Sudangrass, forage sorghums and sorghum-sudangrass crosses as well as johnson grass (all in the genus Sorghum) are often planted for summer pasture and sometimes fed as green chop, silage, or hay. Under certain environmental conditions, especially drought stress, livestock may develop symptoms of prussic acid poisoning when these forages are pastured or fed as green chop.
Death can result from prussic acid poisoning, most commonly when livestock have fed on plants that are very young, stunted by drought or frosted. Cattle and sheep are more susceptible than swine, since they are more likely to consume large quantities of the poison. The danger of prussic acid poisoning is greatest when livestock graze. The greatest number of livestock losses in the summer time occur when grazing drought stressed plants.
What Prussic Acid Is
Most pf the prussic acid in plants exists as a bound, non-poisonous chemical called dhurrin. It is present in most sorghums, but some species and varieties contain less than others.
Also present in the sorghums is 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. A concentration greater than 0.1 percent of dry tissue is considered highly dangerous. The active compound is hydrocyanic acid (HCN). Since prussic acid poisoning is very fast-acting livestock death can occur very quickly. Watch animals closely for any signs of toxicity. Livestock can show symptoms of intoxication within 5 minutes of eating plants with the poison, and may die within 15 minutes. Symptoms of HCN poisoning are gasping, staggering, trembling muscles, convulsions, and death resulting from respiratory failure. The mucous membranes of the mouth and eyes may have a blue coloration as evidence of cyanosis. In cases of recovery, there appears to be no permanent effects.
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 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. Also, young regrowth forage, especially sorghum and sorghum-sudangrass crosses, can be very toxic. As plants mature and plant height increases, the risk of prussic acid poisoning is reduced. However, during times of plant stress such as during drought or frost, toxicity can remain high in maturing plants. Hay maybe be dangerous when cut but becomes safe in time through volatilization of the HCN. Hay stored for two or more months gradually losses its cyanide potential.
Prussic acid can also be found in bahia, corn, cocklebur, white clover and other plants. One problem associated with prussic acid is that it tends to “come and go” in the plant. It may be present for a short time then dissipate.
The following are suggested guidelines when grazing sorghum and sudangrass varieties, crosses and hybrids:
- Do not graze plants in the sorghum family until they are 2-3 feet tall. Sorghum-sudangrass hybrids should be safe to graze of 36 inches or more unless they are drought stressed.
- Sorghum may not be safe to graze until fully headed. Regrowth sorghum should not be grazed until after the plant is completely killed by frost and died.
- Do not graze sorghum, sorghum-sudangrass hybrids or sudangrass during or after a drought, or if the plants show visible signs of moisture stress. Have the plants tested for toxicity levels before grazing.
- Do not graze short regrowth forage following hay or silage harvest or following a period of close grazing.
- Do not graze sorghum or sorghum-sudangrass hybrids following a series of light frost, as the potential for poisoning increases for a short period of time after frosts. Allow 7 to 10 days to pass before grazing after a light frost.
- Do not graze sorghum or sorghum-sudangrass hybrids following a killing frost until the plant has dried, approximately 14 days.
- Do not graze hungry livestock on sorghum or sorghum-sudangrass hybrids. Poisoning potential increases with the amount of high-risk forage consumed. Try to fill up the livestock with hay/feed immediately prior to turning them in to fresh pastures/fields.
- Remove all livestock from the feed source when an animal is found to have died suddenly.
