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MSMA Herbicide Toxicity in Horses

| Updated September 26, 2017

Monosodium Methyl Arsenate (MSMA) is an arsenic herbicide compound sometimes applied to Bermuda grass pastures and hay meadows on the sly. Applying MSMA in an off-label manner is not legal use of the herbicide. Arsenic residue left in hay makes one question at what level it becomes dangerous. Horses are less susceptible to MSMA arsenic poisoning than cattle, but are still at high risk.

Monosodium Methyl Arsenate

Arsenic is a deadly, poisonous chemical element available in yellow, black and gray forms. Its compounds are utilized in pesticides, insecticides and herbicides. Lead arsenate, used as a fruit tree herbicide, has been linked to neurological damage in spray workers. Copper arsenate was used as a coloring agent in sweets well into the 19th century. More recently, monosodium methyl arsenate (MSMA) has replaced lead arsenate in agricultural applications as a less toxic, organic form of arsenic.


Calcium arsenate powder and liquid sodium arsenite herbicide found in old edifices pose highly toxic risks to livestock. Horses and cattle will readily consume grasses and hays sprayed with MSMA. Arsenic compounds damage small blood vessels, affecting the blood supply to major organs. Limited blood supply contributes to inflamed digestive tracts, diarrhea, abdominal pain, thirst and collapse. Because of these symptoms, poisoned horses frequently make their way to nearby water sources. Death generally comes quickly and leaves no outward signs of illness. If a horse survives more than a week after poisoning, the resulting cause of death will ultimately be kidney failure. Arsenic poisoning has no known practical or effective antidote.

Safe Levels

Hay with more than 20 ppm (parts per million) MSMA arsenic should never be fed or sold. No grazing field should ever receive an MSMA spray application, for fear of risking horses’ health. Working with MSMA poisoned grass or hay not only affects a horse’s health, but also the health of everyone who touches it.


The Environmental Protection Agency (EPA) recently stated, “All uses for MSMA, SMA, CAMA and cacodylic acid (organic arsenical herbicides) are ineligible for reregistration.” The main concern is the possibility of organic arsenic herbicides entering the soil and, ultimately, drinking water. MSMA arsenic levels have also raised concern for cancer risks.


Confirming arsenic poisoning requires horse blood, urine, kidney, liver and stomach sample analysis in addition to testing the suspected poison source. Substantiating poisoning claims includes sampling forages sprayed with herbicides or locating evidence anywhere a spill may have occurred.