Drought conditions increase the risk of both nitrate and prussic acid (hydrogen cyanide) toxicity in annual forages. All annual plants can accumulate nitrates under stress, while species such as sorghum, sorghum-sudangrass, sudangrass, shattercane, and johnsongrass can also produce compounds that lead to prussic acid poisoning.
Producers concerned about prussic acid risk may consider planting pearl millet as it does not produce prussic acid. However, it can still accumulate nitrates. Both nitrates and prussic acid can be fatal to cattle. Risk is often greatest during drought or immediately following drought-breaking rains.
Young plants typically contain higher concentrations of prussic acid than more mature plants, but both growth stages can pose a risk under drought stress. Nitrates tend to accumulate in the lower portion of the plant, particularly the stem. In contrast, prussic acid is most concentrated in the leaves. This increases risk during grazing, as cattle preferentially select young, leafy material where prussic acid concentrations are highest.
Nitrogen fertilization can further increase the risk of both nitrate and prussic acid accumulation, particularly when combined with drought stress. Harvest method also impacts risk. Drying forage for hay does not reliably reduce nitrate concentrations, and while it has often been assumed that prussic acid dissipates during curing, recent research indicates that prussic acid can persist at levels that pose a risk to cattle. As a result, drought-stressed forages may remain toxic even after baling and should be managed with the same caution as standing forage.
Ensiling, however, can reduce both nitrate and prussic acid levels by approximately 50%, making it a useful risk management strategy. Even so, silage made from plants with risk of being high in prussic acid or nitrates should always be tested before feeding. Poor ensiling conditions such as incorrect moisture or inadequate packing can limit this reduction.
Before grazing or harvesting drought-stressed forages, testing is strongly recommended. In addition to submitting samples to a laboratory, producers can use simple in-field quick tests to screen for prussic acid risk. This test does not provide exact concentrations but can indicate whether cyanide is present and if risk may be elevated. Learn more here. If testing indicates elevated risk and ensiling is not an option, delaying grazing or hay harvest is often the best approach. Concentrations of both nitrates and prussic acid typically decline within 5 to 7 days after adequate rainfall. Re-testing after this recovery period can help guide safe management decisions.
