Forages at risk for prussic acid poisoning
With parts of the country 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, said 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 said.
He explained that these new shoots are very palatable and will be grazed selectively. However, these fields should not be grazed until a hard frost kills the new shoots or prussic acid poisoning would likely occur.
Garcia said prussic acid is the same as hydrocyanic acid (HCN) and plants of the sorghum species contain a non-toxic compound called dhurrin that is converted to toxic prussic acid by a process called cyano genesis.
“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 said.
He explained 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 said.
Leaves contain twice as much prussic acid as stalks. New, young shoots also are very high in prussic acid potential, Garcia explained.
“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 break-down of dhurrin, and prussic acid evaporates during drying,” he said.
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 and/or sudangrass are going to be ensiled, Garcia said, 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 said. — WLJ