Nitrate concentrations in feeds for livestock depends more on plant species and environmental conditions prior to harvest than on the amount of available nitrogen in the soil. Nitrate is not always toxic to animals. Most forages contain some nitrates. When feeds containing nitrates are consumed by ruminants, nitrates are changed in the rumen to ammonia that is, in turn, converted by bacteria in the rumen into microbial protein. Nitrite is one of the intermediate products in the breakdown of nitrate and is the cause of nitrate poisoning.
Some nitrite is absorbed into the bloodstream.
Nitrite in the bloodstream changes hemoglobin to methemoglobin. Hemoglobin carries oxygen from the lungs to other tissues but methemoglobin is incapable of carrying oxygen. Thus, nitrates become a problem when enough methemoglobin is produced that the oxygen carrying capacity of blood is reduced to a critical level. The toxic level depends both on the amount of nitrate in the feed and how fast the feed that contains nitrate is consumed. For example, it takes about twice as much nitrate to kill a ruminant when the nitrate comes from forages that are eaten over a long period of time compared to a supplement or a drench that is consumed quickly.