Rangeland Noxious Weeds and Invasive Species

Guides to Pacific Northwest Weeds: Washington State Noxious Weed Control Board, Whitman County Noxious Weed Control Board, Missoula County Weed District

Guides to North American Weeds: The Weed Science Society of America, Revegetation Guidelines for Western Montana: Considering Invasive Weeds

Poisonous and Injurious Plants

Poisonous range plants can kill livestock and wildlife and reduce rancher income. Diseases and injuries to livestock are often caused by poisonous plants. Some range plants are poisonous only at certain stages of growth or seasons of the year, while at other times they provide good forage. Knowledge of these plants is essential to the rangeland manager.

Livestock losses are due primarily to an abundance of toxic plants and overgrazing. Many toxic weeds are unpalatable and/or not in dense bunches. Under good range conditions these plants would not pose a significant risk to stock but under poor forage conditions stock may be compelled to eat large quantities of plants they would otherwise avoid. Poor range management, overstocking and little to no knowledge of poisonous plants caused extensive poisoning of stock in the past. Improvements in these areas have lessened poisoning incidences. There is a need for further research on poisonous plants, their life cycles and their interactions with animal physiology. The poisonous plants of primary concern in the west are: Larkspur, Locoweed, Milkvetch, Lupine, and Death Camas. In some cases poisonous plants are palatable and some are abundant in a healthy climax community. In these cases stock and range must be carefully managed to reduce the probability of poisoning. In some cases limited herbicide use or aversion training may be necessary. (Ralphs, 2002)

Stock losses due to poisoning may be underestimated since stock deaths are not always recognized as poisonings. An annual death loss of cattle for 17 western states is 1%. For sheep it is 3.5%. These losses combined with reproductive losses leads to a loss of about $234,257,080 per year. This does not include reductions in animal performance or losses of goats, horses and wildlife. (Nielsen, 1988)

Here are some aids in preventing stock poisoning:

• 
  Know poisonous plants, their characteristics and their effects on livestock and wildlife.
• Do not overuse the range forage. Most poisonous plants are shunned by well-fed grazing animals. Overuse often kills good forage plants while encouraging growth of poisonous plants.
• Graze livestock only in the proper season. Also, it is sometimes possible to prevent poisoning by grazing during that part of the season when
  poisonous plants are not available or are not preferred by livestock.
• Be careful with hungry animals. They are more likely to graze poisonous
  plants during tailing, bedding or after long truck hauls. Poison-free feed
  should be supplied before hungry animals are turned out on ranges with
  poisonous plants.
• Provide ample salt and supplement any minerals known to be deficient
  in the native forage. Animals sometimes eat poisonous plants that they
  would not graze if they had the right kind of feed.
• If possible, use a class of stock not poisoned by the plant present. Cattle can sometimes safely graze ranges containing plants harmful to sheep, and vice versa.
• Use herbicides to kill the poisonous plants.
  
  

Many range plants are mechanically injurious to animals at various times of the year. Members of the cactus family cause injury to grazing animals when their spines puncture the animal’s skin. This causes serious damage during screwworm seasons in the southern states. Needlegrass injures sheep when the seed is mature and begins to shed. Once the needlegrass seed gets into the wool, it may work through the skin. This damages the muscle tissue and lowers the quality of the meat and makes the hide useless for leather. At all other times, the needlegrasses furnish good quality forage to grazing animals.

Grasses with sharp awns or beards may be injurious to livestock. They may furnish grazing early in the spring and again in the fall, but as a general rule, the forage value of heavy-awned grasses is limited.

Revegetating Noxious Weed-Infested Rangeland