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Control poison hemlock in pastures, woodlots

Travis Legleiter Poison hemlock has taken over this hillside
OUT OF CONTROL: Poison hemlock has taken over this hillside in an area where cattle could be present.
Learn to recognize this toxic weed and take control of it.

Turn livestock out in a new pasture and they should be in heaven, right? Don’t be surprised if they’re pulling morningglory vines off the old fencerow. They’re curious creatures with diverse tastes.

Diverse tastes get them into trouble when they sample poison hemlock, a poisonous weed. Victor Shelton, state agronomist and grazing specialist with the Natural Resources Conservation Service, says poison hemlock appears to be showing up frequently in pastures, low areas and roadsides. It’s toxic to both livestock and humans.

Poison hemlock looks a lot like cow parsnip or its cousin, wild parsnip. Cow parsnip is a large plant and isn’t toxic. Wild parsnip is technically edible but can cause rashes in humans. Distinguish poison hemlock compared to parsnips by purplish-colored streaks and spots on the smooth stem, Shelton says. Poison hemlock is a true biennial, so it will set seed the second year, and it’s a prolific seed producer.

Do not handle poison hemlock with bare hands, Shelton warns. According to Purdue University’s toxic plant website, poison hemlock has toxic components that include volatile alkaloids: coniine and gamma-coniine.

A lethal dose for a horse is 4 to 5 pounds of leaves. Cattle may be poisoned with 1 to 2 pounds, and sheep with a half-pound or less. Humans are often poisoned because they mistake the roots for parsnips or leaves for parsley or seeds for anise, Shelton adds.

Poisoning in animals

Pictures shown here should help you identify poison hemlock. If your animals find it first, what symptoms might they show? Within two hours of eating the plant, they tend to become nervous, often tremble and become uncoordinated, Shelton says.

Victor Sheltonpoison hemlock in the vegetative stage

Here is what poison hemlock looks like in the vegetative stage if you get up close.

After the excitement phase, the animal becomes depressed, he says. Heart and respiratory rates slow down, and legs, ears and other extremities become cold. Colic and/or bloating may occur. Even at this stage, the animal may not die. It may remain like this for several hours to days, and then recover, Shelton says.

When animals ingest a lethal amount, they tend to die within five to 10 hours after onset of clinical signs, typically from respiratory failure. Mucus membranes may appear blue. A mousy odor has been reported to emanate from affected animals, Shelton adds.

Control options

The best advice is to keep animals away from poison hemlock. That starts with controlling it. Herbicides are most effective when applied to plants in the first year of growth, or prior to bolting and flowering in year two, explain Bill Johnson and Marcelo Zimmer, Purdue weed control specialists. The closer to reproductive stages, the less effective the herbicide.

Courtesy of Marcelo Zimmer, Purdue Plant and Pest Diagnostic Labpoison hemlock with flowers

Here’s what poison hemlock looks like once it flowers.

In roadside ditches, pastures and waste areas, herbicides containing triclopyr, such as Remedy Ultra or Garlon, or triclopyr plus 2,4-D, such as Crossbow, are most effective. Some herbicides containing dicamba can also be used.

However, dicamba can’t be applied after June 20 through August 31 in Indiana due to volatility issues. Because that’s not the best time to control poison hemlock and dicamba is not the first choice for control, the summer dicamba ban shouldn’t be a big issue for controlling poison hemlock, Johnson concludes.

TAGS: Pasture Weeds
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