When plants take up more nitrogen than they need to build protein, excess nitrogen may build up as nitrate. High nitrogen fertilization generally requires high P and K as well. A high plant population, good growing conditions, as well as effective control of weeds, insects and disease also are conditions which must be met to keep a corn plant from storing nitrates in its leaves or stem. Usually nitrates are not stored in the grain. Shelled corn should be safe to feed, but soft corn or ground ear corn may not be as safe, since nitrates might accumulate in the cob.
Drought appears to be a major factor causing plants and certain weeds to store nitrate.
It can occur even in years of above normal rainfall, if too much nitrogen is applied in any form. Even normally fertilized plants can be dangerous if hit by drought or some other stress which upsets normal growth. Weather factors such as cold temperatures for corn and warm for oats, hail, cloudy weather, etc., can result in nitrate accumulation. The most dangerous situation exists for 3 to 5 days after a rain which follows a prolonged drought. When moisture becomes plentiful, the plants rapidly take up nitrate nitrogen that has been accumulating in the soil during the dry spell.