Reply to Egan et al.: Stewardship for herbicide-resistance crop technology

Egan et al. (1) challenge the use of 2,4-dichlorophenoxyacetic acid (2,4-D) resistance technology (2) in providing farmers a potentially robust and sustainable weed-control system and indicate that we have misrepresented the risk of 2,4-D–resistant weed development. There are several reasons why we disagree with this contention. Closer inspection of the currently characterized biotypes resistant to auxin herbicides referred to by Egan et al. (1) indicates that resistance is frequently incomplete, is complex, and often comes with a phenotypic penalty (ref. 3 and references therein). We proposed several hypotheses for this in our paper based on recent insights into the mechanisms of auxin signaling. Of 28 synthetic auxin-resistant weeds documented (www.weedscience.org), most have limited distribution (<50 ha), even 20–50 y after first discovery. Twelve biotypes do not mention 2,4-D resistance specifically, and only 1 biotype has arisen where typical row cropping is practiced in geographies likely to deploy our technology. Egan et al. (1) also cite a concern for a dicamba-resistant kochia biotype conferred by a single dominant resistance allele (3). This biotype was derived through seven generations of recurrent inbreeding selection with dicamba treatment over at least four generations. Preston et al. (3) indicate that dicamba-resistant kochia, first identified in 1994, has not been a serious crop management problem, despite continued use of the herbicide.

We certainly agree that a single robust mode of action is insufficient to prevent widespread resistance to a given herbicide. Selection studies have shown that, when auxin herbicides are used as the single weed-control agent, resistant biotypes can emerge. Importantly, we do not advocate that 2,4-D–resistance trait technology be used as the sole herbicide treatment; rather, we propose that 2,4-D–resistance genes be stacked with one or more transgenes conferring other herbicide-resistance traits. This will provide growers the opportunity to use multiple modes of action with overlapping spectra of weed control. Stewardship will also include use of robust rates for effective weed control and a recommendation for foundational residual herbicide treatments (4). Use of mechanical weed control is a choice determined by the grower and dictated by economics, weather, and environmental impact (5).

Duke and Powles (4) describe the reasons for the widespread adoption of glyphosate-resistant crops. They recommend reintroduction or maintenance of a diversity of weed-control options, including “herbicide rotations, sequences, combinations of robust rates of different modes of action and use of non-herbicide weed control tools.” This should include the use of soil-residual herbicides, the addition of new modes of action through new transgenes, and the use of nonherbicidal weed-control tools (4).

Our view is that the recommendations of Duke and Powles (4) are consistent with the tenets of weed-resistance management described by the Herbicide Resistance Action Committee and Weed Science Society of America (http://www.wssa.net/Weeds/Resistance/index.htm), which recognizes that the most effective resistance management strategy is one that growers will readily adopt. The transgenes that we described will enable a diversity of highly effective, broad-spectrum chemical modes of action that we anticipate that growers will want to include in their weed-management practices to sustain and complement the convenience and advantages attributed to glyphosate-based weed-control systems (4). We consider this a benefit to the development of sustainable weed-control systems.