Simulation models have been developed for many agronomic applications to understand and predict crop responses to management inputs and changing climate conditions. Increasing drought severity and declines in the Ogallala Aquifer focus interest on models that optimize sustainable water use in the Texas High Plains. Simulation models adapted for perennial grasses can ascertain forage productivity and water use under water-limited conditions. This research compares two models: Agriculture Land Management Alternative with Numerical Assessment Criteria (ALMANAC) and Agriculture Production Systems sIMulator (APSIM) with WW-B.Dahl Old World bluestem [Bothriocloa bladhii (Retz) S.T. Blake] (Bdahl) pastures near Lubbock, TX. Bdahl is an introduced, highly water-efficient and grazing-tolerant perennial C4 grass, which can serve as an optional forage crop for producers facing a dwindling supply of irrigation water. Both models simulate pant growth and biomass productivity via plant physiological and plant-soil hydrological parameters with inputs for weather, management, soil, and plant characteristics. There has been very limited parameter information of the models for forage species. The current research aims to calibrate and compare validations for Bdahl under grazed and non-grazed managements for predicting forage vegetative growth curves, yield, and water use.