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A poisonous weed that is rarely seen

With the wetter spring and summer, sightings of western water hemlock have increased. Prior to 2006 I personally had only seen one occurrence of this highly toxic plant in 25 years of practice. This year in a short time several findings with multiple plants in our practice area a great distance apart have been identified. This is one of the most potent poisonous plants known to cattle. One root bulb can kill a mature cow very quickly. It is important to be on the lookout for this toxic plant and inform your neighbours if it has been sighted in the area.

With the suspicion of any toxic plant 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 bulbous and this distinguishes it from look-alike plants. It is commonly confused with water parsnip, which also has narrow leaves, but they do not have the tooth-like margins and the roots are not bulbous. 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.


Poisonings to water hemlock will generally occur in the early spring as the young shoots are among the first plants to green up, and also in fall as pastures run low. A sun-loving plant, hemlock is usually found out in the open but near dugouts or along streams and other water sources.

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 is well versed in identification. It is important these fieldmen also know this plant is present in your geographic area. Sprayer operators are also excellent reference sources as that is their job to identify weeds in order to select appropriate sprays. 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 precaution. The plant is a perennial so try and 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 prevent rather than treat the disease. Be vigilant in subsequent years in case regrowth from a root bulb can occur and check pastures before livestock are turned in. The seeds are not considered toxic but removing before plants go to seed goes without saying.


Rarely are cattle suffering from hemlock poisoning found 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 the animals are found alive. Treatment by the veterinarian would consist of trying to control the convulsions. No specific antidote exists but depending on the amount consumed low-level poisonings can recover with no long-term effects.

All species of animals could be susceptible but cattle, sheep, goats and bison because of their grazing patterns are most susceptible. They are all less fussy grazers and in conditions of low forage availability will go after these less-desirable plants. Cattle, especially because of the pulling action of their grazing, are most susceptible. Deaths in horses and swine have also been documented.

Fortunately poisonings are very rare because conditions must be right between 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 onto pastures be ever vigilant of 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 where overgrazing has occurred and if we can prevent poisonings by removing some toxic plants in the process so much the better. Fortunately death from water hemlock is rare but it should be considered when there is a sudden death with very little post-mortem findings. †

About the author


Roy Lewis is an Alberta-based veterinarian specializing in large-animal practice. He is also a part-time technical services vet for Merck Animal Health.



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