An understanding of nitrogen (N) assimilation and biomass accumulation of weeds and corn (Zea mays L.) will help develop N management and weed control strategies to improve crop competitiveness. A field study was established to determine the critical weed removal timing at four preplant N application rates (0, 67, 134, and 202 kg N ha-1). Weed removal timings were based on average weed canopy height (0, 5, 10, and 20 cm). The critical weed removal timings were 3.7-, 11.7-, 10.3-, and 15.9-cm weeds when corn was grown at 0, 67, 134, and 202 kg N ha-1, respectively. Results of this study suggest that weeds need to be removed earlier when N is applied at lower rates. In the same study, the total N concentration in common lambsquarters (Chenopodium album L.), common ragweed (Ambrosia artemisiifolia L.), giant foxtail (Setaria faberi Herrm.), and corn was measured at several growth stages. A quadratic model significantly described the relationship between total N concentration and growth stage for most treatment combinations. Total N concentration increased from 5 to 10 cm weed heights and decreased as weed height increased from 10 to 20 cm.Weeds assimilated a large amount of N. A laboratory experiment measured N mineralization from common lambsquarters, common ragweed, and giant foxtail. Weeds were grown in the field at four N rates (0, 67, 134, or 202 kg N ha-1) and collected at two heights (10 or 20 cm). Nitrogen mineralization from weed residue mixed with soil was determined over a 12-week period. Nitrogen mineralization was rapid up to 4 weeks of incubation after which mineralization plateaued. Across treatments, net N mineralization occurred by week 12 of incubation. However, prior to 12 weeks of incubation, N was immobilized by giant foxtail grown under no N application. Weeds that are controlled by 10 cm height may contribute to the available soil N pool. However, this practice is not recommended due to competition between weeds and crops for other factors such as water and light which may reduce yield.A greenhouse study was conducted to evaluate the effect of weed density and N application rate on weed growth and N assimilation of common lambsquarters, a C3 species, and redroot pigweed (Amaranthus retroflexus L.), a C4 species. Study factors included four weed densities (1, 2, 4, and 8 plants pot-1), three N application rates (0, 67, and 134 kg N ha-1). Redroot pigweed shoot biomass was greater than common lambsquarters, indicating that redroot pigweed may be more competitive than common lambsquarters. Weeds grown at low N and low density exhibited greater root biomass compared to weeds grown under higher N application rates and densities. Nitrogen assimilation was greater for redroot pigweed than common lambsquarters.Common lambsquarters is a highly competitive weed in crop production systems and is considered to be highly responsive to N application. A two-year field study was established to measure N assimilation by common lambsquarters and examine the effect of common lambsquarters on corn grain yield. Study factors included common lambsquarters (presence or absence) and sidedress N application rate (0, 56, 112, 168, or 224 kg N ha-1). In 2009, biomass and total N concentration in common lambsquarters increased with N application rate. In 2010, there was no significant difference in common lambsquarters biomass or total N concentration. In both years, corn grain yield was not influenced by the presence of common lambsquarters.
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