Soil Test Note – No. P001
Dr. George V. David, Jr., Extension Livestock
Specialist – Beef Nutrition, Dr. John Jennings, Extension Livestock Specialist
– Forages
PRUSSIC ACID
Hydrocyanic acid (HCN)
or prussic acid is generally not found in healthy plants but is formed by
enzymatic action on compounds called the cyanogenetic glucosides (dhurrin)
when growth is adversely affected. In
healthy plants, the cells containing the glucosides and enzymes (emulsin)
apparently keep these constituents separated.
When growth is checked by adverse environmental conditions such as
moisture stress or frost, the enzymatic action may take place, producing prussic
acid. Breakdown of the plant cells by crushing or
bacterial action in the rumen may also result in prussic acid formation. When dhurrin is broken down in the rumen by
enzymatic action, the cyanide combines with the hemoglobin in the bloodstream. This affects the electron transport system
at the cellular level and prevents the cells from receiving the oxygen from
the blood. The blood is able to transport
oxygen from lungs, but the body tissue can’t take it up and suffocation occurs.
One symptom of prussic acid poisoning is bright red blood since it
is full of trapped oxygen.
Susceptible Plants
The amount of dhurrin found
in plant tissue varies among species. Of
all the plants grown in Arkansas, those belonging to the sorghum category
are the ones most likely to contain potentially toxic levels. Grain sorghum contains the most, followed by
johnsongrass, sorghum-sudan hybrids, then pure sudangrass. Pearl millet is apparently free of the toxin.
HCN may be produced by a few other plant species.
Wild cherry trees produce toxic levels.
Problems with wild cherry poisoning occur most often when animals consume
wilted leaves after trees have been damaged by storms or pruning.
The amount of dhurrin is
higher in young plants than in older ones.
The dhurrin content of leaves is higher than that of the stems. Upper leaves contain more than the lower ones.
Dhurrin concentration decreases as the plants become taller and more
mature. Usually, sorghum-type plants
18 to 24 inches tall are less likely to contain high concentrations of the
toxin. Immature plants and re-growth
following haying or grazing contain the highest levels.
Drought and frost are closely associated with high levels of dhurrin. Plants growing under stress and those that have received nitrogen
rates above 75 pounds per acre in one application may result in more toxin.
Plants have more potential
for producing prussic acid if the soil is high in nitrogen and deficient in
phosphate and potassium. An increase
in prussic acid may result when broadleaf forages or weeds are treated with
2,4-D at rates that stunt the crop growth.
Symptoms of Poisoning
When lethal amounts of
dhurrin are consume and the enzymatic action takes place resulting in prussic
acid poisoning, the dead animals may be found without visible symptoms of
poisoning. Prussic acid-poisoned animals
show an increased rate of respiration, increased pulse rate, gasping, muscular
twitching, and convulsions. Death
may occur from respiratory paralysis.
Susceptible Animals
Ruminant animals – cattle,
sheep and goats – appear to be the most susceptible to prussic acid poisoning.
Reports of poisoning in swine and horses are rare.
Although prussic acid is
not often a problem in horses, feeding sorghum type forage to them may produce
a malady known as “crystitis syndrome.” The
exact cause of the disease is not known, but forage from sorghum-type plants
has been a constant factor in every outbreak. Affected horses exhibit position inccordination,
urine dribbling, and abortion in pregnant mares. Horses should not be grazed on sudan or sorghum-sudan
species since all treatments for crystitis syndrome have met with little success.
Relationship to Nitrate Poisoning
Under certain stress conditions,
especially prolonged drought or cool, cloudy weather, many annual grasses
including the sorghum-sudan hybrids may accumulate high levels of nitrates
in their stems. Many times prussic
acid poisoning is confused with nitrate poisoning since environmental conditions
and animal symptoms of the two disorders are somewhat similar.
Several precautionary measures
may be taken before high nitrate forages are consumed. One of these is the use of propionibacterium.
Most animals which live two hours after the onset of nitrate toxicity
symptoms will recover.
Precautions for Using Sorghums
1. Do not allow animals to graze fields with succulent,
young, short growth. Graze only after
plants reach a height of 18 to 24 inches.
2. Do not harvest or feed drought-damaged plants in any
form within four days following a good rain regardless of height. It is during this period of rapid growth that
an accumulation of HCN in the young tissue and of nitrates in the stems is
most likely to occur.
3. Do not graze wilted plants or plants with young regrowth.
Do not rely on drought-damaged material as the only source of feed.
Keep either dry forage or green chop from other crops available at
all times. Even when this material
is mildly toxic, it can be fed safely to animals receiving some other forage
or grain source. Uneven growth of
hybrids as a result of drought can best be utilized as silage.
4. Do not utilize frost-damaged sorghum as pasture or green
shop during the first seven days after the first killing frost. Delay pasturing until at least seven days or
until the frosted material is completely dried out and paper brown colored.
Do not rely on frosted material as the only source of feed.
The toxin is usually dissipated within 48 hours. Do not graze at night when frost is likely.
5. Do not turn hungry cattle onto a pasture of sorghum
or sorghum-sudan hybrid. Fill them
up on hay first and begin grazing in the late afternoon.
6. Prevent selective grazing of the young regrowth, which
may be highly toxic, by rotational grazing of small pastures which may be
grazed down to a 6-inch stubble within a ten-day period. This will mean cross-fencing to provide short-term
rotational or strip grazing.
7. Silage may contain toxic quantities of prussic acid,
but it usually escapes in gaseous form while being moved an fed. If frosted forage is ensiled, allow fermentation
to take place for at least six to eight weeks before feeding.
8.
The prussic acid potential of
hay decreases during the curing stage and is only dangerous if improperly
cured.
Animal Treatment
Animals affected by prussic
acid poisoning may be treated with sodium nitrate-sodium thiosulfate combinations.
It must be injected intravenously and very slowly; the dosage and method
of administration are critical. Use
a reputable veterinarian to correctly diagnose prussic acid poisoning and
to recommend the proper treatment.
June, 1994
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