resistance to herbicides that can efficiently control most weeds associated with the crop. For example, genes that confer resistance to ALS inhibitors, to which many weed species are already resistant, could be inferior to genes that confer resistance to dicamba, glufosinate, and HPPD inhibitors to produce durable HR corn, cotton, and soybean resistant to two or more herbicides. Similarly, care should be taken to engineer crops for resistance to specific ACCase inhibitors and synthetic auxins that will still be effective in controlling weeds associated with future HR crops.

If crops that are resistant to multiple herbicides—including ALS inhibitors, ACCase inhibitors, synthetic auxins, and glyphosate—are widely planted, continued use of the herbicides in fields that contain weeds already resistant to some of them could involve a risk of selecting for high levels of multiple herbicide resistance. The ability of weeds to evolve biotypes that have multiple herbicide resistance has already been demonstrated in waterhemp populations in Illinois and Missouri that are resistant to three herbicide mechanisms of action (Patzoldt et al., 2005; Legleiter and Bradley, 2008). Evolved multiple resistance will exacerbate problems of controlling some key herbicide-resistant weeds, and local and regional spatially explicit information on the distribution of weeds that are resistant to glyphosate and other herbicides could be useful in helping to manage such a situation (Werth et al., 2008). Tank-mixes and sequencing herbicides rely on redundancy to be effective. Models assessing sequential use of herbicides only, or of herbicides and mechanical weed control, indicate that a low frequency of alleles conferring resistance to herbicides and high weed mortality are critical factors for these strategies to substantially delay the evolution of weed resistant to glyphosate in HR crops (Neve et al., 2003; Neve, 2008; Werth et al., 2008).

In conclusion, regardless of the specific herbicide for which HR crops are genetically engineered, only appropriate stewardship by the grower will delay the evolution of resistance to the herbicide. Resistance management is voluntary in the United States for all pesticides except Bt produced by Bt crops (Berwald et al., 2006; Thompson et al., 2008). Given the rapid increase in and expansion of weeds that are resistant to glyphosate in HR crops, herbicide-resistance management needs national attention. As discussed previously, the rapid evolution of weed resistance to glyphosate has probably been a consequence of growers’ management decisions that favored the use of glyphosate as the primary, if not sole, tactic to control weeds despite efforts in the private and public sectors to strongly recommend alternative strategies (Johnson et al., 2009). Without changes in production practices, the increase in weeds resistant to glyphosate will likely increase weed-management expenses for farmers. The evolution of herbicide resistance and other weed shifts associated with the adoption of GE crops requires the development and use of more



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