The highest levels of nitrate are seen when drought occurs during a period of heavy nitrate uptake by the plant. Nitrates are absorbed from the soil and are normally incorporated into plant tissue as amino acids, proteins, and other nitrogenous compounds, thus nitrates in plants are low. The conversion of nitrates to other compounds occurs primarily in green growing leaves. Under unfavourable growing conditions, especially drought, the conversion process is retarded causing the nitrate to accumulate in stalks, stems and other conductive tissue. If moisture levels improve, the conversion process resumes, and nitrate levels will return to normal after 3 or 4 days.
Ruminant animals can use the nitrogen in nitrates to produce protein.
Microbes in the rumen convert nitrates to nitrites and then to ammonia for incorporation into amino acids. Nitrites are the cause of nitrate toxicity which occurs when the levels of nitrate in the diet overwhelm the animal's digestive system such that the rate of conversion of nitrate to nitrite exceeds the rate of conversion of nitrite to ammonia. Nitrites accumulate and are absorbed from the rumen into blood where it combines with hemoglobin to form methemoglobin which does not carry oxygen. The symptoms of nitrate poisoning are similar to prussic acid poisoning and appear suddenly. Brownish discoloration of the blood is characteristic of nitrate poisoning.