The tallgrass prairie ecosystem was historically maintained through the interaction of climate, fire, and grazing by large ungulates. Efforts are being made to return large ungulates (bison) back into prairie landscapes, but successful introductions can be constrained by suitable habitat as determined by resource availability (plant biomass, forage quality of the plant biomass, and soil carbon and nitrogen), particularly given some of these tracts are former arable land that has been restored to tallgrass prairie. The objective of this study was to quantify whether plant biomass (available forage) differs between prairie types, if forage quality varies across native prairie remnants and restored prairies, and if these differences are related to carbon and nitrogen storage in soil. We sampled native prairie remnants, low seeded-diversity older restored prairies, and high seeded-diversity younger restored prairies at Nachusa Grasslands, Illinois during July 2014. Total plant biomass differed between the three prairie types, with the high seeded-diversity younger-restored prairies having almost twice that of the low seeded-diversity older-restored prairies and more than twice that of the remnant prairies. In regard to forage quality, the percentage of crude protein in dry matter was greater in the remnant prairies compared to both of the restored prairie types. The high seeded-diversity younger restored prairies had the highest lignin concentration in the forage. The fat content of forage was highest in the high seeded-diversity younger restored prairies. Total soil carbon and nitrogen did not differ between the three prairie types at either the 0–10 cm or 10–20 cm depth. The preliminary findings of this study suggest bison grazing within the introduction unit at Nachusa Grasslands will be more intensive within restored prairies as compared to remnants.