Pigweed

VETERINARY TOXICOLOGY

Three cattles are down.The pasture looks different.

You drive out to Cedar Creek Ranch. The animals are weak, reluctant to rise, and knuckling on their hind limbs. A dense stand of tall weedy plants lines the fence row.

Investigate the pasture

What is this plant?

Amaranthus retroflexus: Pigweed

• Summer annual plants that germinate from seeds during late winter through summer.
• Originated in the Americas and then spread to Europe.
• Produces perirenal edema and nephrosis by an unknown mechanism.
• Contains up to 30% oxalate by weight and also accumulates nitrates, especially when mature under stress.
• Produces toxicity when green or dry Pigs are the most commonly poisoned animal, but usually only when first introduced to the plant; cattle and sheep are also affected

Click each hotspot to reveal diagnostic features

How does it cause toxicity?

Two distinct pathways — select each tab to explore

Nitrate Toxicity

Perirenal Edema

Know Your Differentials

Flip each card to see how it compares to Pigweed toxicity

Grass Tetany Hypocalcemia + tetany, but no renal involvement. Normal BUN/creatinine. Spring pasture history. Rapid response to Mg/Ca treatment distinguishes it.

Oak Toxicity Causes oral lesions , pseudomembranes and renal tubular necrosis, but key differentiator is prominent GI signs (hemorrhagic diarrhea, constipation) and acorn/oak ingestion history.

Nitrate-Other Sources Clinically identical to Pigweed nitrate toxicity. Differentiate by source history: corn stalks, sorghum, over-fertilized forages. Treatment is the same.

Leptospirosis Can cause renal failure, but presents with fever, hemoglobinuria, and icterus. Confirm with serology. No plant ingestion history needed.

Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.

Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.

Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.

Use this side of the card to provide more information about a topic. Focus on one concept. Make learning and communication more efficient.

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Nitrate- Other Sources

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Oak Toxicity

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Leptospirosis

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Grass Tetany

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Plant ID

Reddish-green stem, bristly seed spike, red taproot. Found in overgrazed pastures and drought conditions.

What you need to remember

Perirenal Edema

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5–10 day onset. Azotemia, hypocalcemia, hind limb weakness. Renal damage likely advanced when signs appear.

You worked through a complete clinical case. Here's your consolidated reference for Pigweed toxicity.

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First Response

Remove from pasture first — always. Then: fluids, calcium, NSAIDs, producer communication, documentation.

Nitrate Toxicity

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Peracute onset. Chocolate-brown blood. Methylene blue 4–8 mg/kg IV. Time-critical intervention.

Red Taproot

The reddish taproot gives this species its common name: Redroot Pigweed. Diagnostic when uprooted.

Bristly Seed Spike

Dense terminal spike — the most distinctive feature of Amaranthus retroflexus. Look for this at the top of the plant.

Alternate Ovate Leaves

Dull green, slightly hairy, with a distinct pale midrib. Often mistaken for common pigweed or lambsquarters.

Photo by Saara Nafici.

Reddish-Green Stem

Thick, branching, and slightly hairy. The reddish tinge deepens with plant maturity — key ID feature.