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Annual forages are a great resource for producers, but those using sorghums or sudangrass species always need to keep the risk of prussic acid in the back of their mind. At any time of year, young shoots of sorghum species contain prussic acid, which is why recommendations are to allow ample growth (15-18 inches on new plants) before grazing. When ingested by livestock, the cyanide is released can quickly cause lethal results when consumed in high amounts.
This time of year, when a frost occurs, broken cell membranes again release large amounts of prussic acid. Typical practice is to keep animals out of the field for 5-7 days after a freeze for the prussic acid to dissipate. The problem is that each time a new part of the plant is frozen, this five-seven day timer is reset until the entire plant has been killed. This can make grazing difficult in the fall when freezing events occur regularly but are not enough to fully kill the plant.
Another way to ensure that sorghums are safe to graze after a freeze is to do a quick field test. UNL professors Dr. Mary Drewnoski and Dr. Daren Redfearn share the following advice: Using a Quick Field Test to Assess Prussic Acid Risk Producers looking to assess risk in situations where plant growth is borderline or to confirm safety after a killing frost can perform a simple infield test for prussic acid. This test screens for the presence of cyanide but does not provide exact concentrations. It can, however, give an indication of potential risk. To perform the test, you can purchase Cyantesmo paper from CTL Scientific Supply (Item 90604), which can be used for multiple tests and has a shelf life of about two years. You will also need disposable gloves, tape, and ziplock bags. Alternatively, Ward Labs offers kits that include everything needed for three tests.
How to Perform a Quick Field Test:
• Collect the forage: Take a large handful of leaves or other plant material that animals will likely consume. Young shoots and leaves are more likely to contain prussic acid than stems. Collect multiple samples from across the field for a representative assessment.
• Prepare the sample: Place the forage in a ziplock bag until it’s about half full. Tear the leaves into small pieces, mashing them slightly to increase cell injury.
• Attach the test strip: Tape a one-inch strip of Cyantesmo paper to the inside of the bag, near the top, without directly touching the plant material. Always wear gloves when handling the paper.
• Moisten the sample: If the sample is dry (little plant juice), add about one tablespoon of water to the bag, just enough to make it damp.
• Seal and warm the bag: Seal the bag and place it in a warm location, such as on the sun-warmed hood of a vehicle. The heat will help release any cyanide gas present in the plant material.
• Check the test strip: After 10 minutes, check the color of the test strip. If it turns dark blue, the sample is positive for cyanide, indicating a potential risk. If there’s no color change, the sample is negative for cyanide. Some color change indicates some cyanide is present. If only slight color change occurs, wait another 20 minutes and reassess.
• Final assessment: After 30 minutes, a sample causing little or no color change in a sufficiently moist environment likely poses minimal risk of prussic acid poisoning.
Safety Tip: Do not breathe the fumes when opening the bag, as cyanide gas is released.
-Ben Beckman is a livestock systems Extension Educator based out of the Cedar County Extension office in Hartington. You can reach him by phone: (402) 254-6821 or email: [email protected]
