Selection and hybridization have long been used separately or combinationally in the development of cultivars and release of germplasm in switchgrass. Switchgrass is so broadly distributed that enormous genetic variation exists in nature as discussed above. These locally adapted germplasm provide precious raw materials for initial releases of "natural — track" cultivars and named germplasms. Casler (2011) recently provided a comprehensive list of "natural-track" cultivars and germplasms. The Plant Material Centers of the USDA Natural Resource Conservation Service (NRCS) (formerly Soil Conservation Service), State Agricultural Experiment Stations (SAES) and USDA Agricultural Research Service (ARS) laboratories have released most, if not all, of the "natural-track" cultivars and germplasm. Basically, local strains from target geographic regions are collected. Collected accessions are grown in screening nurseries with other available cultivars and germplasm for evaluating important agronomic and adaptive traits. Desirable accessions are further tested in additional environments. Seed of best accessions are increased and released as a new cultivar or a named germplasm. As described by Vogel (2004), release of "natural-track" cultivars and germplasm utilized the natural variation between native ecotypic populations. Selection within individual populations may be applied. For example, ‘Kanlow’ switchgrass was developed using germplasm collected by the Soil Conservation Service at a lowland site near Wetumka, OK. The collection was grown and plants were selected for leafiness, vigor, and retention of green late in season near Manhattan, KS. Seeds of 200 selected plants were increased and released as ‘Kanlow’ by Kansas AES and ARS (Alderson and Sharp 1994). Some other releases are direct seed increase of original collections. ‘Penn Center’ switchgrass was released as a source identified germplasm (USDA NRCS 2010). ‘Southlow Michigan’ switchgrass is a source identified germplasm released by the USDA-NRCS, Michigan Association of Conservation Districts, and the Michigan Department of Natural Resources (Durling et al. 2008). Although "natural-track" releases are genetically similar to or the same as wild plants of the original sources, they provide premier germplasm or populations for developing genetically improved cultivars in breeding programs.

Strong outcrossing sexual behavior and relatively weak asexual reproduction capability of switchgrass require typical recurrent selection procedures used for genetic improvement through hybridization. Vogel and Pedersen (1993) described breeding protocols for outcrossing perennial species. Phenotypic selection within a population consists of preparing seedlings in the greenhouse, transplanting the seedlings into a space-planted nursery, and performing visual selections in year 2 and/or year 3. The selected plants can be used to make new synthetics by poly­crossing with replicated ramets of the selected ones. Seeds of synthetics are grown along with commercial standards for performance and persistence testing at multiple environments (years and locations). ‘Trailblazer’ is an outstanding synthetic cultivar developed by the USDA-ARS and the Nebraska Agricultural Research Division, Department of Agronomy, University of Nebraska (Vogel et al. 1991). It was improved for in vitro dry matter digestibility (IVDMD) by 6% and for beef production by 23% over its source population (Vogel et al. 1991). The success of Trailblazer was evidenced as it was grown over 63,000 ha from 1986 to 1997 (Vogel 2004). Similarly, ‘Shawnee’ was developed with phenotypic selection for IVDMD from a released cultivar ‘Cave-in-Rock’ (Vogel et al. 1996). After two cycles of phenotypic selection, half-sib progenies of 10 superior plants were cross-pollinated to produce Syn 1 seed of ‘Sunburst’ (Boe and Ross 1998). ‘Sunburst’ was improved for seed weight, resulting in superior seedling vigor over other cultivars adapted to the northern Great Plains. Phenotypic recurrent selection with one or more cycles was used to develop ‘Dacotah’, ‘TEM-SLC’, ‘TEM-SEC’, ‘WS4U’and ‘WS8U’ switchgrass (Barker et al. 1990; Tischler et al. 2001; Casler et al. 2006). ‘TEM-LoDorm’ switchgrass was improved through four cycles of phenotypic recurrent selection for reduced seed dormancy (Burson et al. 2009).

Although phenotypic selection within populations was successful for improving IVDMD, reducing seed dormancy, increasing seed weight, and some other traits, the procedure has not been much effective for enhancing biomass yield in switchgrass. Field trials repeatedly indicated narrow-sense heritabilities for biomass yield were low, indicating phenotypic selection is not reliable (Hopkins et al. 1993; Rose et al. 2008; Bhandari et al. 2010 and

2011) . To improve biomass yield, genotypic recurrent selection procedures involving half-sib progeny testing has been used to develop recently released lowland cultivars, ‘Performer’, ‘BoMaster’ and ‘Colony’ by the USDA-ARS and the North Carolina Agricultural Research Service (Burns et al. 2008a, b, and 2010) and ‘Cimarron’ by Oklahoma AES (Wu and Taliaferro

2009) . ‘Cimarron’ switchgrass was developed by poly-crossing seven parents, which were selected from a South Lowland breeding population with one cycle of recurrent selection for general combining ability (RSGCA) after two cycles of restricted recurrent phenotypic selection (RRPS). Details of RSGCA and RRPS used at the Oklahoma State University switchgrass breeding program have been described by Taliaferro (2002). Evidently, half — sib progeny testing is effective in the selection of elite parents for synthetic cultivar development in switchgrass.