Cocklebur and Pigweed Dangers

Two Weeds Every Texas Rancher Needs to Know

Cocklebur and pigweed are two of the most common toxic plants on Texas ranches, and they often show up together in disturbed areas, corrals, and field margins. They've got different toxins and affect cattle differently, but they share some key traits: both thrive in overgrazed and disturbed ground, both are especially dangerous at certain growth stages, and both kill cattle every year.

This article covers identification, toxicity patterns, and management for these persistent pasture invaders.

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Part 1: Cocklebur (Xanthium strumarium)

Why Cocklebur Is Dangerous

Cocklebur contains carboxyatractyloside, a toxin concentrated in the cotyledon (seed leaf) stage. The two-leaf seedlings are 10 times more toxic than mature plants.

Identification

• Cotyledons are 1-2 inches long
• Often emerge in dense mats
• No true leaves yet visible
• Found in low, recently flooded areas
• Large, rough, heart-shaped leaves
• Leaves have 3-5 shallow lobes
• Burrs are the most recognizable feature
• Burrs are oval, 0.5-1 inch, covered in hooked spines
• Contain two seeds
• Brown when mature
• Stick to animal hair (spread mechanism)

Toxicity Timeline

When Poisoning Occurs

• Low areas become saturated
• Cocklebur seeds germinate
• Dense stands of cotyledon-stage seedlings emerge
• Cattle access low areas as water recedes
• Cattle consume toxic seedlings
• Poisoning occurs within 24 hours
• Low spots in pastures
• Around stock tanks after overflow
• Corrals after heavy rain
• Field edges and disturbed areas

Signs of Cocklebur Poisoning

• Weakness
• Reluctance to move
• Nausea (may attempt to vomit)
• Muscle tremors
• Lack of coordination
• Prostration
• Convulsions
• Death (often within 24 hours)

Lethal Dose

• Cotyledon stage: 0.75-1.5% of body weight
• For 1,000 lb cow: approximately 7-15 lbs of seedlings
• A dense mat of seedlings can easily contain this amount

Part 2: Pigweed (Amaranthus species)

Multiple Species Involved

Several pigweed species grow in Texas, all potentially toxic:

• Redroot pigweed (A. retroflexus) - most common
• Palmer amaranth (A. palmeri) - increasingly problematic
• Smooth pigweed (A. hybridus)
• Spiny amaranth (A. spinosus)
• Tumble pigweed (A. albus)

Why Pigweed Is Dangerous

Pigweed accumulates nitrates in its tissues, especially under stress conditions. When cattle consume high-nitrate plants:

• Nitrates are converted to nitrites in the rumen
• Nitrites are absorbed into bloodstream
• Nitrites convert hemoglobin to methemoglobin
• Methemoglobin cannot carry oxygen
• Animal dies from internal suffocation

Identification

• 2-6 feet tall (Palmer can reach 8+ feet)
• Alternate, simple leaves
• Dense flower spikes at top and in leaf axils
• Reddish root (redroot species)
• Tiny dark seeds

Nitrate Accumulation Patterns

• After fertilization (especially nitrogen)
• After herbicide application (before death)
• 3-5 days after rain following drought
• In young, rapidly growing plants
• Under cloudy, cool conditions
• Leaf petioles
• Leaves
• Seeds (lowest)

When Pigweed Poisoning Occurs

• After frost - Stressed plants retain high nitrates
• Herbicide application - Dying plants become more palatable
• Corrals and heavy-use areas - Nitrogen enrichment from manure
• Overgrazed pastures - Pigweed may be only available forage

Signs of Nitrate Poisoning

• Rapid, weak pulse
• Staggering
• Muscle tremors
• Chocolate-brown blood (classic sign)
• Brown discoloration of mucous membranes
• Collapse
• Death (can occur within hours)
• Poor appetite
• Depression
• Abortion in pregnant cows
• Weakness

Lethal Dose

• Depends on nitrate concentration in plant
• Generally 1-2% of body weight of high-nitrate plant
• Drought-stressed pigweed can contain 5-10% nitrate (dry weight)
• Fresh pigweed can kill with smaller amounts than hay

Management Strategies

Preventing Cocklebur Poisoning

• Wait until seedlings develop several true leaves
• Or wait until area dries and seedlings die
• Look for emerging cotyledon-stage seedlings
• Act quickly—danger window is narrow
• Reduce future populations
• 2,4-D is effective on young plants
• Cattle with good forage are less likely to eat seedlings

Preventing Pigweed Poisoning

• Test is inexpensive at diagnostic labs
• Wait if levels are high
• Wait 7-10 days after drought-breaking rain
• Don't graze after frost until plants are dead and decomposed
• Don't graze recently sprayed areas
• Full cattle are less likely to consume large amounts
• Spreading intake over time allows nitrite detoxification
• Herbicides: 2,4-D, dicamba, glyphosate (spot treatment)
• Prevent establishment by maintaining grass

Hay Considerations

• Burrs can cause physical injury
• Generally low risk if mature plants
• Test hay if significant pigweed present
• Can blend with low-nitrate hay to dilute
• Ensiling reduces (but doesn't eliminate) nitrates

Treatment

Cocklebur Poisoning Treatment

• Activated charcoal (if caught early)
• IV fluids
• Glucose administration
• Supportive care

Nitrate Poisoning Treatment

• IV Methylene Blue solution (4% solution, 4-15 mg/kg)
• Reverses methemoglobin formation
• May need repeat doses
• IV fluids
• Oxygen if available

Diagnostic Testing

For Suspected Cocklebur

• Submit rumen contents and liver to diagnostic lab
• Look for cotyledons or burrs in rumen
• Histopathology of liver

For Suspected Nitrate

• Blue color indicates nitrates present
• Kits available from veterinary suppliers
• Submit aqueous humor (eye fluid) from dead animals
• Submit suspect plant material

Comparative Summary

Bottom Line

Cocklebur:

• Cotyledon stage is deadly - Two-leaf seedlings are most toxic
• Watch after flooding - Seedlings emerge in low areas
• Fence off low spots - Keep cattle out for 2-3 weeks
• No antidote - Prevention is essential

Pigweed:

• Stress increases danger - Drought, frost, herbicides
• Chocolate-brown blood - Classic sign of nitrate poisoning
• Antidote exists - Methylene blue; call vet immediately
• Test before grazing - If pigweed is abundant
• Hay retains nitrates - Test pigweed-contaminated hay

Both Plants:

• Indicator of overuse - Both thrive in disturbed, overgrazed areas
• Control before seed set - Reduce future populations
• Healthy grass prevents problems - Good pasture management is best prevention

Related Resources

• Complete Guide to Texas Toxic Plants
• Drought and Increased Toxicity
• Hay Quality Inspection Checklist
• Poisoning First Aid Protocol

References

• Burrows, G.E. & Tyrl, R.J. (2013). Toxic Plants of North America. Wiley-Blackwell.
• Texas A&M AgriLife Extension. (2024). "Cocklebur and Pigweed Poisoning."
• Knight, A.P. (2001). A Guide to Plant Poisoning of Animals in North America. Teton NewMedia.
• Plumlee, K.H. (2004). Clinical Veterinary Toxicology. Mosby.
• Stoltenow, C. & Lardy, G. (1998). "Nitrate Poisoning of Livestock." NDSU Extension.
• Stuart, B.P. et al. (1981). "Cocklebur Intoxication in Cattle." Veterinary Pathology.

"The weeds that grow where water flows—and where cattle go"