Corn yield is determined by soils, weather, seed used and other technology choices. Global population and per capita income growth trends as well as demand from the energy sector have placed great stress on cropland use. Global cropland acres and/or yield per acre will need to increase. Whether new seed technologies have enhanced corn yield is a controversial issue. We study U.S. county corn yields 1964-2008, controlling for location effects, fertilization technologies and weather. We find evidence that trend yield growth has been fastest in the Central Corn Belt, genetic modification technologies have increased trend yield, and this increase has been largest in the Central Corn Belt.