Grazing Johnsongrass After Drought Can Be Deadly
Prussic Acid (Cyanide) Risk in Johnsongrass Regrowth: With recent rains bringing long-awaited relief to drought-stricken pastures across southern Missouri, many producers may begin seeing rapid regrowth of Johnsongrass (Sorghum halepense). While this may seem like a welcome forage opportunity, grazing Johnsongrass immediately after drought and rainfall fed regrowth poses a serious risk of prussic acid (Hydrogen cyanide) poisoning in livestock.
Johnsongrass is known to accumulate hydrogen cyanide (HCN) under certain stress conditions, particularly:
• After drought stress, when plant metabolism is disrupted.
• Following frost or mechanical damage.
• During rapid regrowth, especially in young shoots after a substantial rainfall.
These conditions cause the plant to produce cyanogenic compounds, which can convert to toxic prussic acid in the rumen of grazing animals. Cattle are especially vulnerable, and even small amounts can be fatal.
Nitrate Poisoning Risk After Rainfall: In addition to prussic acid concerns, producers should also be aware of the risk of nitrate poisoning in forage crops following drought and subsequent rainfall.
During drought conditions, plants such as Johnsongrass, Sorghum-Sudan, and other grasses may accumulate high levels of nitrates due to reduced growth and nitrogen uptake. When rainfall occurs and regrowth begins, these nitrates can remain concentrated in the lower parts of the plant, especially in the stems.
Livestock consuming forage with elevated nitrate levels may experience nitrate poisoning, which interferes with oxygen transport in the blood.
Signs of Poisoning in Livestock
Symptoms of prussic acid poisoning:
• Rapid breathing or gasping
• Muscle tremors
• Salivation
• Bright red mucous membranes
• Collapse and sudden death
The smell of bitter almonds is often detected in animals poisoned with cyanide.
Symptoms of nitrate poisoning:
• Labored breathing
• Weakness or staggering
• Bluish or brownish mucous membranes
• Rapid pulse
• Sudden death in severe cases
Best Management Practices
To reduce risk of cyanide and nitrate poisoning:
1. Wait 10–14 days after a rainfall before allowing livestock to graze regrowth.
2. Avoid grazing young shoots less than 18–24 inches tall.
3. Test forage for prussic acid and nitrate levels if possible.
4. Consider haying instead of grazing – cyanide levels may decrease during curing.
5. Supplement with other forages to dilute intake if grazing is necessary.
6. Introduce livestock gradually to suspect forage and provide supplemental feed.
Grazing Safety Checklist
• Delay grazing for 10–14 days after rainfall
• Measure plant height (avoid grazing under 24 inches)
• Test forage for cyanide and nitrate levels
• Monitor livestock for symptoms
• Provide alternative forage or hay
• Consult with Extension or veterinarian before grazing
If you have questions about forage safety, testing options, or pasture management, reach out to your local Extension office or contact.
Joshua Dunn is the Field Specialist in Agronomy at MU Extension, South Central Region
