Agronomic management to improve corn productivity under high-yielding environments

Corn (Zea mays L.) grain yields have increased significantly in the U.S. since the 1930s largely due to genetic improvement and better crop management. Three important management decisions a corn grower makes today are: 1) which hybrid to plant, 2) what population to plant, and 3) what nitrogen (N) program to use. Hybrid selection is a critical management decision made by farmers because for any given year the spread in grain yield among current commercial hybrids that year can be greater than 100 bu acre-1. In addition, hybrids vary substantially in their response to management factors such as population and row spacing. Characterizing hybrids phenotypically for their yield-response to different plant spatial arrangement allows breeders, seed advisors, and farmers to predict which hybrids would have a positive yield-response to increased plant populations and narrower row spacings. Adequate fertility and plant nutrition, especially N, become even more important under these more intensive management systems. Better N placement and timing using the correct source can improve nutrient use efficiency and corn grain yield. For these reasons, the objective of this research was to quantify and predict how agronomic and nutritional management practices can be employed to improve corn productivity under high-yielding environments which encompasses three research areas: How Does Plant Spatial Arrangement Affect Plant Architecture, Growth and Development, and Grain Yield? Narrower row spacings were documented as a viable method to manage greater plant populations by increasing the plant-to-plant spacing within the crop row. As plant population increased, the yield difference due to row spacing increased. Changes in the architecture of the plant in response to narrower row spacings allowed for greater light penetration into the lower canopy when crowded at the higher population. Under competitive environments, (i.e. high plant populations) when resources became limited, plants produced more above-ground biomass at the expense of below-ground biomass. However, plants grown in a narrower spacing allocated more energy to producing below-ground biomass instead of above-ground biomass, subsequently reducing the shoot to root ratio. Which Phenotypic Traits Do Hybrids Possess That Helps Them Yield More When Grown at Increased Populations and Narrower Row Spacings? Of the six hybrids grown, three hybrids tended to be more positive yield-responsive to higher plant populations and narrower row spacings than the other three hybrids. In general, all six hybrids tended to have similar phenotypic responses to plant spatial arrangement. Thus, it was their inherently distinct phenotypic traits that differentiated them in their yield-response to plant population and/or row spacing alterations rather than the plasticity of their traits. The key traits for increased yields under increased plant populations and narrower row spacings were related to capturing more sunlight (leaf angle, leaf width, leaf length, and leaf area per plant), plant size (stover biomass per acre, total above-ground biomass per acre, and plant width), and root weight per plant. What is the Best Nitrogen Source, Placement, and Timing to Improve the Efficiency of Nitrogen Utilization and Grain Yield in Corn? Unfertilized check plots were used to determine the amount of N supplied from the soil during the entire growing season by measuring total N accumulation and grain yield at the R6 growth stage. Site-years that produced corn with low check plot yields tended to yield greater when more of the N was applied upfront at preplant compared to split applications of N. Under-fertilizing corn at an early growth stage resulted in irreversible yield loss that could not be recovered with a sidedress application. However, site-years with high check plot yields achieved greater yields with split applications of N. When making a split application, sidedressing urea ammonium nitrate (UAN) as Y-drop near the crop row was the best method and source of application. In general, the highest yielding N treatments received banded N at preplant as either urea or polyer-coated urea (PCU)