Interactions of Mesotrione and Atrazine in Two Weed Species with Different Mechanisms for Atrazine Resistance

The joint activity of mesotrione and atrazine can display synergistic effects on the control of both triazine-sensitive and site-of-action-based triazine-resistant (TR) broadleaf weeds. The first objective of this study was to evaluate a PRE application of atrazine followed by a POST application of mesotrione for potential interactions in both site-of-action-based TR redroot pigweed and metabolism-based atrazine-resistant (AR) velvetleaf. Results from these sequential experiments demonstrated that synergism was detected in reducing biomass of the TR redroot pigweed but not in the AR velvetleaf with metabolism-based resistance. The second objective was to evaluate the joint activity of mesotrione and atrazine in a tank-mix application in the AR velvetleaf biotype. Greenhouse studies with the AR biotype indicated that synergism resulted from a tank mix with a constant mesotrione rate of 3.2 g ai ha−1 in mixture with atrazine ranging from 126 to 13,440 g ai ha−1. Chlorophyll fluorescence imaging also revealed a synergistic interaction on the AR biotype when 3.2 g ha−1 of mesotrione was applied with 126 g ha−1 of atrazine beginning 36 h after treatment and persisting through 72 h. Nomenclature: Atrazine; mesotrione; redroot pigweed, Amaranthus retroflexus L. AMARE; velvetleaf, Abutilon theophrasti M. ABUTH.