The effect of different inoculum-to-substrate ratios (ISRs) and feed mix (FM) ratios on the kinetics of methane production and yields during anaerobic digestion of spoiled silage mixture (SM) alone or co-digestion with cow manure (CM) was investigated in batch experiments at 37 °C. The silage mixture was prepared from spoiled silages of maize, lucerne and barley in equal proportions of 33% by wet weight. The effect of ISRs of 0.5, 1, 2 and 4 showed that methane yields increased with an increased ISR ratio. At ISRs of 1, 2 and 4, methane yields of 262.18 ± 14.96, 387.77 ± 14.43 and 482.23 ± 38.47 NmL CH4/gVSadded were obtained, respectively. Incubation at ISR 0.5 resulted in low methane yields (174.49 ± 9.29 NmL CH4/gVSadded) due to build-up of volatile fatty acids (VFAs). Further, co-digestion of spoiled SM with CM showed that the highest methane yields of 387.77 and 382.86 NmL CH4/gVSadded were obtained at SM:CM feed mix ratios of 100–0 and 75–25, respectively. The corresponding volatile solids (VS) removal rates were 72.80% and 70.82%, respectively. However, the best synergistic effect was noticed at a SM:CM = 50–50 feed mix ratio. Thus, this study shows that anaerobic digestion of spoiled silages is feasible and co-digestion of spoiled silage mixed with cow manure at a SM:CM feed mix ratio of 75–25 is recommended.