Iowa corn yield from 1960 to 2008 increased an average of 122 kg ha-1 yr-1
(USDA-NASS, 2008), yet, maximum yield per plant, under non-stress conditions, has not
increased (Duvick et al., 2004; Tollenaar and Lee, 2002). When planted under nonstressful
conditions created by extremely low stand densities, hybrid differences were not
attributed to year of hybrid introduction (Duvick, 1997). At low stand densities,
maximum yield per plant is achieved, indicating that there is not necessarily a
relationship between high yield per area and high individual plant yield (Duvick et al.
2004).