The first light frosts are still a few weeks away in Nebraska. However, planning for these events should be considered by beef producers grazing sorghum-related plants. In addition to sorghum, plants such as sudangrass, shattercane, and milo fall under this same spectrum of review as colder temperatures draw near. Following a freeze, these forages can be highly toxic with prussic acid. Drought, pasture clipping, and overgrazing are other events that can cause increased levels of prussic acid.
Sorghum related plants are in a group called cyanogenetic plants. This is due to their production of cyanogenetic glucosides during growth. Freezing and new growth of these plants breaks cell membranes. This causes the freeing of cyanide from its chemical bond becoming prussic acid. Prior to new growth, the intact cyanide and glucosides are not poisonous. Wilting and frost injury of a plant will cause a rapid increase of prussic acid levels and heightens the chances of toxicity in a plant that would regularly be nontoxic.
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Prussic acid toxicity is induced rapidly after consumption. It occurs through physiological processes that combine the toxic agent with hemoglobin to form cyanoglobin. This prevents hemoglobin from carrying oxygen and deprives the peripheral tissues of oxygen. Clinical signs of this toxicity include increased breathing rate, increased pulse, gasping, trembling, blue coloration of the mouth, and convulsions. These indicators are rarely seen because most animals die in a matter of minutes. Morbidity is caused by respiratory paralysis.
Risk of prussic acid poisoning can be mitigated with proper management. Below are health and forage management tips that can allow farmers to utilize sorghum related plants as high-quality annual forages.
- Utilize species that would pose less of a cyanide risk. Hybrids such as sorghum-sudangrass are less likely to produce as high of volumes of prussic acid as sorghum.
- Hold off on grazing plants until they are at least 24-30 inches tall. This avoids the cyanide risk caused from early growth.
- Turn cattle out after feeding cereal grains. This reduces the risk of potentially high intakes of toxic forages, and carbohydrates hinder prussic acid formation.
- Avoid grazing young regrowth.
- Do not graze during or immediately after stressful events that damage plants. This includes drought, hail, or frost. It would be advised to delay grazing for 7-10 days following a plant damaging event to allow prussic acid levels to decline before grazing.
For more information on prussic acid toxicity reach me at my office (402) 624-8007 or more information on Nebraska Beef Extension check out bigredbeeftalk.unl.edu.