Milk urea nitrogen tests on bulk tank milk is being reported from some of the milk marketing coops serving Virginia. A project from the University of Maryland has provided assistance to coops to standardize analysis and reporting of MUN. 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 reflects 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 allthe 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 bulk tank average MUN’s to range between 10 to 14 mg/dl but for most efficient utilization of nitrogen MUN should be below 12. 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 or bulk tank MUN’s are above 12 or below 10 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 utilized and can be used to fine tune thefeeding program and detect ration changes. With high soybean meal prices, overfeeding of protein (most herds need less than 17% protein on a dry basis) should be avoided and more environmentally friendly rations will be the result. Remember, if a cow consumes 50 lbs. of dry matter that is 17% protein that is 8.5 lbs. of protein, which would support more than 80 lbs. of milk. Cows producing greater than 80 lbs. will eat more than 50 lbs. of dry matter and consequently more protein. Grouping cows by production can reduce the need for protein for at least some of the cows in the herd and reduce overfeeding. Also knowing the dry matter intake of each group can reduce the safety factor needed when feeding groups of cows. Testing feeds for nutrient content is also an important “best management practice” in feed management.