1. Economic Analysis Unit, Agricultural Research Council, 1134 Park Street, Hatfield, Pretoria, 0083
2. Agricultural Science and Technology Indicators (ASTI), International Food Policy Research Institute, 1201 Eye Street, NW, 20005 Washington
3. Agricultural Science and Technology Indicators (ASTI), International Food Policy Research Institute, 1201 Eye Street, NW, 20005 Washington, DC
4. Department of Agricultural Economics, Extension and Rural Development, University of Pretoria