Improving nitrogen efficiency and profitability of dairy cattle in the United States

The objectives of these studies were to assess U.S. dairy nutritionists' approaches toward balancing CP in lactating cow diets, and to leverage existing knowledge of postabsorptive AA metabolism through the application of a mathematical ration-balancing model to predict N efficiency through a more accurate postabsorptive amino acid (AA) delivery. In experiment 1, dairy nutritionists (n = 77) that fed a total of 521,000 lactating dairy cows responded to a questionnaire related to demographic information, feelings toward environmental nitrogen (N) excretion, and dietary CP balancing practices. Eighty-nine percent of nutritionists balanced diets based on one or more individual AA requirements of dairy cows. The primary concern with formulation of lower CP diets was the cost per unit of metabolizable protein (MP). In the second study, three treatments were fed to lactating Holstein cows (n = 48) to test proof of concept of NASEM 2021 and a nonlinear optimizer: a control balanced to fulfill all nutrient needs of lactating dairy cows producing 45 kg milk/d using the NRC (2001) dairy model (NRC01), and two diets balanced with a nonlinear optimizer that fulfilled requirements according to the updated NASEM (2021) dairy model and attempted to either: 1) maximize N efficiency through tailored post-ruminal AA supply (NEFF), or 2) maximize income over feed cost (IOFC). A simulation function was written in RStudio (version 2022.02.3) to predict daily animal performance with NASEM 2021. Dry matter intake, milk, and milk components from both the observed data and the simulation data matrices were analyzed as repeated measures (days) in a mixed model to test for both observed and predicted (simulated) differences in treatment means. Income over feed cost was $4.83, $4.77, and $5.12/cow/d for NRC01, IOFC, and NEFF, respectively (P = 0.96). Nitrogen efficiency (%) was greatest for NEFF (33.7), followed by NRC01 (28.9) and IOFC (23.4; P < 0.05 between all treatments). Based upon simulation data, NASEM 2021 predicted relative performance differences between animals that consumed treatments with differing absorbed EAA supplies, although residual analyses revealed that further progress could be made in milk protein (g/d), milk fat (g/d), milk yield (kg/d), and DMI (kg/d) predictions.