Cattlemen should be aware of the dangers certain grasses pose to cattle after a freeze, warns Mark Keaton, Baxter County staff chair with the University of Arkansas Cooperative Extension Service.

“Usually in October, the first killing frost visits Arkansas,” he said. “Crops such as johnsongrass, sudangrass, sorghum-sudangrass, grain sorghum and other sorghums are sensitive to temperatures below 32 degrees. Plant cells of these crops are damaged by frost, and hydrocyanic acid, or prussic acid, is formed.”

Keaton said there’s a chance cattle can be killed by eating only a few pounds of forage from these grasses if they’ve been killed by frost.

Hydrocyanic acid is more abundant in sorghum leaves than in stems. Since young shoots and suckers consist mainly of leaves, they’re more hazardous to eat than mature plants that contain large stems. That’s because stems dilute the harmful effects of this potentially lethal compound found in sorghum leaves, Keaton said.

Hydrocyanic acid released by frost is volatile and vaporizes quickly from frosted sorghum plants, according to Keaton. The same grass is considered safe for grazing after it has thawed from a killing frost for seven to 10 days.

During October, a light frost may occur that burns back only the uppermost leaves of sorghum plants. The lower leaves may remain green until a harder frost occurs several days later. Sometimes, suckers will develop at the base of these plants.

If cattle are removed from these fields immediately after frost and then returned five days later, they may selectively graze the young shoots only. When this occurs, there’s a danger the cattle will consume high levels of the hydrocyanic acid in leafy tissues.

Historically, few cattle fatalities are reported in October from frost-killed sorghum. However, producers should be aware of the hazard, Keaton said. By using some of the following precautions, one may reduce the likelihood of poisoning cattle that consume sorghum forage and johnsongrass.

Keaton offers these tips to safeguard your cattle:

• Remove the cattle from these fields for 14 days each time a non-killing frost occurs on living sorghum crops.

• Don’t allow animals to graze fields with succulent, young, short growth. Graze only after plants reach a height of 18 to 24 inches.

• Don’t harvest or feed drought-damaged plants in any form within four days after a good rain, regardless of height. During this period of rapid growth, hydrocyanic acid accumulates in the young tissue, and nitrates accumulate in stems.

• Don’t graze wilted plants (drought-stressed) or plants with young regrowth. Don’t rely on drought-damaged material as the only source of feed. Keep dry forage from other crops available at all times.

• Don’t use frost-damaged sorghum as pasture during the first seven days after a first killing frost. Delay pasturing for at least seven days or until the frosted material is completely dried out and are the color of brown paper. Don’t rely on frosted material as the only source of feed. The toxin usually dissipates in 48 hours. Don’t turn hungry cattle onto a pasture of sorghum or sorghum-sudan hybrid. Fill them up on hay first and begin grazing in the late afternoon.

• Prevent selective grazing of the young regrowth, which may be highly toxic, by rotational grazing of small pastures that may be grazed down to a 6-inch stubble within a 10-day period. This will mean cross-fencing to provide short-term rotational or strip grazing.

• Remember that hay cured properly will not present any risk from prussic acid poisoning.

• Don’t allow access to wild cherry leaves whether they are wilted or not.

e-mail: ljames@uaex.edu