The herbicides mesotrione and topramezone inhibit 4-hydroxyphenylpyruvate dioxygenase (HPPD) and have efficacy against smooth crabgrass (Digitaria ischaemum). Research was conducted to determine the impacts of soil-applied nitrogen (N) fertilizer on the efficacy of mesotrione and topramezone for smooth crabgrass control.

Dose-response experiments evaluated the response of smooth crabgrass to mesotrione (0, 70, 140, 280, 560 and 1120 g ha^-1) and topramezone (0, 4.5, 9, 18, 36 and 72 g ha^-1) with 0 or 49 kg N ha^-1. Smooth crabgrass was seeded into 10-cm pots filled with a Sequatchie silt loam soil blended with calcined clay in a 3:1 ratio. Treatments were applied with small-plot spray equipment at 280 L ha^-1 to 3- to 5-tiller crabgrass plants. Percent visual control was evaluated 4, 7, 14 and 21 days after treatment (DAT). Aboveground dry biomass was determined 21 DAT. Log-logistic regression analyses were conducted to determine the herbicide dose required to provide 50% visual crabgrass control (I₅₀).

 Further greenhouse research evaluated changes in visual necrosis, weight, chlorophyll and carotenoid pigment concentrations of smooth crabgrass leaf tissue following treatment with mesotrione (280 g ha^-1) and topramezone (18 g ha^-1) with 0 or 49 kg ha^-1. Smooth crabgrass was seeded into 20-cm pots filled with a Sequatchie silt loam soil blended with calcined clay in a 3:1 ratio. Treatments were applied using a spray chamber at 430 L ha^-1 to 3- to 5-tiller crabgrass plants. Leaves present at the time of herbicide application (except for the bud leaf) were marked with indelible ink, designating leaves as those fully emerged before and after herbicide application. Chlorophyll and carotenoid pigments were extracted from leaf tissue harvested 10 days after treatment and quantified via high-performance liquid chromatography. All herbicide treatments were applied with a NIS at 0.25% v/v.

 In dose-response experiments, N application reduced I₅₀ values for mesotrione and topramezone by 50 and 65%, respectively, 21 days after treatment (DAT). Reductions in aboveground biomass with both herbicides were greater when applied following N treatment as well. In leaf-response experiments, N decreased total chlorophyll and xanthophyll cycle pigment concentrations and weight of leaves that emerged after treatment with topramezone. Treatment with N also increased necrosis of leaves emerged after herbicide application in mesotrione-treated plants. Responses of leaves fully emerged before herbicide treatment were not affected by N. Future research should investigate whether increased translocation of these herbicides to meristimatic regions contributed to N-enhanced efficacy.