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Prussic Acid and Greenflax

Green flax 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 eight to 18 per cent crude protein and 40-65 per cent  total digestible nutrients (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.

Prussic acid

Green flax contains a sugar compound called cyanogenic glycoside. This substance itself is not harmful. However, when trauma to the plant occurs, plant cells rupture, allowing the cyanogenic glycoside to mix with the enzyme beta glycosidase (ß-gycosidase), 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 (HCN)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 cyanogenic 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 increased 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.

Levels of toxicity

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 an as fed basis. Numerous sources indicate 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 greater than 1000 mg/kg dry matter. The Merck Veterinary Manual suggests that processed forages and standing plants containing less than 500 mg/kg dry matter are considered safe; 500-750 mg/kg dry matter is cautionary and greater than 750 mg/kg dry matter is extremely hazardous.

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 to a few hours after consumption. Symptoms include laboured breathing, staggering, excessive salivation, convulsions, collapse and death. The smell of bitter almonds is often detected. Prussic acid poisoning diagnosis can be similar to nitrate poisoning by way of clinical signs. Differential diagnosis depends on examining the colour 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 nitrate 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 that receive the treatment usually recover with no further symptoms.

Harvest and feeding management

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. Leaves generally produce two to 25 times more prussic acid than the stem of forages: seeds contain none. 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 sorghum, 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. The concentration of prussic acid is 18 higher in fresh forage than in silage or hay because HCN is volatile and dissipates as the forage dries. If hay is not properly cured, toxin levels of HCN may be present. It has been reported that proper haying and silage techniques can result in greater than 50 per cent of the prussic acid content dissipating. Also, extending the storage time of the forage should reduce the prussic acid to a safe level before being fed. All suspect feeds should be analyzed at a feed analysis laboratory before feeding 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 test 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.

Nitrate levels

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 test 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 bolting) 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.

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