Two pasture growth models that share many common features but differ in model complexity have been developed for incorporation into the Integrated Farm System Model (IFSM). Major differences between models include the explicit representation of roots in the more complex model, and their effects on carbon portioning, nitrogen fixation and transpiration. The overall goal was to develop a model capable of representing growth, competition, and ecosystem carbon fluxes among multiple plant species in pastures while maintaining a relatively simple model structure. Output from the two models will be compared with 12 site years of data from three grazing experiments in central Pennsylvania. Specific parameters of interest include yearly and seasonal forage yield, species composition, gross photosynthesis and net change in soil organic carbon. Results of model comparisons will be presented along with recommendations for future model development.