The core aim of this thesis was to quantify the effects of co-digesting forage silages with animal slurries on methane yields and to investigate if antagonistic or synergistic outcomes occur. In order to complete this assessment, the economic impacts of changing forage silage characteristics, of changing the mixing ratios of forage silage and cattle slurry in binary mixtures (and the presence of synergy or antagonism) and of changing the costs of providing these feedstocks for anaerobic digestion (AD) on the cost of methane production in an on-farm AD facility were accessed. An initial objective, however, was to define an optimal methodology for laboratory-scale anaerobic digestion, specifically to determine the impact of altering the headspace volume within incubation bottles and the overhead pressure measurement and release (OHPMR) frequency on methane yield using a manual manometric biochemical methane potential (mBMP) batch digestion method. Two anaerobic batch co-digestion experiments were conducted with forage silages and animal slurries. In the first experiment, oven-dried perennial ryegrass (harvested at two growth stages) or red clover (harvested at two growth stages) silages and cattle slurry were co-digested. Each binary mixture had synergistic effects which resulted in 2.8-7.5% higher methane yields than predicted from mono-digestion of individual substrates. In the second experiment, cattle slurry (two types) or pig slurry was co-digested with undried perennial ryegrass silages (harvested at two growth stages). Each silage and slurry mixture had antagonistic effects which resulted in methane yields 5.7-7.6% below those predicted from mono-digestion of individual substrates. In the initial experiment and in order to broaden the conditions under which the assessment was made, the biogas and methane yields of cellulose, barley grain, grass silage and cattle slurry were determined in response to three incubation bottle headspace volumes and four OHPMR frequencies. The methane yields of barley, silage and slurry were also compared with those from an automated volumetric method (i.e. AMPTS). Headspace volume and OHPMR frequency effects on biogas yield were mediated mainly through headspace pressure, with the latter having a negative effect on the biogas yield measured but relatively little effect on methane yield. Two mBMP treatments that produced methane yields equivalent to AMPTS were identified. Economic modelling results showed significant impacts of AD feedstock characteristics and their provision cost on the cost of methane production in an AD facility. The feedstock provision cost contributed about half of the total cost of methane production when the AD facility solely operated on grass silage. The total cost of methane produced from mono-digestion of cattle slurry that was supplied free of charge was more than double the cost of methane produced from grass silage. For co-digestion of grass silage and cattle slurry, the total cost of methane production progressively increased as the proportion of slurry in the co-digested mixture increased. Antagonistic and synergistic methanogenesis resulted in a corresponding 6% higher and 5% lower total cost of methane production during co-digestion of grass silage and cattle slurry (at a silage:slurry volatile solids ratio of 0.8:0.2) compared to the binary mixture without these effects.