Accurate prediction of forage biological values and performance with animals fed forages requires accurately accounting for factors that influence animal requirements and feedstuff utilization. The Cornell Net Carbohydrate and Protein System (CNCPS) is an application model that uses a combination of mechanistic and empirical approaches to account for the effects of variation in animal factors and feed carbohydrate and protein fractions on animal performance. Thus, accurate animal and environmental descriptions, DMI, feed carbohydrate, and protein fractions and their digestion rates are required inputs. In 25 growth periods with calves fed high-forage diets, the CNCPS accounted for 74, 81, and 83%, respectively, of the variation in ADG predicted to be supported by the ME, metabolizable protein, and essential amino acid intake, the first-limiting of all three accounting for 81% of the variation with a -1% bias. Thus, the CNCPS can be used to accurately describe forage quality and the effects of changes in forage composition on animal performance. The model was sensitive to variations in NDF, CP, protein solubility, NDF and starch digestion rates, feed and microbial amino acid composition, maintenance protein requirement, body protein amino acid content, and the coefficient of efficiency of use of absorbed protein. Analysis of several trials indicates an improved efficiency of ME use with improved amino acid balances. Uses of the CNCPS discussed include interpreting, planning and applying research, teaching, developing tables of requirements and biological values for feeds, complex nutritional accounting, and predicting performance and profits.