Be Aware of Prussic Acid Danger to Livestock

 BROOKINGS, S.D. -- To avoid prussic acid poisoning of livestock, producers should let sudangrass, sorghum and sorghum-sudangrass reach certain stages of growth before using for forage.

Prussic acid poisoning occurs when the naturally occurring cyanide compounds in plants such as sudangrass, sorghum and sorghum-sudangrass are released as toxic hydrocyanic acid, often called prussic acid.

South Dakota State University Extension Crops Specialist Bob Hall said it's important not to graze or green chop sudangrass until it is at least 18 to 20 inches tall, while sorghum-sudangrass hybrids should not be grazed or green chopped until they are at least 24 to 30 inches tall. Sorghum is generally unsafe for pasture or green chopping until after plants reach maturity.

 More details about how to avoid problems with prussic acid are available in an SDSU Extension publication. SDSU Extension Extra 4016, "Prussic Acid Poisoning of Livestock: Causes and Prevention," is available through county Extension offices. Or find it online at the SDSU Extension drought Web site, http://sdces.sdstate.edu/drought/.

 Drought can keep plants in the high prussic acid stage if sudangrass doesn't manage to grow beyond 18 inches, or if sorghum-sudangrass hybrids don't grow taller than 24 inches.

Complicating the situation is that drought can cause nitrate levels to accumulate in sudangrass, sorghum and sorghum-sudangrass. Rains might help lower the nitrate levels if they arrive in time to cause new growth in the plant, Hall said. But he added that any new growth that follows clipping, drought, frost or grazing can have potentially toxic levels of prussic acid.

"It's those young succulent shoots on that fresh regrowth that comes up that are highly concentrated with the prussic acid," Hall said.

 Potentially hazardous forages still can be used:

* Pasture: Heavy stocking rates of four to six animal units per acre and rotational grazing can reduce the hazard of  prussic acid poisoning when fields are grazed. But be aware that animals by preference will graze leaves and any new shoots the plant produces after drought or frost. These plant parts contain two to 25 times more prussic acid than stems. If there is new growth after a rain or frost, producers may want to consider harvesting the forage rather than grazing it.

* Green chopping: Because green chopping includes the stems, which are much lower in prussic acid than the leaves, it's usually safer than grazing.

* Wilted silage: Silage is generally safe for feeding. It may contain toxic levels of prussic acid while in storage, but much of the poison escapes as a gas when being moved for feeding. Do not feed new silage for at least three weeks after ensiling.

* Hay: Prussic acid content of sudangrass or sorghum-sudangrass hay decreases by as much as 75 percent while curing and is rarely hazardous when fed to livestock.
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Contact: Bob Hall, (605) 688-4760
** For links to other sites showcasing SDSU's work in teaching, research, and Extension, visit http://sdces.sdstate.edu.

Lance Nixon, Editor
AgBio Communications Unit
South Dakota State University
ACC, Box 2231, Rm 200
Brookings, SD 57007
Telephone: (605) 688-4653
Lance_Nixon@sdstate.edu