We assess the potential for increasing the net amount of food calories produced by French agriculture and the possible implications in terms of greenhouse gas emissions and agricultural area allocation. This analysis is based on an agricultural supply model for the European Union mainly with regard to arable crops, meadows, fodder crops, and the main animal products. The model calculates the variations in agricultural greenhouse gas emissions associated with the required level of production. Within the framework of a prospective approach carried out under the technical and economic conditions of the period 2007–2012, we calculate the extent of the changes in an agricultural production system, to which we assign the objective of increasing the net production of calories. In France, for an increase of 40 to 60% depending on the year, three-quarters of meadows would disappear, a large proportion of temporary meadows would switch to cereals, and fallows could exceed 20% of the total agricultural area. These changes would result from the sharp fall in livestock, especially of cattle for meat. The key factor in the analysis is animal feed, which, in addition to the decrease in grass consumption, would lead to a slight increase in fodder and cereals produced and consumed on the farm, and a sharp decrease of around 50% in the purchase of concentrated feeds. The reduction in greenhouse gas emissions is substantial, at least in terms of methane, and could exceed 30% of reference emissions in carbon dioxide equivalent.