1. New South Wales Department of Primary Industries, Beef Industry Centre, University of New England, Armidale, NSW 2351, Australia; and CSIRO Agriculture and Food, FD McMaster Laboratory Chiswick, Armidale, NSW 2350, Australia
2. New South Wales Department of Primary Industries, Wagga Wagga Agricultural Institute, Wagga Wagga, NSW 2650, Australia
3. Department of Animal Science, Cornell University, Ithaca, NY 14853-4801, USA