There are many plants that are poisonous to animals. When forage is plentiful, animals avoid these plants because they may taste bad, have physical barriers such as barbs to discourage consumption, or are in areas where cattle typically do not graze. These plants can cause photosensitivity, abortions, birth defects, have contact irritants or mycotoxins present that reduce animal performance or cause a quick death.
Problems with poisonous plants increase when the forage available on pasture is limited. This can be from overgrazing or moving animals to a new pasture that disrupts their grazing periods.
Once on the new pasture, animals are hungry and will eat anything they can find over a short period of time. If possible, contain the animals in a small area and provide hay or silage the first day to reduce the initial feed consumption on the new pasture.
Some of the most common weeds that cause problems:
Seaside Arrowgrass and Marsh Arrowgrass are found in salt marshes and in saline soil around sloughs. These plants (along with saskatoons and chokecherries) contain hydrogen cyanide in the stems and leaves which causes poisoning. Consumption of 7.7 pounds of fresh arrowgrass can kill a 1,100 pound animal.
Death Camas is a plant that starts growing early in the spring. It can grow throughout the pasture especially in draws and depressions. All parts of the plant are poisonous. Highest concentrations of the steroid alkaloids occur in the vegetative to bud stage. The toxins persist in cut hay. Ingestion of 0.44 pounds of fresh material can kill a 110 pound sheep.
Water Hemlock is considered the most poisonous plant found in low areas. The toxin is contained in the root. When the plant is consumed, the root is often pulled out of the wet soil and is ingested. The cicutoxin acts on the central nervous system, causing convulsions and death from respiratory failure. Death can occur withing 30 minutes of consumption. The toxin can also kill humans. Do not attempt to remove these plants without full protective equipment.
This information was found in the publication authored by Majak, Brooke, and Ogivie. It is worth the time to review. https://www.beefresearch.ca/files/Stock_Poisoining_Plants_of_Western_Canada.pdf
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