1. Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences, PO Box 5003, N-1432, Aas, Norway
2. Poultry Cooperative Research Centre, PO Box U242, University of New England, Armidale, NSW 2351, Australia
3. Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5A8, Canada