It is safe to assume that the average corn yield in the U.S. has increased 5-fold over the last 70 years (NASS, 2004b). For example, in Indiana (Nielsen, 2002), the average corn yield was ∼30 bushels per acre in 1930 and 156 bushels per acre in 2001. The steadily improving yield resulted mostly from the increased fertilizer use and better corn genetics.