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Glyphosate and dicamba herbicide tank mixture effects on native plant and non-genetically engineered soybean seedlings
Olszyk, D., T. Pfleeger, EHenry Lee, AND M. Plocher. Glyphosate and dicamba herbicide tank mixture effects on native plant and non-genetically engineered soybean seedlings. Ecotoxicology. Springer, New York, NY, 24:1014-1027, (2015).
Use of herbicides associated with the potential production of multiple-herbicide resistant crops may have unintended ecological consequences, as shown by studies at NHEERL’s Western Ecology Division, where native plants were treated with simulated drift of glyphosate and/or dicamba. Because weed species are becoming resistant to the widely used herbicide glyphosate, there is a need for crops engineered to contain genes for tolerance to multiple herbicides. Weed management in these crops would require use of multiple herbicides, which could drift to non-target plants under the right conditions. To address these potential impacts, seedlings of seven native plant species common in the midwestern and eastern US and a non-genetically engineered cultivar of soybean were treated with low levels of glyphosate and/or dicamba, and measured for key growth indicators: height and shoot biomass. Low levels of either herbicide alone dramatically reduced growth of many of the species, but simultaneous exposure a second herbicide usually did not produce any additional effect. Species affected most by dicamba were milkweed (which is a key monarch butterflies food source), purple node Joe Purple Pye Weed, Canada goldenrod, and soybean; while Purple Top Tridens was affected more by glyphosate alone. Clearly, non-target plants could be affected from low levels of herbicides, indicating need for risk assessments to consider these potential effects associated with development of multi-herbicide resistant crop technologies.
Weed species are becoming resistant to intensive and extensive use of specific herbicides associated with the production of herbicide resistant crops, e.g., the use of glyphosate for weed management with glyphosate resistant soybeans. To counter this resistance, crops engineered to contain genes for tolerance to both glyphosate and dicamba may be treated with both herbicides to manage weeds resistant to either herbicide. Thus, non-target plants may be subjected to aerial drift from two herbicides used in combination. Of particular concern are native plants which provide resources for wildlife, as well as adjacent crops which have not been genetically engineered for tolerance to herbicides. We evaluated the responses of eight species of native plants to simulated drift of glyphosate and/or dicamba: Andropogon gerardii, Asclepias syriaca, Eutrochium pupureum, Oenothera biennis, Polyganum lapathifolium, Solidago canadenses and Tridens flavus, and non-herbicide resistant soybean (Glycine max, cultivar Oregon 14). Herbicide concentrations representing aerial drift were used, 0.03 or 0.1 x field application rates of 1122 g ha-1 active ingredient (a.i) (831 g ha-1 acid glyphosate) for glyphosate and 562 g ha-1 a.i. for dicamba. Plants were grown in a greenhouse using a modification of the EPA’s vegetative vigor testing protocol. In general, the response to combinations of glyphosate and dicamba was similar to the response to either glyphosate or dicamba alone, depending on species and on which individual herbicide produced the greater effect. Solidago canadenses was the most sensitive species to both herbicides, while A. gerardii was the most tolerant, with only a slight response to glyphosate. The combinations resulted in responses most similar to that from dicamba alone for A. syriaca, G. max, P. lapathifolium, S. canadensis; and from glyphosate alone for T. flavus. The results of this study indicated the need for more data to assess risks to non-target plants from combinations of herbicides.