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Copyright 2013 Rocky Mountain Rider. ALL RIGHTS RESERVED. Reproduction of any editorial material, artwork and photos is strictly forbidden without express written permission of the publisher. For information about reprint rights, please contact the editor;


Noxious or Toxic?

Plant Management for Equine Owners

Selenium-Accumulating Plants Point 

to Toxic Levels of Selenium

 By Dorinda Troutman, RMR Staff Writer


October 2013 issue  


Locoweed (Astragalus) –hundreds of species; Purple locoweed is shown; aka woolly locoweed, woolly milkvetch. Purple, yellow or white flowers. Horses can become addicted to locoweed; poisonous even when not accumulating selenium. Milkvetch (Astragalus). Two-grooved milkvetch is shown. Hundreds of species. Native prairie up to 8,000 feet. White to blue-purple flowers.

     When 21 Venezuelan polo ponies were poisoned by an incorrectly prepared vitamin and electrolyte cocktail in Florida in 2009, it took days to determine the cause: selenium overdose. A compounding pharmacy which produced the cocktail added 5 mg of selenium instead of the .5 mg indicated on the veterinarian’s prescription. A lethal dose of selenium in horses is 3.3 mg/kg.

     Selenium is both poisonous and vital to life. It was discovered as an essential nutrient in 1957, but it was not until 1973 that scientists found out what vital role it plays in nutrition and health.

     On the other side of the coin, selenosis (also called “alkali disease” or “blind staggers”) is selenium poisoning when animals graze on plants that accumulate a toxic amount of the mineral. It is a serious threat to livestock in the Rocky Mountain Region and Great Plains , where selenium content of the soil is highest.

     Selenium soil is usually associated with shale and can be isolated in a pasture, with parts of a pasture having high selenium while other parts do not. Some plants require selenium and these selenium-accumulating plants are considered “indicator species.” These indicator species include locoweeds and milk vetches, woody aster, prince’s plume, and goldenweed. They can all be highly toxic due to their selenium content.


Prince’s plume (Stanleya) aka sentinel of the plains, golden desert plume). Can be five to six feet tall. One species has white flowers. Woody Aster (Xylorhisa, aka smooth woodyaster).

     “In my experience, accumulators don’t cause the poisoning themselves unless an animal is starving,” says Dr. Merl Raisbeck, at the Wyoming State Veterinary Laboratory. “Accumulators have a god-awful odor and taste.

     “There are certain selenium-accumulative plants to watch for that indicate to livestock owners where high levels of selenium are in the soil. They are not foolproof, and I’ve seen bad alkali disease on pastures with no accumulators. Also, other plants growing in the area may be more palatable and still have enough selenium in them to cause chronic selenium toxicosis.”

     Secondary (lower level) selenium-accumulating plants, which are all palatable, include western wheatgrass, barley, wheat, and alfalfa.


     Selenium toxicity can occur in all herbivores, including horses. Horses can have acute, subacute or chronic selenium toxicosis, depending upon the amount of selenium eaten.

     Acute selenium toxicosis is quite rare and occurs when a high dose of selenium is ingested over a short period of time. Symptoms include abnormal movement and nervousness, dark watery diarrhea, elevated temperature, weak and rapid pulse, labored breathing, bloating and abdominal pain, pale, blue mucous membranes and dilated pupils. The symptoms may resemble those of rabies and so extra care should be taken not to contact the animal’s saliva; and call your veterinarian. Death occurs within a few hours to days. There is no known treatment for acute selenium toxicosis.

     Subacute selenium toxicosis (called “blind staggers”) is usually seen in the spring when animals are craving green vegetation. Symptoms include staggering, wandering aimlessly, and head pressing. 


One symptom of chronic selenium toxicosis is separation of the hoof wall. Goldenweed (Oonopsis; aka false goldenweed, many-stem goldenweed).

     Chronic selenium toxicosis (called “alkalai disease”) is caused by smaller amounts of selenium ingested through pasture, hay, or water, usually over a longer period of time. Symptoms include weight loss, hair loss (especially on mane and tail), and lameness. In severe cases the hoof wall may slough off.

     For subacute and chronic selenium toxicity, treatment includes preventing the horse from eating plants or drinking water with selenium and supportive therapy with a high protein diet. Work closely with your veterinarian and farrier to achieve the best results for recovery.


     To prevent selenium poisoning, learn to identify selenium-accumulating indicator plants; be careful of your sources of hay and test if high selenium level is a problem in your area; and contact your veterinarian if any symptoms appear in your horses.


     Photos shown are of most common selenium-accumulating indicator plants in our region.



Copyright 2013 Rocky Mountain Rider. ALL RIGHTS RESERVED. Reproduction of any editorial material, artwork and photos is strictly forbidden without express written permission of the publisher. For information about reprint rights, please contact the editor;

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