1476 A life cycle assessment case study of the carbon footprint of high performance Irish, UK and USA dairy farms D. O’Brien 1 , J.L. Capper 2 , P.C. Garnsworthy 3 , C. Grainger 1 and L. Shalloo 1 1 Livestock Systems Research Department, Animal & Grassland Research and Innovation Centre, Teagasc, Moorepark, Fermoy, Co. Cork, Ireland. Corresponding author: D. O’Brien, Tel.: +353 25 42671; fax: +353 25 42340.E-mail address: donal.o’brien@teagasc.ie 2 Department of Animal Sciences, Washington State University, Pullman, USA 99164 3 The University of Nottingham, School of Biosciences, Sutton Bonington Campus, Loughbor- ough, LE12 5RD, United Kingdom Abstract: Life cycle assessment (LCA) is the accepted approach to simulate and compare carbon footprint (CF) of milk. The objective of this study was to apply LCA to compare CF of high per- formance confinement and grass-based dairy farms. Physical performance data from research herds were used to quantify CF of a high performance Irish grass-based dairy system and a top performing UK confinement dairy system. For the USA confinement dairy system, data from the top 5% of herds of a national database were used. Life cycle assessment was applied using the same dairy farm greenhouse gas (GHG) model for all systems. The model estimated all on and off-farm GHG sources associated with dairy production until milk is sold from the farm in kg of carbon dioxide equivalents (CO 2 -eq) and allocated emissions between milk and meat. The CF of milk was calculated by expressing GHG emissions attributed to milk per t of energy corrected milk (ECM). The comparison showed the CF of milk from the Irish grass-based system (837 kg of CO 2 -eq/t of ECM) was 5% lower than the UK confinement system (884 kg of CO 2 -eq/t of ECM) and 7% lower than the USA confinement system (898 kg of CO 2 -eq/t of ECM) when no GHG emissions were allocated to meat. However, without grassland carbon sequestration, the grass-based and confinement dairy systems had similar CF per t of ECM. Additionally, using different emission algorithms or methods to allocate GHG emissions between milk and meat af- fected the relative difference and order of dairy system CF. This indicates that further harmoniza- tion of several aspects of the LCA methodology is required to compare CF of divergent dairy systems. Relative to recent reports that assess the CF of milk from average Irish, UK and USA dairy systems, this case study indicates that top performing herds of the respective nations have CF about 30% lower than average systems. Although, differences between studies are partly ex- plained by methodological inconsistency, the comparison suggests that there is potential to re- duce the CF of milk in each of the nations by implementing practices that improve productivity. Keywords: CF, grass, confinement, milk production