Minimizing the Prussic Acid Poisoning Hazard in Forages
Rhykerd and K. D. Johnson
Agronomy Department, Purdue University
Cooperative Extension Service
West Lafayette, IN 47907
Sudangrass, forage sorghums and sorghum-sudangrass crosses (all in the genus Sorghum)
are often planted for summer pasture and sometimes fed as green chop, silage or hay. Under
certain environmental conditions, livestock may develop symptoms of prussic acid poisoning
when these forages are pastured or fed as green chop.
Death can result from prussic acid poisoning, most commonly when livestock have fed on
plants that are either very young, stunted by drought or frosted. Cattle and sheep are
more susceptible than swine, since they are more likely to consume large quantities of the
What Prussic Acid Poisoning Is
Most of the prussic acid in plants exists as a bound, non-poisonous chemical called
dhurrin. It is present in most sorghums, but some species and varieties contain less than
Also present in the sorghums is a material called emulsion, which under certain
conditions can react with dhurrin to form prussic acid (also referred to as hydrocyanic
acid). If plants are damaged, such as by freezing, chewing or trampling, the
emulsion-dhurrin reaction is enhanced, freeing sufficiently larger quantities of poison
(cyanide) to cause a potentially hazardous condition.
Prussic acid is extremely poisonous. A concentration greater than 0.1 percent of dry
tissue is considered highly dangerous.
Signs of Prussic Acid Poisoning
The signs of prussic acid poisoning appear suddenly--i.e., within 15-20 minutes after
animals consume the "tainted" forage. These visual symptoms include staggering,
labored breathing, spasms and foaming at the mouth. Affected animals then often lie
prostrate and thrash about. Treatment must be administered quickly to prevent death.
Factors Affecting Prussic Acid Content In Plants
Species. The vegetative portion of all sorghums contains prussic acid.
Generally, however, prussic acid content in sudangrass is about 40 percent less than in
most other sorghums. As a group, the sorghum-sudangrass hybrids have more prussic acid
than sudangrass. Crosses have now been developed, however, that contain extremely low
quantities. As a precaution, plant those hybrids known to be lower in prussic acid.
Johnsongrass, shattercane and sorghum almum could contain dangerous levels of prussic
acid, and may be hazardous in pure stands or as contaminants in sudangrass or
sorghum-sudan cross pastures. Fence rows contaminated with these weedy species might also
be hazardous. Plants of the Prunus genus, such as wild black cherry trees (P.
serotina), chokecherry (P. virginiana) and pin-cherry (P. pennsylvanica)
are potential problems and should be eliminated from grazing areas.
Pearl millet, another summer pasture crop, does not contain toxic levels of prussic
Plant Parts In the sorghums, leaf blades normally contain higher prussic
acid levels than leaf sheaths or stems, the heads are low in prussic acid, and the seeds
contain none. Upper leaves have more prussic acid than older leaves. Tillers and branches
("suckers") have the highest levels, because they are mostly leaves and not
Maturity. Highest prussic acid levels are reached before the boot stage. As
plants mature, the stalks make up a greater proportion of the plant, causing prussic acid
content in the total forage to decrease. However, the hazards associated with poisoning
may decrease only slightly with age if animals selectively graze those plant parts that
are high in prussic acid.
Drought. Severe drought is probably the most common cause of prussic acid
poisoning. Drought-stricken plants are hazardous to feed because they are mostly leaves.
Sorghum grazed or fed as green chop in the heart of a drought may retain high levels of
Freezing. Cold weather may kill only the tops of sorghum plants, leaving the
lower portion alive. The unbound prussic acid in this forage does not decline until
wilting begins. The forage is usually considered safe to pasture or feed as green chop 5-6
days after a killing frost. New shoots emerging from unkilled portions of the plant are
apt to be high in prussic acid. Therefore, this forage should not be used until that new
growth reaches a height of 2 feet.
Fertilizer. The excellent yield potentials of sudangrass, sorghum-sudangrass
crosses and forage sorghums can only be attained by applying high rates of nitrogen
fertilizer (e.g., 200 pounds per acre or more). However, if high N rates are applied to
soils deficient in phosphorus and potassium, prussic acid levels usually increase.
Therefore, to reduce the hazard of prussic acid poisoning, maintain phosphorus and
potassium levels according to soil test report recommendations. Also consider
split-applying heavy N rates into 2-4 applications.
Herbicides. 2,4-D may cause prussic acid content to increase in forages. The
effect may last several weeks.
Safe Feeding of Potentially Hazardous Forages
Pasture. The risk of prussic acid poisoning can be reduced by feeding ground
cereal grains to the animals before turning them out to graze. Carbohydrates in the grain
tend to inhibit the emulsion from hydrolyzing dhurrin, which causes prussic acid
formation. The chance of problems on pasture can be further reduced by using heavy
stocking rates (4-6 head per acre) and rotational grazing.
Deaths on pasture are partially caused by cattle selectively grazing leaves and shoots.
These plant parts may contain 2-25 times more prussic acid than stems. Cattle may also
avoid frost-damaged leaves and shoots, grazing instead the young suckers lower on the
plant that could contain lethal levels of prussic acid. Therefore, if new shoots develop
after a frost. the crop should not be grazed until this new growth is 2 feet tall.
In most cases, grain sorghum stubble can be safely pastured because cold weather is
likely to have killed the plants before they are grazed However, the stubble should be
observed carefully for dangerous suckers that may develop after the main stalks have been
killed. Sorghum that has wilted and dried 5-6 days after being killed by frost is
considered safe for grazing.
Green Chop. Green chop forage is usually safer than the same material used for
pasture because it is not selectively grazed. Whereas in the case of pasture only the
leaves may be eaten, with green chop material the total plant is consumed. Stems act as
safety devices `diluting' the high prussic acid content of leaves.
Silage. Sorghum silage is generally safe for feeding. Although it could contain
toxic levels of prussic acid while in storage, much of the poison escapes as a gas during
fermentation and when being moved for feeding. However, as a precaution, do not feed new
silage for at least 3 weeks after harvesting and storing.
Hay. The prussic acid content of sorghum hay decreases as much as 75 percent
while curing and is rarely hazardous when fed to livestock.
A Final Precaution. Rather than expose the entire herd to danger, use test
animals for brief periods when the silo is freshly opened or when turning onto
Treatment For Prussic Acid Poisoning
If large quantities of forage high in prussic acid are consumed rapidly. death can
occur within a few minutes. However, the usual situation is that the animals consume
smaller quantities of the forage over a longer period, causing first salivation, then a
gradual increase in respiratory rate, followed by staggering, falling, severe convulsions
and finally death within 45 minutes. Generally, animals that survive 2 hours after the
onset of symptoms will recover.
Obviously, immediate treatment by a veterinarian is necessary to save the animals.
Treatment includes administering sodium nitrite and sodium thiosulfate.
Poisoning caused by prussic acid is somewhat similar to nitrate poisoning. In fact, the
treatment for prussic acid involves the inducement of a degree of nitrate poisoning
(methemoglobinemia) by administering sodium nitrite. Simultaneous treatment with sodium
thiosulfate converts the newly formed cyanmethemoglobin to thiocyanate and hemoglobin,
which permits the blood to again transport oxygen normally.
Cooperative Extension work in Agriculture and Home Economics, state of Indiana, Purdue
University, and U.S. Department of Agriculture cooperating; H. A. Wadsworth, Director,
West Lafayette, IN. Issued in furtherance of the acts of May 8 and June 30, 1914. Purdue
University Cooperative Extension Service is an equal opportunity/equal access institution.