Evaluation of models for balancing the protein requirements of dairy cows

Most diets for dairy cattle in the US are formulated using the mathematical model developed by the National Research Council (NRC). This model is simpler than more mechanistic models and is largely empirical. Based on the research reviewed in this paper, the simpler empirical approach is recommended for routine diet formulation at the present time. Under typical conditions using feed tables for most feed descriptions, the NRC model was more accurate than the Cornell Net Carbohydrate and Protein System, and, in its present form, the model developed by Baldwin et al. (4) is too difficult to run routinely in the field. However, the more mechanistic approaches are recommended to investigate diet and animal interactions under nonstandard environmental conditions, animals, or feeds. Because the NRC model does not address many of the potential limitations of some diets or management conditions, more complex models are needed to identify why some herds appear to underperform. Mechanistic models can be used to study or explain nutritional or physiological concepts and to develop and test research hypotheses. Ultimately, it is the fundamental research in nutrition modeling that enables advances in routine diet formulation procedures. Nonetheless, models used routinely to balance diets need to be as simple and as accurate as possible.