Prussic Acid Poisoning Could be a Problem in Late Summer and Early Fall
Livestock Update, September 2002
John B. Hall, Extension Animal Scientist, Beef, VA Tech
Members of the sorghum family such as grain sorghum, forage sorghum, sudex, Sudangrass and Johnsongrass may produce high level of prussic acid during times of stress. Wild cherry, choke cherry, peach, apple and elderberry trees can also form prussic acid. Prussic acid also known as hydrogen cyanide is extremely toxic to mammals in high amounts. This cyanide compound quickly inhibits the animal's ability to use oxygen and death occurs in a matter of minutes to hours.
The plants mentioned above produce sugars in the leaves and stems, which contain the cyanide ion. Drought stressed plants produce high levels of these sugars. When the plant is damaged or chewed these sugar combine with other enzymes in the plant to produce prussic acid. Normally, plant growth keeps the concentration of these sugars low enough that the animal can detoxify the prussic acid. However, concentrations of prussic acid reach toxic levels in drought stressed, frost damaged, or storm damaged plants.
To avoid prussic acid poisoning, producers should not graze cattle on drought stressed or frosted forages from the sorghum family. Additionally, cattle should be kept away from wilted cherry leaves from newly cut or storm damaged trees. The prussic acid danger in forages is reduced after several weeks of significant regrowth following a drought or one to two weeks following a killing frost. Dried cherry leaves do not pose a danger. Also, prussic acid content of the forage can be reduced by 50% through the hay curing or ensiling process. So making hay or silage from these crops may be the best option.
Tests for prussic acid are available through the state diagnostic labs or other laboratories. Forages should contain less than 50mg% on a dry basis to be safe to feed. Pasture samples need to be taken fresh and frozen. Remember summer annuals may also be high in nitrates. For more information on prussic acid poisoning in livestock or sampling procedures contact your county extension agent.