Metabolism and utilisation of non-protein nitrogen compounds in ruminants: a review

Unique stomach physiology allows ruminants to utilise any form of dietary nitrogen (N) as a protein source. Ruminant N requirements can be divided into N requirements for microbial fermentation and host requirements for amino acids. Ruminant diets often contain non-protein nitrogen (NPN) compounds, a group of components that are not proteins but can be converted to proteins by rumen microbes. Therefore, NPN compounds in ruminant diets serve as alternative sources of ammoniacal nitrogen (NH3-N), which is necessary for the synthesis of microbial protein (MCP) in the rumen. MCP is an important component of metabolizable protein for ruminants and, under certain production conditions, may be the only amino acid source for synthesis of tissues and products. In this manner, ruminants utilise NPN compounds without having enzymes for their degradation. The use of NPN compounds in ruminant nutrition allows for an increase in feed consumption and utilisation, as well as the substitution of protein supplements, which in turn leads to more cost-effective production. However, abrupt supplementation or excessive consumption of NPN could negatively affect MCP synthesis, feed digestion, and animal production. On the other hand, it may lead to animal poisoning and the release of large amounts of N into the environment. Knowledge of N metabolism in the rumen, as well as optimization of its utilisation, sets the conditions for the presumed beneficial effects of NPN compounds in ruminant diets, without adverse effects on animal health and the environment. Therefore, the aim of this study was to summarize the recent literature data on the metabolism and utilisation of NPN compounds in ruminants.