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Published October 21, 2013, 11:41 AM

Forages at risk for prussic acid poisoning

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.

By: SDSU Extension Service, SDSU Extension Service

BROOKINGS, S.D. — 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, says Alvaro Garcia, South Dakota State University Extension dairy specialist.

“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,” Garcia says.

He explains that these new shoots are very palatable and will be grazed selectively. But, these fields should not be grazed until a hard frost kills the new shoots or prussic acid poisoning would likely occur.

Garcia says prussic acid is the same as hydrocyanic acid (HCN) and plants of the sorghum species contain a nontoxic 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,” Garcia says.

He explains that 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,” Garcia says.

Leaves contain twice as much prussic acid as stalks. New, young shoots also are very high in prussic acid potential, Garcia explains.

“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,” he says.

Because freezing disrupts plant cell walls, it leads 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 or sudangrass are going to be ensiled, Garcia says, it is important to wilt it to decrease hydrocyanic acid concentration, and to 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,” Garcia says.

For more information about managing prussic acid in livestock forages, contact Garcia at alvaro.garcia@sdstate.edu or 605-688-6488 or Tracey Renelt, SDSU Extension dairy field specialist at 605-882-5140 or tracey.renelt@sdstate.edu.

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