Effect of surfactants on the efficacy of clethodim and glyphosate tank-mixures in the control of Abutilon theophrasti Medik. and Chenopodium album L

Broadleaf weeds, Abutilon theophrasti Medik. and Chenopodium album L. are a major problem in glyphosate tolerant soybeans in the US. By applying compatible herbicides with different modes of action together, better efficiency is achieved, weed spectrum is expanded and the cost of weed control is reduced. This study was conducted with several goals: (1) to test the possibility of controlling weed species A. theophrasts and C. album with clethodim and glyphosate tankmixures, (2) to study the impact of surfactants on the efficacy of applied herbicides and (3) to determine whether synergistic and/or antagonistic interactions occur between clethodim and glyphosate in the control of velvetleaf and common lambsquarters. The study included 29 treatments. Treatments included glyphosate formulated without a surfactant (Touchdown HiTech), clethodim (SelectMak) formulated without a surfactant, glyphosate formulated with a surfactant (Roundup PoverMak) and formulations of both surfactant-free herbicides in a tank-mixures with the following surfactants: ATPlus DRT-EPS 0.25% vv-¹, ATPlus DRT 6000 0.25% vv-¹ and 0.5% vv-¹, Tveen 20 0.25% vv-¹, Tveen 22 0.5% vv-¹, Tveen L5-15 0, 5% vv-T, Tveen L10-10 0.5% vv-¹ and Tveen L15-5% vv-¹. The study was arranged as a completely randomized block design with four replications and two independent experimental runs. The effects of treatments on A. theophrasti showed that there were no significant interactions in most treatments. Treatments with clethodim and ATPlus DRT 6000 showed a synergistic effect, while treatments with clethodim and Tween L10-10 and clethodim and Tween L15-5 showed antagonistic effects on A. theophrasti. Two antagonistic reactions were observed in treatments where surfactants Tween 22 and Tween L5-15 were added to glyphosate formulated without a surfactant (Touchdown HiTech). None of the surfactants had an impact on the activity of clethodim in the case of C. album. However, when the surfactants were added to glyphosate, synergism occurred. That was confirmed in treatments with glyphosate formulated with surfactants. Synergistic interactions have also been confirmed when surfactants were added to the tank-mixures of glyphosate and clethodim, with the exception of ATPlus DRT-EPS.