1. Agricultural Systems Unit, Walloon Agricultural Research Centre, 6800 Libramont, Belgium
2. Institute for Food and Resource Economics, University of Bonn, Nußallee 21, 53115 Bonn, Germany
3. INRAE, UMR Herbivores, F-63122 Saint-Genès-Champanelle, France
4. Council for Agricultural Research and Economics, 26900 Lodi, Italy