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Effects of mixtures of dicamba and glyphosate on nontarget plants
Citation:
Olszyk, D., T. Pfleeger, E Henry Lee, M. Blakeley-Smith, T. Shiroyama, AND M. Plocher. Effects of mixtures of dicamba and glyphosate on nontarget plants. American Chemical Society, Annual Meeting, DC, Washington, August 20 - 24, 2017.
Impact/Purpose:
New technologies are being develop using mixtures of herbicides to effectively manage weeds in multiple herbicide resistant crops such as soybean and cotton. In order to insure the safety of these new herbicide formulations and uses, the US Environmental Protection Agency must assess environmental risks, including potential risk to nontarget plants. A key question in these assessments is whether the mixture’s effects can be predicted based on the toxicity of each herbicide, or if there could be unexpected enhancement (synergistic) or decreases (antagonism) of effects due to interactions between chemicals. Impacts to nontarget plants would be especially important if they include threatened or endangered species, or if they provided important ecosystem services such as providing wildlife habitat. To address the critical need for ecological data relevant to mixtures of herbicides, we have conducted controlled studies using dicamba and glyphosate alone and in combination at rates at or below those which could be present adjacent to agricultural fields under drift conditions. There were few significant interactions between the herbicides, instead responses to combinations of glyphosate and dicamba primarily resembled the responses to either of the herbicides. Those species most sensitive to dicamba, e.g., broad-leaf plants, had mixture responses similar to dicamba alone; while species more sensitive to glyphosate, e.g. grasses, had mixture responses similar to glyphosate alone. This study provides the EPA with important new information to address potential ecological risks on mixtures of herbicides, especially that the effects of mixtures on plants may be similar to those of the individual herbicides.
Description:
New technologies are being developed using mixtures of herbicides to manage a broader variety of weeds in multiple herbicide resistant crops such as soybean and cotton. As part of its regulation of pesticides, the US Environmental Protection Agency considers environmental risks, including impacts to nontarget plants potentially exposed to pesticide drift- including mixtures of herbicides. To provide information on possible effects of mixtures of herbicides on nontarget plants, we have conducted greenhouse and field studies using dicamba and glyphosate alone and in combination at rates of 0.01 to 0.2 x field application rates of approximately 830 and 563 g HA-1 for glyphosate (as acid glyphosate) and dicamba, respectively. Growth and reproductive endpoints for a variety of native plants were evaluated. Our findings indicated that responses to combinations of glyphosate and dicamba resembled the responses to either of the herbicides with a few exceptions of an interactive response. Those species most sensitive to dicamba, e.g., broad-leaf plants, had mixture responses similar to dicamba alone; while species more sensitive to glyphosate, e.g. grasses, had mixture responses similar to glyphosate alone.