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Precautions for Grazing and Harvesting Forages After a Frost

There are several warm-season forage species grown in Florida that can accumulate toxic compounds after experiencing stress conditions or frost.  The toxins produced are called cyanogenic glucosides which are readily converted to prussic acid (i.e. hydrogen cyanide).  Grasses in the sorghum family, Johnsongrass, shattercane, chokecherry, black cherry, indiangrass and elderberry are all plants capable of producing these compounds.  Black cherry is a very common tree species found in wooded areas and along fence lines in Florida.  Pastures should be regularly scouted for black cherry and fallen limbs should be moved to an area where livestock cannot access them.  The levels of these toxic compounds increase when trees are damaged and leaves turn brown.  Below is a table that lists the potential toxicity of plants after a frost/freezing event.

Plant/Species Potential Cyanide Poisoning
Sudangrass varieties low to intermediate
Sudangrass hybrids intermediate
Sorghum-sudangrass hybrids and forage sorghum intermediate to high
Grain sorghum high to very high
Piper sudangrass low
Pearl and foxtail millet rarely cause toxicity

Most of the plants listed, when grown in Florida, are produced in the summer months when frost events are not an issue.  However, in some cases sorghum that is chopped for silage will remain in the field as stubble throughout the winter.  Grazing this stubble after a freeze event could be hazardous to the livestock.  Ruminant livestock are more susceptible to poisoning by prussic acid and should be monitored for the following symptoms if the plants listed above are grazed after a freeze.  Symptoms include: excess salivation, labored breathing, staggering, convulsions, collapse and sometimes death.  Symptoms can occur within minutes of consuming plants with high concentrations of prussic acid.

Forage plants that have underwent a freeze event can still be cut for hay or chopped for silage.  The toxic compound exists as in a gaseous form and is released during the drying process of hay production and is lost during the ensiling process.  If the concentration of prussic acid was extremely high at the time of chopping some amount may remain after the ensiling process.  Samples should be submitted to a lab for testing when in doubt.

Do not graze plants with prussic acid poisoning potential…

  • …on nights when frost is a possibility.
  • …for 5 to 7 days after a killing frost.
  • …for 2 weeks after a non-killing frost.
  • …when new growth appears after a frost.

Consult your local Extension agent if you think that you may have plants with prussic acid accumulation.  Samples can be submitted to a forage testing laboratory to confirm the presence of the toxin.

References

Barnhart, S. K., 2008.  Fall Frost Effect of Forage. Iowa State University. Integrated Crop Management. https://crops.extension.iastate.edu/cropnews/2008/10/fall-frost-effects-forage

Sulc, M. 2015. Precautions for Harvesting Forages After a Frost. Ohio State University Extension. C.O.R.N. Newsletter. 2015-34. https://agcrops.osu.edu/newsletter/corn-newsletter/2015-34/precautions-harvesting-forages-after-frost

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