The cyanogenic “potential” of plants is affected by the type (species and variety) of the plant, weather, soil fertility and stage of plant growth. ![]() Why is Johnsongrass and other Sorghum species only risky at certain times of the season but safe in others? The cyanogenic glycosides are used by the plant as protection from grazing animals, insects, and parasites when the plant is most vulnerable. Regrowth (for example, after a light frost) contains extremely high dhurrin concentrations. Peak concentrations occur in the first week after germination, declining markedly once the plant reaches approximately 2 ft in height. Dhurrin concentrations are highest in the leaves, particularly new growth. When plant cells are damaged, the plant enzymes can reach dhurrin and cleave it, releasing cyanide gas (abbreviated as HCN). Sorghum species including Johnsongrass, sorghum, sudan grass and hybrid sorghum-sudan contain the cyanogenic glycoside “dhurrin”. These cyanogenic glycosides and the enzymes necessary to convert them to free cyanide gas are separated in different locations within the plant cells. Where does the cyanide come from in a plant? Certain plants contain compounds called “cyanogenic glycosides” which are not toxic by themselves but only when the plant is damaged. No matter which name is used, cyanide is one of the most rapid and deadly toxins that affects cattle. Hydrogen cyanide was first isolated from a blue dye (Prussian blue) and because of its acidic nature, it became known by the common name “prussic acid”. Prussic acid, cyanide, or hydrocyanic acid are all terms relating to the same toxic substance. Usually within the month of October when the first frosts are expected in KY, the questions begin regarding the risk of prussic acid poisoning from Johnsongrass (Sorghum halepense) after frost and when is it “safe to graze again”. ![]() ![]() When sorghum species including Johnsongrass, sorghum, sudan grass and hybrid sorghum-sudan is frosted, the plant cells are damaged and the plant enzymes can reach dhurrin and release cyanide gas.
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