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Posted 25 September 2006. Forage and Grazinglands. Be Alert for Prussic Acid in Forage Sorghums South Dakota State University. Brookings, South Dakota (September 19, 2006) - With parts of South Dakota experiencing the first cold weather of fall, producers should be aware that forage sorghum, sorghum-sudan hybrids, and sudangrass all have the potential to produce prussic acid poisoning in livestock when stressed by factors such as frost.
If the frost is light and only kills the upper few leaves, the plant may attempt to regrow by putting out a new shoot from the base of the plant. These new shoots are very palatable and will be grazed selectively. These fields should not be grazed until a hard frost kills the new shoots or prussic acid poisoning would likely occur. Prussic acid is the same as hydrocyanic acid (HCN). South Dakota State University Extension Forage Specialist Peter Jeranyama said plants of the sorghum species contain a non-toxic compound called dhurrin that is converted to toxic prussic acid by a process called cyanogenesis. “The toxifying action of prussic acid is almost immediate and death can occur within 15 to 20 minutes. In general, cattle and sheep are more susceptible to prussic acid poisoning than horses and pigs,” Jeranyama said. Large amounts of prussic acid may be released via cyanogenesis in a short period of time when sorghum plant tissue is injured by wilting, freezing, cutting, or trampling. “In general, forage sorghums tend to be highest in prussic acid potential, followed by sorghum-sudan hybrids, then sudangrass, which is usually safe,” Jeranyama said. Leaves contain twice as much prussic acid as stalks. New, young shoots also are very high in prussic acid potential. As plants mature or age, the amount of dhurrin decreases. Field curing liberates 50 to 70 percent of the prussic acid. Conditioning helps increase liberation of prussic acid because it causes enzymatic breakdown of dhurrin, and prussic acid evaporates during drying. SDSU Extension Dairy Specialist Alvaro Garcia added that freezing disrupts plant cell walls leading to a quick release of hydrocyanic acid. Wilting the forage for five to six days before feeding helps reduce its concentration, and makes it a safer feed for cattle. Feeding green chop to cattle is usually safer than grazing as there is less leaf selection by the animals. If sorghum and/or sudangrass are going to be ensiled, Garcia said, it is important to wilt it (it decreases hydrocyanic acid concentration) and allow it to ferment undisturbed for three weeks or more before feeding it. To achieve a desirable fermentation, make sure there's adequate compaction and overall management of the ensiling process. Sudangrass preserved as hay is usually considered safe as the hydrocyanic acid drops by as much as 75 percent during the drying process. For more information about managing prussic acid in livestock forages, contact SDSU Extension Forage Specialist Peter Jeranyama at (605) 688-4754. Or refer to SDSU Extension Extra 4016, "Prussic Acid Poisoning of Livestock: Causes and Prevention." The publication is available through county Extension offices, or online. Contacts: Peter Jeranyama Lance Nixon, Editor |
