Prussic acid can kill livestock

Cattle Market & Farm Reports, Editorials
Sep 13, 2004
by WLJ
With all the recent cold weather and frost in the northern Corn Belt, a possibility exists that the forage sorghums will accumulate toxic amounts of prussic acid. Prussic acid is the same as hydrocyanic acid (HCN), or cyanide, which is poisonous.
All forage sorghums, sorghum-sudan hybrids, and sudangrass can produce prussic acid through a process called cyanogenesis.
Prussic acid is toxic in livestock and death can occur within 20 minutes if large enough amounts of the forage are consumed.
Large amounts of prussic acid can be produced quickly when sorghum plants are injured by stress such as wilting, freezing, cutting, or trampling. Paul Peterson, an extension agronomist at the University of Minnesota, said forage sorghums tend to be highest in prussic acid potential, followed by sorghum-sudan hybrids and sudangrass.
"Leaves contain twice as much prussic acid as stalks," he said. "New, young shoots, also are very high in prussic acid potential."
Drought, frost, or any climatic condition that halts normal growth usually causes increased prussic acid release.
Field curing releases 50 to 70 percent of the prussic acid. Conditioning the forage increases release because it causes enzymatic breakdown and prussic acid volatilizes during drying.
Peterson and Marcia Endres, an extension animal scientist at the University of Minnesota, said in a report all forage sorghums can be fed safely when harvested as dry hay, regardless of growth stage, and all sorghums can be ensiled and fed with safety when harvested after heading.
If harvested earlier, drying in the field is necessary for best ensiling and lowering of prussic acid potential.
Sorghums can be grazed safely about five days after frost if the frosted plants have dried out.
Frosted sorghum forage plants are dangerous until the material has dried. If there is a killing frost, Peterson and Endres said in their report, wait two weeks before grazing sorghums.
"If new shoots develop after a partial frost killing, don't graze any sorghum until complete frost killing occurs," the report said. "The new shoots are especially high in prussic acid potential and may be preferred by livestock." — Daniel Davidson, DTN Agronomist