<P><B>Abstract</B></P> <P>This study investigated the effects of mixtures of agricultural wastes and energy crop on solid digestate quality and biogas production. The feedstock mixtures of dairy manure and switchgrass (DM:SG) had the similar lag phase with the control feedstock of dairy manure, while was shorter than the feedstock mixtures of dairy manure and corn stover (DM:CS). Under the stable digestion conditions, the mixture of DM:SG at the mixture ratio of 80:20 had the highest methane production of 138 mL/g total solids (TS) loading; the mixtures of DM:SG and DM:CS at the mixture ratio of 60:40 had the highest VS reduction of 25.8%; and the mixture of DM:SG at the mixture ratio of 60:40 had the highest cellulose and xylan reduction of 40.4 and 40.7%, respectively. Two bacterial phyla (<I>Firmicultes</I> and <I>Bacteroidetes</I>) and three archaeal genera (<I>Methanosarcina</I>, <I>Methanobrevibacter</I>, and <I>Methanobacterium</I>) were the abundant microbial communities in all tested digestions. The statistical analysis concludes that anaerobic digestion can homogenize the feedstocks to generate solid digestates with uniform-format carbohydrate composition and similar mono-sugar conversion. The mean cellulose and xylan contents of the solid digestates were 26.6% and 15.2%, respectively. The corresponding mean glucose and xylose conversions of the solid digestates were 82.3% and 98.7%.</P> <P><B>Highlights</B></P> <P> <UL> <LI> AD can be used as a pretreatment unit to generate a uniform-format cellulosic feedstock. </LI> <LI> The mixture of dairy manure and switchgrass had better methane production. </LI> <LI> More corn stover and switchgrass in the mixture led higher TS and VS reduction. </LI> <LI> Microbial communities shifted greatly to adapt to different feedstock conditions. </LI> <LI> Uniform-format solid digestates are amendable for fuel and chemical production. </LI> </UL> </P>