U of M Extension Home : U of M Home

Minnesota Crop News > 2001-2008 Archives

Forage sorghum, sorghum-sudan hybrids, and sudangrass all have the potential to produce prussic acid poisoning in livestock. Prussic acid is the same as hydrocyanic acid (HCN). Plants of the sorghum species contain a non-toxic compound called dhurrin that is converted to toxic prussic acid by a process called cyanogenesis. Pearl millet does not cause prussic acid poisoning. The toxifying action of prussic acid is almost immediate and death can occur within 15-20 minutes. In general, cattle and sheep are more susceptible to prussic acid poisoning than horses and pigs.

Large amounts of prussic acid may be released via cyanogenesis in a short period of time when sorghum plant tissue is injured by wilting, freezing, cutting, or trampling. The amount of dhurrin in a plant varies with genetics, plant part, plant age, climate, moisture supply, and soil fertility. In general, forage sorghums tend to be highest in prussic acid potential, followed by sorghum-sudan hybrids, then sudangrass, which is usually safe. Leaves contain twice as much prussic acid as stalks. New, young shoots also are very high in prussic acid potential. As plants mature or age, the amount of dhurrin decreases. Drought, frost, or any climatic condition that halts normal growth usually causes increased prussic acid release.

Field curing liberates 50-70% of the prussic acid. Conditioning helps increase liberation of prussic acid because it causes enzymatic breakdown of dhurrin, and prussic acid evaporates during drying. Never bale material with prussic acid until it contains less than 18 percent moisture. Ensiling may provide some liberation of prussic acid. To be safe, don’t expect ensiling to liberate prussic acid unless plants are conditioned and wilted before ensiling. However, ensiling should be done before the plant material reaches 50% moisture.

Management tips:

• Sudangrass and sorghum-sudan hybrids growing under normal conditions are safe to graze or green chop after they have reached ~18-24”.
• Forage sorghums may not be safe for grazing or green chopping until headed.
• All sorghums (forage sorghum, sorghum-sudan, and sudangrass) can be fed safely when harvested as dry hay, regardless of growth stage.
• All sorghums can be ensiled and fed with safety when harvested after heading. If harvested at earlier stages, partial field drying is necessary for best ensiling, and some lowering of prussic acid potential can be expected.
• All sorghums having adequate growth for safe grazing before frost can be grazed safely ~5 days after frost if the frosted plants have dried out. This means sudangrass or sorghum-sudan hybrids over 18-24” tall, or headed forage sorghums. Frosted sorghum is very dangerous until it has dried out, so wait until the plant material is dry.
• Wait 2 weeks after a killing frost before grazing sorghums that are too short for safe grazing before frost.
• If new shoots develop after a partial frost killing, don’t graze any sorghums until complete frost killing occurs. The new shoots are especially high in prussic acid potential and may be preferred by livestock.