1. Nutrition and Food, Animal Sciences Group, P.O. Box 65, NL-8200 AB Lelystad, The Netherlands
2. Crop and Weed Ecology Group, Wageningen University, Wageningen, The Netherlands
3. Present address: Applied Research, Animal Sciences Group, Wageningen University and Research Centre, P,O, Box 2176, NL-8203 AD Lelystad, The Netherlands