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Grasslands, shrublands, and savanna ecosystems worldwide are often grazed by domestic livestock. These vegetation types are critical for human flourishing and are vulnerable to overuse and degradation when local socio-economic conditions or misunderstanding of plant ecology leads to overuse. On large grazing areas, whether used as private property or as common pool resources, the ability of plant communities to retain rangeland health attributes of soil stability, hydrologic function, and biotic integrity depend on both stocking rate and careful application of patterns of grazing timing, duration, severity, and frequency. Sustainable stocking rates depend on judicious allocation of available forage. Historical stocking rate tools have assumed that land managers have accurate information on forage quantity and that a static sustainable stocking rate can be developed. But in non-static arid and semi-arid ecosystems already defined by resource scarcity and prone to threshold events driven by abiotic variables, the inherent interannual variability of precipitation and unpredictable net primary herbaceous production pose particular challenges for pastoralists. Washington State University Extension, in partnership with the University of Arizona and the United States Forest Service, developed a free grazing decision support tool that incorporates historical forage production and variability with user-defined animal behavior parameters and spatial distribution to estimate livestock terrain use. We show how StockSmart allows stocking calculations and grazing planning based on spatially-explicit estimates of available forage rather than total forage. It also allows testing infrastructure investments against resulting increases in forage availability. These considerations are critical for avoiding ecological state changes through overgrazing into degraded but stable conditions.