Managing Forages to Minimize Prussic Acid Poisoning
David W. Koch, Extension Agronomy Specialist, University of WyomingAlthough not as common as nitrate poisoning in Wyoming, livestock producers need to
be aware of conditions in which prussic acid can be toxic to livestock. Conditions that produce
high nitrates can also increase prussic toxicity potential.
Prussic acid or hydrogen cyanide poisoning arises from the release of emulsin, which is
found primarily in plant tissue of the sorghum family, and interaction with dhurrin, also found in
these same crops. Damage to plants, such as freezing, trampling, chewing results in the
interaction of these two plant compounds and the creation of hydrogen cyanide (HCN).
Species. Grain and forage sorghums are most likely and sudangrass least likely to have high
prussic acid potential. Most sorghum-sudangrass hybrids now have low prussic acid potential,
like the sudangrass parent, although varieties can vary. It is worth inquiring when purchasing
seed. A few weedy sorghums also have prussic acid potential, however, they are not common in
Wyoming.
Plant parts. Leaf blades are generally highest, followed by leaf sheaths, stems and heads.
Upper leaves have a greater amount than lower leaves. Tillers, or “suckers” have a higher
potential because they are mostly leaf tissue.
Plant maturity. As plants mature, stems or stalks make up more of the total forage; however, if
livestock are allowed to selectively graze the hazard declines little. Overall, the prussic acid
potential seems to decline after the boot stage.
Drought. Any stress that disrupts normal growth can contribute to prussic acid toxicity.
Probably the most common cause of prussic acid poisoning in sorghums is drought. Drought-
stricken plants consist mainly of leaves. Animal poisoning can result from grazing or green chop
feeding. Regrowth following drought can have deadly concentrations.
Freezing. Sorghum is resistant to fall light frosts. Initial frosts may kill only the tops of
sorghum plants, leaving the lower portions alive. New shoots can later emerge and are likely to
be high in prussic acid potential.
Fertilizer. High nitrogen fertilizer, along with low soil phosphorus and potassium can increase
cyanide hazard.
Animals. Ruminants are more susceptible than horses and swine. Cud chewing and rumen
bacteria enhance HCN release.
What can be done to avoid prussic acid poisoning?
Grazing management. Wait until forage is at least 18-24 inches tall before initial grazing. Be
particularly cautious if drought slows or stunts growth. Chances of prussic acid poisoning can be
reduced by heavy stocking rates and rotational grazing. This reduces selective grazing of leaves.
Ground corn or other cereal grain can be fed prior to turn out. Grain carbohydrates tend to
inhibit emulsin from hydrolyzing dhurrin and forming hydrogen cyanide. Feeding hay before
turnout reduces intake of sorghum forage and dilutes the cyanide. In order to avoid poisoning of
frost regrowth, wait until at least five days after a frost that kills new shoots. Another precaution
with grazing live vegetation is to turn in a test animal(s), rather than the whole herd, to determine
if there may be a problem.
Green chop. Chopping eliminates the problem with selective grazing and the chopping helps
release prussic acid before utilization; however, material that is very high in prussic acid
potential can still pose a hazard, since forage is usually fed shortly after chopping. If nitrate
accumulation is suspected, green chop should not be allowed to heat up, as nitrate toxicity
increases under these conditions.
Silage. Sorghum that has been ensiled is generally safe for livestock feeding. Much of the
poisonous gas escapes during fermentation and in the process of movement to feed bunks. Silage
should not be fed for at least 3 to 4 weeks after ensiling, however.
Hay. Prussic acid potential declines substantially during cutting and curing. Prussic acid is
rarely a problem with hay feeding. One practice that has proven successful in Wyoming is
swathing, raking into windrows and leaving forage in the field for fall and winter grazing. The
cutting tends to preserve higher quality than leaving forage standing and since precipitation
declines through the fall there is generally little weather damage. Animals eat the forage as well
as fed hay, saving the cost of baling, hauling and feeding. There is less leaf loss, compared to
waiting until the crop dries up following a killing frost.
Symptoms and treatment.
Death can occur within minutes if large quantities of forage with high prussic acid
potential are consumed. Animals consuming smaller amounts over time will show, in
progression, excessive salivation, increased respiratory rate, staggering, falling, severe
convulsions, then death. Animals can survive once symptoms have begun if removed from the
forage. Treatment usually involves administering sodium nitrite and sodium thiosulfate by a
veterinarian. Forage high in prussic acid potential can also be high in nitrates and the animal
symptoms can be similar. See bulletin, "Managing forage to reduce nitrate poisoning of
livestock."
Lab analysis
Suspect forages can be analyzed for prussic acid or hydrogen cyanide potential hazard.
See your University Extension Educator in your county for assistance in obtaining a
representative sample and lab analysis.
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