Veterinary advice: anything green is not edible
Plant poisoning is a common problem throughout North America, causing significant losses due to sudden death, reproductive failure, low growth rates, contamination of animal products (milk, meat) and physical damage.
Recognizing poisonous 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 article Poisonous native plants, uses milkweed (Asclepias species) as a classic example. Milkweed contains an irritating milky sap and therefore unpleasant but also toxic.
“Other compounds found in plants that are potentially toxic to animals are normal plant components 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 proteins. Plants such as locoweeds (Astragalus and Oxytropis species) are poisonous because they have developed a mutually beneficial relationship with specific fungi (endophytes) which, when growing in the plant, produce a poisonous alkaloid toxic to horses and cattle. . There are many native plants, over 200 in total, potentially toxic to livestock, but eating a few bites rarely poisons an animal, ”says Knight.
Every summer, prairie ranchers lose animals to poisonous plants. Some poisonous plants are good fodder when eaten in small amounts, as the poison is eliminated or diluted as quickly as it is ingested. Most poisonous plants are only dangerous when eaten in large quantities and can be harmless at certain times of the year. However, a few are extremely toxic, even in small amounts, throughout the year.
Prevention of livestock losses depends on a good knowledge of poisonous plants and the seasons when they are most dangerous. Good management will often prevent poisonous plants from being a problem. When investigating sudden livestock death on pastures, producers and veterinarians should keep common poisonous plants in mind.
When cattle are dragged long distances, animals tend to eat anything green along the trail and may graze on poisonous plants. The scarcity of palatable forage, lack of water or lack of salt can cause animals to graze on 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 can have unexpected problems. Check the pasture carefully before grazing. Feeding hay full of weeds can also be poisonous.
Western hemlock: The most poisonous plant in North America. It is toxic to all livestock and humans. A 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. In late summer, the toxin is confined to the roots, but a grazing animal can pull them out if the soil is wet. Western hemlock grows in moist areas such as springs or the edges of streams.
Arrow grass by the sea: The second largest plant in Alberta. Arrowgrass is an erect, grass-like plant with a spike-shaped flower stalk. It grows in salt marshes and alkaline swamps, where it is underwater for part or all of the growing season. It can be found alone or in patches. Young leaves are the most poisonous, and as little as two kg of plant material is fatal if eaten in a short time. Arrow grass begins to grow earlier in the spring than pasture grasses, and cattle hunting for green plants can then graze on it. Animals can also eat the plants throughout the year if they are deprived of salt, as arrowgrass has a high salt content.
Camas of death: A small, herbaceous perennial with grass-like leaves. The many flowers are small and creamy yellow. The plant grows from a small onion-like bulb. It begins to grow early in the spring, before most grasses, and flowers in May. All parts of the plant are poisonous, especially the bulb. It can be grazed in early spring when livestock are looking for new growth.
Milkvetch: Narrow-leaved Milkvetch is a branched herb that grows one to two feet tall. The leaves are compound, with very narrow leaflets. The cream-colored flowers open in early June. It is found in open grasslands and along roadsides, often on more sandy soils.
Two-Grooved Milkvetch is a sturdy, multi-stemmed plant one to three feet tall. It has a distinct and unpleasant odor. It also has compound leaves, with many small leaflets. The flowers are dark purple and showy. It grows in semi-humid sites in southern Alberta. It is often found in new road ditches. Both milkvetches accumulate selenium, which causes problems for livestock when grazed.
Smothered Cherry: Eating leaves on branches cut from cultivated red cherry trees sometimes poisons livestock. The poisonous substance of the Chinese cherry tree, hydrocyanic acid, is mainly found in the leaves. But all parts of the plant contain cyanogenic glycosides, except ripe berries. Wilted leaves are more poisonous than when they are fresh. The leaves become less toxic as the growing season progresses.
Chokecherry and arrowgrass both contain hydrocyanic acid. Rumen microorganisms readily hydrolyze cyanic glycosides to release hydrogen cyanide (HCN) – a highly toxic chemical. Cyanide blocks the action of the enzyme cytochrome oxidase which prevents hemoglobin from releasing oxygen into the tissues. Death quickly results from anoxia. Cyanide glycoside concentrations increase when the plant is stressed by drought or frost.
Larkspur: Cattle deaths from larks foot poisoning have plagued the industry for over a century. As members of the delphinium family, the 60 wild species are grouped together based on their size at maturity and preferred growing location. Tall and low lark’s foot species contain many alkaloid compounds. The most important toxin can cause muscle paralysis leading to respiratory failure and death. Affected cattle can exhibit a variety of symptoms, including muscle weakness, staggering, difficult breathing, bloating and inability to stand. Unlike cattle, sheep are relatively unaffected by the consumption of larkspur.
The famous quote from Paracelsus sums it up best: “All things are poison and nothing is without poison; the dose makes a poison.
Dr. Ron Clarke is preparing this column on behalf of the Western Cattle Practitioners Association. Suggestions for future articles can be emailed to Canadian cattlemen or WCABP.