1. Animal and Veterinary Science Group, Scotland's Rural College, King's Buildings, Edinburgh, EH9 3JG, UK
2. Division of Genetics and Genomics, The Roslin Institute, R(D)SVS, University of Edinburgh, Easter Bush, Midlothian, EH25 9RG, UK
3. UMR 1213 Herbivores, Institut National de la Recherche Agronomique Genès, F-63122 St. Genés Champanelle, France