Urea is the major end product of protein nitrogen metabolism. It is synthesized in the liver from ammonia, which is produced by amino acid de-amination. Milk urea nitrogen is the fraction of milk protein that is derived from blood urea nitrogen (BUN).
When cows consume feed containing protein, if bacteria cannot capture the ammonia and convert it to microbial protein, the excess ammonia is absorbed part of the protein is degraded to ammonia by rumen microbes across the rumen wall. Because ammonia can shift blood pH, the liver converts ammonia to urea to be excreted or recycled. Urea diffuses freely across cell membranes; therefore MUN concentrations represent blood urea.
If MUN values are high, the herd is possibly wasting feed protein along with excreting excess nitrogen into the environment. If MUN values are too low, the rumen bacteria yield can be reduced thereby limiting milk production and milk protein yield.
The milk required for the test can be taken at any time during the day.