Predicting forage quality in switchgrass and big bluestem

Predicting forage quality would help producers schedule hay harvesting to o btain 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 w eight (MSW) could be used to predict in vitro digestible dry matter (IVDDM) , crude protein (CP), and neutral-detergent fiber (NDF) of 'Trailblazer' sw itchgrass (Panicum virgatum.) and 'Pawnee' big bluestem (Andropogon gerardi i 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 l-wk interv als in 1990 and 1991 and at 2-wk intervals in 1992 and 1993. They were morp hologically classified as MSC and MSW and analyzed for 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 predic ted by MSC and MSW, Big bluestem IVDDM 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 determi nate growth habit of these species, Morphological development accurately pr edicted forage quality in many instances. Although no universal parameter a dequately predicted concentrations of IVDDM, CP, and NDF, it was possible t o accurately predict quality with readily available environmental data and measures of plant maturity.