Dairy Pipeline: December 2002
Charles C. Stallings
Extension Dairy Scientist, Nutrition
Milk urea nitrogen testing has now been around for several years and most DHI labs will offer an analysis (United DHI will conduct this measurement). Pennsylvania DHI reports results on a web site (http://126.96.36.199/mun/mun.html) maintained by the University of Pennsylvania's Center for Animal Health and Productivity. Summaries of all cows that have been tested since September 1995 are included. They find that first lactation cows averaged 12.9 mg/dl plus or minus a standard deviation of 3.8. The standard deviation gives the range for 2/3's of the cows tested. In other words 1/3 fall outside this range (9.1 to 16.7 for first lactation cows). Second lactation cows averaged 13.2 plus or minus 4.0 (9.2 to 17.2) and third and later lactation cows averaged 13.1 plus or minus 4.1 (9.0 to 17.2). Urea is a small molecule that travels dissolved in water. In other words urea will be in blood, urine, and milk at approximately the same concentrations. Urea is a product of protein degradation and does reflect the protein status of the animal. Over or under feeding can result in high or low levels of MUN, respectively. Also high levels of rumen degradable protein can result in elevated MUN. Energy intake also can have an impact. If there is not enough energy present in the rumen to utilize all the nitrogen that is available some will pass into the blood and be transformed into urea in the liver. Jersey's have between 1 to 2 mg/dl more MUN than Holsteins. Typically expect herd average MUN's to range between 10 to 14. Individual cows will be outside of this range and factors such as feed and water intake, time of eating relative to sampling, and level of production will all have an influence. If herd average MUN's are elevated or depressed outside of this range, check total protein intake, rumen degradable protein intake, and ruminally available energy. MUN concentrations do give an indication of how efficiently protein or nitrogen is utilization and can be used to fine tune the feeding program and detect ration changes.