Droughts, Frost Raise Prussic Acid Poisoning Concerns

By Ellen Crawford, North Dakota State University

Livestock may suffer from prussic acid poisoning after feeding on sudangrass, forage sorghums or sorghum-sundangrass crosses under certain environmental conditions.

Producers are concerned about the recent frost because livestock most commonly die from prussic acid poisoning when they’ve fed on plants that are very young, stunted by drought or frosted. Cattle and sheep are more susceptible than swine because they are more likely to consume large quantities of the poison, according to J.W. Schroeder, North Dakota State University Extension Service dairy specialist.

Producers often plant sudangrass, forage sorghums and sorghum-sudangrass crosses for summer pasture and sometimes feed them as green chop, silage or hay.

Most of the prussic acid in plants is a bound, nonpoisonous chemical called dhurrin. It is in most sorghums, but some species and varieties contain less than others. Sorghums also have a material called emulsion, which under certain conditions can react with dhurrin to form prussic acid (also referred to as hydrocyanic acid). If plants are damaged, such as by freezing, chewing or trampling, the emulsion-dhurrin reaction is enhanced, freeing sufficiently larger quantities of poison (cyanide) to cause a potentially hazardous condition.

“Prussic acid is extremely poisonous,” Schroeder warns. “A concentration greater than 0.1 percent of dry tissue is considered highly dangerous.”

The signs of prussic acid poisoning appear suddenly, usually 15 to 20 minutes after animals consume "tainted" forage. Symptoms include staggering, labored breathing, spasms and foaming at the mouth. Affected animals then often lie prostrate and thrash about. They must receive treatment quickly to prevent death.

The vegetative portion of all sorghums contains prussic acid. The prussic acid content in sudangrass generally is about 40 percent less than in most other sorghums, and the sorghum-sudangrass hybrids have more prussic acid than sudangrass. However, crosses have been developed that contain extremely low quantities.

Pearl millet, another summer pasture crop, does not contain toxic levels of prussic acid.

In the sorghums, leaf blades normally contain higher prussic acid levels than leaf sheaths or stems, the heads are low in prussic acid and the seeds contain none. Upper (newer) leaves have more prussic acid than older leaves. Tillers and branches (suckers) have the highest levels because they are mostly leaves and not stalk material.

Sundangrass, sorghum and the sorghum-sudangrass crosses reach their highest prussic acid levels before the boot stage. As plants mature, the stalks make up a greater proportion of the plant, causing prussic acid content in the total forage to decrease. However, the hazards associated with poisoning may decrease only slightly with age if animals selectively graze the plant parts that are high in prussic acid.

Freezing weather may kill only the tops of sorghum plants, leaving the lower portion alive. The unbound prussic acid in sorghum does not decline until wilting begins. The forage usually is considered safe for livestock to graze or eat as green chop five to six days after a killing frost, Schroeder says.

Deaths on pasture partially are caused by cattle selectively grazing leaves and shoots. These plant parts may contain two to 25 times more prussic acid than stems. Cattle also may avoid frost-damaged leaves and shoots, instead grazing the young suckers lower on the plant that could contain lethal levels of prussic acid. Therefore, if new shoots develop after a frost, do not allow livestock to graze the crop until this new growth is 2 feet tall.

Producers who intend to allow their livestock to graze on these forages can reduce the risk of prussic acid poisoning by feeding the animals ground cereal grains before turning them out to graze. Producers can reduce the chance of problems on pasture further by using heavy stocking rates (four to six head per acre) and rotational grazing.

In most cases, livestock can graze on grain sorghum stubble because cold weather likely kills the plants before they are grazed. However, producers should observe the stubble carefully for dangerous suckers that may develop after the main stalks have been killed. Sorghum that has wilted and dried five to six days after frost kills it is considered safe for grazing.

Green-chop forage usually is safer than the same material used for pasture because livestock do not graze the green chop selectively. In the pasture, livestock may eat only the leaves, but they’ll consume the whole plant with green chop. Stems act as safety devices, “diluting” the high prussic acid content in leaves. Nonetheless, green chop can have some risk if producers do not wait for it to wilt.

“Ensiling sorghum generally is a safer approach for feeding,” Schroeder says. “Although ensiled sorghum could contain toxic levels of prussic acid while in storage, much of the poison escapes as a gas during fermentation and when it is being moved for feeding. However, as a precaution, do not feed new silage for at least three weeks after harvesting and storing.”

The prussic acid content of hayed forages such as sorghum hay decreases as much as 75 percent while curing, which makes the hay rarely hazardous when fed to livestock.

“As a final precaution, have a sample of the feed tested by a lab,” Schroeder advises.