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Poisonous Native Plants
August 2009
Contact: Joanne Littlefield
Assistant Director-Community Relations
Phone: (970) 491-4640
Joanne.Littlefield@colostate.edu
As Colorado wages war on noxious weeds, it is important to recognize that some native plants or wild flowers can invade pastures and rangeland and cause poisoning in livestock and horses. Native plants (wild flowers) are not listed on the Colorado Noxious Weed List but can be toxic to animals that eat them.
Anthony Knight, a professor of large animal veterinary medicine and plant toxicology at Colorado State University, emphasizes that most animals will not preferentially eat poisonous plants, but will do so if they are hungry. Animals may be forced into eating large quantities of a particular plant that may be poisonous if they have no available forages as occurs when pastures or range lands are overgrazed.
It’s not uncommon for animals to eat small quantities of toxic plants as they graze and not be unaffected. To paraphrase Paraceleus, “It’s the dose that makes the poison,” said Knight. “A few weeds or wild flowers in a pasture are not a good reason to use herbicides or some other drastic measure to get rid of them.” Avoiding overgrazing will allow grasses to grow and compete with weeds to prevent them taking over a pasture.
Plant poisoning is more likely to occur in early spring before the grasses become available, or in late summer when grasses have been eaten and weeds remain “However, plant poisoning may occur in livestock at any time of year , as even in winter , the hay animals are fed may contain poisonous plants, Knight explains. It is important to check hay before feeding it to animals, and ideally purchase certified weed-free hay.
According to Knight, landowners with livestock need to be aware of noxious weeds and those common native wild flowers that may pose a threat to their animals. In Colorado and many western States, there are some important toxic native plants that can cause poisoning of livestock.
Water Hemlock (Cicuta maculata)
Probably the most lethal wildflower in Colorado is the water hemlock. It has hollow stem, white flowers, tuberous root system and grows in wet marshy conditions. Water hemlock typically grows up to four feet and looks like the wild carrot. The plant is extremely poisonous to animals and people, “An ounce or two of the hemlock root is sufficient to kill an adult horse,” Knight said. There is no specific treatment and animals typically die within hours of eating the plant. Water hemlock is the one native plant that should not be allowed to grow in animal pastures!
Senecio, Groundsel (Senecio species)
Varieties of Senecios grow in Colorado, and typically produce their yellow blooms in late summer. All parts of the plant contain alkaloids, that when overgrazed cause irreversible liver disease. Once the liver is extensively damaged, it cannot detoxify break-down products as normal and these cause a severe dermatitis of white skinned areas in the presence of sunlight referred to as photosensitization.
Death Camus (Zigadenus species)
Death Camus is a common Colorado native wildflower. It is a member of the lily family and looks like a wild onion. The plant contains complex alkaloids in the bulb and new growth that act on the central nervous system causing respiratory paralysis and death. Death Camus is problematic to livestock in the spring as it emerges before grasses. There is no treatment, and animals are often found dead. It’s important to recognize the plant and control it by digging out the onion-like bulb or spot spraying with an appropriate herbicide.
Milkweeds (Asclepias species)
Several species of Milkweed are native to Colorado. The narrow-leafed milkweed is more likely to cause poisoning in animals. This variety has thin grass-like leaves with white flowers. It grows in hay meadows especially in Western Colorado. Milkweeds retain their toxicity when dried, so it’s important to check hay for milkweed pods before feeding it to animals.
Locoweeds (Astragalus and Oxytropis species)
Locoweeds are native wildflowers that have caused more economic loss to the livestock industry than any other group of plants. There are many species of locoweed that cause a wide range of signs including abnormal behavior, abortions, fetal deformities, weight loss and eventual death. The most common and problematic locoweeds are those with white and purple flowers. In drought seasons the plants are dormant, but flourish in years when rainfall produces ideal growing conditions. Locoweeds produce a toxic alkaloid in conjunction with a fungus (endophyte) growing in the plant that affects the brain of horses and animals causing neurologic abnormalities. The word locoism meaning “crazy,” in Spanish aptly describes the severe changes in behavior seen in affected horses.
Larkspur (Delphinium species)
As native wild flowers in the Rocky Mountain region, larkspurs are responsible for more cattle deaths than any other native plant. With showy blue/purple flowers with a characteristic spur off of the back of the flower, larkspur species can be found at high altitude as well as on the prairies. The plants contain highly toxic alkaloids especially pre-flowering, that act on the nervous system causing paralysis and death in a matter of hours. Cattle are highly susceptible to larkspur poisoning, while horses are rarely affected, and sheep are quite resistant to larkspur poisoning. Where larkspur are abundant in cattle grazing areas, care should be taken to avoid having cattle graze the larkspur especially before it blooms. Sheep may be used as biological controls to graze the larkspur before cattle have access to the range.
There are many other native plants with toxic potential if allowed to become invasive in pastures. Plants such as buttercups, sneeze weeds, night shades, skunk cabbage, and lupine amongst others may become a problem locally. For further information on other poisonous plants in Colorado, go to www.southcampus.colostate.edu/poisonous_plants. To access fact sheets about pasture weed management go to www.ext.colostate.edu/PUBS/NATRES/03105.html, or www.ext.colostate.edu/pubs/natres/03106.html.
Updated Tuesday, September 13, 2011
