In ruminant animals such as cattle, sheep and goats, nitrate is converted to nitrite by bacteria in the rumen. This nitrite is then changed to ammonia.
Excess ammonia is absorbed by the blood and passed in the urine as urea.
This occurs when the nitrate breakdown system is in balance and no surplus of nitrites accumulate. In contrast, monogastric animals such as horses and pigs, convert nitrate to nitrite in the intestine, closer to the end of the digestive tract, where there is less opportunity for the nitrites to be absorbed by the blood. It is this difference in the site of conversion that makes nitrate poisoning of much less concern for monogastric animals than it is with ruminants. When ruminants consume a high nitrate feed, some of the nitrate cannot be immediately converted to nitrite and finally to ammonia. This causes both nitrate and nitrite to accumulate in the rumen.