Abstract 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 CH 4 and H 2 S 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 CH 4 potential than dairy manure on a volatile solids basis. While no synergistic effect on CH 4 production resulted from co-digestion, increasing the radish fraction in the co-digestion mixture significantly increased CH 4 production. Initial H 2 S production increased as the radish fraction increased, but the sulfur-containing compounds were rapidly utilized, resulting in all treatments having similar H 2 S concentrations (0.10–0.14%) and higher CH 4 content (48–70%) in the biogas over time. The 100% radish digester had the highest specific CH 4 yield (372 ± 12 L kg −1 VS). The co-digestion mixture containing 40% radish had a lower specific CH 4 yield (345 ± 2 L kg −1 VS) but also showed significantly less H 2 S production at start-up and high quality biogas (58% CH 4 ). Results from BMP2 showed that the radish harvest date (October versus December) did not significantly influence radish C:N mass ratios or CH 4 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 CH 4 production of manure digesters in the fall/winter.