USING MILK UREA NITROGEN TO PREDICT NITROGEN-EXCRETION AND UTILIZATION EFFICIENCY IN LACTATING DAIRY-COWS

Because animal agriculture has been identified as a major source of no npoint N pollution, ways to reduce the excretion of N by production an imals must be examined. The objective of this research was to develop and evaluate a mathematical model that integrates milk urea N to predi ct excretion, intake, and utilization efficiency of N in lactating dai ry cows. Three separate digestibility and N balance studies (10 diets, 40 cows, and 70 observations) were used to develop the model, and 19 independent studies (93 diets) were used for evaluation. The driving v ariables for the model were milk urea N (milligrams per deciliter), mi lk production (kilograms per day), milk protein (percentage), and diet ary crude protein (percentage). For the developmental data set, the mo del accurately predicted N excretion and efficiency with no significan t mean or linear bias for most predictions. Residual analysis revealed that a majority of the unexplained model error was associated with va riation among cows. For the independent data set, model prediction err or was approximately 15% of mean predictions. A mean of at least 10 co ws was determined to be appropriate for model predictions. Target milk urea N concentrations were determined from expected urinary N excreti on for cows that were fed according to National Research Council recom mendations. Target values calculated in this manner were 10 to 16 mg/d l, depending on milk production. Milk urea N is a simple and noninvasi ve measurement that can be used to monitor N excretion from lactating dairy cows.