COLUMBIA, Mo. – Poison hemlock, a plant that can kill livestock, is one of the first weeds to appear in Missouri pastures, says Kevin Bradley, University of Missouri Extension weed scientist.

It also can cause birth defects in the offspring of pregnant livestock, especially when eaten in its earlier growth stage by animals in their first trimester of pregnancy, says MU Extension veterinary toxicologist Tim Evans.

Poison hemlock grows in ditches, roadways, fencerows, shaded areas of pastures and in no-till cropping systems. During its first year, it forms an upright flower stalk. It bolts in its second year. It can grow to 10 feet tall.

It is commonly confused with the wild carrot. Its white flowers form in umbrella-shaped clusters like those of Queen Anne’s lace. Both have hollow stems, but poison hemlock’s purple-mottled stem is hairless. Queen Anne’s lace also rarely grows taller than 2 feet and poses no danger to humans or livestock.

Control poison hemlock with pasture herbicides early while it is in the rosette stage and before the plant forms flowers, says Bradley. Early control is important because mature plants with flowers do not respond as well to chemical control.

Nonchemical control options include hand weeding, mowing and tillage, Bradley says. In pastures, growth regulator herbicides and products with 2,4-D, dicamba or 2,4-D plus triclopyr are options.

Bradley says research has also shown that dicamba plus glyphosate may offer more consistent control than 2,4-D plus glyphosate, but glyphosate can only be used in a spot-spray scenario or where injury to desirable forages can be tolerated.

Every part of this biennial contains gamma-coniceine, coniine and other toxic compounds that are harmful even in very small amounts, says Evans. These toxins affect cattle most, particularly with respect to birth defects, but they also affect sheep, goats, swine, horses and other domestic animals.

Evans says these toxins affect the nervous system like nicotine, initially causing tremors that may progress to total paralysis and suffocation. Symptoms usually appear within an hour of ingestion, and the animal can die in a few hours.

In ancient Greece, poison hemlock was used to execute condemned prisoners, including the philosopher Socrates. Its potent toxins also can irritate the skin, so wear protective gear including gloves, long sleeves, pants and eye protection when working in areas with this plant.

Cows that eat poison hemlock during the first trimester of pregnancy may give birth to calves with skeletal deformities, a condition known as crooked calf syndrome, says Evans. The offspring of pigs, sheep and goat can exhibit similar signs.

The leaves of immature poison hemlock plants appear to be most palatable to livestock. Animals usually avoid eating mature plants, but this is not the case when poison hemlock is accidentally incorporated into hay, says Evans.

To identify poison hemlock, visit the MU Plant Sciences Weed ID guide at https://weedid.missouri.edu.

For more information

“Weed of the Month: Poison Hemlock,” MU Integrated Pest Management, https://ipm.missouri.edu/croppest/2012/2/Weed-of-the-Month-Poison-Hemlock.

“Plants Poisonous to Livestock,” MU Extension publication G4970, https://extension.missouri.edu/publications/g4970.

Photo

https://www.invasive.org/browse/detail.cfm?imgnum=1391472
Poison hemlock. John D. Byrd, Mississippi State University, Bugwood.org. Shared under a Creative Commons Attribution 3.0 license.