Estimating Crop Yield Potential At National Scales.

World population will increase 35% by 2050, which requires doubling crop yields on existing farm land to avoid expansion of agriculture into remaining rainforests, wetlands, and grasslands. Whether this is possible depends on closing the gap between yield potential (Yp, yield without stress) and current average farm yields in both developed and developing countries. Quantifying the yield gap is therefore essential to inform policies and prioritize research to achieve food security without environmental degradation. Previous attempts to estimate Yp have been too coarse, too general, and opaque. We therefore developed a protocol to overcome these limitations. Simulated Yp estimates required specific information on crop management and +20 years of observed daily climate data from weather stations in major crop production areas. National Yp estimates were weighted by potential production within 100-km of reference weather stations (RWS). Sensitivity analysis found this protocol substantially reduces uncertainty of Yp estimates.  In simulating Yp of irrigated rice (China), irrigated and rainfed maize (USA) and rainfed wheat (Germany), more than 50% of total harvested area fell within 100-km of RWS.  This protocol is appropriate for any country and all crops for which a robust simulation model is available. Results suggest average farm yields plateau when they reach 75-85% of estimated national Yp, which appears to occur for rice in China, wheat in Germany, and irrigated maize in USA. Prediction of when average crop yields will plateau in other countries is now possible based on the estimated Yp ceiling using this protocol.