Potential of Annual Cereal Crops to Serve as Fuel Ethanol Feedstock and Livestock Feed

Increased public concern about global warming and over-reliance on foreign petroleum oil has led to the development of renewable and clean energy in the United States. Ethanol blended gasoline burns more efficiently and can contribute to reduction in greenhouse gasses emission (Wang et al. 1999). Ethanol production in the US has increased rapidly in recent years with a total production of 3.9 billion gallons in 2005 (RFA 2007), with maize as the major feedstock for fuel ethanol production. Since little maize grain is produced in Montana due to the cool weather and short growing season, alternative feedstocks need to be explored for fuel ethanol production. There is an abundant but underutilized supply of agricultural residues and herbaceous grasses available in Montana. In 2006, for example, Montana produced 5.2 million tonnes (t) of wheat and 0.9 million t of barley. Over 9 million t of residues were left behind as a by-product of these crops. In addition, Montana farmers also produced 5.8 million t of hay (Montana Department of Agriculture 2006). The annual and/or perennial grasses and cereal forage crops may serve both as livestock feed and lignocellulosic feedstock for fuel ethanol production. Using winter annual triticale and sweet sorghum or pearl millet for double-cropping (two harvests per year) will allow biomass production to be further increased in the northern Great Plains. The advantage of using annual forage crops for fuel ethanol feedstock is that farmers do not need additional machinery and technologies for crop production. Although sweet sorghum and pearl millet are tropical originated plants, these crops also have potential for adaptation to temperate climate (Lueschen et al. 1991). Due to the complex structure of the plant cell wall, pretreatment is needed to effectively convert lignocellulosics to fermentable sugars by enzymatic hydrolysis. Various pretreatment methods have been used to open up the lignocellulosic multicomponent matrix for making the carbohydrate components more accessible to hydrolytic enzymes (Lynd 1996; Wyman 1999). Based on the composition analysis of feedstocks, Chen et al. (2007b) used NaOH for triticale hay and straw, and H2SO4 for sweet sorghum and pearl millet hay pretreatment, respectively. The maximum xylan solubilization (78%–81%) by H2SO4 and maximum lignin reduction (75%–85%) by NaOH was achieved with treatment at 2.0% (w/v) at 121°C and 15 psi for 60 min. However, chemical free alternatives need to be explored to make the pretreatment process environment friendly. Ensilage is an ancient method used by farmers to preserve forage crop for centuries (Wilkinson et al. 2003; Weinberg and Ashbell 2003). The fermentation during ensiling produces lactic acid, which results in a low pH environment to prevent degradation of carbohydrates in the feedstocks (Linden et al. 1987). In modern agriculture, a silage additive containing a combination of hemicellulase, fungal alpha-amylase, bacterial alpha-amylase, and cellulase, depending on the product and manufacture, is added to fresh forage during ensiling process to enhance fermentation and break down of plant cell wall structures (Linden et al. 1987; Henderson 1993). This process is called enzyme-assisted ensiling (ENLAC) (Schmidt et al. 1997). The acidic environment produced by ensiling also serves as a pretreatment that can result in enhanced conversion of biomass to sugars (Richard et al. 2001). Linden et al. (1987) reported that ensiling fresh sorghum results in hydrolysis of 70% cellulose to fermentable sugars. Using ENLAC process as a feedstock storage and in-situ pretreatment method may provide some advantages over other commonly used pretreatment methods, such as reduced energy input during pretreatment and ability to preserve and utilize fresh biomass in areas where drying of biomass feedstock is prevented by weather conditions.