Cellulosic Ethanol Production of Forage Sorghum Under Deficit Irrigation.

Recent emphasis on renewable energy sources has led to an increased demand for biofuel feedstocks.  However, current production of biofuels such as ethanol (EtOH) utilizes food crops such as corn grain and sugarcane, which are typically cultivated on fertile soils that have few limits on plant growth.  The finite amount of this land will be needed to feed a growing global population, making marginal lands not relied upon for food production a more feasible option to provide biofuel feedstocks.  In the semiarid regions such as the Southern High Plains of the U.S., cropping options are limited due to climate and the limited irrigation.  However, drought tolerant, high residue crops such as sorghum are well adapted to these areas and can provide feedstocks for EtOH production.  This study evaluated the biomass yields and potential cellulosic EtOH production of sorghum cropping systems under limited water conditions.  The systems studied consisted of 2 high-tonnage yield forage sorghum cultivars with different cell wall composition (typical and reduced lignin content) under 2 deficit irrigation levels (rain-fed non irrigated and supplemental irrigation at 2.88 mm day^-1) and 2 biomass removal rates (100% and 50% removal).  During the 2009 growing season, both varieties produced sufficient biomass for EtOH production under both water levels.  Under supplemental irrigation, biomass (49%) and EtOH yield (65%) was higher, a result of 18% higher cellulose content in the irrigated biomass.  A variety with a 36% lower lignin content produced less biomass (26%), but similar EtOH yield on a land area basis due to more efficient conversion of this biomass to EtOH.  Overall, 3993 and 2417 kg ha^-1 EtOH was produced for the irrigated and non-irrigated treatments, respectively.  This study showed that a drought tolerant crop such as sorghum can produce sufficient biomass for EtOH production under marginal growing conditions.