Inorganic Herbicides and Organic Arsenicals
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. Their use has been mostly curtailed in developed countries.
The use of inorganic arsenicals as herbicides has been reduced greatly because of livestock losses, environmental persistence, and their carcinogenic potential. These compounds can be hazardous to animals when used as recommended. Ruminants (including deer) are apparently attracted to and lick plants poisoned with arsenite.
The highly soluble organic arsenicals can concentrate in pools in toxic quantities after a rain has washed them from recently treated plants. Arsenicals are used as desiccants or defoliants on cotton, and residues of cotton harvest fed to cattle may contain toxic amounts of arsenic. 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. Poisoning may be expected from smaller doses if consumed on successive days.
Dimercaprol (3 mg/kg for large animals, and 2.5–5 mg/kg for small animals, IM, every 4–6 hr) is the recommended therapy. Sodium thiosulfate also has been used (20–30 g, PO, in ~300 mL of water for cattle; one-fourth this dose for sheep); however, a rationale for its use is not established, and it may be unrewarding. (Also see Arsenic Poisoning.)
Ruminants apparently can metabolize ammonium sulfamate to some extent and, in some studies, exposed animals made better gains than did control animals. However, sudden deaths have occurred in cattle and deer that consumed treated plants. Large doses (>1.5 g/kg) induce ammonia poisoning in ruminants. Treatment is designed to lower rumen pH by dilution with copious amounts of water to which weak acetic acid (vinegar) has been added.
Borax is toxic to animals if consumed in moderate to large doses (>0.5 g/kg). Poisoning has not been reported when borax was used properly but has occurred when it was accidentally added to livestock feed and when borax powder was scattered in the open for cockroach control. Principal signs of acute poisoning are diarrhea, rapid prostration, and perhaps convulsions. An effective antidote is not known. Balanced electrolyte fluid therapy with supportive care is indicated.
Many 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 signs of toxicity appear. The minimum lethal dose is 1.1 g/kg for cattle, 1.54–2.86 g/kg for sheep, and 5.06 g/kg for poultry. Ingestion results in hemolysis of RBCs and conversion of Hgb to methemoglobin. Treatment with methylene blue (10 mg/kg) 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 it is present in the body. Blood transfusions may reduce some of the tissue anoxia caused by methemoglobin; IV isotonic saline can hasten elimination of the chlorate ion. Mineral oil containing 1% sodium thiosulfate will inhibit further absorption of chlorate in monogastric animals.