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Title:   Author  item
                                            BREKKE, BRENT - Iowa State Universityitem
                                            Edwards, Jodeitem
                                            KNAPP, ALLEN - Iowa State University
  Submitted to: Crop Science
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 10/17/2010
                                    Publication  Date: 9/1/2011
                                    Citation: Brekke, B.H., Edwards, J.W., Knapp, A. 2011. Selection and adaptation to high plant density in the Iowa Stiff Stalk synthetic maize (Zea mays L.) population. Crop Science. 51(5):1965-1972.Interpretive Summary: Maintaining or increasing present rates of genetic improvement in corn hybrid performance will require corn breeders to better understand the basis for past gains and to expand the diversity of currently available corn lines and populations. However, nearly all diverse sources of corn breeding populations are very poorly adapted to the high plant densities that farmers currently plant. We evaluated the improvement in adaptation to high plant density in populations that have been selected over 70 years of corn breeding in order to identify specifically how these corn populations have become adapted to high plant densities. We have found that populations that have undergone long-term selection have the highest grain yield at the highest plant densities while at the same time do not differ among densities in their rates of lodging, test weight, or grain moisture. We found little improvement with selection in grain yield when planted at low plant density. The results suggest that expanding diversity of populations used in corn breeding will require adapting populations with no breeding history to high plant densities as a first step. Utilization of available diversity of corn populations for agricultural purposes has been extremely difficult, and thus new approaches are needed.
                                Technical Abstract: 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, and stalk and root lodging 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. Root lodging has remained unchanged. 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.


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