People pretty well agree that an “acceptable” milk urea nitrogen level is in the 11 to 15 range. But there can be variation between herds and within herds.
A MUN value of 11 mg/dL may be normal for one herd, while a value of 13 mg/dL may be normal for another. Now, a study by researchers at Virginia Tech and Purdue University has quantified how much variation may be due to overfeeding protein, all else being equal. Researchers studied the MUN levels in five commercial herds, plus the Virginia Tech herd. They found that when other variables are held constant, a one-percentage unit change in dietary protein concentration results in a 1.1 mg/dL change in MUN. However, this may be too steep of a curve for some herds. If a herd is at 17.3 MUN and wants to get down to 12, it would have to reduce dietary crude protein by 4.8 percentage points (4.8 x 1.1 = 5.28, the difference between 17.3 and 12).
Depending on where crude protein is to begin with, a 4.8-percentage-point reduction in crude protein could hurt milk production. Therefore, “one should assess all aspects of the operation before resorting to reduced protein feeding to achieve an MUN goal,” the authors write in the December edition of Journal of Dairy Science. Genetics is one factor that influences a herd’s MUN level.
“In the absence of selection measures, a producer may have inadvertently selected for a herd of cows that are predisposed to elevated MUN concentrations,” the authors say. They go on to discuss variation within herds and setting MUN targets that take this variation into account.