Effect of fertilizer nitrogen on weed emergence and growth

The timing of nitrogen (N) fertilizer application may influence germination, emergence, and competitiveness of weeds. Research was conducted to determine the influence of total inorganic soil N (N_it) on the germination, emergence, and growth of five weed species. In a greenhouse experiment, seed of five weed species were exposed to four levels of N, and seed germination was measured. In the field, urea ammonium nitrate (UAN 28%) was applied at multiple rates at three spring timings, and N_it, weed emergence, and growth were measured for 21 to 35 d after application (DAA). Germination of the four dicotyledonous and single grass species was not stimulated by 450 ppmw of N compared with the untreated control. In the field, N_it of 112 or 168 kg N ha^-1, measured at 7 and 21 DAA, was always greater than N_it in the untreated control. The duration of the available N pulse in the upper 8 cm of soil was dependent on N application rate and timing. At 8 to 16 cm of soil depth, N_it was greater when 168 kg N ha^-1 was applied compared with no N at 21 and 35 DAA in 2004. Emergence of common lambsquarters increased as N application rate increased for each application date in 2003, but not in 2004. Emergence of ladysthumb increased with N application rate for the April 15, 2003, date; emergence of giant foxtail increased with N application rate for the April 6, 2004, date. Weed biomass was always greater when 168 kg N ha was applied compared with no N, and at four of six N application dates, when 112 kg N ha^-1 was applied. This research shows that spring N fertilizer applications increase N_it and weed growth, but the influence of N on weed emergence is dependent on the weed species, seed source, and environmental conditions.