The spent wheat straw from horse stall bedding has lower cellulose and hemicellulose contents, but higher volatile fatty acid content than raw wheat straw. Biogas production from solid-state anaerobic digestion (SS-AD) of spent wheat straw and raw wheat straw was compared in this study. The SS-AD tests were conducted at 22% total solids (TS) content using inoculum from a liquid AD system at three feedstock-to-inoculum (F/I) ratios of 2.0, 4.0, and 6.0. Daily methane yields of spent wheat straw peaked 8 and 3 days earlier than those of raw wheat straw at F/I ratios of 2.0 and 4.0, respectively. The highest methane yield of 150.0 L/kg volatile solids (VS) was obtained from spent wheat straw at an F/I ratio of 4.0, which was 56.2% higher than that of raw wheat straw. The corresponding cellulose and hemicellulose degradation of spent wheat straw was 24.1% and 49.4% higher than those of raw wheat straw, respectively. (C) 2011 Elsevier Ltd. All rights reserved.