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Nitrates & Prussic Acid in Forages

by Sarah Kenyon, Ph.D Field Specialis in Argonomy, MU Extension

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Several forage crops and some weeds should be monitored for nitrates to avoid livestock losses.
Common plants include: Johnsongrass, sorghum, sudangrass, green graze, and corn. Nitrates accumulate in response to drought, frost, overgrazing, or N fertility.
The nitrates present in the forage convert to nitrite in the rumen. Nitrite in the blood converts hemoglobin to methemoglobin, which doesn’t bind oxygen. Methemoglobin formation is associated with chocolate brown blood. Nitrate poisoning often results in death due to lack of cellular oxygen.
All plants take up nitrates, but not all plants develop toxic levels. Poisoning happens when excess nitrogen builds up in the lower part of some forages, which can happen despite good fertility and management decisions.
To avoid nitrate poisoning, do not cut hay less than 8 inches or force animals to graze more than 8-10 inches to the ground. Wait five days after a “good” rain to graze.
If you suspect high nitrate levels, immediately move cattle to other pastures and call a veterinarian. Death from nitrate poisoning usually occurs within four hours. Often the first sign of poisoning is death of livestock.
Nitrates present in hay are stable and will not dissipate; only the green, growing plant can convert the nitrates to a safe form. Therefore, hay containing these forages should be tested for nitrates. In silage, nitrate concentration can dissipate 20%-50% over time.”
Most MU Extension centers offer diphenylamine-sulfuric acid spot tests. Call your local center before bringing in a sample. Cut stem at ground level to test. Producers with positive results benefit from a quantitative forage analysis.
Prussic acid, or cyanide poisoning, can also be a problem in Johnsongrass, sorghum, sudangrass, and wild cherry. Prussic acid is not present in millet. During stress cyanogenetic glucosides are released by the plant which inhibit the action of the enzyme that links oxygen with red blood cells resulting in asphyxiation. The blood will be a bright cherry red because oxygen is present but it is unable to bind.
Prussic acid can volatilize in forage; therefore, one solution is to cut the forage for hay. Levels of cyanide greater than 2 milligrams per kilogram (2 ppm) of dry plant tissue are considered potentially dangerous. Prussic acid levels are highest in young, leafy tissue, whether in initial growth after planting or regrowth after clipping.
For more information, see the MU Extension publication “Nitrate Problems in Livestock Feed and Water,” available for free download at extension2.missouri.edu/g9800.
For more information please call the MU Howell County extension office at 417-256-2391.

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