Managing Warm Season Annuals After Killing Frost

If you or anyone you know has used warm season annuals for forage, you made a very good choice! But you have also probably heard of the concern of prussic acid. What is prussic acid and why is it a topic of discussion this time of year?

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What is Prussic Acid?

                Prussic Acid is a compound that exists in warm season forage plants such as forage sorghum, sorghum sudangrass, and sudangrass. While these are often harvested and grazed during the summer, this time of year can also be common. The concerns with prussic acid arise when we start having killing and non-killing frosts. When these types of plants are killed from a frost, the cells rupture releasing the prussic acid as a gas that will be harmful to livestock. Here are some procedural guidelines to follow this time of year if you have these crops:

Grazing:  After a killing frost, keep livestock from grazing for 10-12 days

Mechanical Harvest: At, or before, a killing frost, the forage can be chopped or baled at any time the moisture is correct for the harvest type; however, do not feed forage for 30-40 days after harvesting.

What happens during those wait times?

                Since it is released as a gas, the forage dries and the prussic acid dissipates through volatilization. This makes the concentration levels in the tissue safe for livestock consumption. It is important that the killing frost we are speaking of is truly killing to the entire plant. If the frost only kills part of the plant, it can attempt to regrow by sending out new shoots that can also higher prussic acid concentrations.

Still have concerns?

                Forage samples can be sent in and tested for prussic acid concentrations at most forage sampling facilities. The cost of these can range from $50-$60, but can serve as insurance and be helpful in relieving worries about what you are feeding. Simply ask for a Prussic Acid Test. Keep in mind that since prussic acid volatizes as a gas, this is a time sensitive test that often needs overnight shipping to the testing facility.

Overall, the likelihood of prussic acid poisoning is very controllable by using the guidelines above. Feel free to give the office a call if you would like any additional information!

Bean, Brent, and Kim McCuistion. "Avoiding Prussic Acid." Sorghum Checkoff, 1 Nov. 2017.