LAUSR

energy from cellulosic feedstocks is fourto seven-fold greater than for ethanol produced from corn (Zea mays L.) grain (Farrell et al., 2006; USDOE, 2016). The USDOE has identified sustainable production of regionally adapted feedstocks as a research priority for developing the cellulosic bioenergy industry (USDOE, 2015). Although crop residues will supply the bulk of potential cellulosic bioenergy feedstocks in the near future, energy crops are predicted to supply a growing portion of the supply beginning in the 2020’s, potentially surpassing crop residues by 2030 (USDOE, 2016). With corn occupying a large land area in the United States (USDA–NASS, 2014), corn residues have been identified as a primary source of cellulosic feedstock (Dhugga, 2007; USDOE, 2016). Karlen et al. (2014) reported that removing half to all corn residue resulted in stover yields of 4 to 7 Mg ha–1 in a meta-analysis of results from 239 site–yr. Karlen et al. (2014) indicated that residue removal improved yields in no-till situations in the central Corn Belt, but Varvel et al. (2008) documented a reduction in corn yield with 50% residue removal in eastern Nebraska. Johnson et al. (2014) estimated that roughly 6 Mg ha–1 of residue needed to be returned to the soil to maintain soil organic carbon levels. Karlen et al. (2014) and Propheter and Staggenborg (2010) documented that any amount of residue removal increased nutrient export from the field, implying a greater fertilization requirement in situations where the residue is harvested for bioenergy production. The various types of sorghums [Sorghum bicolor (L.) Moench] have several potential advantages over corn relative to biomass production (Mathur et al., 2017; Nghiem et al., 2016). Forage and sweet sorghums maximize biomass yield at N fertilizer rates less than those required to maximize biomass Long-term Biomass and Potential Ethanol Yields of Annual and Perennial Biofuel Crops