The plant density at which Zea mays L. hybrids achieve maximum grain yield has increased throughout the hybrid era while grain yield on a per plant basis has increased little. Changes in plant traits including grain yield, moisture, test weight, stalk and root lodging, flag leaf angle, anthesis-silking interval(ASI), plant height, tassel branch number, and total number of leaves have been well characterized in comparisons of commercial hybrids representing different eras of hybrid maize production but have yet to be examined in a recurrent selection program.

The objective of this experiment was to determine if direct selection for grain yield and agronomic performance in the Iowa Stiff Stalk synthetic population has indirectly improved adaptation to high plant density. Material from an unselected base population, Iowa Stiff Stalk Synthetic (BSSS), was compared to the most advanced cycles of selection from two different recurrent selection programs at seven Iowa locations in 2008 and 2009.

Populations were compared at densities of 38,300, 57,400, 77,500, and 95,700 plants ha-1. Treatments were replicated twice at each location and arranged in a split plot design. Increasing density in advanced populations led to increased yield unlike the yield decrease seen in less advanced populations at high density, indicating an adaptation to high plant density. Increasing density in advanced populations did not increase grain moisture, test weight, or stalk lodging supporting our hypothesis of increased adaptation to high plant density in more advanced populations. Root lodging has remained unchanged.

Advanced populations had reduced ASI. Plant density did not affect flag leaf angle which became more vertical in advanced populations. Increasing plant density in advanced populations increased plant height while not effecting ASI or tassel branch number; supporting our hypothesis of increased adaptation to high plant density.