Management of Nitrate and Prussic Acid in Forage Crops

The two factors of drought conditions and export markets are focusing recent attention on nitrates and prussic acid in forages. During normal years, nitrate-nitrogen (NO3-N) taken up by crops is quickly converted into protein. When forages grow under drought stress, however, they tend to accumulate nitrate. Alternative forages such as sorghums and sudans tend to accumulate even higher levels, with the added problem of potentially toxic prussic acid levels. Nitratenitrogen in excess of 1400 ppm is considered to be potentially toxic to ruminants. Surveying samples submitted to Servi-Tech Laboratories this past summer shows a range of <100 ppm to >19,000 ppm NO3-N, with an average of 2200 ppm. Accurate determination of nitrate levels is critical to management of high-nitrate forages. Prussic acid poisoning can occur when animals are fed sorghum or its close relatives. Stress such as drought or frost leads these plants to produce higher levels of dhurrin, a compound containing hydrogen cyanide (HCN). Cyanide is toxic when levels exceed 300-500 ppm HCN on a dry basis. Factors contributing to high nitrate and prussic acid levels are discussed. Recommendations are presented to provide management guidelines to utilizers of these forages.