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Weed & Pest

With Greening of Range Comes Danger of Poisonous Plants

Written by Rachel Mealor
    With green up of rangelands and pastures, learning to identify poisonous plants in your area that could be potentially harmful to livestock is important.
    Poisonous plants occur in many places throughout pastures, so why don’t animals always die from them? Likewise, why do poisonous plants seem to be very toxic one year, but not the next? According to research from the Behavioral Education for Human, Animal, Vegetation,
and Ecosystem Management (BEHAVE) group at Utah State University (www.behave.net), the potency of a toxin strengthens as the stress level in the animal increases. Stress intensifies a toxin’s action on the animal, likely by reducing the body’s ability to detoxify or dilute the toxic properties of the plant. Stress can result from various situations, such as predators, herders, weather, new environments, etc.
    Body condition of livestock can also play a role on the effect poisonous plant consumption can have on an animal. Thin animals or animals in poor body condition are often more susceptible to toxic plants and more likely to suffer from their effects than animals in average or better condition. This may be due in part to a greater body mass of the animals in better condition in which to dilute plant toxins.
    Livestock losses can often be prevented through proper livestock and pasture management. Surveying each of your pastures for plants that could be potentially harmful to your livestock, before moving your livestock into them for grazing, is suggested. Continue plant monitoring activities during the time animals graze each pasture.  
    Managers should never knowingly allow hungry or thirsty animals to graze areas heavily infested with poisonous plants. If poisonous plants are present around watering areas, animals may actively graze on these succulent plants; this high consumption of the poisonous plant coupled with a large intake of water can have a deadly impact on the grazing animal. In areas where poisonous plants are found, assure an adequate level of desirable forage is available for grazing prior to animal turn out. Animals that have a choice of other highly palatable or nutritious plants are less likely to consume poisonous plants. Having supplemental feed and water available upon arrival to animals being trailed or transported to new ranges can also help prevent losses that could occur due to plant toxicity.
    The belief weeds are more poisonous than other plants is untrue, as some poisonous plants are native. Another incorrect assumption is that poisonous plants occur only as a result of bad management. Some poisonous plants are part of the native plant community found on Western rangelands (such as tailcup lupine or little larkspur). Their presence is natural and does not reflect poor management. There are large expanses of well-managed rangelands that exhibit natural populations of poisonous plants. In these areas, it is up to the resource manager to develop grazing plans that will minimize the effect of these plants on animals using these pastures.
    Here are several examples of poisonous plants to look for. Further information and photographs of these poisonous plants and others can be found in A.P. Knight and R.G. Walter’s book A Guide to Plant Poisoning of Animals in North America. Information about these plants and others is also online at http://plants.usda.gov/index.html.
    Death camas (Zigadenus spp.) is one of the first plants to green up in the early spring. Often found growing among wild onion, death camas can be differentiated as its leaves are not hollow like those of wild onions nor do they smell like an onion. Sheep are most likely to be affected by eating death camas, but, occasionally, cattle and horses are affected. Its habitat ranges from moist mountain valleys to dry, sandy hills and plains. Animals can be poisoned by consuming leaves, stems, and flowers that contain toxic alkaloids. The bulbs can cause severe illness in humans when eaten. Typical symptoms observed in affected animals include salivation, nausea, vomiting, muscular weakness, and staggering gait. Keeping animals out of areas where death camas is found, particularly in early spring when other green growing forages are unavailable, can prevent consumption of this plant.
    Larkspur (Delphinium spp.) probably causes more cattle losses in the western United States than any other native plant species. Like death camas, the toxic substances found in larkspurs are alkaloids. Cattle are very susceptible to larkspur poisoning, as they find this plant palatable. Sheep and horses can potentially be affected, especially if subjected to sudden physical activity after consuming large amounts of larkspur. Larkspur is commonly grouped into tall and low varieties according to their growth habit. Tall larkspurs grow in deep, moist soils at high altitudes and may reach seven feet in height. Tall larkspurs generally start growth in May or June. Low larkspurs grow at lower elevations in drier rangeland, rarely reaching a height of more than two to three feet. Low larkspurs begin growing in early spring and are often the only green forage available for grazing on the rangeland at that time. It is seldom eaten after reaching maturity. Leaves are divided into deep lobes and are usually confined to the bottom half of the plant. Larkspurs have showy, ornately spurred blue to deep purple flowers. Common signs of poisoning include uneasiness, increased excitability, staggering and falling, muscle twitching, regurgitation, and bloat. If possible, keep cattle off ranges infested with larkspur until the larkspur is mature and dries out. If avoidance is not feasible, then knowledge of the plant’s growth habits and management of livestock grazing, plant selectivity, and behavior will help producers manage the problems associated with grazing cattle in areas infested with larkspur.
    Poison hemlock (Conium maculatum) is poisonous to a variety of animals, including birds, wildlife, cattle, sheep, goats, pigs, horses, and man. People are generally poisoned when they mistakenly eat hemlock for plants such as yampa (Perideridia gairdneri), parsley (Petroselinum crispum), and Queen Anne’s lace (Daucus carota). Livestock are not attracted to this plant due to its strong odor; however, they will eat it if no other forage is available or if it is incorporated in hay or silage. Poison hemlock can be found along roadsides, ditches, cultivated fields, and waste areas particularly where the ground is moist. Poison hemlock begins growth in early spring. It is a coarse upright plant around four to six feet in height. It has a hollow stem usually marked with small purple spots (especially near the base). Leaves are delicate, similar to parsley. Signs of poisoning are similar in all species and develop within an hour of the plant being eaten. Salivation, abdominal pain, muscle tremors, and incoordination will occur first, followed soon by trouble breathing, dilated pupils, weak pulse, and finally respiratory paralysis and coma without convulsions precede death. Destroying poison hemlock plants by mowing or with herbicides prior to the seed stage greatly reduces the chances of hemlock becoming an invasive weed and problem to livestock.
    As you plan your turn out dates and pasture rotations for the summer, remember to keep poisonous plants in mind. For more information on poisonous plants and other grazing or management issues, please contact me or your local county extension educator. Contact information for University of Wyoming Cooperative Extension Services (UW CES) county extension offices is available online at http://ces.uwyo.edu/Counties.asp.
    Rachel Mealor is the UW CES range specialist. She can be reached at 307-332-1840 or This email address is being protected from spambots. You need JavaScript enabled to view it..