Numerous studies have reported conversion of various agricultural residues and processing wastes to sugars and to ethanol using a variety of pretreatment and fermentation technologies. As shown in Table 9.3, high overall yields of xylose, glucose, and other sugars have been realized through pretreatment followed by enzymatic hydrolysis for the following feedstocks: corn stover, barley straw, and corn fiber. On the other hand, much lower yields have been reported for flatpea hay, wheat straw, and sugarcane bagasse. Unfortunately, it is challenging to compare many of these results accurately due to changes in methods, enzyme formulations, analytical approaches, and data reporting. However, one team did systematically investigate bioconversion of corn stover, which has the highest availability in the United States (see Figure 9.1), for different pretreatment operations in combination with the same enzymes and using the same methods (Wyman et al. 2005a). In this case, these leading pretreatments that spanned pH values from about 1.2-11 realized over 90% overall yields of xylose and glucose. Some work has also been conducted in determining the fermentability of the sugars released to ethanol as it is vital to obtain high ethanol yields in the fermentation step (Wyman et al. 2005b). In this case, inhibitory effects of compounds produced or released during up-stream processing by operations such as pretreatment must be avoided or overcome. In addition, high ethanol yields must be realized from all pentose and hexose sugars that tend to be preva­lent in agricultural residues. Furthermore, some pretreatments such as controlled pH produce substantial amounts of oligomeric sugars that many organisms cannot ferment to ethanol directly, and steps must be introduced to either hydrolyze these to monomers and possibly dimmers, or organisms must be employed that can fully utilize these soluble polymers (Yang and Wyman 2008).