Windrow composting of cattle manure is a significant source of gaseous emissions that include ammonia and the greenhouse gases of carbon dioxide, methane, and nitrous oxide. A process-based model was developed to simulate different carbon (C) and nitrogen (N) processes, such as mineralization, immobilization, volatilization, etc., occurring during composting, and their effects on and interactions with environmental conditions within the windrow. Comprehensive measurement data from a published independent cattle manure composting study were used in refining and assessing the model. Out of 15 measured parameters available for model evaluation, six were also used in refining model constants. Simulation results indicate that the model performed reasonably in predicting environmental conditions (e.g., moisture), C and N balance within, and gaseous emissions from static (i.e., no turning) and turned windrows. At the end of 99 days of composting, predicted C (50%) and N (31%) losses, mainly as gaseous emissions, for the static windrow compared well with measured losses (45% and 19%, respectively) - the discrepancy observed for N loss was likely due to non-simulation of the finished compost cover that was included on the experimental windrow. The model performed better in simulating the turned windrow, with predicted C (78%) and N (55%) losses very close to those measured (77% and 57%, respectively). The performance of the model was further assessed using another published independent study on a turned windrow, with predicted C (43%) and N (40%) losses comparable to measured losses (53% and 42%, respectively). The process-based compost model is incorporated in a version of the Integrated Farm System Model (IFSM), a whole-farm simulation model used to evaluate the performance, environmental impact, and economics of crop, dairy, and beef production systems. The new component will enable whole-farm evaluation of the impacts of implementing manure composting strategies.