Joint action of the effects of atrazine and mesotrione can lead to synergistic herbicidal activity in broadleaf weed species. The objective of these experiments was to determine if specific rates are required to provide synergistic joint activity between mesotrione and atrazine in both triazine-sensitive (TS) and triazine-resistant (TR) redroot pigweed. Herbicide rates were evaluated in TS and TR redroot pigweed in two experiments: a dose response of mesotrione alone and in mixture with a constant rate of atrazine and a dose response of atrazine alone and in mixture with a constant rate of mesotrione. Results from these experiments revealed that synergism was detected in the TS pigweed when 56 g ai ha⁻¹ mesotrione was mixed with 126 g ai ha⁻¹ atrazine. In the TR pigweed, synergism was detected when mesotrione rate at 10 to 56 g ha⁻¹ was mixed with a constant rate of atrazine at 126 g ha⁻¹. Additionally, when mesotrione was held constant at 10 g ha⁻¹, synergism was detected in mixture with atrazine from 31 to 3556 g ha⁻¹ in TR pigweed. Furthermore, in TR pigweed, analysis of slope deviation across the dose-response curves of mesotrione with and without atrazine revealed a divergence that increased in magnitude as the rate of mesotrione increased. In other words, increased synergism was observed with increased mesotrione rate in the TR pigweed, which was also supported by biomass reduction and atrazine-like injury to the leaves. An additional experiment investigated synergism between mesotrione and bromoxynil in both TS and TR pigweed. Mesotrione at 10 g ha⁻¹ was synergistic when paired with bromoxynil from 70 to 210 g ha⁻¹ in both the TS and the TR pigweed.

Nomenclature: Atrazine; bromoxynil; mesotrione; redroot pigweed, Amaranthus retroflexus L. AMARE