1. Department of Agricultural and Biological Engineering; University of Florida; Gainesville FL 32611 USA
2. Department of Geological Sciences and WK Kellogg Biological Station; Michigan State University; East Lansing MI 48824 USA
3. International Maize and Wheat Improvement Center (CIMMYT); Apdo. Postal 6-641 Mexico City 06600 Mexico
4. Department of Earth System Science; Stanford University; Stanford CA 94305 USA
5. Center on Food Security and the Environment; Stanford University; Stanford CA 94305 USA