Poisonous Plants on Pastures

Cathie Erichsen Arychuk, P.Ag.
Bison Production Specialist
AAFRD, Fairview


Each year, livestock producers in Alberta lose animals to poisonous plants.  Recognizing problem plants and understanding management opportunities to reduce the risk of poisoning are important tools in preventing a serious problem on your pastures.

Poisonous Plants on Pastures

Every summer, livestock producers in Alberta lose animals to poisonous plants.  About 200 species of plants have been implicated in livestock poisonings in Alberta. Luckily, only a couple commonly cause problems.

Plants are considered poisonous if they contain toxic substances in sufficient amounts to harm livestock.  Depending on the type of poison, and the amount eaten, an animal's reaction can range from reduced performance to death.  Plants are not easily classified as poisonous or not.  Some poisonous plants are good forage when eaten in small amounts because the poison is eliminated or diluted as rapidly as it is ingested.  Most poisonous plants are dangerous only when consumed in large quantities and even then may be harmless at certain times of the year.  However, a few such as western water hemlock are extremely poisonous even in small amounts.

Preventing livestock losses depends on a good knowledge of poisonous plants and the seasons when they are the most dangerous.  Good management will often prevent poison plants from being a problem.  Most poisonous plants are native to range and pasture lands.  Stock do not normally graze these plants because they are unpalatable or few in number.

Common Poisonous Plants

Western Water Hemlock
Water hemlock is the most poisonous plant in North America.  It is toxic to all livestock and to humans.  One root can kill a cow.  Most livestock losses occur in the spring, when the toxin is present in all parts of the plant.  By late summer, the toxin is confined to the roots, but these can be pulled out by a grazing animal if the soil is wet.  Western water hemlock grows in wet areas such as springs, or stream edges.

Seaside Arrowgrass
Arrowgrass is the second most troublesome plant in Alberta.  Arrowgrass is an erect, grass-like plant, with a spikelike flower stalk.  It grows in salt marshes and alkaline sloughs, where it is underwater for part or all of the growing season.  It can be found singly or in patches.  Young leaves are the most toxic, and as little as 2 kg of plant material is fatal if eaten over a short time period.  Arrowgrass starts to grow earlier in the spring than pasture grasses, and stock hunting for green plants may graze it then.  Animals may also eat the plants throughout the year if they are deprived of salt, as arrowgrass has a high salt content.

Death Camas
Death camas is a small perennial herb with grasslike leaves.  The many flowers are small, and creamy yellow coloured.  The plant grows from a small bulb, resembling an onion.  It starts growing early in the spring, before most grasses, and it blooms in May.  All parts of the plant are poisonous, the bulb most of all.  It may be grazed in early spring when stock are hunting for new growth.

Milk Vetch
Narrow-leaved milk-vetch is a branched herb that grows from 1 to 2 feet tall.  The leaves are compound, with very narrow leaflets.  The cream-coloured flowers open in early June.  It is found on open prairies and roadsides, often on sandier soils.  Two-grooved milk vetch is a stout, many stemmed plant 1 to 3 feet tall.  It has a distinct, unpleasant odour.  It also has compound leaves, with many small leaflets.  The flowers are deep purple and showy.  It grows on semi-moist sites, across Southern Alberta.  It is often found in new road ditches.  Both milk-vetches accumulate selenium, which causes problems in livestock when grazed.

Most cases of livestock poisoning from plants can be prevented.  Livestock are occasionally trailed from place to place over long distances.  While on the move, animals tend to eat any green plant along the trail and may graze poisonous plants.  Scarcity of palatable forage, lack of water, or lack of salt may cause animals to graze material that might otherwise be rejected.  Grazing too early in the spring, before forage species have produced much growth increases the likelihood of poisoning.  New pastures may have unexpected problem areas.  Check the pasture carefully before grazing it.

Several publications to help you identify and manage poisonous plants are available.  Stop in to your local Alberta Agriculture, Food and Rural Development office to pick one up.

Sign up to our e-newletter today to keep up to date with us!

* indicates required