Use of Milk or Blood Urea Nitrogen to Identify Feed Management Inefficiencies and Estimate Nitrogen Excretion by Dairy Cattle and Other Animals

The objective of this review is to describe recent research findings related to the use of milk or blood urea nitrogen to identify inefficiencies in protein nutrition and estimate nitrogen excretion. A mathematical model was developed to integrate milk urea nitrogen (MUN) and milk composition to predict urinary and fecal excretion, intake, and utilization efficiency for nitrogen in lactating dairy cows. This model was subsequently used to develop target MUN concentrations for lactating dairy cattle fed according to National Research Council recommendations. Further research identified a change in measurement of MUN by Dairy Herd Improvement Associations, and subsequently resulted in adjustments to the model. Target MUN concentrations for most dairy herds bulk tank samples are between 8 to 12 mg/dl. Urinary nitrogen (g/d) can be estimated as 0.026 times MUN (mg/dl) times body weight (kg) for dairy cattle. A similar approach can be used with blood or plasma urea nitrogen. Because blood or plasma urea is higher than MUN, the coefficient relating blood urea to urinary N is lower than for MUN. Urinary nitrogen (g/d) can be estimated as 0.013 times MUN (mg/dl) times body weight (kg) for cattle, sheep, goats and horses. However, pigs and rats were found to be more efficient at clearing urea from the blood, and therefore, higher coefficients are used to relate blood urea concentration to N excretion rate for these species. Several extension and field research projects using MUN have been implemented. Bulk tank samples are used to identify herds with either chronic or occasional herd nutrition problems. Herds with high MUN have been found to be at risk for over feeding protein and herds with low MUN have been found to be at risk for under feeding protein.