Milk urea nitrogen target concentrations for lactating diary cows fed according to National Research Council recommendations

The objectives of this study were to develop and evaluate a mathematical mo del to predict milk urea N and to use this model to establish target concen trations. A mechanistic model to predict milk urea N was developed using ra w data from 3 studies (10 diets, 40 cows, and 70 observations) and was eval uated with 18 independent studies (89 treatment means). For the independent literature data set, the model prediction error was approximately 35%; the majority of the error was due to variation among experiments. A mean of at least 25 cows was determined to be necessary for reliable model prediction s. This model, which uses such data as protein intake and milk production, was used to predict milk urea N concentrations when cattle are fed accordin g to National Research Council recommendations. Target values calculated in this manner for a typical lactation were 10 to 16 mg/dl, depending on days in milk. Target concentrations were sensitive to changes in milk productio n and amount of N intake and were relatively insensitive to body weight, pa rity, and grouping strategy. Analysis of data from the Lancaster Dairy Herd Improvement Association (n = 133,057) indicated that cows in the region we re being fed diets containing approximately 17% crude protein, regardless o f parity. A comparison to target milk urea N concentrations for this data i ndicated that cows were being fed 8 to 16% more protein than recommended by the National Research Council. Target milk urea N concentrations have been established, and dairy farmers now have a definitive way to interpret milk urea N concentrations.