1 Animal Nutrition Group, Wageningen University and Research, PO Box 338, 6700 AH Wageningen, the Netherlands.
2 PEGASE, Agrocampus Ouest, INRA, 35590 Saint Gilles, France.
3 School of Agriculture, Policy and Development, University of Reading, PO Box 237, Reading RG6 6AR, United Kingdom.
4 Institute of Nutritional Physiology, Leibniz Institute for Farm Animal Biology (FBN), 18196 Dummerstorf, Germany.
5 Department of Animal Science, Aarhus University, PO Box 50, 8830 Tjele, Denmark.
6 Animal Nutrition Group, Wageningen University and Research, PO Box 338, 6700 AH Wageningen, the Netherlands. Electronic address: jan.dijkstra@wur.nl.