Cattle producers need to think twice before turning herds out onto drought-stricken pastures, according to University of Arkansas (UA) System Division of Agriculture forage experts. 

When drought occurs, grasses can poison cattle two ways: with accumulated nitrates or prussic acid, also known as cyanide.

Here are several forages to monitor:

Sudangrass or sorghum-sudan hybrids accumulate nitrates. When the grass is eaten, the cow’s digestive process converts the nitrate to nitrite. Nitrite can bind to red blood cells, preventing them from carrying oxygen to tissues. The animal eventually suffocates.

In addition to packing nitrate, johnsongrass can kill another way.

 “I’ve taken samples of johnsongrass hay that will top the relative feed value chart, while also cranking out an excellent yield,” says Robert Seay, UA Benton County Extension staff chair. “However, the same heat and drought conditions that help highlight favorable johnsongrass traits also elevate the level of prussic acid, or cyanide in the plant.

“Fortunately, this toxin dissipates when johnsongrass is cut and cured as during normal haying,” he says.

Johnsongrass and sorghum have the highest prussic-acid potential when the plants are less than 18” tall, or when wilted from drought or frost, adds John Jennings, UA forage specialist.  “Current conditions have caused short and wilted plants which increases toxicity risk."

Another grazing hazard in drought: perilla mint, also called rattlesnake weed or purple mint. “Although cattle dislike mint, when preferred forages aren’t available, grazing animals will eat it,” Seay says.

Wild-cherry leaves are another potentially deadly hazard, says Johnny Gunsaulis, UA Washington County Extension agent. Cherry leaves and stems, once wilted, contain cyanide.

“When producers run cattle in areas they hoped would be hayfields, the cattle are probably going to start ranging into the wooded areas more,” he says.

For more on forages, visit www.uaex.edu or contact your county Extension office. For additional stories on drought-stricken forages, read "Strategies To Manage Southeastern Pastures, Cattle In Drought," "Death By Prussic Acid In Drought-Stressed Forage" and "Prussic-Acid Cattle Deaths Isolated, Experts Say."