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Frequently Asked Questions - Prussic
Acid and Green Flax
Murray
Feist
Ruminant Nutritionist, AKC, SAFRR
September 2005
Can
green flax be used for livestock feed?
Flax crops delayed by weather and damaged by frost
can result in incomplete seed development, and in some cases, regrowth
and reflowering may have occurred on the green plant. While the green flax
plant can potentially contain 8-18% crude protein and 40-65% TDN, there
are several points that need to be considered, including the potential
for toxicity, if producers are thinking about harvesting the flax as forage.
What
is prussic acid and how is it formed in green flax?
Green flax contains
a sugar compound called cyanogenic glycoside. This substance itself is
not harmful. When trauma to the plant occurs, plant cells rupture, allowing
the cyanogenic glycoside to mix with the enzyme beta glycosidase (ß-gycosidase)
and triggering a reaction that converts the cyanogenic glycoside into
a new compound called hydrocyanic acid (hydrogen cyanide) or prussic acid.
When consumed, the hydrogen cyanide is absorbed into the bloodstream and
acts to prevent the oxygen in the blood from being released from the haemoglobin
to tissue cells. As a result, the animal dies of asphyxiation. Rumen bacteria
can also create a prussic acid poisoning scenario. Certain bacteria in
the rumen producing a ß-gycosidase enzyme will also convert green
flax cynogenic glycosides into prussic acid even without a frost occurring.
The trauma on the green plant caused by trampling and chewing releases
the cyanogenic glycoside into the rumen environment where the enzyme is
present. This too will result in the creation of prussic acid, asphyxiation
and death of the animal if the concentration of prussic acid is high enough.
Other factors such as drought, wilting and chemical desiccation can also
result in prussic acid levels in flax. Levels of prussic acid tend to
remain high for several weeks post frost/spraying or other trauma (e.g.
livestock trample while grazing, crushing, etc.) on the plant, and a longer
period of time may be required for proper reduction of prussic acid levels.
What levels of prussic acid are toxic to livestock?
The toxicity level
of prussic acid has been referenced at varying levels. One source from
the University of Saskatchewan indicated that toxicity can occur should
the level of prussic acid exceed 200 mg/kg (ppm) on a dry feed basis.
Another sources from Kansas State indicates that the feed is generally
safe for consumption at 0-500 mg/kg dry matter, potentially toxic and
should be blended at the 600-1000 mg/kg dry matter and dangerous to cattle
at >1000 mg/kg dry matter. The Merck Veterinary Manual suggests that
processed forages and standing plants containing <500 mg/kg dry matter
are considered safe; 500-750 mg/kg dry matter is cautionary and >750
mg/kg dry matter is extremely hazardous.
What
are the symptoms of prussic acid poisoning?
Reactions to prussic acid poisoning are swift and lethal. Livestock poisoned
from prussic acid usually tend to die within several minutes from consumption.
Symptoms include laboured breathing, staggering, excessive salivation,
convulsions, collapse and death. Prussic acid poisoning diagnosis can
be similar to nitrate poisoning by way of clinical signs. Differential
diagnosis depends on examining the color of the blood. Livestock poisoned
by nitrates will have a chocolate, dark-coloured blood sample, whereas
livestock poisoned by prussic acid will have a bright red blood sample
which does not clot quickly. If an animal affected by prussic acid does
not die from the poisoning, it can be treated with an injection of sodium
nitrates and sodium thiosulfate by a veterinarian. The sodium nitrate
releases the cyanide from the haemoglobin in the blood, freeing it up
to re-attach itself to the sodium thiosulfate for excretion in the urine.
Livestock fortunate to receive the treatment usually recover with no further
symptoms.
Can
harvest and feeding management influence prussic acid levels?
There are several factors which can influence the level of prussic acid
in flax. Stage of growth, crop fertilizer regime and harvest management
is extremely important. When the plant is green and growing, the highest
concentrations of prussic acid will be found in leaves and seeds. In the
event of frost, new leaves growing on the flax will contain higher levels
of free prussic acid than the more mature leaves and stems and possibly
the seeds. Based on data for sorgham, another crop that can accumulate
prussic acid, crops at risk will be located on fields that have soils
high in nitrogen and low in phosphorus and potassium. With regards to
harvest management, free prussic acid dissipates more rapidly then the
prussic acid in the stems and leaves. It has been reported that proper
haying and silage techniques can result in >50% of the prussic acid
content dissipating due to enzyme activity and release of free cyanide
in a gas form. Also, extending the storage time of the forage should reduce
the prussic acid to a safe level before being fed. It should be noted
that a prussic acid test from a feed analysis laboratory would be required
before allowing access to the harvested forage.
If the opportunity
to swath graze is possible, producers should exercise caution particularly
within several weeks post swathing/cutting. Avoid turning hungry livestock
out onto the swaths for approximately one week post frost. Risk can be
minimized by feeding hay or grain as it will help to dilute the amount
of prussic acid content in the stomach. Low levels of prussic acid can
be detoxified under these circumstances. Again, a prussic acid analysis
from a feed analysis laboratory is required before attempting to swath
graze a flax crop. Standing flax should NOT be considered for grazing
as the act of trampling and chewing will release the cyanogenic glycoside
and the ß-gycosidase enzymes, creating a toxic environment for the
livestock.
Are
nitrate levels a concern in feeding flax straw?
Another nutritional issue with flax is nitrate accumulation after a frost.
By theory, one can minimize the threat of nitrate accumulation after a
frost by allowing the plant to regrow for a period of several days to
a week. This assumes that the plant has not been killed by the frost.
This strategy would allow the plant to metabolize the nitrates. If the
plant has been killed by the frost, the crop should be harvested as soon
as possible and then sampled and tested for nitrates prior to use. Following
common nitrate management recommendations is advisable. If nitrate content
is deemed to be a concern, a nitrate analysis by a feed analysis laboratory
is recommended.
Final
Decision
The cutting of green flax requires some decision making. First, consideration
must be given to the current feed availability. Second, the level of crop
maturity needs to be assessed. Early stages of growth (prior to bolling)
may be very nutritious, while later stages may become too fibrous to be
of practical use. While green flax is nutritionally comparable to canola
hay, harvesting the flax when green may be hard on the cutting equipment
and may require crimping to help the crop dry down enough for baling/harvesting.
Third, the nitrate and prussic acid issues need to be considered as it
may force the blending of the feed. And finally, processing the feed with
a bale processor or feeding it as silage may be necessary to minimize
wasting by the livestock.
For more information,
please contact the Agriculture Knowledge Centre
at 1-866-457-2377.
Links
1. Kansas
State Forage Facts: Prussic Acid Poisoning
2. The
Merck Veterinary Manual, 1991.
3. www.ForageBeef.ca
4. Natural Toxicants in Feeds, Forages and Poisonous Plants. P.R. Cheeke,
1998. 2nd Edition. pp 279-281.
5. Clinical and Diagnostic Veterinary Toxicology, G.D. Osweiler et al,
1985, 3rd Edition, pp 455-458.
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