Anaerobic digestion (AD) of animal manure is traditionally classified as a treatment to reduce the environmental impacts of odor, pathogens, and excess nutrients associated with animal manure. This report shows that AD also changed the composition of manure fiber and made it suitable as a cellulosic feedstock for ethanol production by increasing the cellulose content, reducing the particle size, and enhancing the digestibility. The solid digestate from an anaerobic digester (AD fiber) was assessed for ethanol production in this paper. AD fiber from two types of digesters was used in this study, a plug-flow reactor (PFR) and a completely stirred tank reactor (CSTR). Switchgrass and corn stover were used as controls for comparison to a more researched energy crop and agricultural residue. Dilute alkali and dilute acid pretreatment methods were compared for effectiveness of ethanol production. Using the most effective dilute alkali pretreatment conditions (2% sodium hydroxide, 130°C, and 2 h), enzymatic hydrolysis of 10% (dry basis) pretreated AD fiber from a plug flow reactor (PFR) produced 51 g/L glucose at a conversion rate of 90%. The ethanol fermentation on the hydrolysate had a 72% ethanol yield. The results indicated that 120 million dry tons of cattle manure available annually in the U.S. can generate 63 million dry tons of AD fiber that can produce more than 1.67 billion gallons of ethanol. Integrating AD with biorefining will make significant a contribution to cellulosic ethanol production.