Substances such as inorganic arsenicals (sodium arsenite, arsenic trioxide), organic arsenicals (methyl arsonate, methyl arsonic acid), sodium chlorate, ammonium sulfamate, borax, and many others were formerly used on a large scale. These older herbicides are nonselective, generally cheaper, more toxic, and more likely to cause problems than newer compounds. However, poisonings associated with these compounds are uncommon in countries that have limited or halted their use. Brief descriptions of these compounds are listed below.
Arsenical Toxicity in Animals
The use of inorganic arsenicals as herbicides has decreased greatly because of livestock losses, environmental persistence, and carcinogenic potential. These compounds can be hazardous to animals when used as recommended. Ruminants are apparently attracted to and lick plants poisoned with arsenite.
Arsenicals are used as desiccants or defoliants on cotton plants, and residues of cotton harvest fed to cattle can contain toxic amounts of arsenic. The highly soluble organic arsenicals can concentrate in pools in toxic quantities after a rain has washed them from recently treated plants. Clinical signs and lesions caused by organic arsenical herbicides resemble those of inorganic arsenical poisoning. Single toxic oral doses for cattle and sheep are 22–55 mg/kg. Smaller doses may result in poisoning if consumed over a period of days (1).
Dimercaprol (3 mg/kg for large animals, and 2.5–5 mg/kg for small animals, IM, every 4 hours for 2 days; every 8 hours on day 3; and every 12 hours for the next 10 days) is recommended (2). For large animals, thioctic acid (50 mg/kg, IM, as a 20% solution, every 8 hours) has been used in combination with dimercaprol. Sodium thiosulfate also has been used in cattle (20–40 mg/kg, IV, every 8 hours and 80 mg/kg, PO, every 24 hours) (3). One-fourth this dose has been suggested for sheep; however, a rationale for its use is not established, and it may be unrewarding. (Also see Arsenic Poisoning in Animals.)
Borax Toxicity in Animals
Borax is toxic to animals if consumed in moderate to large doses (> 0.5 g/kg) (4). Proper use of borax has not been reported to result in intoxications. However, accidental incorporation into livestock feed and application of borax powder in the open for cockroach control have resulted in poisonings. Principal clinical signs of acute poisoning include fasiculations, diarrhea, rapid prostration, and seizures. An effective antidote is not known. Balanced electrolyte fluid therapy with supportive care is indicated.
Sodium Chlorate Toxicity in Animals
Cases of chlorate poisoning of livestock have occurred both from ingestion of treated plants and from accidental consumption of feed to which it was mistakenly added as salt. Cattle sometimes are attracted to foliage treated with sodium chlorate. Considerable quantities must be consumed before clinical signs of toxicity appear. The minimum lethal dose is 1.1 g/kg for cattle (5, 6), 1.54–2.86 g/kg for sheep, and 5.06 g/kg for poultry (7).
Ingestion of chlorate results in hemolysis of RBCs and conversion of Hgb to methemoglobin. For ruminants, treatment with methylene blue 1% solution (4–15 mg/kg, IV, every 6 hours) must be repeated frequently because, unlike the nitrites, the chlorate ion is not inactivated during conversion of Hgb to methemoglobin and is capable of producing an unlimited quantity of methemoglobin as long as chlorate is present in the body (8). In one case study, cattle were successfully treated with methylene blue 1% at an even lower dose (1 mg/kg) (9).
Blood transfusions may decrease some of the tissue anoxia caused by methemoglobin; IV isotonic saline solution (0.9% NaCl) can hasten elimination of the chlorate ion. Mineral oil containing 1% sodium thiosulfate will inhibit further absorption of chlorate in monogastric animals.
Key Points
Incidents of poisonings involving inorganic and arsenical herbicides are increasingly rare due to limited use and availability of these compounds.
Poisonings involving inorganic and arsenical herbicides are most likely to occur through the misuse or incorrect storage of product.
For More Information
Gupta RC, Gupta PK. Toxicity of herbicides. In: Gupta RC, ed. Veterinary Toxicology: Basic and Clinical Principles. 4th ed. Elsevier. 2025:565-579.
Also see pet owner content regarding herbicide poisoning.
References
Plumlee K. Clinical Veterinary Toxicology. Mosby; 2004.
Talcott PA, Peterson ME. Small Animal Toxicology. Saunders; 2012.
Bertin FR, Baseler LJ, Wilson CR, Kritchevsky JE, Taylor SD. Arsenic toxicosis in cattle: meta‐analysis of 156 cases. J Vet Intern Med. 2013;27(4):977-981. doi:10.1111/jvim.12124
Sisk DB, Colvin BM, Bridges CR. Acute, fatal illness in cattle exposed to boron fertilizer. J Am Vet Med Assoc. 1988;193(8):943-945.
Moore GR. Sodium chlorate poisoning in cattle. J Am Vet Med Assoc. 1941;99:50-52.
Harwood PMA. Poisoning in dairy cattle by chemicals in common use. Vet Rec. 1953;65:291-292.
McCulloch EC, Murer HK. Sodium chlorate poisoning. J Am Vet Med Assoc. 1939;95:675-682.
Allison CD, Wenzel J. Nitrate Poisoning of Livestock. Guide B-807. New Mexico State University.
Blakley BR, Fraser LM, Waldner C. Chlorate poisoning in beef cattle. Can Vet J. 2007;48(10):1071.
