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Formulating diets for intestinal unavailable nitrogen using blood meal in high-producing dairy cattle.

The high cost of protein feeds and growing concern for the environment have motivated dairy producers and nutritionists to focus their attention on reducing nitrogen (N) losses on dairy farms. It is well recognized that reducing the N content of cattle diets is the single most important factor to increase the efficiency of N use. However, effectively lowering the N content of diets requires the nutritionist to know the availability of N in feeds so as to not negatively affect milk production or overfeed N. To provide reliable data for nutritionists, a new assay to estimate unavailable N in the intestine (uN) was developed. To determine whether uN could be used as a replacement for acid detergent insoluble nitrogen (ADIN) in diet formulation, we conducted a replicated pen study to evaluate the effect of total-tract uN on the performance of high-producing dairy cattle. One hundred twenty-eight cattle that were 97 to 147 d in milk at the beginning of the experiment were allocated into 8 pens of 16 cows, and pens were randomly allocated to 2 dietary treatments. Cattle were fed 1 of 2 isonitrogenous and isocaloric diets that were also equal in neutral detergent fiber, deviating only in the inclusion of 2 different blood meals (BM) used in each diet. The uN contents of the 2 BM were 9% (low uN) and 34% (high uN) total N content as predicted by the assay, whereas when measured as ADIN, no difference in indigestibility was observed. The inclusion of BM was on an isonitrogenous basis, and the predicted difference in uN was 39 g/d or 5.8% of N intake, representing the formulated difference in available N between the 2 treatments. There was no effect of uN on dry matter or N intake, which averaged 27.3 kg/d and 668 g/d for both treatments, respectively. Milk yield and energy-corrected milk were 1.6 and 1.9 kg/d greater for cows fed the low uN diet compared with those fed the high uN diet. The lower uN diet was also associated with greater milk protein yield, greater milk fat yield, and greater milk urea N. The Cornell Net Carbohydrate and Protein System (version 6.5) was used to evaluate the application of the uN measurement by replacing ADIN in BM with the uN value in the inputs for the BM. All other cow and feed chemistry data were inputted as measured in the experiment. The predictions of metabolizable protein-allowable milk demonstrated that using the uN values in place of ADIN increased the accuracy of the prediction and enabled the model to predict the first-limiting nutrient provided all other feed, cattle, and management characteristics were also defined.