A New Approach for Calculating Metabolizable Protein Requirements by Lactating Dairy Cows

This article describes a new system for calculating metabolizable protein (MP) requirements by lactating dairy cows. The system was developed with 133 treatment means from 36 scientific publications. The nonlinear relationship between total protein output (scurf + endogenous urinary + metabolic fecal + milk) and MP supply was used to construct a response model determining changes in protein output with varying supplies. The efficiency of MP utilization predicted by the model decreased nonlinearly with supply, ranging from 0.85 to 0.43 as protein supply increased from 0.78 to 3.28 kg/day (1.72 to 7.22 1b/day). The combined MP requirement (i.e., lactation plus maintenance) was defined as the MP supply needed to predict a given total protein output in the estimated response curve. A requirement function was constructed by solving the estimated nonlinear response curve in terms of the MP supply. This function directly computes the supply needed for a given total protein output while accounting for a variable efficiency at different protein supply levels. For protein outputs below 1.1 kg/day (2.42 1b/day), the calculated requirements were lower than the ones from the current Northern American feeding system for dairy cows. Conversely, for total protein outputs beyond 1.1 kg/day (2.42 1b/day), the calculated requirements were higher than predicted by current feeding systems. Finally, one example is presented with the detailed use of the new system for calculating the combined MP requirements.