Response of three annual weeds to corn population density and nitrogen fertilization timing

The limited understanding of soil nitrogen (N) effects on crop yield loss to weeds prevents the refinement of integrated weed management to consider soil fertility. A 3-yr field experiment compared ammonium nitrate applications before corn emergence (PRE N) or in late June (POST N) on the growth of giant foxtail, velvetleaf, common waterhemp, and grain corn. The N responses were studied in corn population densities of 5.4 and 7.9 plants m(-2) to understand how crop density might influence N effects on yield loss. In late June of each year, leaf areas were 25% (corn), 64% (velvetleaf), and 41% (giant foxtail) lower with POST N than with PRE N. Common waterhemp leaf area in late June was unaffected by N timing. Corn population density did not affect weed shoot dry weights evaluated in late June. Corn and velvetleaf heights in late July were up to 19 and 48% lower, respectively, with POST N compared with PRE N. Common waterhemp and giant foxtail heights in late July were unaffected by N timing. Corn yield was decreased 13 to 18% by giant foxtail with POST N compared with PRE N. Velvetleaf seed yield was lower, but giant foxtail seed yield was higher, with POST N than with PRE N. Velvetleaf and giant foxtail seed yields were 23 to 56% and 30 to 62% lower, respectively, with the high corn population density compared with the low population density. In competition between corn and common waterhemp, neither N timing nor corn population affected corn yield loss or common waterhemp seed production. N management may alter weed competitiveness and weed seed production in corn, but fertilization recommendations may vary with the weed species that are present.