1. Adaptation Physiology Group, Department of Animal Sciences, Wageningen University and Research Centre, P.O. Box 338, 6700 AH Wageningen, The Netherlands
2. Animal Breeding and Genomics Centre, Department of Animal Sciences, Wageningen University and Research Centre, P.O. box 338, 6700 AH Wageningen, The Netherlands
3. Department of Equine Sciences, Veterinary Faculty, Utrecht University, P.O. Box 80.163, 3508 TD Utrecht, The Netherlands