Forage quality of native warm-season grasses in response to nitrogen fertilization and harvest date

The objectives of this research were to determine the effect of N levels and plant maturity on the nutritive quality of four warm-season grasses, and to evaluate relationships between forage quality and morphological development. Indiangrass (Sorghastrum mutans L. Nash), big bluestem (Andropogon gerardii Vitman), Eastern gamagrass (Tripsacum dactyloides L.), and switchgrass (Panicum virgatum L.) were grown in four replications, applied with 3N levels (0, 65, and 140 kg/ha), and harvested on ten dates at approximately 2-week intervals for first seven harvest dates and 3-week intervals for last three harvest dates. For all species, harvesting at early maturity increased (P<0.05) forage quality including increased in vitro dry matter digestibility (IVDMD) and crude protein (CP), and decreases in neutral detergent fiber (aNDF) concentrations, whereas allowing forage to reach later maturity reduced forage quality with declines in IVDMD and CP and increases in aNDF concentration. The aNDF concentration in warm-season grasses with the exception of big bluestem increased with N fertilization at 65 kg/ha, while N fertilization at 140 kg/ha had no consistent effect on aNDF concentration of the four grass species across both years. Nitrogen fertilization increased (P<0.05) the IVDMD value across four grass species in 2006, whereas it decreased (P<0.05) the IVDMD value for switchgrass and did not affect (P>0.05) the IVDMD value for the other three species in 2007. The concentration of CP for all four warm-season grass species increased (P<0.05) at higher rates of N fertilization. Therefore, although there were variations in the effect of N fertilization on aNDF and IVDMD making drawing conclusions difficult, N fertilization could improve forage quality for warm-season grasses through increasing the concentration of CP. For big bluestem, switchgrass, and indiangrass, forage quality parameters showed a quadratic relationship to mean stage by count (MSC), with coefficients of determination ≥0.96 for IVDMD, 0.88 for aNDF, and 0.88 for CP, respectively. However, predicting forage quality changes based on MSC did not provide reasonable estimates (P>0.05) for eastern gamagrass.