Forages such as johnsongrass, sorghum, sudangrass and sorghum-sudangrass hybrids can contain toxic concentrations of hydrocyanic acid (HCN) after a frost event or during periods of drought. Cyanide or HCN is commonly referred to as prussic acid, hence the name prussic acid poisoning. These forages contain cyanide in a bound form at all times. However, when a stress event such as frost or drought occurs, it causes damage to plant cell structures. This releases enzymes that allow cyanide to be released in a free form. The following provides a list of factors that influence cyanide accumulation in forages:
- Plant maturity – Young plants contain more HCN per unit weight than older, more mature plants with a significant amount of growth.
- Plant parts – There is more HCN in plant leaves than stems.
- Fertilizer – The use of high nitrogen rates (greater than 70 pounds of nitrogen per acre) will increase plant potential for prussic acid accumulation.
- Drought – Drought-stressed plants may have high levels of free HCN because a lack of water does not allow plants to grow out of the high prussic acid stage. Drought conditions may limit plant growth, keeping them small, and HCN can be present in a greater concentration as a proportion of the plant dry matter.
- Frost – Depending on the severity of the frost event, cold weather may only kill a portion of HCN-accumulating forages. Unbound HCN does not decline in these plants until wilting begins. Young, tender regrowth from unkilled plants may be high in prussic acid.
Prussic acid levels increase due to stress on the plant and deteriorate over time, unlike nitrate accumulation in forages. Avoid grazing areas where known cyanide-containing forages are present during periods of potential stress and immediately following a frost or periods of drought. Generally, within a five-day period after frost, high levels of prussic acid have degraded within the plant.
Do not turn hungry cattle into pastures containing cyanide-accumulating forages following an environmental stress event such as drought or frost. Provide hay and supplement to cattle to provide “gut fill” before giving animals access to these pastures. Use rotational grazing management to prevent selective grazing of lush, young cyanide-accumulating plants. If forages are baled as hay or ensiled, prussic acid presence will decrease and is generally safe for feeding. As a precaution, do not feed newly ensiled forage for at least three weeks after ensiling.
