A new protein requirement system for dairy cows

Accurate prediction of protein requirements for maintenance and lactation is needed to develop more profitable diets and reduce N loss and its environmen-tal impact. A new factorial approach for accounting for net protein requirement for maintenance (NPM) and metabolizable protein (MP) efficiency for lacta-tion (EMPL) was developed from a meta-analysis of 223 N balance trials. We defined NPM as the sum of the endogenous protein fecal and urinary excretion and estimated it from the intercept of a nonlinear equation between N intake and combined total N fecal and uri-nary excretion. Our model had a strong goodness-of-fit to estimate NPM (6.32 + 0.15 g protein/kg metabolic body weight; n = 807 treatment means; r = 0.91). We calculated the EMPL as a proportion of the N intake, minus N excreted in feces and urine, that was secreted in milk. A fixed-EMPL value of 0.705 + 0.020 was pro-posed. In a second independent data set, nonammonia-nonmicrobial-N and microbial-N ruminal outflows were measured, and the adequacy of the MP prediction (51 studies; n = 192 means treatments) was assessed. Our system based on the fixed-EMPL model predicted the MP requirement for lactation and maintenance with higher accuracy than several North American and European dairy cattle nutrition models, including the INRA (2018) and NASEM (2021). Only the NRC (2001), CNCPS 6.5, and Feed into Milk (2004) mod -els had similar accuracy to predict MP requirement. Our system may contribute to improve the prediction for MP requirements of maintenance and lactation. However, most refined predictive models of intestinal digestibility for rumen undegradable protein and mi-crobial protein are still needed to reduce the evaluation biases in our model and external models for predicting the MP requirements of dairy cows.