Is My Forage Sorghum, Sorghum x Sudangrass, or Sudangrass Safe to Graze?

Annual forages like sorghum, sorghum x -sudangrass, and sudangrass can accumulate nitrates, which should be evaluated before grazing. However, another potential risk is prussic acid poisoning (cyanide), which can be lethal if ingested.

Forage sorghum (also known as cane) and grain sorghum (commonly referred to as milo) pose the greatest risk, with sorghum x -sudangrass having a reduced risk, but still a concern. Sudangrass has the lowest risk of the sorghums, but it is not a negligible risk. Prussic acid poisoning can occur in certain scenarios, such as new growth or following a frost. If these plants are fertilized with high rates of nitrogen fertilizers, the risk of prussic acid increases further. Shoots shorter than 15 to 18 inches are especially dangerous, particularly for the sorghums.

To minimize risk, wait until sorghum and sorghum x sudangrass plants are at least 24 inches tall and sudangrass is at least 18 inches tall before grazing. If you have regrowth in late summer that does not meet these plant height thresholds, you can either wait seven days after a killing frost before grazing or test the plants to assess prussic acid levels.

Frost poses a significant risk because prussic acid forms in plants when cell membranes are damaged by freezing. This allows chemicals within the plant cells to combine and produce cyanide. After seven days, any cyanide generally dissipates into the air. However, each freeze that does not completely terminate plant growth results in an additional seven-day countdown. A killing frost is typically considered to be around 24°F. However, dense stands may require a longer, sustained cold period to achieve a full termination of growth.

To mitigate this risk, one strategy is to pull cattle off pastures when frost is forecast and then wait seven days after a killing frost before grazing. However, the period between the first frost and a killing frost can sometimes be prolonged, with frost-free intervals where grazing might seem feasible. During these windows, testing the forage for cyanide can provide valuable information, helping producers determine if it is possible to safely graze before the final killing frost occurs.

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 in-field 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:

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.

For more details on this process, you can watch a helpful how-to video on quick field tests here.