Integration of anaerobic co-digestion of grass silage and cattle slurry within a livestock farming system in Ireland: Quantification of greenhouse gas emission reduction and nutrient flow

Pasture-based beef systems can provide cattle slurry and grass silage for anaerobic digestion (AD). However, sustainable production of these feedstocks within a livestock farm presents challenges. This study examined the farmland area required to provide slurry and grass silage for a 40 GWh biomethane plant, quantified greenhouse gas (GHG) emissions reduction and analyzed digestate management. Results indicated that 130 farms of 50 ha and a livestock unit (LU) of 2.1 LU/ha were required to meet the feedstock requirements of the AD plant. Assigning 15 % of the farmland to produce grass silage for AD decreased GHG emissions by 24 % compared to conventional beef farming. Depending on soil nutrient status, returning digestate to the farmland reduced the application of nitrogen, phosphorus and potassium fertilizers by up to 65 %, 33 % and 56 %, respectively. Future research should analyze the economic implication of biomethane production from these feedstocks at farm and supply chain scales.