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Prussic acid
Alicia Boor
Alicia Boor

After sorghum harvest, many producers want to move their cattle onto milo stalks to graze. With an early hard freeze, it is a good idea to be aware of prussic acid in sorghum, and if you are concerned, have your forage tested. A quick test now can give you peace of mind when you turn your livestock out to utilize the stalks and grain that are in the field.

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. 


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.

If you have any questions, or would like more information, you can contact me by calling 620-793-1910, by email at or just drop by the office located at 1800 12th street in Great Bend.

Alicia Boor is the Agriculture and Natural Resources agent with K-State Research and Extension – Cottonwood District. Contact her by email at or call 620-793-1910.