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.

The objectives of this study were to develop and evaluate a mathematical model to predict milk urea N and to use this model to establish target concentrations. A mechanistic model to predict milk urea N was developed using raw data from 3 studies (10 diets, 40 cows, and 70 observations) and was evaluated 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 predictions. This model, which uses such data as protein intake and milk production, was used to predict milk urea N concentrations when cattle are fed according 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 production and amount of N intake and were relatively insensitive to body weight, parity, and grouping strategy. Analysis of data from the Lancaster Dairy Herd Improvement Association (n = 133,057) indicated that cows in the region were being fed diets containing approximately 17% crude protein, regardless of parity. A comparison to target milk urea N concentrations for this data indicated 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.

Improving the efficiency of feed N utilization by dairy cattle is the most effective means to reduce nutrient losses from dairy farms. The objectives of this study were to quantify the impact of different management strategies on the efficiency of feed N utilization for dairy farms in the Chesapeake Bay Drainage Basin. A confidential mail survey was completed in December 1998 by 454 dairy farmers in PA, MD, VA, WV, and DE. Nitrogen intake, urinary and fecal N, and efficiency of feed N utilization was estimated from survey data and milk analysis for each herd. Average efficiency of feed N utilization for milk production by lactating dairy cows (N in milk/N in feed x 100) was 28.4% (SD = 3.9). On average, farmers fed 6.6% more N than recommended by the National Research Council, resulting in a 16% increase in urinary N and a 2.7% increase in fecal N. Use of monthly milk yield and component testing, administration of bovine somatotropin (bST), and extending photoperiod with artificial light each increased efficiency of feed N utilization by 4.2 to 6.9%, while use of a complete feed decreased efficiency by 5.6%. Increased frequency of ration balancing and more frequent forage nutrient testing were associated with higher milk production, but not increased N utilization efficiency. Feeding protein closer to recommendations and increasing production per cow both contributed to improving efficiency of feed N utilization.

The hypothesis of this field study was that providing farmers with information regarding their herd's milk urea nitrogen (MUN) would result in more accurate feed management and a change in MUN toward target values. All dairy herd bulk tanks (n = 1156) in the Maryland and Virginia Milk Producers' Cooperative were tested for MUN each month for six months ending in May 1999. Farmers (n = 454) who returned a survey were provided with the results of their MUN analysis each month along with interpretive information. Survey results indicated that most (89.5%) dairy farmers did not routinely use MUN prior to participating in the project, but most (88%) extension agents and nutritionists in the region recommended it. The average MUN across all farms in the study increased in the spring, but the increase was 0.52 mg/dl lower for farmers receiving MUN results than for those who did not participate in the program. Farmers who indicated they increased dietary crude protein (CP) due to low MUN started with MUN values that were 3 mg/dl below target but ended with target values. Farmers who indicated that they decreased CP due to high MUN began the project with high MUN but decreased it by 1 mg/dl compared to non-participating farmers. At the end of the project, 30% of farmers responding to a follow-up survey indicated they would use MUN analysis in the future. Providing MUN results and interpretive information to farmers was documented to change feeding practices and subsequent MUN results.

Reducing nitrogen (N) excretion by dairy cattle is the most effective means to reduce N losses (runoff, volatilization, and leaching) from dairy farms. The objectives of this review are to examine the use of milk urea nitrogen (MUN) to measure N excretion and utilization efficiency in lactating dairy cows and to examine impacts of overfeeding N to dairy cows in the Chesapeake Bay drainage basin. A mathematical model was developed and evaluated with an independent literature data set to integrate MUN and milk composition to predict urinary and fecal excretion, intake, and utilization efficiency for N in lactating dairy cows. This model was subsequently used to develop target MUN concentrations for lactating dairy cattle fed according to National Research Council (NRC) recommendations. Target values calculated in this manner were 8 to 14 mg/dl for a typical lactation and were most sensitive to change in milk production and crude protein intake. Routine use of MUN to monitor dairy cattle diets was introduced to dairy farms (n = 1156) in the Chesapeake Bay watershed. Participating farmers (n = 454) were provided with the results of their MUN analyses and interpretive information monthly for a period of 6 months. The average MUN across all farms in the study increased in the spring, but the increase was 0.52 mg/dl lower for farmers receiving MUN results compared to those who did not participate in the program. This change indicated that participating farmers reduced N feeding compared to nonparticipants. Average efficiency of feed N utilization (N in milk / N in feed x 100) was 24.5% (SD = 4.5). On average, farmers fed 6.6% more N than recommended by the NRC, resulting in a 16% increase in urinary N and a 2.7% increase in fecal N compared to feeding to requirement. N loading to the Chesapeake Bay from overfeeding protein to lactating dairy cattle was estimated to be 7.6 million kg/year. MUN is a useful tool to measure diet adequacy and environmental impact from dairy farms.