Drought stress and forage could be a lethal combination, according to Tom Troxel, University of Arkansas animal scientist. Two Arkansas cows recently died of prussic-acid poisoning caused by a cyanide compound found in several types of grasses when those grasses are stressed by drought or frost.

“Of all the plants grown in Arkansas, those belonging to the sorghum category are most likely to contain potentially toxic levels,” he says. “Grain sorghum contains the most, followed by johnsongrass, sorghum-sudan hybrids and then pure sudangrass.”

Johnsongrass may be the plant to watch most since it grows wild throughout Arkansas and infests many grazed areas, Troxel says. It’s believed to be the culprit in the two deaths.

“Usually, sorghum-type plants that are more than 18-24” tall are less likely to contain high concentrations of the toxin,” Troxel says. “Immature plants and regrowth following haying or grazing contain the highest levels. Close grazing for several years usually eliminates johnsongrass from pastures,” and, he adds, “millet is free of the toxin.”

Prussic-acid poisoning symptoms include anxiety, progressive weakness and labored breathing. When lethal amounts of the acid are consumed, death often occurs rapidly.

Ruminant animals – cattle, sheep and goats – appear to be the most susceptible to prussic-acid poisoning. Reports of poisoning in swine and horses are rare.

Troxel offers the below precautions for using sorghums or johnsongrass:

  • Don’t let animals graze fields with succulent, young, short growth. Graze only after plants reach a height of more than 18-24”.
  • Don’t graze drought-damaged plants in any form, regardless of height, within four days following a good rain. During this period of rapid growth, prussic acid is likely to accumulate in young tissue and nitrates in stems.
  • Don’t graze wilted plants or plants with young regrowth. Don’t rely on drought-damaged material as the only source of feed. Keep dry forage or greenchop from other crops available at all times. Utilize uneven growth resulting from drought as silage or hay.
  • Don’t turn hungry cattle onto pastures of sorghum, sorghum-sudan or johnsongrass. Fill them up on hay or other forage first; begin grazing them in late afternoon.
  • To use potentially toxic forage, harvest it as hay or silage. Prussic-acid levels decline in stored forages. Well-cured hay is safe to feed, and if forage likely to have high prussic acid is ensiled, it is usually safe to feed three weeks after silo fill.

For more about drought management, contact your county Extension office or visit www.uaex.edu.