By Kim Ricardo
Drought creates several challenges for ranchers, from reduced forage availability to declining pasture quality. It can also create concerns about forage toxicity issues resulting from excess nitrate and prussic acid accumulation. Sorghum related forages particularly, can accumulate compounds that produce prussic acids, otherwise known as hydrocyanic acid, under environmental stress. When livestock consume forages containing elevated levels of prussic acid, cyanide poisoning can occur quickly and, in some cases, become fatal.
Prussic acid risk deserves particular attention when drought-stressed plants receive moisture and new growth occurs. As moisture comes and cool-season forages experience some late-season green-up, producers may be tempted to turn livestock into pastures as forage conditions improve. However, this regrowth following periods of environmental stress does not always come risk free. Knowing which forage species pose the greatest risk for prussic acid poisoning and taking appropriate management precautions before grazing or feeding them can help reduce the risk of potential livestock losses.
Know Which Forages Pose a Risk
Prussic acid concerns are generally associated with certain species in the sorghum family. Forage sorghum, sorghum-sudangrass, sudangrass hybrids, and johnsongrass, for example, all contain cyanogenic compounds that can release prussic acid when the plant has been damaged. Typically, prussic acid is most heavily concentrated in the leaves and is greater in immature, leafy plant regrowth.
However, it is important to recognize that not every plant, field, or pasture carries the same level of risk. Therefore, before considering grazing or feeding forages that might bring prussic acid concerns, consider recent weather and management practices. If there is question to whether a pasture may be at risk of high prussic acid accumulation, utilizing a forage test is the safest, most insurable way to avoid risk and prevent losses from occurring.
Pay Attention to Stress and Regrowth
Because prussic acid accumulation typically follows periods of environmental stress of planta damage, fields that have been hayed or drought-stressed pastures where forage growth has been stunted are of greatest risk for prussic acid concern. Plants grown in soil with high levels of nitrogen and low levels of phosphorus and potassium are also at elevated risk for prussic acid accumulation. Frost can also increase risk of prussic acid poisoning.
Source : sdstate.edu