No-tillage field studies were conducted in 1999 and 2000 at Columbia, MO, to determine the interaction of grass weed interference and side-dressed N fertilization on corn and weed growth, corn yield, and the N content of the soil and plant biomass at various intervals early in the growing season and at harvest. Ammonium nitrate (112 kg N ha−1) was applied before planting. A herbicide was applied to the entire experimental area before planting to control winter vegetation and to reduce broadleaf weed emergence. A mixture of large crabgrass, giant foxtail, and barnyardgrass was allowed to reinfest the experiment after corn planting and was sprayed with glyphosate when the weeds were 8, 15, 23, or 31 cm tall. Each removal date treatment was duplicated; one series received 45 kg N ha−1 side-dressed when the corn was approximately 60 cm tall, and the other series received no additional N. Corn dry weight and the N content of the corn biomass early in the growing season were similar between the weed-free control and treatments with grass interference up to 23-cm height. Side-dressed N applications before grass weeds were controlled did not increase the early-season N content of the corn biomass in 1999, but the N content of the grass weeds increased by 11 kg ha−1. In 2000, side-dressed N increased the N content of the corn and grass biomass by 13 and 18 kg ha−1, respectively. This suggests that grass weeds should be controlled before the side-dressed N applications to ensure that the N applied can be used by the corn rather than by the grass weeds. Grass weed interference beyond 15-cm height reduced corn yield by at least 1.13 Mg ha −1 and N content in the corn biomass at corn harvest by at least 30 kg ha−1.
Nomenclature: Glyphosate; corn, Zea mays (L.) ‘Dekalb 626RR’; giant foxtail, Setaria faberi (L.) Herrm. SETFA; large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG.