Given on the 16th June at FACCE-JPI Macsur LiveM conference "Modelling Grassland-Livestock Systems under Climate Change" 15-16th June, Potsdam, Germany Farm systems modelling is an important tool to quantify and understanding the impacts of management decisions on farm productivity and environmental burdens, such as greenhouse gas emissions. Predicted emission intensity varied little between models, from 0.98 to 1.02 carbon dioxide equivalents (kg milk)-1, corresponding to a variation of about ±5%. This similarity disguised much larger variations in the underlying sources. For the two largest sources (enteric methane and soil nitrous oxide), which accounted for on average 55% and 26% of the total GHG emissions respectively. The differences and limitations of the inter-comparison are discussed and ways forward are suggested.