1. Crop and Weed Ecology, Plant Sciences Group, Wageningen University, Haarweg 333, NL-6709 RZ Wageningen, The Netherlands
2. Biological Farming Systems, Plant Sciences Group, Wageningen University, Marijkeweg 22, NL-6709 PG Wageningen, The Netherlands
3. Teagasc, Johnstown Castle Research Centre, Wexford, Ireland