Smooth crabgrass (Digitaria ischaemum) is an annual grassy weed found throughout the United States. Mesotrione and topramezone are herbicides that have activity against smooth crabgrass by inhibiting 4-hydroxyphenylpyruvate dioxygenase. Research was conducted to determine the effects of nitrogen (N) fertility on multi-tiller smooth crabgrass control with mesotrione and topramezone. Field experiments were arranged in a 2 x 5 factorial with mesotrione (0.28 kg ha^-1) and topramezone (12 g ha^-1) applied with 5 rates of nitrogen fertilizer derived from ammonium sulfate (0, 12, 25, 37 and 49 kg ha^-1). Greenhouse experiments were conducted to determine the effect of N on smooth crabgrass control with various rates of mesotrione and topramezone. Treatments were arranged in a 2 x 2 x 6 factorial with two rates of nitrogen (0 and 49 kg ha^-1) derived from urea applied with six rates of mesotrione (0, 70, 140, 280, 560 and 1120 g ha^-1) and topramezone (0, 6, 12, 24, 48 and 96 g ha^-1). In field experiments, few differences in smooth crabgrass control were detected between mesotrione and topramezone.  Application of N fertility increased smooth crabgrass control with both herbicides, although few differences were detected between the 25, 37 and 49 kg ha^-1 rates of N.  In greenhouse experiments, 49 kg N ha^-1 reduced IC₅₀ values for both mesotrione and topramezone and caused greater reductions in aboveground biomass relative to the untreated. Additional research is needed to determine the effects of N source and application timing on smooth crabgrass control with these herbicides.