Forage radish, a winter cover crop, was investigated as a co-substrate to increase biogas production from dairy manure-based anaerobic digestion. Batch digesters (300 cm(3)) were operated under mesophilic conditions during two experiments (BMP1; BMP2). In BMP1, the effect of co-digesting radish and manure on CH4 and H2S production was determined by increasing the mass fraction of fresh above-ground radish in the manure-based co-digestion mixture from 0 to 100%. Results showed that forage radish had 1.5-fold higher CH4 potential than dairy manure on a volatile solids basis. While no synergistic effect on CH4 production resulted from co-digestion, increasing the radish fraction in the co-digestion mixture significantly increased CH4 production. Initial H2S production increased as the radish fraction increased, but the sulfur-containing compounds were rapidly utilized, resulting in all treatments having similar H2S concentrations (0.10-0.14%) and higher CH4 content (48-70%) in the biogas over time. The 100% radish digester had the highest specific CH4 yield (372 +/- 12 L kg(-1) VS). The co-digestion mixture containing 40% radish had a lower specific CH4 yield (345 +/- 2 L kg(-1) VS) but also showed significantly less H2S production at start-up and high quality biogas (58% CH4). Results from BMP2 showed that the radish harvest date (October versus December) did not significantly influence radish C:N mass ratios or CH4 production during co-digestion with dairy manure. These results suggest that dairy farmers could utilize forage radish, a readily available substrate that does not compete with food supply, to increase CH4 production of manure digesters in the fall/winter. (C) 2015 Elsevier Ltd. All rights reserved.