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Poison hemlock lurks in pasture, hay bales

Mindy Ward Poison hemlock plant with smooth purple stems
PURPLE POISON: Cattle farmers and other livestock producers need to inspect pastures and hayfields for poison hemlock. The key is to look for smooth, purple stems.
The toxic plant can cause birth defects and death in cattle, sheep, hogs and goats.

Pasture weed identification can be difficult, and in some cases, if you are wrong, it can prove debilitating or even deadly for livestock.

Poison hemlock is one pasture plant often confused with wild carrots. Tim Evans, University of Missouri College of Veterinary Medicine toxicology lead, says consuming poison hemlock early or in hay can cause birth defects or crooked calf disease.

“The thing we need to realize is this plant is extremely toxic,” he says. “The good news is generally when it's in its mature stages, it's generally not very palatable to most livestock. Most of the time, they're not going to go ahead and eat it. Unless for some reason it gets incorporated into hay, then the animal no longer has a choice in the matter, it's going to go ahead and consume it.”

Identifying poison hemlock  

When determining whether a plant is poison hemlock or a wild carrot, look at the stem.

The first thing you notice about a poison hemlock stem is that it is smooth, says University of Missouri Extension agronomist Gatlin Bunton. “That is the biggest giveaway; there’s no hairs on it," he says. The second is that it is has a purple color. Bunton says that to the contrary, the wild carrot stem has many fine hairs and is always green.

Poison hemlock is a biennial, so it grows year after year. The first year of it looks much like a rosette, low to the ground, with a small bush-like plant. By the second year, it is bolting and flowering. “And then is when it gets pretty hard to control,” Bunton says.

Courtesy of Michigan State UniversityA calf with crooked limbs

CALF CONCERNS: Pregnant beef cows may have already eaten enough poison hemlock to cause problems for their future calf. Crooked calf disease can create problems with limbs, but also cleft palates.

The flowers look similar to wild carrot but are generally smaller. They are a flat-top flower made up of a lot of different smaller flowers. Still, at the flowering stage, it is easier to tell the difference because the purple stem is very visible.

Evans says poison hemlock is toxic, whether livestock graze it in the pasture or farmers bale it and feed it later.

Ingesting hemlock is toxic

Poison hemlock is made up of compounds that act like nicotine, Evans explains. There are two different types of intoxications that can be associated with poison hemlock. One is when livestock have what starts out as tremors and then eventually causes paralysis leading to death. The second has to do with the compounds in the plant, particularly in the early growth stages, being teratogenic, causing birth defects in calves, pigs, sheep and goats.

How an animal reacts depends on how much and when the poison hemlock was ingested. Symptoms include dilation of the pupils, reduced heart rate, coma, trembling, nervousness and difficulty breathing, and can occur as quickly as in two hours.

Evans is more worried about, for instance, cows consuming it in the early growth stage and during their early reproductive stage. For cattle, consuming it during the first 90 days of the first trimester can result in birth defects.

“It doesn’t take much to cause those adverse effects,” he warns. “And at that stage, you might not know there’s a problem if you haven’t seen it and the animals get into it.”

Barry Rice,, Bugwood.orgPlants shown in the rosette stage

TREAT EARLY: The best time to rid pastures or hayfields of poison hemlock is when it is in the rosette stage. Chemical or manual removal works well.

When the offspring are born, they have what Evans calls “multiple congenital contractures,” or what some call crooked calf disease. “You’ll see problems in their limbs; you may see a deformity in the back as well,” Evans explains. Some calves are born with cleft palates.

Whether a calf lives or dies depends on the severity of condition. Evans says those calves with cleft palate are prone to pneumonia, but those with a minor defect, like bowed legs, can live for quite some time. “It may be something the animal can survive,” he says, “but it is certainly not going to thrive.”

Both men agree ridding pastures of the poison hemlock will solve potential problems.

Destroying poison hemlock in pastures

Whenever poison hemlock reaches its second year of growth, it becomes difficult to control, Bunton says. Applying herbicides at the rosette stage are going to yield the best results.

He recommends Grazon P+D, or GrazonNext and including Remedy — that triclopyr component offers good control when looking to a broadcast spray option. For spot treatments, 2, 4-D or glyphosate plus dicamba works. Farmers also can manually remove poison hemlock with a shovel.

It is important to treat poison hemlock before going to seed. Mowing pastures can help, but it does not remove the plant from the field.

“The biggest concern is that we recognize that if there is a small amount in hay that animals get into,” Evans says, “it is a potential risk.”

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