Prussic acid can pose risk to certain grazing livestock
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Prussic acid is creeping its ugly little head once again.
Many producers in the area took advantage of May rains and rolled up a precious hay stock.
However with worsening conditions producers are considering to graze those stressed hay meadows instead of waiting for a second crop, simply out of necessity.
Prussic acid in forages can pose a significant risk to certain grazing livestock.
Some plants like sorghums, millets and a few others produce cyanogenic glucosides (prussic acid) as they grow. Most times it is Johnsongrass in a Bermuda grass hayfield that poses the greatest risk.
Under normal conditions, certain plants are actively growing and healthy, they contain low levels of prussic acid because the compound breaks down over time, thus eliminating toxic accumulations.
Symptoms of prussic acid poisoning
One of the symptoms of prussic acid poisoning is death of your livestock. Death can take place in a matter of minutes once livestock are turned into a pasture whose plants may contain prussic acid.
Death is caused when the plant produces glucosides or sugar compounds that break down in the rumen, frees the cyanide from the sugar and forms hydrocyanic acid.
The HCN combines with hemoglobin to form cyanoglobin, which does not carry oxygen and basically suffocates the animal.
Horses, hogs and other nonruminant animals are less affected by prussic acid because their stomachs convert the prussic acid to less toxic formic acid and ammonium chloride.
Sampling for prussic acid
Prussic acid accumulates mainly in leaves, with highest concentrations in new growth. Because livestock usually eat leaves before stems, samples taken for prussic acid analysis should be largely comprised of leaves. This is especially true when sampling fields where cattle will be allowed to graze.
To sample standing forages, selectively remove the newest upper leaves from 10 to 12 plants in different areas of the field.
Place leaves in a paper bag, place the bag on dry ice in a Styrofoam cooler and ship overnight to the testing laboratory. The test is only $6, but the shipping process is considerably more.
The testing laboratory can evaluate the prussic acid concentration either qualitatively or quantitatively. But due to the extreme variability in prussic acid sampling, shipping methods and rapid field changes limit the value of knowing the level of prussic acid in that particular sample.
A simple positive or negative test result will generally suffice for a management decision.
Prussic acid management
1. In standing forages that contain prussic acid, livestock should not be allowed to consume it until the level declines to the point that it is no longer detectable, which is only known through additional tests.
2. Standing forages can be green chopped and ensiled. Prussic acid is enzymatically converted to free cyanide, which escapes when silage is removed. Another benefit of green chop/silage systems is that the effect of prussic acid is diluted when livestock aren't free to select only leaves but also stems which will contain much lower levels.
3. Standing forages also can be cut, field cured and baled. Bales should still be sampled even though field cured, due to the chance prussic acid might still be prevalent at some level. Bales can be made even safer by grinding them to combine leaves and stems.
Brian Yanta is the Goliad County Extension Agent.
