Great Summer Forage — If You Manage the Risk
Sorghum-sudan grasses are among the most valuable summer forages in Texas, producing excellent feed during the hot months when cool-season grasses are dormant. But these same forages can kill cattle under certain conditions. The cyanide (prussic acid) poisoning potential of sorghum-sudan hybrids and johnsongrass has to be understood and managed if you want to use these forages safely.
This article covers the science behind sorghum toxicity, high-risk conditions, and management practices to safely use these important forages.
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Understanding the Toxin: Cyanogenic Glycosides
The Cyanide Mechanism
Sorghum species contain dhurrin, a cyanogenic glycoside stored in plant cells. When plant cells are damaged (by grazing, frost, wilting, or cutting), enzymes release hydrogen cyanide (HCN), also called prussic acid.
- Chewing and rumen breakdown release HCN
- HCN is rapidly absorbed into bloodstream
- Cyanide blocks cellular oxygen use
- Cells cannot utilize oxygen, even though blood is full of it
- Animal dies from cellular asphyxiation
Species and Varieties
- Forage sorghum
- Sudan grass
- Sorghum-sudan hybrids
- Johnsongrass
- Columbus grass
- Pearl millet (does NOT contain dhurrin—safe alternative)
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High-Risk Conditions
Growth Stage Risks
- Rapid growth concentrates dhurrin
- Leaves have highest HCN levels
- Never graze until plants exceed 18-24 inches
- Tillers emerging after main stalk removed
- Each regrowth cycle restarts the risk
- Wait until regrowth reaches 18-24 inches
- Lower concentration per pound of forage
- Still can be dangerous under stress
Environmental Stress Risks
- Growth stops but HCN production continues
- When rain returns, levels remain high initially
- Wait 7-10 days after drought-breaking rain
- Releases stored HCN
- Wilted, frost-damaged plants are extremely dangerous
- Most cattle deaths occur 1-5 days after first frost
- Dying plants can be highly toxic
- Don't graze treated areas until plants are dead and decomposed
- HCN accumulation
- Young plants especially affected
- Especially in combination with drought
- Don't over-fertilize forage sorghums
Recognizing Cyanide Poisoning
Time Course
Cyanide poisoning is extremely rapid—among the fastest-acting plant poisonings.
- Symptoms appear within 15-30 minutes of consumption
- Death can occur within minutes to a few hours
- Rapid progression from first symptoms to death
Clinical Signs
- Rapid breathing
- Salivation
- Muscle tremors
- Difficult breathing despite rapid respirations
- Gasping
- Later: cyanotic (blue) mucous membranes
- Convulsions
- Collapse
- Death
The Paradox of Cyanide
Animals are breathing rapidly but still dying—because their cells cannot use the oxygen in their blood. The blood may appear bright red (oxygenated) because oxygen isn't being extracted.
Post-Mortem Findings
- Blood may be bright red (cherry red)
- Bitter almond smell from rumen contents (if checked quickly)
- Few other specific findings
- Diagnosis often based on history and circumstances
Testing for HCN
Field Testing
- Available from diagnostic labs
- Can be done on-farm
- Qualitative (positive/negative) or semi-quantitative (color comparison)
- Macerate (crush) plant tissue
- Place in container with picrate paper
- Incubate briefly
- Color change indicates HCN presence
Laboratory Testing
- Quantitative HCN analysis
- Submit fresh plant samples
- Keep samples frozen until delivery
- Results typically within 24-48 hours
When to Test
- Before grazing young plants
- After frost
- During or after drought
- Before grazing regrowth
- When you have any doubt
Safe Management Practices
Grazing Management
- This applies to initial growth AND regrowth
- Rotational grazing allows regrowth monitoring
- After rain, wait 7-10 days
- Test before returning cattle
- Monitor regrowth height
- Fill cattle with other hay before turning out
- Start grazing in afternoon (lower HCN than morning)
- Watch cattle closely for first few hours
- Have treatment supplies available
Hay Making
- Hay is generally safer than fresh forage
- Must be completely dry before baling
- Rained-on hay may not dry properly—test it
- Cure completely before baling
- Store 6-8 weeks before feeding if concerned
- Test if any doubt
Silage
- Wait at least 3 weeks before feeding
- Well-fermented silage is significantly safer
- Still test if starting material was high-risk
Treatment
Emergency Treatment
- Sodium nitrite solution
- Sodium thiosulfate solution
- Both given IV together
- Methemoglobin binds cyanide (removes from cells)
- Sodium thiosulfate converts bound cyanide to thiocyanate
- Thiocyanate is excreted in urine
- Multiple doses may be needed
- Animal that responds may relapse
- Treat again if symptoms return
- Keep antidote kit on-site during high-risk periods
Having Treatment Available
If grazing sorghum-type forages during any risk period:
- Have treatment kit from veterinarian
- Know how to administer (or have vet on call)
- Time is everything—minutes matter
Pearl Millet: The Safe Alternative
Pearl millet (Pennisetum glaucum) is an excellent alternative summer forage that does NOT contain cyanogenic glycosides.
Pearl Millet Advantages
- No prussic acid risk at any growth stage
- Can be grazed at any height
- Safe after frost
- Good production in hot weather
- Tolerates sandy soils
Pearl Millet Limitations
- May not yield as much as sorghum-sudan
- Less drought tolerant than sorghums
- Fewer varieties available
Consider Pearl Millet When
- You cannot monitor grazing closely
- You have inexperienced help
- You want peace of mind
- Pastures have variable growth stages
- You've had problems with sorghum management
Johnsongrass: The Wild Card
Johnsongrass (Sorghum halepense) is a perennial sorghum that grows wild throughout Texas. It presents the same cyanide risks as planted sorghums.
Johnsongrass Concerns
- Cannot be eliminated from pastures easily
- Grows in mixed stands with other forages
- New growth after mowing is high-risk
- Frost affects it just like planted sorghums
Managing Johnsongrass
- Scout pastures for johnsongrass patches
- Know where it grows on your ranch
- Apply same precautions as sorghum-sudan
- Consider patch fencing during high-risk periods
- Herbicide control in hay fields
Economic Considerations
Value of Sorghum-Sudan
- Excellent summer production
- Heat and drought tolerance
- Lower water requirement than corn
- Good rotational grazing potential
Cost of Losses
- Single cow death: $1,500-2,500+
- Multiple death incident: devastating
- Reputation damage if selling bred stock
- Emotional toll
Risk Management
- Testing is inexpensive: $15-25 per sample
- Treatment kits: $50-100
- Fencing for management: variable
- Pearl millet seed slightly higher but no risk
Case Example: Post-Frost Disaster
- Four dead cattle found
- Two more staggering, breathing heavily
- One surviving animal died within 30 minutes
- Cattle had grazed frost-wilted sorghum
- Classic cyanide poisoning timeline
- One treated and survived with IV antidote
- Immediate removal of remaining cattle
- No further deaths
Quick Reference: When NOT to Graze
- [ ] Within 7-10 days after frost
- [ ] During severe drought stress
- [ ] Within 7-10 days after drought-breaking rain
- [ ] When regrowth is short
- [ ] After herbicide application
- [ ] When you haven't tested and conditions are questionable
Bottom Line
- Height is critical - Never graze plants under 18-24 inches
- Frost is deadly - Remove cattle before frost; wait 7-10 days minimum after
- Drought increases risk - Remove during severe stress; wait after rain
- Regrowth restarts the risk - Each cutting cycle requires waiting for height
- Testing is cheap insurance - $15-25 to test vs. $2,000+ cow
- Treatment exists but requires speed - Have antidote on-site during risk periods
- Pearl millet is safe - Consider for peace of mind
- Johnsongrass is wild sorghum - Same rules apply to wild stands
Related Resources
- Complete Guide to Texas Toxic Plants
- Fall Acorn and Frost Dangers
- When to Call the Vet Flowchart
- Poisoning First Aid Protocol
References
- Texas A&M AgriLife Extension. (2024). "Prussic Acid Poisoning in Cattle."
- Burrows, G.E. & Tyrl, R.J. (2013). Toxic Plants of North America. Wiley-Blackwell.
- Undersander, D.J. et al. (2003). "Sorghums, Sudangrass, and Sorghum-Sudan Hybrids." Focus on Forage. University of Wisconsin.
- Knight, A.P. (2001). A Guide to Plant Poisoning of Animals in North America. Teton NewMedia.
- Kahn, C.M. (ed). (2010). The Merck Veterinary Manual (10th ed). Merck & Co.
- USDA NRCS. (2024). "Managing Sorghum-Sudan for Grazing."
"The frost that kills the grass can kill your cattle too"
