Since the introduction of hybrid corn in the 1930s, the corn grain yield trend has been increasing in Iowa and across the U.S. Corn Belt. In Iowa, there has been an increase of corn plant densities of approximately 400 plants per acre per year since 2000. Corn grain yield and yield components are greatly influenced by plant densities. Corn grain yields respond to plant densities with a curvilinear or quadratic response if plant densities are increased to supra-optimum densities.