1. Adaptation Physiology Group, Department of Animal Sciences, Wageningen University, P.O. Box 338, 6700 AH, Wageningen, The Netherlands
2. Denkavit Nederland B.V., P.O. Box 5, 3780 BA, Voorthuizen, The Netherlands
3. Animal Nutrition Group, Department of Animal Sciences, Wageningen University, P.O. Box 338, 6700 AH, Wageningen, The Netherlands