June 29, 2006
Potential High for Nitrate and Prussic Acid Poisoning
There is concern for the potential of high nitrates and prussic acid levels in the drought areas of North and South Dakota, according to Duane Berglund, North Dakota State University Extension agronomist. Drought increases the potential of nitrate accumulation in forage plants or certain crops harvested for hay. Also, those plants that tend to contain prussic acid will have higher, more dangerous levels.
Crop plants known to accumulate nitrates include oats as hay, straw or stubble, corn as silage or stalks that are grazed, immature barley, wheat as pasture or hay, pearl or proso millets, flax and the sudan-sorghum complex of forages.
The forage crops know to cause prussic acid poisoning includes sudangrass, forage sorghum-sudans, and sorghum varieties or hybrids and crosses. Piper sudangrass (an old variety) possesses the least of the poisoning potential when compared with the sudangrass hybrids and sorghum-sudangrass crosses. The forage sorghum hybrids have the greatest potential for prussic acid content and poisoning potential.
“In corn, nitrates accumulate in the lower portion of the plant when stresses reduce the crop yield to less than the supplied nitrogen fertility level,” Berglund says. “Nitrates are responsible for lethal silo gas and interfere with the blood’s ability to carry oxygen when fed to animals. When chopping stressed corn plants, a 12-inch stubble should be left. If it rains, allow three days before resuming chopping. Plants that recover from stress situations eventually will convert nitrates to a nontoxic form.”
Prussic acid accumulates in sorghum and sundangrass that grows rapidly following stress. Poisoning occurs when animals graze young sorghum plants, drought-stunted plants or frost-damaged plants. Sorghum plants are poisonous after a frost that kills the tops, but not the crown or when new growth begins following a rain. When new shoots develop after a light frost, cattle should not be allowed to graze.
“Weeds consumed as forages under drought conditions also can be another source for nitrate poisoning,” Berglund says. “Among the species that can accumulate dangerous levels of nitrates are: Canada thistle, curly dock, jimsonweed, kochia, lambsquarter, various nightshades, redroot pigweed, smartweed, Russian thistle and wild sunflowers. Producers need to closely monitor livestock that may be feeding on these weeds during drought conditions.”
Other drought-related resources can be found at www.ag.ndsu.edu/drought/.