Weed species such as kochia, lambsquarters, sunflower and pigweed are routinely high in nitrate. Under certain environmental conditions, wheat, corn, soybeans, Johnsongrass and other plants accumulate toxic levels of nitrate. Nitrate content generally is highest in young plant growth and decreases with maturity. Sorghums and sudangrass are exceptions because concentrations remain high in mature plants. Nitrate will accumulate due to excessive nitrogen fertilization or if the plants are stressed due to drought, frost, hail or disease.
Nitrates accumulate at night, on cloudy days and when temperatures are cool.
Rain following a drought will also cause a rapid buildup of nitrate levels. After a drought-ending rain, one to two weeks are required for nitrate concentrations to be reduced to safe levels. Nitrate levels are highest in the roots and stems, levels are lower in the leaves, and almost no nitrate accumulates in the flower and seed. Ensiling will reduce nitrate levels by 40 to 60 percent, however, forages with extremely high nitrate concentrations at harvest may still be dangerous and should be analyzed before feeding. Nitrate concentrations do not decrease over time in forage harvested as hay.