Not all toxic plants are equally dangerous, and many factors, such as stage of growth, part of the plant consumed and quantity, play a role in whether animals are affected.
With the ongoing weather variability the past few years, pasture conditions have been highly inconsistent. Some are thriving, while others are struggling due to overgrazing, flooding or drought.
“When pastures thin out, they leave space for opportunistic weeds to move in,” Shelby Gruss, Iowa State University Extension forage specialist, says. “While all weeds can compete for light, nutrients and moisture, some pose a greater threat due to toxicity risks for livestock.”
Below is a list of some of the most common toxic pasture weeds in Iowa and the Midwest, with tips on identification:
Poison Hemlock (Conium maculatum)
Toxic Part: Entire plant, especially leaves and stems
Risk: Highly toxic to all livestock (and humans); small amounts can be fatal
ID Tips: Finely divided, fern-like leaves; purple-spotted stems; umbrella-shaped white flower clusters
Notes: Often found in ditches and low-lying areas; do not mow when flowering — can release toxins and spread seeds
Common look-a-like: Queen’s Anne Lace/wild carrot
Wild Parsnip (Pastinaca sativa)
Toxic Part: Sap causes skin irritation when exposed to sunlight (phytophotodermatitis)
Risk: Mainly a risk to humans (and pets) through skin contact; generally avoided by livestock, but ingestion can cause photosensitivity and severe sunburn
ID Tips: Yellow umbrella-shaped flowers; hairy and grooved stems; leaves look like celery or parsley
Notes: Wear gloves and long sleeves when handling; mowing before seed set helps control
Common look-a-like: Queen’s Anne Lace/wild carrot
St. Johnswort (Hypericum perforatum)
Toxic Part: Entire plant
Risk: Contains hypericin, which causes photosensitivity (sunburn-like skin damage) in light-skinned livestock such as sheep, cattle, goats and horses; severe cases can lead to skin sloughing, blindness or death
ID Tips: Upright perennial with small flowers; the flowers have five bright yellow petals that often have black dots on the petal margins; leaves are opposite, oval and have translucent spots when held up to light; woody base; grows 1' to 3' tall in clumps
Notes: Becomes more problematic in overgrazed or low-fertility areas. Most toxic when flowering. Often avoided unless forage is limited
Common Milkweed (Asclepias syriaca)
Toxic Part: Leaves and stems
Risk: Contains cardiac glycosides; toxic in large amounts, especially to cattle and sheep
ID Tips: Thick oblong, opposite arranged leaves; milky sap; large round pink flower clusters; stems covered with dense pubescence
Notes: Generally unpalatable unless forage is scarce; monitor pasture for adequate forage growth
Black Cherry (Prunus serotina) / Johnsongrass (Sorghum halepense)
Toxic Part: Wilted leaves (Black Cherry); young regrowth and frost-damaged tissue (Johnsongrass)
Risk: Both produce cyanide (prussic acid), which can be fatal within minutes
ID Tips (Cherry): Tree with shiny leaves, dark bark, and white flowers
ID Tips (Johnsongrass): Tall grass with prominent midrib, purple panicle seeds, spreads rhizomes
Notes: Do not allow livestock access to wilted branches or frosted regrowth; delay grazing after frost or drought
Common look-a-like (Johnsongrass): Sorghum species
White Snakeroot (Ageratina altissima)
Toxic Part: Entire plant
Risk: Causes “milk sickness” in humans through milk from affected cattle; toxic to cattle, goats, horses
ID Tips: Opposite triangular leaves with serrated edges (sharply toothed) with three main veins; white flowers in flat-topped clusters
Notes: Common in wooded pastures or shaded edges; control early; avoid grazing densely infested areas
Common Look-a-like species: bonesets, other snakeroots
Horsenettle (Solanum carolinense)
Toxic Part: Berries and leaves
Risk: Contains solanine; can affect the nervous system and digestive tract
ID Tips: Spiny stems and leaves; small, star-shaped purple flowers; yellow berries when ripe
Notes: Toxic when consumed in large amounts; spot spray or dig out to reduce spread; berries are particularly toxic
Cocklebur (Xanthium strumarium)
Toxic Part: Seeds and seedlings
Risk: Toxic to pigs, sheep and cattle; seedlings are especially dangerous
ID Tips: Rough stems with darks pots; sandpaper-like, triangular leaves with serrated leaf edges; hooked burs that cling to fur and clothing
Notes: Can cause liver damage; avoid overgrazing, as seedlings are more likely to be eaten when forage is limited
Nitrate Accumulators
Some plants can accumulate toxic levels of nitrates, especially after drought or heavy fertilization:
Common culprits: Pigweed, lambs quarters, Johnsongrass, and thistles
Risk: Nitrate poisoning reduces oxygen transport in blood — can be fatal
Management Tip: Have suspect forages tested before grazing or feeding; elevated risks following fertilization, and following drought breaking rains
Horsetail (Equisetum spp.)
Toxic Part: Entire plant
Risk: Contains thiaminase; can cause neurological symptoms, especially in horses
ID Tips: Hollow, jointed stems; no true leaves; resembles a bottle brush
Notes: Grows in poorly drained soils and along ditches; improve drainage and limit access
“The first step to weed management is identifying the species you have in your field,” Gruss says.
If you’re unsure about a plant in your pasture, contact your local field specialist.
“When in doubt, better safe than sorry — keep animals away until you’ve confirmed safety,” she summarizes. “Typically, animals will avoid toxic species when there is something else to graze, but eliminating the toxic weed species is ideal. If using herbicides, please read and follow all herbicide directions and grazing restrictions.”
More information on common pasture weeds can be found here: Recognizing and managing common pasture weeds.
More information can be found here: 2025 Herbicide Guide: Iowa Corn and Soybean Production for herbicides labeled for grass pasture, hayfield (table 6) and alfalfa and legumes (table 7).
