Synthesizing Rangeland Processes for Decision-Making Using the Gpfarm-Range Model.

A modeling approach that assesses impacts of alternative management decisions prior to field implementation would reduce decision-making risk for rangeland and livestock production system managers. The Great Plains Framework for Agricultural Resource Management – Rangeland model (GPFARM-Range) was developed as a decision support tool that synthesizes field-scale hydrology, forage, carbon-nitrogen, and cattle processes. The model was shown to have strategic and tactical functionality for predicting forage production (77% – 94% explained variability of with-in-season forage biomass) and selecting cattle stocking rates in Northern mixed-grass prairie. The model was shown to give acceptable simulations of soil organic carbon and nitrogen (< 5% relative root mean square error, RRMSE during calibration; < 20% RRMSE during validation). Also, the model adequately simulated the effects of elevated [CO₂] (720 ppm in field enrichment experiments on shortgrass steppe, 1997-2001) on both peak standing crop and soil water (within ± 10% bias; index of agreement > 0.70). GPFARM-Range captured observed trends in increased C₃ grass biomass and reduced plant transpiration under elevated [CO₂]. The model synthesizes the complex interactions among rangeland management, climate, forage, and livestock production. Contributions of Dr. Lajpat R. Ahuja in facilitating multi-disciplinary collaborations to achieve these synthesis results are highlighted.