Weed resistance development and management in herbicide-tolerant crops: experiences from the USA
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- Owen, M.D.K. J. Verbr. Lebensm. (2011) 6(Suppl 1): 85. doi:10.1007/s00003-011-0679-2
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The evolution of weeds in USA agroecosystems predates herbicide tolerant (HT) crops by several decades. However, given the unprecedented adoption of genetically engineered (GE) HT crops, particularly in maize, cotton and soybean and the concomitant use of glyphosate, the evolution of glyphosate-resistant (GR) biotypes of agronomic important weeds now represents a significant threat to the sustainability of the GE HT trait and the herbicide. Notably, GR biotypes of horseweed (Conyza canadensis), common waterhemp (Amaranthus tuberculatus), Palmer pigweed (Amaranthus palmeri), giant ragweed (Ambrosia trifida) and johnsongrass (Sorghum halepense) have increased dramatically in frequency in conjunction with the adoption of crop production systems based on GE HT crops. Furthermore, other shifts in weed communities have been observed. It must be emphasized that the GE HT crops did not directly cause these weed shifts. The industry was responsible initially for these changes in weed communities given their initial positions that the glyphosate-resistant weed problems were unlikely to evolve and the recommendations to use glyphosate exclusively for weed management. However, decisions by growers to use glyphosate must be factored in as imparting selection pressure on the weed communities resulting in weed management problems, particularly when university recommendations strongly suggested not to use glyphosate in this manner. The key is now to provide growers with sufficient information to convince them to adopt integrated weed management programs and utilize best management practices. Given the presumed and real benefits of convenience and simplicity of the GE HT crop production systems; this will be a difficult task.
KeywordsGlyphosate Glyphosate-resistant crops Glyphosate-resistant weeds Management
Weeds are the most important pest complex impacting mankind. Herbicide resistance in weed populations has contributed to the costs incurred by growers but should not be considered a topic or concern specifically focused upon the relatively recent introduction of genetically engineered (GE) herbicide tolerant (HT) crops. However, the incredible global change in the agricultural landscape attributable to the GE HT technology has indeed significantly impacted the evolution of herbicide-resistant (HR) weeds. Currently, more than 340 HR weed biotypes in more than 190 different plant species have been reported (Heap 2010). Nevertheless, concerns about GE HT crops have dramatically changed how HR weeds are now perceived by agriculture, society and the federal government (Arntzen et al. 2003; Benbrook 2003; Ervin et al. 2010).
Despite considerable effort on many fronts, the problems associated with GE HT crops and HR weeds seem to be largely without resolution attributable, in part, to the general unwillingness of growers to recognize the implications of their management tactics, the unrealistic marketing by the herbicide and seed industries, and the erroneous belief that new technologies and tactics will be available in the short-term future (Owen et al. 2009). As a result, weed populations with evolved resistance to glyphosate continue to increase (Owen 2009b).
2 Current situations with herbicide-resistant weeds
The HR weed biotypes are increasing globally (Heap 2010). While grower adoption of GE HT crops has been identified as the primary problem causing the increase in HR weeds by many advocacy groups, the evolution of herbicide resistance predates GE HT crops by more than 3½ decades (Duke 2005; Ryan 1970). Weeds have evolved resistance to 20 herbicide mechanisms of action and 60 different weed species have been reported to have evolved HR biotypes in the USA (Heap 2010). The current number of HR weeds includes 350 HR biotypes represented by 195 species and are now estimated to infest over 400,000 fields (Heap 2010). However, given the voluntary nature of these reports, the information may not provide an accurate nor current description of the extent of HR weed populations.
The evolution of multiple and cross resistances to herbicides is becoming increasingly more common. For example, common waterhemp populations with cross resistances to ALS inhibiting herbicides and multiple resistances to glyphosate are widely distributed throughout the Midwest (Owen 2009a; Patzoldt et al. 2005). Common waterhemp populations in Kansas and Iowa have evolved multiple resistances to PPO and ALS inhibiting herbicides (Falk et al. 2005; Owen 2009a). In Illinois, populations of smooth pigweed (Amaranthus hybridus) and kochia (Kochia scoparia) have evolved multiple resistances to PSII and ALS inhibiting herbicides (Foes et al. 1999; Maertens et al. 2004). Multiple resistances to ALS inhibiting herbicides and glyphosate are also reported in horseweed (Conyza canadensis) (Davis et al. 2009).
2.1 Glyphosate resistant weeds
It is clear now that weed populations with glyphosate resistance are occurring more rapidly and are widely distributed in USA maize, cotton and soybean GE HT systems despite grower knowledge have about the evolution of GR weed biotypes (Johnson et al. 2009b). Currently there are 20 weed species with evolved resistance to glyphosate and 12 species in the USA of which at least seven weed species have evolved resistance in GE HT crop production systems in at least 22 states (Heap 2010).
Growers have not demonstrated that they are fully aware of the factors which affect the evolution of GR weed populations and generally the level of their concern is low (Johnson and Gibson 2006). A typical grower response to GR weeds can be described as “waiting for the inevitable to occur” and then mitigating the problem rather than addressing it proactively.
2.2 Implications of glyphosate resistant weeds
Given the predominance of GE HT maize, cotton and soybean cultivars and the use of glyphosate, the implications of evolved glyphosate resistance is significant in the USA. The historic lack of integrated weed management (IWM) by growers has sped the frequency of the problems (Boerboom et al. 2009). Importantly, the likelihood of growers adopting cultural and mechanical strategies is low (Green and Owen 2010; Kruger et al. 2009). Furthermore, given the cross and multiple resistances that have evolved in important weeds, the numbers of herbicides that will provide effective control are limited. Lastly, consider that it is very unlikely that new herbicides with new modes of action will be available within ten to 15 years (Green and Owen 2010).
Another implication of GR weed biotypes is the impending increases in tillage to manage these weed populations (Johnson et al. 2009a). In many instances (i.e. no tillage cotton production in the Mississippi Delta), the only tactic available to manage GR weed populations is tillage (Steckel and Gwathmey 2009). Tillage has major economic implications to growers, as well as ecological, environmental and societal concerns in general (Ervin et al. 2010). The economic considerations for growers reflect the additional equipment that may be required to implement tillage practices, the increased requirement for petroleum fuels and the added personnel and time necessary to accomplish the tillage.
3 Management of herbicide resistant weeds
There can be no question that the effective management of weeds is the most important consideration for the production of crops and herbicides are the key tactic for weed control (Gianessi and Reigner 2007). However, ill-advised herbicide use will inevitably result in HR weed populations (Owen 1997). HR weed biotypes are a consequence of the effectiveness and consistency of herbicides in managing weed complexes and clearly a demonstration of “Darwinian evolution in fast forward”. The GE HT crops allowed growers to adopt weed management programs based on simple and convenient systems and one herbicide (glyphosate) which predictably selected for GR weed biotypes despite claims by industry that this would not occur (Bradshaw et al. 1997). As a result of these simple and convenient weed management tactics, recent concern for the management of GR weeds is particularly important in agriculture in the USA.
Educational programs provided by land grant university extension now focus largely on the management of HR weeds (Scott et al. 2009). Surveys developed several years ago, but likely still depicting a current portrait of USA growers, indicated that generally growers are not overly concerned about GR weeds (Johnson et al. 2009b). It is presumed that as the frequency of GR weed populations increase, grower awareness and response will also increase. Importantly, the increasing frequency of GR weed populations also has social costs and externalities generally not considered (Marsh et al. 2006). These include reduced use of conservation tillage, failure of the current GE HT-based crop production systems and other environmental issues (Marsh et al. 2006).
Industry has approached resolving the problem of HR weeds with the development of crops with GE HT resistances to different herbicides, and in some instances, with multiple resistances to several herbicides within the crop cultivar. This tactic is unlikely to resolve the HR weed problem in the long term (Green and Owen 2010). The management of HR weeds must include as many tactics as possible (Green and Owen 2010). Consider that on approximately 67 and 27 % of the GE HT soybean and maize, respectively, only glyphosate is used (Ervin et al. 2010). Thus, the inclusion of other herbicides on these crop acres could greatly help mitigate the pending GR weed problems. Growers suggested that herbicide rotation was an important tactic to manage these GR weed problems (Foresman and Glasgow 2008).
4 Herbicide resistance: implications and costs
Herbicide resistance in weeds has historically been a threat to global food production (Moss and Rubin 1993). Currently, the evolution glyphosate resistance has further threatened the productivity of global agriculture and reinforced the need to adopt IWM practices (Powles 2008). Furthermore the practices adopted by growers to mitigate HR weed populations will be costly; these costs must be considered against the benefits, whether immediate or longer term (Orson 1999).
4.1 Implications and grower perceptions
Surveys have been conducted to assess grower perceptions about the issues and implications of herbicide resistance, weeds, tillage, and integrated approaches for weed management (Kruger et al. 2009; Llewellyn et al. 2002, 2004). For example, a robust survey conducted in six states in the USA reported that growers in the Midwest are more likely to adopt crop rotation to address weed management problems than growers in the Southeast (Shaw et al. 2009). Indiana, USA growers considered weeds as the most important pest complex when compared to diseases and insects, however they were not overly concerned about the evolution of GR weed populations (Gibson et al. 2005; Johnson and Gibson 2006).
The perceptions of growers from Australia were very different that Midwest growers. Surveys reported that in the Australian grain-growing regions, growers had awareness of HR weeds and ascribed a high economic cost to herbicide resistance (Llewellyn et al. 2002). Growers in Australia were adopting IWM and perceived that these tactics had economic value (Llewellyn et al. 2004). However, practices that included alternative herbicides were perceived to have the highest economic return on investment and growers felt that a stock of new herbicides would be developed in the short term to better manage the building HR weed populations (Llewellyn et al. 2002, 2004).
4.2 Herbicide resistance costs
Herbicides are used on more than 90 % of crop acres in the USA annually and represent 65 % of the pesticide expenditures by growers (Gianessi and Reigner 2007). If herbicides were not available, USA crop productivity would decline by a minimum of 20 %. When herbicide use results in the evolution of HR weed biotypes, this too has associated costs in reduced production, higher expenditures for herbicides and dramatic increases in the weed seedbank (Peterson 1999). The difficulty is changing grower perspectives and practices to make the necessary adjustments in weed management programs. How this can be accomplished on the scale necessary to address the pending issues of evolved HR weed populations in the USA remains to be seen.
Weeds represent the most economically important pest complex to global food production and also significantly impact mankind at all levels, from health perspectives to the pursuit of recreation. Interestingly, the better weed management becomes, the more difficult it becomes to manage weeds. This conundrum reflects the diversity of weed genomes facilitating their continuing adaptation to all forms of selective practices imparted by man during the practice of plant production (Barrett 1983; De Wett and Harlan 1975). During the last five decades, herbicides have been an important component for effective weed management. As a result, biochemical adaptation or mimicry has become an increasingly important problem.
Recent weed management tactics have taken a slightly different path and focus on the use of herbicides that are selective to crops due to genetic engineering (Duke and Powles 2008). The use of glyphosate in GE HT crops has provided exceptional control of many weeds and represents the most important change in technology in the history of agriculture (Owen 2009b). However, it was inevitable that weed populations would again rise to the genetic challenge and resistance to glyphosate would evolved despite suggestions otherwise (Bradshaw et al. 1997; Neve 2007). Certainly the costs of managing GR weeds are important but other costs to the environment are also significant (Cerdeira and Duke 2006). These costs represent the loss of conservation tillage practices, increased soil erosion and declining water quality (Cerdeira and Duke 2006). It is thus surprising, with so many benefits at risk due to the evolution of HR weeds, that growers in the USA are not yet fully engaged in adopting mitigation practices but rather appear to be waiting for the problems to evolve locally (Johnson and Gibson 2006; Owen 2008; Webster and Sosnoskie 2010).
The ability to effectively manage HR weeds including those resistant to glyphosate is well-studied and tactics readily available to growers (Beckie 2006). Models clearly demonstrate that the adoption of a diverse management approach (IWM) to controlling weeds can prolong the utility of the GE HT cultivars and glyphosate (Werth et al. 2008). Proactive management of GR weed populations is economically sustainable and provides important stewardship for the GE HT traits; IWM must be implemented immediately to maintain these crop production systems.
Conflict of interest
The author Micheal D. K. Owen declares that the manuscript was not sponsored and that he has no conflict of interest.