Periodically there are concerns about possible toxicity to livestock from high levels of nitrates in forages. Nitrate poisoning is generally caused by animals eating too much forage that is high in nitrates, which have not been changed to protein in the plant. It can also be caused by animals eating too much urea or nitrogen fertilizer that has been spilled in the field or left where animals can find and eat it.
Nitrate fertilizer is quite palatable, especially to cattle.
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.