Three experiments were conducted to identify mechanisms that might explain variations in weed responses to nitrogen (N) fertilization. The first experiment investigated the responses of two crop and six weed species to N fertilization rate and shading in a growth chamber environment. The dry weight and leaf area responses, among species, to N supply and shading were positively correlated with their maximum relative growth rates with the high N fertilization rate. Relative growth rates, among species, were negatively correlated with mean seed weights.;The second experiment was conducted in field plots to investigate the effects of N fertilization timing and corn (Zea mays L.) population density on giant foxtail (Setaria faberi Herrm.), velvetleaf (Abutilon theophrasti Medicus), and common waterhemp (Amaranthus rudis Sauer) competition with corn. Giant foxtail reduced corn yield when no N fertilizer was applied until late June (POST N) but not when N fertilizer was applied prior to corn emergence (PRE N). Velvetleaf seed yield was greater with the PRE N application than the POST N application, but giant foxtail seed yield was lower with PRE N than with POST N. The high corn population density reduced velvetleaf and giant foxtail seed yields, compared to the low corn population density. Corn, velvetleaf and giant foxtail seed yields were associated with their heights and light interception in mid-season. N fertilization timing and corn population density did not affect common waterhemp.;A third experiment investigated the interactions between corn and velvetleaf seedlings grown in a replacement series, as affected by N fertilization rate, in a glasshouse environment. N effects on seedling size of each species were positively correlated with the population density of that species. Relative yield responses suggested that velvetleaf size was reduced by corn competition and that antagonism might have occurred. A growth analysis of corn and velvetleaf seedling monocultures identified N responses. The relative growth rates of velvetleaf were greater than those of corn. The net assimilation rate of velvetleaf was lower with the high N supply than with low N, perhaps due to an increase in self-shading with high N.