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.

Nomenclature: Common waterhemp, Amaranthus rudis L. Sauer AMATA; giant foxtail, Setaria faberi L. Herrm. SETFA; velvetleaf, Abutilon theophrasti L. Medicus ABUTH; corn, Zea mays L. ‘Pioneer 33V08’ ZEAMD.