Plant poisoning is a common problem throughout North America, causing significant losses from sudden death, reproductive failure, poor growth rates, tainting of animal products (milk, meat) and physical damage.
Recognizing toxic plants and understanding the effects of toxins on animals is an important aspect of good range management. Plant poisoning can be largely avoided.
Plants contain a variety of toxic compounds that deter herbivores and insects. Dr. Anthony Knight, College of Veterinary Medicine and Biological Sciences, Colorado State University, in his paper Poisonous Native Range Plants, uses milkweed (Asclepias species) as a classic example. Milkweed contains a milky sap that is an irritant and therefore distasteful but is also poisonous.
“Other compounds found in plants, potentially toxic to animals, are normal components of plants essential for plant growth. Nitrates and cyanogenic glycosides, for example, are found in a wide variety of plants and are essential in the formation of plant protein. Plants such as locoweed (Astragalus and Oxytropis species) are poisonous because they have developed a mutually beneficial relationship with specific fungi (endophytes) that, when growing in the plant, produce a toxic alkaloid poisonous to horses and livestock. There are numerous native range plants, over 200 in total, potentially poisonous to livestock, but rarely does eating a few mouthfuls poison an animal,” explains Knight.
Every summer, livestock producers across the Prairies lose animals to poisonous plants. 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 then may be harmless at certain times of the year. However, a few are extremely poisonous even in small amounts, year-round.
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. When sudden death of livestock on pasture is being investigated, producers and veterinarians should keep common poisonous plants in mind.
When livestock are trailed over long distances, animals tend to eat anything green 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. Feeding hay full of weeds can also be toxic.
Western water hemlock: The most poisonous plant in North America. It is toxic to all livestock and humans. One root can kill a cow. It contains a toxin called cicutoxin, a violent convulsant, which acts as a stimulant in the central nervous system 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 a grazing animal can pull these out if the soil is wet. Western water hemlock grows in wet areas such as springs, or stream edges.
Seaside arrowgrass: The second-most troublesome plant in Alberta. Arrowgrass 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. Young leaves are the most toxic, and as little as two kg of plant material is fatal if eaten over a short time. 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: A small perennial herb with grass-like leaves. The many flowers are small, and creamy yellow. 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 is hunting for new growth.
Milkvetch: Narrow-leaved milkvetch is a branched herb that grows from one to two 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 milkvetch is a stout, many-stemmed plant one to three 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 milkvetches accumulate selenium, which causes problems in livestock when grazed.
Chokecherry: Eating leaves on branches trimmed from cultivated chokecherry trees sometimes poisons cattle. The toxic substance in chokecherry, hydrocyanic acid, is found principally in the leaves. But all parts of the plant contain cyanogenic glycosides, except the ripe berries. Wilted leaves are more toxic than when fresh. Leaves become less toxic as the growing season advances.
Chokecherry and arrowgrass both contain hydrocyanic acid. Rumen micro-organisms readily hydrolyze cyanic glycosides to free hydrogen cyanide (HCN) — a highly poisonous chemical. The cyanide blocks the action of the enzyme cytochrome oxidase that prevents hemoglobin from releasing oxygen to the tissues. Death results rapidly from anoxia. Concentrations of cyanide glycosides increase when the plant is stressed by drought or frost.
Larkspur: Cattle deaths from larkspur poisoning have plagued the industry for well over a century. As members of the Delphinium family, the 60 wild species are grouped based on height at maturity and preferred growing location. Both tall and low larkspur species contain many alkaloid compounds. The most significant toxin can cause muscular paralysis leading to respiratory failure and death. Affected cattle can show a variety of symptoms including muscular weakness, staggering, laboured breathing, bloat and inability to stand. In contrast to cattle, sheep are relatively unaffected by larkspur consumption.
The famous quote by Paracelsus summarizes it best: “All things are poison and nothing is without poison; the dose makes a poison.”