To mitigate the negative environmental impact of farming, it is important that diets are formulated to accurately match requirements. For that, an adequate characterization of feed composition and its variability is crucial. The original Cornell Net Carbohydrate and Protein (CNCPS) feed carbohydrate and protein fractionation schemes were evaluated and modified to improve predictions of the rumen degradable protein (RDP), rumen undegradable protein (RUP) and microbial protein supply. For carbohydrates, a new expanded scheme was developed; the CA1 is volatile fatty acids (VFA), CA2 is lactic acid, CA3 is other organic acids, CA4 is sugars, CB1 is starch, CB2 is soluble fiber, CB3 is available neutral detergent fiber (NDF), and CC is unavailable NDF. The expanded scheme accounted for more variation in changes in silage quality and non-fiber carbohydrate composition. The CNCPS and National Research Council (NRC) protein schemes were evaluated using Monte Carlo techniques. Both schemes shared similar limitations including (1) the range of RDP and RUP was over-predicted; (2) the methods used to estimate degradation rates had low accuracy and repeatability, and (3) the assumptions underlying the kinetic models were too restrictive to mimic ruminal digestion. The CNCPS protein scheme was revised and alternative schemes were developed. Predictions of RDP and RUP were improved by assigning rates obtained with the inhibitory in vitro system to a combined insoluble protein B fraction, or by redefining A and B1 fractions as the non amino-N and amino-N in the soluble fraction, respectively. Urea recycled to the rumen may represent an important source of N for microbes. A dynamic mechanistic model was developed to be used as a component of ration formulation models to predict N recycling to the GIT and urinary urea N. Recycling processes were modeled as positive feedbacks, while renal excretion was modeled as a negative feedback. Both processes were assumed to be regulated by N intake. Model simulations suggested that accurately accounting for urea recycled to the rumen reduces degradable nitrogen needed in the diet, and the use of the NRC 1985 empirical equation to predict urea recycling to the rumen may greatly underestimate recycling in lactating dairy cows.