SIDEDRESS NITROGEN APPLICATION RATE AND COMMON LAMBQUARTERS EFFECT ON CORN GRAIN YIELD. L. E. Bast*, W. J. Everman, D. D. Warncke; Michigan State University, East Lansing, MI (336)

Early postemergence herbicide applications may result in weed re-infestations from late- emerging species, which compete with corn for soil nitrogen. A field study was conducted in irrigated corn (Zea mays) at the Montcalm Research Farm 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 randomized complete block design was used. Factors included the presence or absence of common lambsquarters and 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 V6/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 corn 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 0 to 56 kg N ha^-1 was applied. There was no difference in grain yield when 56-224 kg N ha^-1 was applied. In 2010, there was no difference in nitrogen assimilation by common lambsquarters and grain yield was not impacted by the presence of common lambsquarters. Grain yield was greatest when 168 kg N ha^-1 was applied. Our 2009 results indicate that the presence of later-emerging common lambsquarters may reduce corn grain yields; however, these effects were not mitigated with sidedress nitrogen applications. In 2010, weather conditions were more favorable for corn growth development than in 2009. Common lambsquarters had no impact on yield in 2010, indicating that the crop may have been able to out-compete weeds better than in 2009.