Amaranthus retroflexus (redroot pigweed) poisoning in cattle.

Amaranthus retroflexus (redroot pigweed)-induced nephrotoxicity was diagnosed in 6 herds of cattle from 3 counties in southwest Missouri. Forty-eight cows and calves died and another 35 were clinically affected. Serum urea nitrogen concentration, determined in 4 affected calves, was between 55 and 284 mg/dl, and serum creatinine concentration was between 6.7 and 29.9 mg/dl. Postmortem examination of affected cows and calves revealed amber-colored fluid in peritoneal cavities and retroperitoneal perirenal edema. Histologic examination of kidney sections revealed widespread degeneration and necrosis of proximal and distal tubules. Compared with archived kidney sections from cattle with Quercus (oak) poisoning, distal renal tubules were more severely affected. Oak poisoning also was associated with a higher prevalence of interstitial fibrosis and renal tubular oxalosis. We concluded that ingestion of the aerial and leafy portions of pigweed by cattle in drought situations does not necessarily lead to nitrate-induced sudden death associated with consumption of the nitrate-containing stems.