November 4, 2015 - Now is the Time to be Aware of HCN in Frosted Forage



Wednesday, November 4, 2015 - When harvesting forages following frost it is always best to have the forage analyzed to insure levels of HCN are safe. Ensiling will allow the forage to respire and reduce the amount of Frost covered leavesHCN. However, any forage suspected of being high in HCN should be tested before feeding.


  • When frost occurs, the plant cells that accumulate cyanide rupture, releasing the poison.
  • After a killing frost, wait at least four to five days to allow the HCN gas to disperse before allowing animals to graze.
  • New growth after a frost or drought will have very high levels of HCN and be extremely dangerous. Producers should wait until new growth is 18-24 inches tall before grazing or green chopping the forage.
ppm HCN

Effect on animals


Generally safe; should not cause toxicity.


Potentially toxic; should not be the only source of feed.

1,000 and above

Dangerous to cattle and usually will cause death.

Source: Kansas State University Cooperative Extension Service

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