Extension by Rachel MealorWritten by Rachel Mealor
By Rachel Mealor, UW Extension Range Specialist
It seems as though many parts of Wyoming have seen very little rain these past few months. Off course, the rain is known for showing up right after many have cut their hay! This May and June have seemed unseasonably dry, especially compared to last year. These dry conditions can impact reservoir’s levels and the amount of water available for irrigation and diminish growth of many native grass species that make up plant communities on our rangelands. However, a less obvious consequence can be livestock poisoning from animals consuming plants they may not normally eat, or at least consume very little of, when there are other plants to choose from. This year there has been some concern regarding plant toxicity, especially in areas known for experiencing selenium and sodium accumulation.
A number of plants accumulate selenium in sufficient amounts to be toxic if livestock consume them. Many of these plants are found in selenium rich areas and often require selenium for growth. Plants that are probably familiar to many that fit this description are certain Astragalus species, prince’s plume and some woody asters. These species are described as indicator plants and may accumulate up to 3,000 parts per million (ppm) selenium. Plants that do not require selenium to grow, but still accumulate selenium, are called secondary selenium absorbers. Secondary selenium absorbers include native range and crop plants such as western wheatgrass, barley, alfalfa and wheat.
Two-grooved milkvetch (Astragalus bisulcatus), is a selenium-accumulating plant and although it is not palatable to most livestock, it has been found in a few cases of selenium toxicosis. Some suggest that even though selenium-accumulating plants are not readily eaten, they can contribute to selenium toxicosis by making selenium in the soil available to neighboring secondary selenium-accumulating plants that may be more palatable to animals.
Selenium is actually required for most animals and concentrations of 0.3 ppm are recommended for most livestock that are meat- or food-producing animals. When livestock ingest large amounts of selenium (>400 ppm) it can lead to acute selenosis. Oral selenium doses between one and five milligrams per kilogram body weight are considered toxic levels. Chronic poisoning, oftentimes called alkali disease, results when between five and 40 ppm of selenium are in the diet for several weeks or months. Some symptoms of acute poisoning include lethargy, abnormal posture or unsteady gait, diarrhea or death. Some signs of chronic poisoning are rough hair coat, emaciation, lameness or joint stiffness, overgrown or deformed hooves or reproductive losses in cattle.
Greasewood is another plant that has the potential to become a concern. The plant toxins, sodium and potassium oxalates, are found in the leaves with lower concentrations in the other plant parts. The toxin amount varies quite a bit depending on where the greasewood is growing (from 10 to 22 percent of plant dry weight). Toxicity increases as the growing season progresses. Greasewood is a woody perennial shrub that livestock can generally consume in moderate amounts with other forage and is actually fairly palatable. Death occurs when livestock consume large amounts of greasewood in a short amount of time. This year is of particular interest as the drought has resulted in limited growth of other forage in areas where greasewood is found. However, livestock losses usually occur in the fall and winter when cattle and sheep eat large amounts of leaves that have fallen off the plant from early spring growth. Signs of poisoning may develop four to six hours after a toxic amount is consumed by livestock. One gram oxalate per kilogram body weight is lethal in sheep or 1.5 to 2.0 kilograms for a cow. For the plant to be lethal, it must be consumed rapidly without other forages to dilute the toxin. Animals can be conditioned to oxalate toxins by feeding oxalates at low doses for several days. Conditioned animals must consume from 30 to 50 percent more than animals that have not been conditioned. Signs of poisoning include depression, weakness or reluctance to move, rapid breathing, drooling or coma.
It is important to note the danger of allowing thirsty or hungry animals to graze areas that are heavily infested with toxic plants. Special attention should be given to animals that have been trailed and then placed in a pasture with toxic plants. In areas where toxic plants are found, assure adequate levels of desirable forage for grazing prior to turning animals out. Animals that have a choice of other palatable or nutritious plants are less likely to consume toxic plants. However, this year cool season grasses may have grown very little due to such dry conditions! This could be an issue if one of the only palatable plants for livestock to eat contains toxic compounds. Having supplemental feed and water available to animals upon arrival after being trailed or transported to new ranges can also help prevent losses that could occur due to plant toxicity.
For more information on toxic plants please visit the Agricultural Research Service website at ars.usda.gov/Main/site_main.htm?modecode=54-28-20-00.