1. Moredun Research Institute, Penicuik EH26 0PZ, UK
2. Estación Experimental Mercedes, Instituto Nacional de Tecnología Agropecuaria (INTA), Corrientes W3400, Argentina
3. Biomathematics and Statistics Scotland, Edinburgh EH9 3FD, UK
4. Royal (Dick) School of Veterinary Studies, University of Edinburgh, Edinburgh EH10 5HF, UK