1. Adaptation Physiology Group, Wageningen Institute of Animal Sciences, Wageningen University, P.O. Box 338, 6700 AH, Wageningen, The Netherlands
2. Laboratory for Physiology and Immunology of Domestic Animals, Department of Biosystems, Division of Livestock-Nutrition-Quality, Catholic University Leuven, Kasteelpark Arenberg 30, B-3001 Leuven, Belgium