The system presented is designed to be used as a daily management aid for adjusting nutrient requirements and utilization over wide variations in cattle, feed, management and environmental conditions. It was assembled from separate submodels of physiological and metabolic functions: intake; rumen carbohydrate fermentation and protein degradation; intestinal digestion and absorption; excretion; heat production; nutrient partitioning and utilization for maintenance, growth, lactation, and reserves, using National Research Council recommendations and additional data. A mix of mechanistic and empirical approaches were used, depending on the data available, variables needed to drive the equations, and level of aggregation needed for practical application. Each submodel was tested on independent data of beef and dairy cattle performance. Many intermediate variables are described and presented, so the system can be easily applied in a microcomputer spreadsheet.