1. Department of Agricultural and Food Economics, Agriculture and Food Science Centre, Queen's University Belfast, Newforge Lane, Belfast BT9 5PX, Northern Ireland
2. Department of Land Economy, University of Cambridge, 19 Silver Street, Cambridge CB3 9EP, England
3. Biometrics Division, Department of Agriculture and Rural Development (NI), Agriculture and Food Science Centre, Newforge Lane, Belfast BT9 5PX, Northern Ireland