Oxalates and nitrates are present in pigweed. Oxalates and/or possibly other unidentified compounds in Amaranthus spp. cause kidney tubular nephrosis and death of the animal. The soluble oxalates in the plant are absorbed from the gastrointestinal tract and bind with calcium in the blood to produce insoluble calcium oxalate. The calcium oxalate is then filtered by the kidneys where it causes an oxalate nephrosis and kidney failure.
Pigweed can have as much as 30% oxalate in the dried plant and ruminants eating large amounts of the plant are very likely to be poisoned especially if they have not been exposed to high levels of oxalates previously.
Annual plants, with stout, erect stem 30-150 cm tall, usually much-branched and hairy. The leaves are petiolate, ovate to lanceolate and acute at apex. The flowers are monecious in densely crowded spikes 8-20 cm wide, borne in pannicles. The flowers are greenish with long, spine-tipped bracts. Many small shiny black seeds are produced.
Decreased rumen activity. Pigs may develop abdominal distension due to the accumulation of ascitic fluid.
Muscle weakness and muscle tremors, ataxia, knuckling at the fetlock joints, recumbency.
There is no specific treatment other than supportive therapy. Administration of insulin, glucose, and fluids intravenously may help manage the hyperkalemia and renal failure. If the animals urine output stops and creatinine levels remain high despite agressive fluid therapy, the prognosis is very poor. Nitrate poisoning should be treated with methylene blue.
Death may result from increased serum potassium levels.
Renal failure. Marked increase in serum creatinine, blood urea nitrogen and potassium.
The most characteristic postmortem finding is the accumulation of hemorrhagic fluid around the kidneys in the retroperitoneal space. Nephrosis is often present without the presence of oxalate crystals, suggesting that Amaranthus spp. may contain other as yet undefined toxic substances.
Pigweed poisoning may appear very similar to Halogeton, oak poisoning, and ethylene glycol poisoning and should be considered in the differntial diagnosis.
Oxalate levels remain high in the dried plant. If there is a lot of pigweed remaining in the pasture it is best to cut, gather and burn or remove the plant material from where animals may have access to it.
See Kochia weed for comments on managing nitrates in forages for cattle.