Signs and lesions of poison hemlock poisoning:

  • Nervous trembling
  • Neuromuscular stimulation followed by depression and paralysis
  • Ataxia, especially lower and hind limbs
  • Salivation
  • Lack of coordination
  • Dilation of the pupils
  • Rapid, weak pulse
  • Respiratory paralysis
  • Coma
  • Death
  • Convulsions have been reported
  • Occasionally bloody feces and gastrointestinal irritation

Skeletal birth defects and cleft palate in calves and piglets if cows or sows eat poison hemlock during susceptible stage of gestation: 40th to 100th days for cows, 30th to 60th days for sows

Poison hemlock (Conium maculatum) can be found growing throughout the U.S. Sheep, cattle, swine, horses and other domestic animals are poisoned by eating a small amount. It is also extremely poisonous to humans.

Poison hemlock is sometimes confused with western waterhemlock--a more deadly plant--because the names are similar. (See waterhemlock chapter in this volume.) Poison hemlock has a number of common names, including deadly hemlock, poison parsley, spotted hemlock, European hemlock, and California or Nebraska fern.

Roots of poison hemlock may be mistaken for wild parsnips and eaten by people. The stem of poison hemlock has purple spots on it.

All parts of poison hemlock--leaves, stem, fruit and root--are poisonous. Leaves are especially poisonous in spring up to the time the plant flowers. Fresh leaves are unpalatable, so livestock seldom eat hemlock when other feed is available. The tox­ic compounds are coniine, γ‑coniceine and related piperidine alkaloids.

Where and when poison hemlock grows:

Because of its attractive flowers, poison hemlock was brought to the U.S. from Europe as a garden plant but has escaped cultivation and can be found growing in many pastures and in some areas on rangeland. Poison hemlock is found at roadsides, along fences and ditch banks, on edges of cultivated fields, along creekbeds and irrigation ditches, and in waste areas. It may invade fields or pastures.

Poison hemlock is a biennial and belongs to the carrot family. It starts growing in early spring but does not flower until its second year. In favorable locations it may be a perennial. Poison hemlock harvested with hay can be toxic to livestock and produce birth defects.

How poison hemlock affects livestock:

Poison hemlock ingestion is often fatal. Sheep may be poisoned by eating as little as 4-8 oz. of green leaves. Cattle that eat 10-16 oz. may be affected. Signs usually appear within an hour after an animal eats the plant. Animals die from respiratory paralysis in 2 to 3 hours. Convulsions, which are common in waterhemlock poisoning, seldom occur with poison hemlock.

Skeletal deformities or cleft palate may be induced in offspring of cows, sheep, goats and pigs if poison hemlock is ingested by the mother during susceptible stage of gestation: 40th to 100th days in cows and 30th to 60th days in sheep, goats and pigs. Palate and skeletal deformities in calves are indistinguishable from the lupine-induced crooked calf disease.

How to reduce loss:

Avoid stressing poisoned animals that are not recumbent. For recumbent animals, support respiration and treat with activated charcoal and a saline cathartic. Gastric lavage may be beneficial, with atropine therapy to control parasympathetic signs.

Animals that recover seldom show lingering effects.

Research results show that poison hemlock may be controlled by treating plants before they begin to bud with 2,4-D plus dicamba (2.5 lbs. + 1 lb. ae/acre). Repeat applications may be needed.