Urea and nitrate poisoning as a source of non-protein nitrogen in ruminant diets: a review

Abstract

The inclusion of non-protein nitrogen (NPN) as a partial replacement for true proteins is economically viable and does not compete with human food. However, excessive consumption of NPN by ruminants can be lethal due to poisoning. In this sense, the aim of this review was to describe the use of NPN in ruminant feeding and the consequences of ammonia and nitrate poisoning, including the poisoning mechanisms, recommended levels, prevention measures and treatments. Urea and nitrate are sources of NPN, and the toxic components are ammonia and nitrite, respectively. Excessive consumption of urea increases the hydrolysis of urea into ammonia, surpassing the use by the rumen microbiota. When the absorption of ammonia into the bloodstream exceeds the liver's capacity to assimilate it into urea, acute intoxication occurs, damaging the central nervous system. Nitrate poisoning occurs when consuming contaminated forage or water. The rumen microbiota reduces nitrate to nitrite and, subsequently, to ammonia. However, when nitrite reduction exceeds ammonia reduction, nitrite is absorbed into the bloodstream and oxidizes the iron atom in hemoglobin, transforming it into methemoglobin, which loses its ability to transport oxygen to tissues. It is concluded that ammonia and nitrate poisoning is influenced by the content of rapidly fermenting carbohydrates. Therefore, attention to the quality of nutrition and herd management is recommended to prevent outbreaks of poisoning.