MUN stands for milk urea nitrogen. The liver converts excess nitrogen from protein or ammonia to a small molecule called urea. The urea is filtered from the blood and excreted by the kidneys. Blood urea nitrogen or BUN (sometimes referred to as PUN for plasma or SUN for serum) is a measure of the amount of nitrogen in the blood (or circulating body fluids). Some of the urea in circulation will diffuse into the milk. This can be measured in the milk as MUN. As a result of this circulation, MUN is well correlated with average BUN over the course of a day, and both reflect the amount of nitrogen excreted in urine per day or excess nitrogen absorbed into the body.
Dairy diets containing excessive amounts of any form of protein will increase the amount of urea (nitrogen) in the bloodstream. This will cause an increase in milk urea as well. If diets are formulated efficiently, most of the nitrogen fed will be captured by the rumen microbes, and less will escape into the bloodstream. Measuring MUN can help determine the efficiency of rumen fermentation.
