Agricultural activities are important sources of atmospheric nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2). In the European Union, the agricultural sector contributes approximately 10% of the total emitted greenhouse gases (GhG). We search for evaluating GhG emissions at the farm level and desiging mitigation options from a biotechnical point of view. As GhG emissions occur at several stages within the farm operation, it is essential to combine nutrient cycling and farm working through an integrated farm approach to reach a full accounting of processes. Our proposal comprises the following characteristics: - Our whole farm approach (FarmSim) combines empirical and mechanistic modelling through describing of above and below ground C and N fluxes ; with respect to GhG, emissions can be also calculated with emissions factors, comparable to the IPCC methodology when no model is available yet as for manure management for instance. - Rebuilding farmers' practices description from available data (on farm and through farms'network databases) needs to choose a level of information to translate real situations to generic ones. It requires thus to choose a set of farm components and compatible spatio-temporal scales in respect with the data needed by the models. - At the moment, we coupled two models allowing to quantify the emissions of the different land uses (in our case PaSim, a grassland ecosystem model and CERES-EGC, a crop model). - The system boundaries are limited to the farm gate, or may include pre- and post-chain effects according to more and less high inputs and the difficulty to forecast produce fate if databases are available. - Building plausible scenarios requires to control a set of farm operation coherences to account for standing mitigation options. In a first step, we used a case study formalized by the French Livestock Institute (IE) and some farm working data from an experimental dairy farm belonging to the French Institute for Agricultura