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Predicting forage quality would help producers scheduled hay harvesting to obtain desired hay quality. Our objective was to determine if growing degree day (GDD), day of the year (DOY), mean stage count (MSC), and mean stage weight (MSW) could be used to predict in vitro digestible dry matter (IVDDM), crude protein (CP), and neutral-detergent fiber (NDF) of ‘Trailblazer’ switchgrass (Panicum virgatum.) and ‘Pawnee’ big bluestem (Andropogon gerardii Vitman) grown in Nebraska and Kansas. This field study was conducted from 1990 to 1993 at Mead, NE on Typic Argiudoll soils and from 1992 to 1993 at Manhattan, KS on Aquic Argiudoll soils. Plants were sampled at 1-wk intervals in 1990 and 1991 and at 2-wk intervals in 1992 and 1993. They were morphologically classified as MSC and MSW and analyzed IVDDM, CP, and NDF. Switchgrass IVDDM and CP were best predicted by GDD models, which accounted for 86 and 91% of the variation, respectively, whereas NDF was best predicted by MSW and CP was best predicted by GDD, which both accounted for 90% of the variation. Mean stage weight accounted for 74% of the variability in big bluestem NDF. The DOY model adequately predicted forage quality due primarily to the determinate growth habit of these species. Morphological development accurately predicted forage quality in many instances. Although no universal parameter adequately predicted concentrations of IVDDM, CP, and NDF, it was possible to accurately predict quality with readily available environmental data and measures of plant maturity.