Minnesota Crop News > 2001-2008 Archives
August 24, 2004
Early Frost Means Be Alert to Prussic Acid in Forage Sorghums
Paul Peterson and Marcia Endres, Extension Agronomist and Extension Animal Scientist. University of Minnesota
Forage sorghum, sorghum-sudan hybrids, and sudangrass
all have the potential to produce prussic acid poisoning
in livestock. Prussic acid is the same as hydrocyanic acid
(HCN). Plants of the sorghum species contain a non-toxic
compound called dhurrin that is converted to toxic prussic
acid by a process called cyanogenesis. Pearl millet does
not cause prussic acid poisoning. The toxifying action
of prussic acid is almost immediate and death can occur
within 15-20 minutes. In general, cattle and sheep are
more susceptible to prussic acid poisoning than horses
Large amounts of prussic acid may be released via cyanogenesis
in a short period of time when sorghum plant tissue is
injured by wilting, freezing, cutting, or trampling. The
amount of dhurrin in a plant varies with genetics, plant
part, plant age, climate, moisture supply, and soil fertility.
In general, forage sorghums tend to be highest in prussic
acid potential, followed by sorghum-sudan hybrids, then
sudangrass, which is usually safe. Leaves contain twice
as much prussic acid as stalks. New, young shoots also
are very high in prussic acid potential. As plants mature
or age, the amount of dhurrin decreases. Drought, frost,
or any climatic condition that halts normal growth usually
causes increased prussic acid release.
Field curing liberates 50-70% of the prussic acid. Conditioning
helps increase liberation of prussic acid because it causes
enzymatic breakdown of dhurrin, and prussic acid evaporates
during drying. Never bale material with prussic acid until
it contains less than 18 percent moisture. Ensiling may
provide some liberation of prussic acid. To be safe, don’t
expect ensiling to liberate prussic acid unless plants
are conditioned and wilted before ensiling. However, ensiling
should be done before the plant material reaches 50% moisture.
- Sudangrass and sorghum-sudan hybrids growing under
normal conditions are safe to graze or green chop after
they have reached ~18-24”.
- Forage sorghums may not be safe for grazing or green
chopping until headed.
- All sorghums (forage sorghum, sorghum-sudan, and sudangrass)
can be fed safely when harvested as dry hay, regardless
of growth stage.
- All sorghums can be ensiled and fed with safety when
harvested after heading. If harvested at earlier stages,
partial field drying is necessary for best ensiling,
and some lowering of prussic acid potential can be expected.
- All sorghums having adequate growth for safe grazing
before frost can be grazed safely ~5 days after frost
if the frosted plants have dried out. This means sudangrass
or sorghum-sudan hybrids over 18-24” tall, or headed
forage sorghums. Frosted sorghum is very dangerous until
it has dried out, so wait until the plant material is
- Wait 2 weeks after a killing frost before grazing
sorghums that are too short for safe grazing before frost.
- If new shoots develop after a partial frost killing,
don’t graze any sorghums until complete frost killing
occurs. The new shoots are especially high in prussic
acid potential and may be preferred by livestock.