Sweet Sorghum as a Bioenergy Crop for the US Great Plains

Abstract

Sorghum (Sorghum bicolor L. Moench), including sweet sorghum, is widely adapted to diverse and often marginal crop production environments. Sweet sorghum stalks have high sugar content compared with other sorghum types and has potential for producing ethanol to be mixed with gasoline or for producing ethyl tert-butyl ether, an octane additive to gasoline. Sweet sorghum was introduced to the United States for syrup production in the 1850s (Winberry, 1980). Production peaked following sugar shortages during World War II at about 136 million L yr-1 of syrup in 1946 (Hunter & Anderson, 1997), but thereafter declined because of low sugar prices and inadequate production efficiency. Sweet sorghum can be competitive with corn (Zea mays L.) and grain sorghum for ethanol yield when grain yield is less than 9 Mg ha-1, and is comparatively efficient in nitrogen use (Smith & Buxton, 1993). Sweet sorghum can easily substitute for corn or grain sorghum in many cropping systems. Currently, most ethanol produced in the U.S.A. is from the starch of corn grain with the support of federal subsidies. Energy gains with production of ethanol from grain are modest, typically ranging from 30 to 130% depending on N use efficiency, ethanol plant efficiency, and the efficient use of the distillers grain co-product. Sweet sorghum can be produced at less cost than corn, often with higher energy gains (Smith & Buxton, 1993). Rather than producing starch, sweet sorghum carbohydrates are stored in the stalk as sugar, with sugar concentrations of 8-20% (Rains et al., 1990). Conversion of sugar to ethanol requires less energy than starch as much energy is used to depolymerize the starch. Sweet sorghum has demonstrated potential to produce up to 6000 L ha-1 of ethanol in Iowa and Colorado U.S.A. (Smith & Buxton, 1993), equivalent to ethanol from approximately 20 Mg of corn grain. However, estimated ethanol yields were on average 33% more with grain of corn and grain sorghum compared with sugar of sweet sorghum for seven rainfed site-years in Nebraska U.S.A. (Wortmann et al, 2010). Seasonal availability, the need to transport and store much mass, and storability of sweet sorghum constrain sweet sorghum as a bio-energy crop. In planning for bio-fuel production, long-term sustainability of cropping systems must be considered. Sustainability of a cropping system is very much dependent on production environment and resource availability. In one study comparing the sustainability of different bioenergy crops, sweet sorghum, along with oil palm (Elaeis guineensis L.) and sugarcane (Saccharum spp.) for biofuel, were found to be more sustainable in comparison to