Eagle County Healthy Landscapes: Safeguarding livestock from poisonous plants | VailDaily.com
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Eagle County Healthy Landscapes: Safeguarding livestock from poisonous plants

Denyse Schrenker
Special to the Daily
Russian knapweed is found throughout Eagle County and is allelopathic, meaning it exudes a toxic substance in the soil that prevents competing plants from being able to grow. It is only toxic to horses and causes the muscles in their lips, jaws, and tongue to become permanently frozen so they cannot chew.
Special to the Daily

Poisonous plants can cause a variety of problems in livestock such as a really bad sunburn, muscle spasms, difficulty breathing, and death. Some may cause very mild reactions while others can result in sudden death.

Not all parts of the plant may be poisonous, like Rhubarb. While we make the stems into delicious pies and jams, eating the leaves could kill you. Some plants are more poisonous under certain conditions, like after a freeze or during drought, and some only cause problems in certain livestock species. 

So what is it about certain plants that makes them poisonous? Although all plants produce secondary compounds, some which can be toxic, poisonous plants accumulate these toxic compounds at levels dangerous to livestock. Palatability is also an important aspect of poisonous plants. A plant might be toxic, but if livestock thinks it’s about as tasty as a toddler finds vegetables appetizing, then it does not pose much threat. 

In addition to potentially harming Seabiscuit, poisonous plants cause large economic losses. Estimates indicate that the annual death rate due to plant poisoning for cattle, sheep, and horses is somewhere around three to five percent. For the western United States, the monetary value for death and abortion in livestock from eating poisonous plants exceeds $340 million. Economic loss is much more significant than loss due to death because it accounts for lower weaning weights or birth deformities, smaller calf and lamb crops, weight loss and poor performance in animals, and the increased costs of controlling the plant and fencing. 

Learning to identify poisonous plants is key to preventing heartache and financial loss. Although there are many native plants that are poisonous to livestock, such as the appropriately named Death Camas, we will focus on the non-native varieties. For a complete list of plants found in Colorado that are poisonous to livestock, please visit https://csuvth.colostate.edu/poisonous_plants

The Colorado Noxious Weed Act outlines certain criteria a non-native plant must pass in order to be declared noxious. Being poisonous to livestock is one of those criteria.

  • Boucingbet (Saponaria officinalis) is not frequently a problem for livestock poisoning, but it causes more issues as an invasive weed.
  • Common tansy (Tanacetum vulgare), which was introduced from Europe in the 1600s to be used as an ornamental, and its use for embalming and as an insect repellant, is toxic to livestock but they find it very unpalatable so poisoning is rare.
  • Common St. Johnswort (Hypericum perforatum) is a weed well known as a treatment for mild depression — in fact, it outsells Prozac 20-1 in Germany, but it also contains a compound that causes extreme photosensitivity in white pigmented skin. The skin of livestock that have grazed St. Johnswort or contaminated hay will itch and become red, swollen and sore, and may peel or come off in large sheets. Eyelids may swell and blindness can occur in severe cases.
  • Halogeton (Halogeton glomeratus) most frequently poisons sheep but it can cause death in cattle as well. Symptoms of poisoning include muscle tremors, reluctance to move, drooling, and rapid, shallow respiration. Death typically occurs within 9 to 11 hours after consuming a lethal dose.
  • Russian knapweed (Acroptilon repens), which is found throughout Eagle County, is allelopathic, meaning it exudes a toxic substance in the soil that prevents competing plants from being able to grow. It is only toxic to horses and causes the muscles in their lips, jaws, and tongue to become permanently frozen so they cannot chew. Horses should never be grazed in fields known to have this weed because they find it quite tasty. It is distinguished from other noxious knapweeds by its smooth rounded bracts that are all one color (the bracts are found on the enlarged base of the flower where it joins with the stem).
  • Poison hemlock (Conium maculatum) is extremely toxic to all livestock, wildlife, and humans. Death can occur due to respiratory failure in animals that consume a lethal dose within two to three hours. This weed has lacy, fern-like leaves that resemble parsley and clusters of tiny white flowers. Crucial to identifying poison hemlock are the purple spots found on its stem.

Denyse Schrenker is a CSU Extension agent in Eagle County specializing in horticulture and small acreage. Eagle County Healthy Landscapes is a collaboration between Eagle County Vegetation Management, Open Space, and the CSU Extension office to help inform residents of best management practices for healthy landscapes and to provide resources for weed mitigation, collaborative land management, and local flora. Visit http://www.eaglecounty.us/weeds/ for guides, event details and other resources.


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