1. Respectively Department of Agricultural Economics, University of Pretoria, Pretoria 0001, Republic of South Africa and Centre for Research into Economics and Finance in Southern Africa, London School of Economics, Hougton St., London WC2A 2AE, United Kingdom and Department of Agriculture and Food Economics, University of Reading, 4 Earley Gate, Whiteknights Road, Reading RG6 2AR, United Kingdom.