Effect of Sidedress Nitrogen Application Rate and Presence of Common Lambsquarters On Corn Grain Yield.

Early postemergence herbicide applications may result in weed re-infestations by late- emerging species, which compete with corn for soil nitrogen.  A field study was conducted in irrigated corn at the Montcalm Research Station near Entrican, MI in 2009 and 2010 to examine the effect of sidedress nitrogen application rate and the presence of common lambsquarters (Chenopodium album) on corn grain yield.  A split-plot, completely randomized design was used with the main factor being the presence or absence of common lambsquarters and subplot factor consisted of 5 sidedress nitrogen application rates of 0, 56, 112, 168, and 224 kg N ha^-1.  At planting, 78 kg N ha^-1 was applied to all treatments.  At the V7 corn growth stage, weed re-infestation was simulated by transplanting common lambsquarters (2-5 cm height) into corn rows at 5 plants m^-1 and nitrogen fertilizer was applied.  Weed density was maintained throughout the growing season.  Corn and weed chlorophyll measurements were recorded at silking.  Above-ground common lambsquarters biomass was collected from two 1.5 m sections, fresh and dry weights recorded, and analyzed for total nitrogen content.  Grain yield was determined at harvest.  In 2009, chlorophyll content of corn and common lambsquarters increased with sidedress nitrogen application rate.  Common lambsquarters biomass was greatest when 168 and 224 kg N ha^-1 was applied and contained the highest percentage of nitrogen when 112-224 kg N ha^-1 was applied.  The presence of common lambsquarters and sidedress nitrogen rate both influenced grain yield, but there was no significant interaction.  Grain yield decreased by 251 kg ha^-1 when common lambsquarters were present in the corn rows.  Grain yield increased when 56 kg N ha^-1 was applied compared to no nitrogen application; however, there was no difference in grain yield at rates between 56-224 kg N ha^-1.  Our 2009 results indicate that the presence of later-emerging common lambsquarters may reduce corn grain yield, and these effects were not mitigated with sidedress nitrogen application.