Milk Urea Nitrogen, an Easy Monitoring Tool

Is there too much protein in a dairy cow utilizing? Milk urea nitrogen (MUN) is a simple test that is routinely measured when milk is picked up at the farm. Milk urea nitrogen is a byproduct of protein breakdown in the cow. When there is more protein in the ration, and therefore in the rumen (the first stomcah compartment,) than the microbial population in the rumen can use that protein gets broken down to ammonia, and the ammonia is absorbed into the bloodstream. Ammonia floating around in the blood is toxic to the cow so she converts ammonia to urea, which is then excreted in urine and milk. Most of the urea ends up in the urine contributing to the majority of nitrogen found in manure, which can lead to air and water quality concerns.

The higher the concentration of MUN the less efficiently the cow is using the nitrogen in her feed, which means more is being excreted in urine. This represents not only a waste of ration protein (nitrogen), but also a waste of energy that could be going to milk production because it takes energy to convert excess protein to urea.

So, what should the concentration of MUN be in milk? Assuming the ration is balanced well in other areas a concentration as low as 8-10 mg/dl can be okay, but 10-14 mg/dl is still considered normal. Any values consistently above are a red flag that the protein concentration of the ration is too high. Generally, a ration crude protein concentration of 16% would be associated with normal MUN levels, and should be sufficient to meet the protein demands of a lactating cow.