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Population and specific hybrids were made between populations and genotypes of switchgrass, Panicum virgatum L., and their progeny were evaluated for heterosis in space-transplanted fi eld trials in eastern Nebraska for a 3-yr period. ‘Kanlow’ (lowland tetraploid) x ‘Summer’ (upland tetraploid) hybrids exhibit midparent heterosis for second- and third-year biomass yields for both population and individual plant hybrids. These data and previously reported molecular marker data indicate that lowland-tetraploid and upland-tetraploid switchgrasses represent different heterotic groups that can potentially be used to produce F1 hybrid cultivars. Hybrids produced from cultivars and experimental strains developed from upland-octaploid germplasm originating from spatially separated western and eastern regions of the original tallgrass or an adjacent forested ecoregion did not exhibit heterosis for any trait evaluated. These results suggest that these upland populations evaluated were from the same or closely related large germplasm pools or heterotic groups. A method for developing F1 switchgrass hybrid cultivars utilizing the gametophytic self-incompatibility mechanism of the species is described.