1. Crop and Weed Ecology Group, Wageningen University, P.O. Box 430, NL-6700 Wageningen, The Netherlands
2. Biological Farming Systems Group, Wageningen University, Marijkeweg 22, NL-6709 PG Wageningen, The Netherlands
3. Teagasc, Johnstown Castle Research Centre,Wexford, Ireland
4. Department of Animal Science, Cornell University, Ithaca, NY 14853, USA