What is the toxic limit of HCN in sorghum?

November 26, 2025 •

4 min read

According to agricultural extensions, concentrations of hydrogen cyanide (HCN) in sorghum above 750 parts per million (ppm) on a dry matter basis are considered very dangerous for livestock. Understanding what is the toxic limit of HCN in sorghum is crucial for livestock producers to prevent fatal poisoning, a risk associated with feeding certain sorghum varieties.

Quick Summary

This guide details the specific toxic and safe levels of hydrogen cyanide (HCN) found in sorghum forages, explaining how concentration varies with plant maturity and environmental stress. It covers the symptoms of prussic acid poisoning in animals and outlines management practices, including cutting strategies and testing, to mitigate toxicity risks effectively.

Key Points

Toxic Limit: Concentrations of hydrogen cyanide (HCN) in sorghum are considered toxic between 500-750 ppm and very dangerous above 750 ppm on a dry matter basis.
Stress Increases Risk: Drought, frost, and high nitrogen fertilization can cause a significant increase in HCN levels in sorghum plants.
Maturity Matters: Immature plants and new regrowth contain the highest concentrations of HCN; delaying grazing until the plant is over 18-24 inches tall is a key safety measure.
Grazing is High-Risk: Grazing presents the highest risk of HCN poisoning compared to other methods, as animals may selectively eat high-risk leaves and shoots.
Silage is Safest: The ensiling process is the most effective method for reducing HCN risk, as fermentation allows the toxic gas to dissipate.
Test Before Feeding: When in doubt, especially after environmental stress, testing forage samples is the most reliable way to determine if HCN levels are safe for livestock.
Watch for Symptoms: Early signs of HCN poisoning include rapid breathing, frothing, and muscle tremors; immediate action is required if symptoms appear.

Understanding Hydrogen Cyanide (HCN) in Sorghum

Sorghum plants naturally contain cyanogenic glycosides, specifically a compound called dhurrin. When the plant's cells are damaged through chewing, wilting, or freezing, enzymes are released that break down dhurrin and produce hydrogen cyanide (HCN), also known as prussic acid. While low levels of HCN can be detoxified by an animal's body, high concentrations overwhelm the system, causing cellular asphyxiation and potentially rapid death. This is why knowing what is the toxic limit of HCN in sorghum is a critical aspect of responsible livestock management.

Factors Influencing HCN Levels

Several factors can significantly influence the concentration of HCN in sorghum forage:

Plant Age: Younger, immature plants and new regrowth typically contain higher concentrations of dhurrin and thus higher potential for HCN toxicity. As the plant matures, the toxin concentration decreases. Grazing should be delayed until the sorghum is at least 18 to 24 inches tall.
Environmental Stress: Conditions such as drought, frost, and extreme heat can stress the plant and increase HCN production. A non-killing frost can be particularly dangerous as it damages plant cells and stimulates new growth with high HCN levels. Waiting 7-10 days after a killing frost is recommended before grazing or feeding.
Genetics and Variety: The potential for cyanogenic glycoside production is genetically regulated, meaning different sorghum varieties and hybrids have inherently different HCN potentials. Some newer hybrids are being developed to be dhurrin-free, effectively eliminating the risk.
Fertilization: High nitrogen fertilization, especially in soils low in phosphorus, can lead to increased HCN concentration in the plant. Diligent soil testing and balanced fertilization can help manage this risk.

Recognizing Prussic Acid Poisoning

Symptoms of HCN poisoning are often rapid and severe. Producers should be vigilant for these clinical signs:

Rapid and labored breathing
Frothing at the mouth
Muscle tremors or twitching
Staggering or incoordination (ataxia)
Excessive salivation
Bright red mucous membranes initially, followed by blue discoloration (cyanosis) later
Convulsions
Collapse and death, sometimes occurring within minutes to hours of consumption

Management Strategies to Mitigate HCN Risk

Implementing safe feeding practices is the most effective way to prevent HCN poisoning. Here are several recommended strategies:

Comparison of Forage Utilization Methods


Method of Use	Risk Level	Safety Considerations
Grazing	Highest	Animals can selectively graze high-risk leaves and immature regrowth. Wait for sufficient plant maturity and avoid after frost or drought.
Green Chop	Moderate	Chopping mixes plant parts, diluting the highest concentrations. Still carries risk if chopped too early or under stress conditions.
Hay	Low to Moderate	Curing hay does not eliminate the risk, and testing is recommended for hay from potentially stressed plants. Toxic levels can persist.
Silage	Lowest	The ensiling process, through fermentation, is highly effective at reducing HCN potential by releasing the compound as a gas. Requires proper ensiling techniques.

Safe Feeding Guidelines

Delay Grazing: Never allow animals to graze immature sorghum. Wait until plants are at least 18-24 inches tall.
Monitor Regrowth: After a non-killing frost or harvest, monitor regrowth closely, as new shoots will be high in HCN. Wait for the plant to be completely dead for 7-10 days after a killing frost.
Test Suspect Forage: If environmental stress has occurred, test the forage before feeding. Several laboratory methods, such as the Sodium Picrate test, can estimate HCN levels.
Dilute Toxic Feed: If forage contains moderate HCN levels (e.g., 500-750 ppm on a dry matter basis), it can be diluted by mixing it with a low-prussic acid feed, such as grain.
Avoid Hungry Animals: Ensure animals are well-fed before introducing them to sorghum pasture to prevent them from consuming large amounts of high-risk forage too quickly.
Consider Genetics: If possible, choose low-HCN or dhurrin-free sorghum varieties to minimize risk.

Conclusion

For livestock producers, knowing what is the toxic limit of HCN in sorghum is a vital piece of information that can save an entire herd. While sorghum provides a valuable and nutritious forage, its potential to produce deadly hydrogen cyanide, particularly under stress, cannot be overlooked. The established threshold of 500-750 ppm HCN (dry matter basis) as toxic and over 750 ppm as very dangerous serves as a critical benchmark for risk assessment. By adhering to best practices—including delaying grazing, testing suspect feed, and utilizing safer preservation methods like ensiling—producers can effectively manage this risk and ensure the safety and health of their animals.

References

Frequently Asked Questions

A concentration of hydrogen cyanide (HCN) above 750 ppm on a dry matter basis is considered very dangerous for cattle and should not be fed.

Yes, drying or curing sorghum to make hay can reduce HCN content, but it does not eliminate the risk entirely. If the initial HCN level was very high, it may still remain at toxic levels, and testing is still advised.

After a killing frost, wait 7 to 10 days before grazing to allow the HCN to dissipate. After a non-killing frost, monitor for regrowth and wait for it to reach at least 18-24 inches tall or until a killing frost occurs.

Young, immature sorghum plants contain higher concentrations of the cyanogenic glycoside dhurrin, which is the precursor to HCN. As the plant matures, the dhurrin concentration decreases.

You can have a representative sample of your forage tested at a qualified laboratory. Some on-farm tests, like the sodium picrate test, can also provide a qualitative indication of toxicity.

Early signs include rapid breathing, frothing at the mouth, excitement, and muscle tremors. Symptoms can appear within minutes to a few hours of consuming the toxic forage.

Yes, ensiling sorghum is one of the safest options. The fermentation process during ensiling effectively reduces HCN potential by releasing the gas from the forage.