1. Institut National de Recherche Agronomique (INRA), Centre de Clermont-Ferrand-Theix, Unité de Recherches sur les Herbivores, 63122 Saint-Genès Champanelle, France
2. Wageningen Institute of Animal Sciences, P.O. Box 338, NL-6700 AH Wageningen, The Netherlands
3. Animal Sciences Group, Wageningen University and Research Centre, Nutrition & Food, P.O. Box 65, NL-8200 AB Lelystad, The Netherlands