Economic and greenhouse gas emissions assessment of grass silage under different management scenarios in Irish grassland systems

Feed provision is the principal variable cost and a key source of greenhouse gas emissions (GHGE) on Irish grassland farms. Grass silage is the predominant conserved forage in Irish livestock systems and management choices such as cutting frequency, conservation method and fertilisation strategy affect both costs and emissions. The Grange Feed Cost Model (GFCM), developed by Teagasc, is a simulation model which integrates agronomic outputs and resource use to calculate total feed cost from crop establishment to feeding; however, it does not include environmental indicators. The objective of this study was to augment the existing model with a GHGE module enabling scenario-based assessment. The economic module accounts for variable and fixed costs and reports results per hectare (ha), per tonne (t) of dry matter fed (DM) and per GJ of metabolisable energy fed (ME). To implement the GHGE module, ISO 14040/44 principles were followed using an inventory-based cradle- to-feeding boundary. The global warming potential (GWP) was calculated using IPCC (AR6) 100-year characterisation factors for carbon dioxide (CO2), methane and nitrous oxide. Emission factors for upstream inputs were derived from established Life Cycle Inventory databases (Ecoinvent and Agri-footprint) or the literature. Forage production scenarios included perennial ryegrass only (PRG), harvested once or twice, and PRG plus red clover (RC) harvested twice or three times, annually. In terms of management, two conservation methods (pit or baled silage) and two fertilisation strategies (inorganic fertiliser with slurry (WS) or without slurry (WO)) were evaluated. Under the one-cut PRG pit scenario, GHGE were 1.53 t CO2-eq/ha (31.6 g CO2-eq/MJ ME) for WS and 1.55 t CO2-eq/ha (31.9 g CO2-eq/MJ ME) for WO. Total costs were 307 €/t DM (28.9 €/GJ ME) and 325 €/t DM (30.6 €/GJ ME), respectively. In the two-cut PRG pit scenario, GHGE increased to 2.59 t CO2-eq/ha (32.4 g CO2-eq/MJ ME) under WS and 2.70 t CO2-eq/ha (33.8 g CO2-eq/MJ ME) under WO, with total costs of 339 €/t DM (32.2 €/GJ ME) and 354 €/t DM (33.6 €/GJ ME), respectively. Baled silage increased GHGE and cost per ha by approximately 7% but reduced GHGE and cost per MJ of ME fed by 3% compared with pit conservation. Across PRG scenarios, slurry use reduced GHGE by 3% and total costs by 4%. RC swards exhibited markedly lower GHGE. Two-cut RC pit silage produced 1.36 t CO2-eq/ha (16.2 g CO2-eq/MJ ME fed), while baled silage produced 1.42 t CO2-eq/ha (14.4 g CO2-eq/MJ ME fed). Total costs were 251 €/t DM (23.8 €/GJ ME fed) for pit and 252 €/t DM (23.7 €/GJ ME fed) for baled silage. Three-cut RC produced 1.69 t CO2-eq/ha for pit and 1.74 t CO2-eq/ha for baled silage, at a total cost of 267 €/t DM and 258 €/t DM, corresponding to 24.5 and 25.4 €/GJ ME fed, respectively. For the conditions considered, the most favourable combined economic and environmental performance was observed for RC two-cut pit silage WS. The least favourable economic performance was associated with PRG two-cut baled silage WO, which showed the highest costs per hectare and per unit of energy fed. The highest GHGE per MJ ME fed was observed for PRG two-cut pit silage WO. Compared with PRG, RC reduced GWP by over 50%, while maintaining competitive or lower costs per unit of energy. Overall, systems with higher harvest frequency and greater input use increased GHGE and costs per hectare, while differences per ME fed remained limited. Integrating economic and GHGE metrics highlights how functional unit selection shapes mitigation assessment in grassland systems.