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Western water hemlock is a deadly killer of cattle

This year’s wet spring has seen increased sightings of western water hemlock — one of the most poisonous plants known in cattle production.

Prior to 2006, I had only seen one occurrence of this highly toxic plant in 25 years of practice. This year, several multiple plants have been found a great distance apart in our practice area. One root bulb can kill a mature cow very quickly, and so it’s important to be on the lookout for this toxic plant and inform your neighbours if found.

As with any toxic plant, accurate identification is critical. Water hemlock has narrow leaves with sharp tooth-like margins. The flowers are small, white and in umbrella-like clusters. The roots are very bulbous, which distinguishes it from look-alike plants such as water parsnip, which also has narrow leaves, but lacks tooth-like margins and bulbous roots. Cow parsnip is also very common in our area, but it is generally a larger plant and has very large fan-like leaves. In drier conditions, cattle and other livestock can graze cow parsnip and it actually has pretty good feed value. Poisonings to water hemlock generally occur in the early spring from its young shoots, which appear before much else is growing. Hemlock likes wetter conditions so is often found around dugouts, streams and other water sources. It generally does not like a lot of shade so is often in the open. The late fall — when other vegetation is sparse — is the other critical time when poisonings occur from eating the bulbous roots. The plant in its entirety can be pulled out easily which is how livestock, especially cattle, gain access to the roots.

If you have problems identifying this or other potentially toxic or noxious weeds there are several sources for advice. The local agricultural fieldman or crop specialist are well versed in identification. It is important these ag fieldmen also know this plant is present in your area. Sprayer operators are also well versed in weed identification. Veterinarians are well trained in the treatment of the poisonings and could reference pictures of the toxic plants.

Water hemlock control involves manual removal, as plant numbers are generally low, close to a water source and there can be a fair distance between plants. The poison is toxic to humans so use gloves when picking and do not cut into the bulbous roots. Protective eyewear would also be a wise precautionary measure. The plant is a perennial, so try to pull the entire root out. This is generally easy especially on the bigger plants by grasping right at the base of the plant. Any small leaf shoots should also be removed. Dispose by incinerating, desiccating or composting. As with all poisonings, it is far better to be preventive than treat the disease. Be vigilant in subsequent years in case of regrowth, and check pastures before livestock are turned out. The seeds are not considered toxic, but removing before plants go to seed goes without saying.

Rapid death

Rarely would you find livestock from hemlock poisoning alive as death can occur within 15 minutes. Most are reported as sudden deaths around water sources. Here veterinarians must rule out other causes of sudden death such as blue-green algae poisoning, anthrax, blackleg or bloat. Many of these toxins appear to be increasing in frequency. Convulsions and other nervous signs such as frothing and clamping of the jaws are observed if animals are found alive. Treatment by a veterinarian would consist of trying to control the convulsions. No specific antidote exists, but depending on the amount consumed, animals can recover from low-level poisonings with no long-term effects.

All species of animals are vulnerable but because they are less fussy grazers, cattle, sheep, goats and bison are especially at risk. Cattle, because of the pulling action of their grazing, are most susceptible. Deaths in horses and swine have also been documented. Fortunately poisonings are very, very rare because conditions must be right between the stage of plant growth and the lack of other available pasture. Rotational grazing systems where large numbers of animals are forced onto a small area could actually increase likelihood of exposure to hemlock if it was present.

When walking pastures, look for what species of grasses, forbs and weeds are present. This gives us clues as to the health of the pasture, where production can be improved, and where overgrazing has occurred. If we can prevent poisonings by removing some toxic plants in the process, so much the better.

About the author


Roy Lewis practised large-animal veterinary medicine for more than 30 years and now works part time as a technical services veterinarian for Merck Animal Health.


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