July 3, 2002

Prussic Acid Poisoning a Concern When Feeding Emergency Forages

Alternative forages could be deadly for livestock, a specialist at North Dakota State University warns.

"Some producers have planted alternative forages as emergency winter feed because of drought or other problems," notes Greg Lardy, NDSU Extension livestock specialist. "Producers need to remember that prussic acid poisoning can be a concern when grazing or haying sorghum, sudangrass or related species."

Prussic acid, also known as hydrocyanic acid or hydrogen cyanide, can accumulate under a number of growing conditions, Lardy notes. Sorghums and related species contain the compound in their outer layer of cells. When those cells are damaged by drought, freezing, chewing , or rumen fermentation, prussic acid is released. Once plants are eaten, cyanide is absorbed into the bloodstream and binds to enzymes in cells, preventing hemoglobin from transferring oxygen.

"The animal dies from asphyxiation," Lardy says. "And death occurs very quickly, often in minutes." Signs include excessive salivating, difficulty breathing, staggering, convulsions and collapse.

Grain sorghums are potentially more toxic than forage sorghums or sudangrass. Indian-grass, chokecherry and flax can also cause prussic acid poisoning. Hybrid pearl millet and foxtail millet generally have very low levels of cyanide. Cattle are more susceptible to prussic acid poisoning than sheep. High concentrations can affect horses.

To avoid prussic acid poisoning, Lardy offers the following recommendations:

• Have forage tested for prussic acid before grazing or feeding.
• Feed hungry cattle before allowing them to graze forages that may contain high levels of prussic acid. That's because hungry animals often rapidly consume the most palatable new growth where prussic acid accumulations are heaviest.
• Turn cattle out late in the day when prussic acid levels are lowest. Cattle don't develop immunity, but they can detoxify low levels of prussic acid.
• Avoid grazing animals on troublesome plants after a light frost or after a drought-ending rain. Wait several days after a killing frost before grazing. New growth after frost or drought stress is often dangerously high in cyanide. A killing frost ruptures cells, allowing the cyanide to dissipate during the following days.

Chop or ensile plants that are high in prussic acid. Ensiling tends to reduce concentrations, but suspect forages should be analyzed for cyanide to make sure they're safe.

"Young, rapidly-growing plants generally have high levels of prussic acid. Higher concentrations of cyanide are found in young leaves than in old leaves or stems," Lardy says. "New forage growth following a drought or frost is dangerously high in cyanide. Any stressful condition that retards plant growth may increase prussic acid content." Plants grown in soils high in nitrogen, but low in phosphorus and potassium tend to have high concentrations.

"The best guard against prussic acid poisoning is to know where to expect it and how to avoid it," Lardy says. "Call on your veterinarian or extension agent for help if you need it." Producers who suspect their livestock have been affected by prussic acid should contact a veterinarian immediately. The condition can be treated if problem is diagnosed soon enough or the poison dosage isn't lethal.

###

Source: Greg Lardy (701) 231-7660, glardy@ndsuext.nodak.edu
Editor: Tom Jirik (701) 231-9629, tjirik@ndsuext.nodak.edu