1. Economic Research Service; USDA; 355 E Street, SW Washington, DC 20024 USA
2. World Agricultural Economic and Environmental Services; 3215 S. Providence Rd., Suite C Columbia MO 65203 USA
3. Department of Agricultural and Applied Economics; Texas Tech University; Lubbock TX 79409 USA
4. School of Agricultural Economics and Rural Development; Renmin University; Beijing 100872 China