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Agronomy Day 2007

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Synergistic Herbicide Tank Mixes for Weed Management in Corn

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Josie Hugie Josie Hugie
Graduate Student
Department of Crop Sciences
217-244-6882;
hugie@illinois.edu
Dean Riechers Dean Riechers
Associate Professor
Department of Crop Sciences
217-333-9655;
riechers@illinois.edu
AJ Woodyard AJ Woodyard
Graduate Student
Department of Crop Sciences
217-244-6882;
awoodyar@illinois.edu

Herbicide tank mixes, typically combining two or more herbicides with different modes of action, are frequently used to broaden the weed control spectrum, obtain residual activity, and control resistant biotypes. An example of a common postemergence tank mix for corn is Callisto and atrazine for broadleaf weed control. Despite the advantages mentioned above for herbicide tank mixes, there are some potential drawbacks that also need to be considered, including incompatibility and the potential for detrimental herbicide interactions. For example, antagonism may occur when one herbicide inhibits the uptake of the other herbicide, or when a contact herbicide is mixed with a systemic herbicide, resulting in poor weed control. An example of antagonism that has been noted with postemergence tank mixes in soybean includes the postemergence grass herbicides (ACCase inhibitors) when tank mixed with contact burn herbicides (PPO inhibitors). Another unwanted interaction (in this case resulting in crop injury) may occur between agrochemicals when certain postemergence herbicides, such as Accent or Beacon, are applied to corn in combination with organophosphate insecticides.

A unique form of herbicide interaction that provides enhanced weed control in corn is a Callisto plus atrazine tank mix. Callisto, a pigment inhibitor, when tank mixed with a low rate of atrazine, a photosynthetic inhibitor, displays synergistic herbicidal activity on waterhemp, pigweeds, and lambsquarters, meaning the injury observed in the target plant is greater than the expected sum activity of the combined herbicides. In addition to improving weed control with the Callisto plus atrazine tank mix, this mixture may be used as a strategy to prevent or delay the development of weed resistance to Callisto. Resistance may be delayed through combining these herbicides with two distinct target sites and modes of action. Thus, combining herbicides having synergistic activity offers not only a valuable weed management tool for corn, but may also reduce the selection for herbicide resistant populations and allow for the reduction of atrazine rates through gaining higher activity per unit of active ingredient.Additionally, the synergism between Callisto and atrazine has been documented in atrazine-resistant pigweed biotypes, suggesting that when combined with Callisto, atrazine has some level of activity in these atrazine-resistant weeds. Greenhouse studies have also shown that a preemergence treatment of atrazine followed by a postemergence treatment (four weeks later) of Callisto displays synergistic activity in the atrazine-resistant biotype. Current greenhouse, field and laboratory studies are aimed at determining the underlying physiological basis for the interaction between Callisto and atrazine.