All plants contain some nitrate, but excessively high levels are likely to occur in forages having been grown under stress conditions, such as when corn fertilized for high grain yield is stunted by drought and is alteratively harvested for silage. Other plants, such as sudangrass, sorghum, pearlmillet, oats, orchardgrass and tall fescue, can also accumulate nitrates at high levels. Most weeds commonly found in corn also accumulate toxic levels of nitrate, including redroot pigweed, common lambsquarters, ragweed, velvetleaf, witchgrass, Canada thistle and black nightshade. High rates of nitrogen fertilization and drought conditions are the most important factors contributing to nitrate buildup in plants. Generally there is a direct response in plant nitrate concentration to increasing fertilizer nitrogen.
Nitrate accumulation is greater from nitrate fertilizers than from ammonium sulfate or urea.
Nitrate accumulation also is greater with delayed applications of fertilizer. The highest levels of nitrate accumulate when drought occurs during a period of heavy nitrate uptake by the plant. A drought during or immediately after pollination is often associated with the highest accumulation of nitrates. Ex- tended drought prior to pollination is not necessarily a prelude to high accumulations of nitrate. The resumption of normal plant growth from a heavy rainfall will reduce nitrate accumulation in corn plants, and harvest should be delayed for 3 to 4 days after the rainfall.