Glyphosate-tolerant spring wheat currently is being developed and most likely will be the first major genetically engineered crop to be marketed and grown in several areas of the northern Great Plains of the United States. The public has expressed concerns about environmental risks from glyphosate-tolerant wheat. Replacement of traditional herbicide active ingredients with glyphosate in a glyphosate-tolerant spring wheat system may alter ecological risks associated with weed management. The objective of this study was to use a Tier 1 quantitative risk assessment methodology to compare ecological risks for 16 herbicide active ingredients used in spring wheat. The herbicide active ingredients included 2,4-D, bromoxynil, clodinafop, clopyralid, dicamba, fenoxaprop, flucarbazone, glyphosate, MCPA, metsulfuron, thifensulfuron, tralkoxydim, triallate, triasulfuron, tribenuron, and trifluralin. We compared the relative risks of these herbicides to glyphosate to provide an indication of the effect of glyphosate when it is used in a glyphosate-tolerant spring wheat system. Ecological receptors and effects evaluated were avian (acute dietary risk), wild mammal (acute dietary risk), aquatic vertebrates (acute risk), aquatic invertebrates (acute risk), aquatic plants (acute risk), nontarget terrestrial plants (seedling emergence and vegetative vigor), and groundwater exposure. Ecological risks were assessed by integrating toxicity and exposure, primarily using the risk quotient method. Ecological risks for the 15 herbicides relative to glyphosate were highly variable. For risks to duckweed, green algae, groundwater, and nontarget plant seedling emergence, glyphosate had less relative risk than most other active ingredients. The differences in relative risks were most pronounced when glyphosate was compared with herbicides currently widely used on spring wheat.
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