Pigweed (Amaranthus retroflexus) toxicosis in cattle

Four of approximately 15 dry cows introduced on a 10-acre fescue-clover-orchard grass-Dallis grass pasture in East Tennessee became recumbent. Clinical findings included depression, muscle tremors, increased heart and respiratory rates, hypocalcemia, hypomagnesemia, hyperkalemia, azotemia, and elevated creatinine phosphokinase. Three cows recovered; 1 died. Differential diagnoses considered were hypocalcemia, hypomagnesemia, rabies and toxicoses from Amaranthus retroflexus (pigweed), Quercus spp (oak), Cassia spp (senna) oxalate (Aspergillus niger or flavus), mycotoxicosis, lead, arsenic or insecticides. Pigweed toxicosis was confirmed based on clinical and postmortem findings, partially ingested pigweed in the pasture, and ruling out other possible causes. Several factors probably contributed to this incident: since the cattle were newly introduced to the pasture, the cattle may have been attracted to the pigweed in the new pasture and became addicted to it; their rumen microflora had little time to acclimate to the pigweed; and dry weather produced poor forage quality thus forcing the cows to eat the pigweed.