Answer provided by Larry Chase, professor and extension specialist in dairy nutrition at Cornell University.
Q: Besides rumen efficiency, what can my producers learn from their milk urea nitrogen (MUN) levels?
A: Currently, a number of milk cooperatives provide MUN values to producers for each load of milk shipped. This means that many farms will have 20 or more MUN values per month. This is a bulk tank herd value that can be used as a tracking tool. It has limitations as a diagnostic tool to evaluate the MUN status of individual cows or production groups.
One option is to graph bulk tank MUN values and look for deviations from the herd baseline value. If there are changes in MUN, this could be a signal that something has changed and that additional investigation is warranted.
Another option is to look at the variation in MUN values within a month in the herd.
We recently looked at the standard deviation (a measure of variability) in daily bulk tank MUNs in 1,500 herds from two milk cooperatives. The average MUN values were 11.8 and 12.6 mg/dl for the two cooperatives. About 45 percent of the MUN values were between 11 and 13 mg/dl, while 25 percent of the MUN values were less than 11 and 30 percent were more than 13. The average standard deviation in these samples was about three MUN units. Even though there was a large range in variability observed, 78 percent of the monthly averages had a standard deviation of 0.5 to 1.4 MUN units. A larger standard deviation equals more variability.
If a herd has high variability in the MUN values within a month, this could be a sign of inconsistency in daily feeding management. Additional work is needed to better define the relationship between variability in bulk tank MUN values and herd management. The first step for producers and their advisers is to evaluate the variability in bulk tank MUN values in their herd. This can be done graphically or with actual calculations. If variability is observed, then an evaluation of daily feeding-management practices could be examined. Are forage dry matters checked and feed ingredient amounts adjusted? Has a change in silos taken place or has nutrient content of the forage changed within the silo? These are just examples of questions that could be evaluated.