Prussic acid in sorghum following a hard freeze

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The following article was written by Brent Bean, Ph.D., Director of Agronomy at the United Sorghum Checkoff Program and is reprinted here with his permission. – Kathy Gehl, eUpdate Editor

You will likely be getting some questions regarding prussic acid or hydrogen cyanide (HCN) in sorghum following what will unfortunately be a hard freeze in many regions at the start of this week in October.

Below are some key considerations:

1. Prussic acid (HCN) poisoning is more of a concern when grazing sorghum than when harvested for hay or silage because HCN will dissipate in harvested forages if properly ensiled/cured. For grazing it is best to wait approximately seven days after the hard freeze to graze.

2. Sorghum silage - Most of the HCN will dissipate within 72 hours following warm weather after a hard freeze. However, if HCN levels are high at the time of harvest, wait at least four weeks before feeding the forage. The HCN will volatilize during the fermentation and feed mixing process.

3. Hay - The curing process for hay will allow the HCN to dissipate as a gas, reducing the HCN content to safe levels.

Testing for Prussic Acid

1. If high prussic acid concentrations are suspected prior to grazing or at harvest, forage should be tested before grazing or feeding. There are quantitative and qualitative tests available to learn more about the potential for prussic acid poisoning in a particular forage.

2. If HCN levels exceed 200 ppm on an ‘as-is’ basis or 500 ppm on a dry basis, the forage should be considered potentially toxic and should not be fed as the only source of feed to animals.

3. Contact the forage lab that will conduct the HCN analysis prior to sending in samples so that proper handling procedures can be followed. There are commercial labs available to producers in Kansas to handle this type of testing.


For a more complete discussion, see the Sorghum Checkoff and other links below:

USCP - Avoiding Prussic Acid

University of Nebraska - Cyanide Poisoning

Texas AgriLife - Nitrate and Prussic Acid Poisoning


To monitor the freeze conditions in Kansas, go to the Kansas Mesonet Freeze Monitor tool:

For more information on how to use the Freeze Monitor, please read the recent eUpdate article, “Fall has arrived and the Mesonet freeze monitor returns”, in Issue 712.

Tags:  sorghum freeze