Nitrates are present in all plants, but normally their concentrations are not excessive. Under normal growing conditions, nitrate from the soil is absorbed by the roots of forage plants, and is supplied to the upper portions of the plant (primarily leaves) where it is converted into plant protein. However, adverse environmental conditions (such as drought), sudden weather changes (cool, cloudy weather), leaf damage (due to hail, frost, or herbicides), or heavy fertilization with nitrogen, can cause plants to develop and retain potentially dangerous levels of nitrate. The lower stalks and stems at the base of the plant are the site of accumulation. Grains, seeds and leaves do not accumulate significant amounts. Nitrate levels will remain high until there is new leaf growth. Plants with high stem-to-leaf ratios are more likely to cause nitrate intoxication. Levels of nitrate will remain high until there is new leaf growth, increasing photosynthesis that provides the necessary energy to utilize the excess nitrate. Hay will remain a hazard because toxicity is unchanged by drying, but the nitrate concentrations in ensiled forage crops may be reduced by up to 60 percent with proper fermentation and microbial degradation.
Arnold, Michelle; Gaskill, Cynthia; Lehmkuhler, Jeff; and Smith, S. Ray, "Nitrate Poisoning" (2014). Agriculture and Natural Resources Publications. Paper 165.