1. Animal Nutrition Group, Wageningen University, P.O. Box 338, NL-6700 AH Wageningen, The Netherlands
2. Animal Sciences Group, Wageningen University and Research Centre, Lelystad, The Netherlands
3. Present address: Laboratory of Microbiology, Wageningen University, Wageningen, The Netherlands
4. Applied Plant Research, Wageningen University and Research Centre, Lelystad, The Netherlands