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Danger stalks spring pastures

Vet Advice with Dr. Ron Clarke

Every spring and summer, livestock producers incur losses as a result of poisonous plants. Acute deaths often go undiagnosed. The more than 200 poisonous species of plants on Canadian range cause chronic illness and debilitation, decreased weight gain, abortion, birth defects, poor reproductive performance, and photosensitization.

Some poisonous plants are nutritious when eaten in small amounts, dangerous only when consumed in large quantities, or consumed during certain times of year. Conditions like overgrazing, trucking, trailing, corralling, or introduction onto new range trigger changes in grazing patterns and, ultimately, to the ingestion of poisonous plants.

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Signs of toxicity are typically dose-related so clinical signs vary between individual animals in a herd and between ages. Knowing when useful forages become poisonous is an important element of range management.

The definitive diagnosis of plant poisonings is often difficult. Helpful information covers:

  • Familiarity with the types of poisonous plants growing in a specific area.
  • Understanding conditions under which livestock may be poisoned.
  • Knowledge of local soil deficiencies or excesses, which may complicate plant toxicities or confuse interpretation of clinical signs.
  • Knowledge of syndromes associated with each species of poisonous plant in the area.
  • Understanding the times through the grazing season problems might arise.
  • A detailed history of the animal(s): age, breed, etc.
  • A record of management changes already instituted or planned.
  • Clinical examination of affected animals or necropsy of mortalities.

Common causes of acute poisoning:

Western water hemlock (Cicuta douglasii)

Common names include: beaver-poison, children’s-bane, snakeweed and musquash-poison. Water hemlock is the most poisonous plant in North America. It is toxic to all livestock and humans. 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 that can be pulled out from moist soil and eaten by a grazing animal. Western water hemlock grows in wet areas like springs or stream edges. It can be confused with wild carrot, wild parsnip or wild ginseng, non-poisonous members of fleshy root plants.

All plants from the genus Cicuta contain cicutoxin. Roots maintain toxicity even after drying.

Water hemlock is a large native biennial with the umbrella-shaped clusters of small white flowers typical of the carrot family. Correct identification is important, both to reduce the risk of livestock consuming it, and to minimize tedious removal of patches of similar-looking but innocuous species. The following reference at foragebeef.ca ‘Stock-poisoning Plants of Western Canada‘ is an excellent source of information .

When cut vertically, chambers in the stem base and in the thicker roots exude yellowish, aromatic, and extremely poisonous oil. The odour has been described similar to raw parsnip or parsley. Symptoms of poisoning appear rapidly, usually within 30 minutes after ingestion. The first symptom is excessive salivation and frothing. Frothing is followed by tremors, uneasiness, and violent convulsions. Severe abdominal pain and colic is common. Between convulsive seizures animals become recumbent. Due to the rapid onset of symptoms, treatment is usually unsuccessful. It only takes a piece of the root the size of a walnut to kill a 1,200-pound cow or horse.

Seaside arrow-grass (Triglochin palustre)

Arrow-grass is the second most troublesome plant in Alberta.  It is an erect, grass-like plant, with a spike-like 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 and emits a pleasant aromatic smell when bruised. Young leaves are the most toxic, and as little as two kilograms of plant material is fatal if eaten over a short time. Arrow-grass starts to grow earlier in the spring than pasture grasses, and stock hunting for green plants may graze it. Animals may seek the high salt content of arrow-grass if deprived of salt through the grazing season. The toxic ingredient is cyanide (prussic acid), a deadly poison. Cyanide ions interfere with cellular respiration resulting in the body’s tissues being unable to use oxygen.

Other cyanogenic species include sorghum, chokecherry, pin cherry, wild black cherry.

Rupturing plant cells by cutting, wilting, freezing, drought, crushing, trampling, chewing or chopping induces cyanide formation. Once consumed, cyanide rapidly enters the blood stream. Poisoned animals are often found dead. Clinical signs, when observed, occur in swift succession — rapid and laboured breathing, excitement, generalized muscle tremors, staggering, and collapse. The mucous membranes are usually bright pink, and the blood a bright cherry red.

Two forms of chronic cyanide poisoning in domestic animals exist: hypothyroidism due to disruption of iodide uptake by thyroid cells; and chronic cyanide induced neuropathy (progressive paralysis) with bladder problems.

Death camas (many varieties)

Death camas is a small perennial herb with grass-like 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. It may be grazed in early spring and cause severe losses when stock hunt for new growth. Death camas contains toxic steroidal alkaloids.

Most cases of livestock poisoning from plants can be prevented.  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 produce ample growth also increases the likelihood of poisoning.

About the author

Columnist

Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).

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