Biomass from permanent grasslands and meadows can be exploited for biogas production, because this substrate is abundant and does not compete with food production. In the present study, the biogas productivity of meadow grass silage, harvested in two different seasons (early and late Cut), was investigated. The grass silage was mechanically pretreated with different methods to increase its biodegradability. It was found that the early cut of non-treated meadow grass silage led to higher methane production [294 mL of CH4/g of volatile solids (VS)] compared to the corresponding non-treated meadow grass silage from the late cut (282 mL of CH4/g of VS). Moreover, it was found that the application of two mesh grating plates, as the pretreatment method, greatly enhanced the methane production in early and late cut silage in a range of 15 and 17%, respectively, compared to the non-treated grass silage. The methane productivity from pretreated meadow grass silage, harvested at fall (late cut), was further examined in a co-digestion process with three different types of livestock manure (mink, poultry, and cattle). The silage was co-digested with manure in five different manure/silage mixing ratios in terms of organic matter. The results showed that the optimum silage concentration in the co-digestion mixture with manure, for the highest methane yield, was strongly dependent upon the chemical composition of the manure. More specifically, the ammonia concentration of manure and the C/N ratio of the co-digestion mixture were found to be the key parameters for an improved biomethanation process.