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 nonpoint N pollution, ways to reduce the excretion of N by production animals must be examined. The objective of this research was to develop and evaluate a mathematical model that integrates milk urea N to predict excretion, intake, and utilization efficiency of N in lactating dairy 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 variables for the model were milk urea N (milligrams per deciliter), milk production (kilograms per day), milk protein (percentage), and dietary crude protein (percentage). For the developmental data set, the model accurately predicted N excretion and efficiency with no significant mean or linear bias for most predictions. Residual analysis revealed that a majority of the unexplained model error was associated with variation among cows. For the independent data set, model prediction error was approximately 15% of mean predictions. A mean of at least 10 cows was determined to be appropriate for model predictions. Target milk urea N concentrations were determined from expected urinary N excretion for cows that were fed according to National Research Council recommendations. Target values calculated in this manner were 10 to 16 mg/dl, depending on milk production. Milk urea N is a simple and noninvasive measurement that can be used to monitor N excretion from lactating dairy cows.