Milk urea nitrogen (MUN) should be used for lactating cows rather than blood urea nitrogen (BUN) as it is more reliable and represents nitrogen efficiency over a longer range in time (1 to 3 days). Average MUN is between 10 and 16 mg/dl; however, when the ration balance of protein and rumen fermentable carbohydrate is optimum, MUN will typically range from 8 to 11 mg/dl. If you are feeding legume forage or pasture, you'd expect MUN to be higher than the minimum for NRC formulations.
If MUN is high, it could be caused by too much total protein, high levels of soluble and/or degradable protein, low levels of rumen fermentable carbohydrate (poor ammonia capture by the bacteria), and/or a poor rumen environment (such as low rumen pH) reducing microbial capture of ammonia.
In the total diet, we recommend 35 to 38% of total protein as RUP, 63 to 65% as RDP, and 39% of total protein as soluble. If soluble protein in the ration is below 30% of ration protein, adding urea is recommended.
Mike Hutjens, Extension Dairy Specialist, University of Illinois - Illini DairyNET
Rick Kohn, Professor, University of Maryland
