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Switchgrass (Panicum virgatum L.) and big bluestem (Andropogon gerardii Vitman) are important warm-season grasses in livestock production systems in the central and eastern USA. The objectives of this study were to quantify the morphological development of ‘trailblazer’ switchgrass and 'Pawnee' big bluestem and to evaluate day of the year (DOY) and growing degree day (GDD) as predictors of switchgrass and big blnestem morphological stage. Pure stands of each species were sampled at weekly intervals in 1990 and 1991 at Mead, NE, and classified as to mean stage count (MSC) and mean stage weight (MSW). Prediction equations for MSC and MSW were developed based on DOY and GDD. The validation study was harvested at 2-wk intervals in 1992 and 1993 at Mead, NE, and Manhattan, KS, and classified as to MSC and MSW. Switchgrass and big bluestem MSC and MSW were related linearly in all environments. Linear DOY calibration equations accounted for 96% of the variation in switchgrass MSC across four environments, which indicates that switchgrass development was related to photoperiod and that general management recommendations could be based on DOY in the central Great Plains. Quadratic GDD calibration equations accounted for 83% of the variation in big bluestem MSC across four environments, which indicates that big bluestem development is more difficult to predict and management recommendations in the central Great Plains should be based on morphological development (which is best predicted by GDD). The comprehensive growth staging system gave repeatable results for quantifying the morphological development of switchgrass and big blnestem. The morphological development of switchgrass and big bluestem can be reliably predicted for adapted cultivars in the central Great Plains during years with near-normal precipitation using DOY and GDD because of the determinate growth habit of these grasses.