Enhanced Efficacy of HPPD-Inhibiting Herbicides for Annual Grassy Weed Control.

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.