1. USDA Economic Research Service, 805 Pennsylvania Ave, Kansas City, MO 64105, USA
2. University of Connecticut, Agricultural and Resource Economics, Storrs, CT, USA
3. North Carolina State University at Raleigh, Department of Crop and Soil Sciences, Raleigh, NC, USA
4. Council for Scientific and Industrial Research, Crops Research Institute, Accra, Ghana
5. Kwame Nkrumah University of Science and Technology, College of Science, Kumasi, Ghana