1. Department of Veterinary and Animal Sciences, University of Copenhagen, Grønnegaardsvej 3, 1870 Frederiksberg C, Denmark.
2. Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences, P.O. Box 5003, 1432 Ås, Norway.
3. School of Animal Biology, University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia.
4. Department of Animal Science, Aarhus University, Blichers Allé 20, 8830 Tjele, Denmark.