 100 species occurring across North America. While appearance can vary by species, most milkweeds share several identifying characteristics that make the genus easily recognizable in pastures, hayfields, roadsides and disturbed ground.

The most recognizable feature is the thick, milky sap that quickly oozes from broken leaves or stems. Plants typically grow upright from 2 to 5 feet tall and typically form colonies through underground rhizomes or spreading root systems.

Leaves are usually arranged opposite one another on the stem and tend to be thick, oval or lance-shaped with a prominent center vein. Depending on species, leaves may be broad and waxy or narrow and slender. Common milkweed (Asclepias syriaca), one of the most widespread species, has broad leaves and stout stems, while whorled milkweed species have much finer, grass-like leaves. Unlike hemp dogbane, stems of all milkweed species are finer and softer rather than woody.

Flower clusters are another key identifying feature. Blooms form in rounded, umbrella-like clusters and are commonly pink, purple, white or greenish in color. Flowering generally occurs from late spring through summer depending on geography and species. After flowering, milkweed produces elongated seed pods filled with flat brown seeds attached to silky white fibers that disperse by wind when pods split open.

All aboveground portions of the plant should be considered toxic to livestock including cattle and horses. Milkweed contains cardiac glycosides, also called cardenolides, that can affect the heart, nervous system and digestive tract of cattle and other livestock. Toxicity levels can vary significantly among milkweed species with some western species, such as whorled milkweed and woollypod milkweed, more potent than others.

In general, poisoning is relatively uncommon when adequate forage is available as milkweed is generally avoided by cattle due to its bitter taste. However, cattle may consume the plant when little other favorable forages are available such as in drought stressed or overgrazed pastures, or when animals unable to sort through feedstuff such as in baled hay from infested pastures. Clinical signs can include depression, weakness, labored breathing, staggering, bloating, muscle tremors, irregular heartbeat and, in severe cases, death.

Where it is found

Milkweed species are distributed across most of the United States, although specific species differ by region. The genus is especially common in prairies, rangelands, roadsides, hayfields, ditches, and stressed or overgrazed pastures.

Most milkweed species thrive in full sun and well-drained soils. Common milkweed is widespread throughout the Midwest, Great Plains, Northeast and portions of the eastern United States, while species such as showy milkweed and whorled milkweed are more commonly associated with western rangelands and semi-arid regions. Sandy soils, dry uplands and disturbed pastures often make for ideal growing conditions.

Milkweed pressure may increase in undermanaged grazing pastures where desirable forage populations have declined. Because many species spread through underground root systems in addition to seed production, infestations can persist and gradually expand over time if left unmanaged. Wind-dispersed seed also allows new plants to establish along fence lines, roadsides and pasture borders.

Although milkweed is widely recognized as an important pollinator and monarch butterfly host plant, ranchers and livestock producers should still monitor its presence in grazing systems and hay production fields due to the toxicity risk it poses to cattle and other livestock species. Particular attention should be paid during drought years, forage shortages or before cutting hay from infested areas.

How to treat it

Apply GrazonPD3™ herbicide at 2.66 pints per acre to actively growing milkweed less than 4 inches tall.

GrazonPD3™ is a Restricted Use Pesticide. GrazonPD3 is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. GrazonPD3 is not for sale, distribution or use in Nassau and Suffolk Counties in New York State. Always read and follow label directions.