New Weapons on the Way to Battle Wicked Weeds

Article ID: 606528

Released: 26-Aug-2013 7:00 AM EDT

Source Newsroom: American Chemical Society (ACS)

EMBARGOED FOR RELEASE: Sunday, Sept. 8, 2013, 3:30 p.m. Eastern Time
Note to journalists: Please report that this research was presented at a meeting of the American Chemical Society.

A press conference on this topic will be held Sunday, Sept. 8, at 10:30 a.m. in the ACS Press Center, Room 211, in the Indiana Convention Center. Reporters can attend in person or access live audio and video of the event and ask questions at

Newswise — INDIANAPOLIS, Sept. 8, 2013 — A somber picture of the struggle against super-weeds emerged here today as scientists described the relentless spread of herbicide-resistant menaces like pigweed and horseweed that shrug off powerful herbicides and have forced farmers in some areas to return to the hand-held hoes that were a mainstay of weed control a century ago.

The reports on herbicide resistance and its challenges, and how modern agriculture is coping, were part of a symposium on the topic at the 246th National Meeting & Exposition of the American Chemical Society (ACS), the world’s largest scientific society. The meeting with almost 7,000 scientific and other reports continues through Thursday in the Indiana Convention Center and downtown hotels. Abstracts of the symposium presentations appear below.

Costs of weed control have doubled or more in some areas and crop yields have suffered, according to experts.

“The problems associated with herbicide-resistant weeds are spreading and intensifying, especially weed species resistant to multiple products, including the mainstay of 21st century agriculture, the herbicide glyphosate,” said Bryan Young, Ph.D., who spoke at the symposium. He is with Southern Illinois University in Carbondale.

“More than 200 individual weed species have been confirmed resistant to at least a single herbicide, with infestations covering millions of acres in the United States and 60 other countries. It is spreading beyond soybeans and cotton. Weed management in corn has become more and more difficult in recent years due to herbicide-resistant weeds.”

Farmers, he pointed out, are not battling the mild-mannered dandelion or snow thistle that home gardeners visualize at the mention of “weed.” Rather, the battle involves nightmares like Palmer amaranth pigweed, which has been termed the master blueprint for the perfect weed. Under good conditions, Palmer amaranth grows an inch or more a day to heights approaching 10 feet with a stem tough enough to damage farm equipment. It crowds out crops and drains moisture and nutrients from the soil. Resistant plants thrive despite multiple soakings with glyphosate, and a single plant may produce almost 1 million seeds to perpetuate the menace.

Young said that growers are responding to such challenges by integrating alternative herbicides into their weed control programs, herbicides that work a different way and thus sidestep the resistance. They also are turning to herbicides that have residual activity in the soil, preventing weed seeds from growing into a new generation of weeds. When those measures fail, farmers are turning to cover crops to block weed growth and tilling the soil to kill emerged weeds or bury viable seeds deep below the soil surface.

Herbicides, however, remain the most effective tools for managing weeds in terms of overall control and for cost efficiency, Young emphasized. They have other benefits, such as reducing the need for plowing and other soil tillage — which is costly in terms of energy use and may contribute to soil erosion. And a number of products are on the way to help. They include new herbicide formulations that work in ways that sidestep the resistance mechanisms in today’s weeds. And they include crop seeds with genetic traits that enable farmers to apply herbicides to their fields without harming the crops.

“We must remember that herbicides or herbicide-resistant crop traits don’t create herbicide-resistant weeds,” Young said. “Rather, the use and management of these technologies to gain control of weeds by practitioners determines the risk of herbicide-resistant weeds evolving. We need to be better stewards of herbicides to reduce the impact of herbicide-resistant weed species.”

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Complexity of weed management over the next five years

The era of glyphosate-resistant crops enabled, arguably, the most simplified and robust weed management system ever implemented by producers of major row crops. However, current trends over the next five years will necessitate the adoption of much more complex weed management tactics as no single technology or practice will translate into broad-scale effectiveness. More specifically, the complexity of weed management will potentially involve the pervasiveness of weeds with resistance to multiple herbicide groups, the selection of crop seed from several herbicide-resistant trait packages, the increased use of diverse herbicide chemistry, and an increased awareness to prevent off-target movement of herbicides. Glyphosate-resistant weed species currently infest millions of acres in the major crop production regions of the US, and the spread of resistance is anticipated to continue if altered management tactics are not implemented. The private industry sector anticipates the regulatory approval and commercialization of herbicide-resistant crop traits over the next five years, such as resistance to the herbicides 2,4-D, dicamba, isoxaflutole, and mesotrione with various combinations of these traits with resistance to glyphosate and glufosinate. Thus, weed management options will be driven, or limited, by the seed selected for planting well in advance of the weeds emerging in the fields. The integration of multiple herbicides with soil residual or foliar activity will require knowledge of herbicide site of action groups, soil factors, crop rotation restrictions, herbicide interactions, and optimal foliar application techniques that optimize herbicide efficacy while reducing the potential for off-target movement. Thus, the glyphosate-resistant crop technology that allowed for the most simplistic method for controlling weeds over the past 15 years will be followed by complex decisions on seed selection, herbicide combinations, application methods, and herbicide stewardship over the next five years.

Herbicide resistance by weeds: Its effects on and off the field

Resistance to herbicides has been a reality for several decades. In the past, products with new modes of action entered into the market and provided a ready fix for problems that arose. Beginning in the mid-to-late-1990s, crops containing genetically-engineered glyphosate resistance exploded onto the market and rapidly became the dominant and then the only weed control measure in the majority of American cotton, soybean, and corn crops. The number of alternative products used for weed control dropped significantly and resulted in a high level of selection pressure for resistance. With the advent of resistance to glyphosate in these crops, the effectiveness of this exceptional tool for weed control has been severely impacted. This development has, in some cases, rekindled the need for alternative weed control methods. For example, in Southern cotton fields, hand-hoeing is being implemented at considerable cost with the express purpose of reducing the contribution of single, escaping resistant weeds to the soil weed seed bank. Agriculture is at a crossroads with few new products in the pipeline and none with new modes of action foreseeable in the near future. Opportunities exist for new chemical weed control solutions, but they must pass ever higher hurdles for weed control and in their toxicological environmental profiles. Finding new solutions is not easy. We must preserve all the current tools while searching for new ones.

New tools for sustainable management of herbicide resistant weeds: The Enlist™ Weed Control System

Herbicides are essential to today's high-production agriculture, and in many areas, herbicide-tolerant (HT) crops broaden the utility of these products. Since the mid 1990s a dominant HT system has enabled widespread use of glyphosate across multiple crops. However, the continuous use of glyphosate without accompanying resistance management best practices has led to an increase in glyphosate-resistant weed species that threatens the future use of glyphosate and the benefits it has enabled. Dow AgroSciences is developing the Enlist™ Weed Control System which is based on a new family of herbicide tolerance traits, innovations in herbicide formulations, and novel approaches to technology stewardship. This system will add greater diversity to weed management programs and help sustain the gains made with glyphosate-tolerant crops by enabling more flexible use of an innovative form of 2,4-D and other herbicides in maize, soybean, and cotton. Field studies have confirmed >90% control of a wide range of weed species (including many biotypes that are resistant to glyphosate) with a combination of a new 2,4-D plus glyphosate. Crops containing Enlist™ traits can withstand over twice the amount of 2,4-D plus glyphosate required for weed control with no significant impact on yield. New weed management technologies such as Enlist™ will require proper stewardship, including resistance management best practices, to ensure they remain viable over a long period of time. Effective resistance management programs must be based on scientific principles as well as compatibility with established production systems. Factors influencing the development of best management practices will be discussed. (™Trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow. Regulatory approvals are pending for the Enlist™ herbicide solution and crops containing Enlist™ herbicide tolerance traits. The information presented here is not an offer for sale. Always read and follow label directions. ©2013 Dow AgroSciences LLC)

Strategies for sustainable weed management

Weeds continue to evolve as agriculture places selection pressures on them. Weeds have shifted towards annual species in response to traditional tillage and towards shallow-emerging species with reduced tillage. From the 1960's to 1980s, species shifted when selective herbicides controlled some, but not other species. More recently, changes in growth habit (such as germination time) and herbicide resistance have occurred within species. To date, there are approximately 400 instances of resistance with species and herbicide modes of action groups ( With Monsanto's Roundup Ready systems, resistance in troublesome species such as Amaranthus palmeri, Amaranthus rudus, and Conyza canadensis has required proactive changes to weed control programs. With consultation from the academic community and other manufacturers, Monsanto launched the Roundup Ready PLUS platform which is based on Best Management weed control recommendations that include the use of residual herbicides and additional modes of action with glyphosate. The PLUS platform also provides growers with incentives to use residual and alternative MOA herbicides. Additionally, Monsanto and other companies are developing new trait-based herbicide systems that will allow new modes of action to be used in crops where they were not previously available to improve weed control further and to manage and prevent resistance. Currently, Monsanto is developing soybean and cotton with resistance to dicamba, a well established, broadleaf-controlling herbicide. Monsanto is also working to develop other herbicide traits and herbicides. The entire agricultural industry needs to remain vigilant, continue to develop weed control programs with diverse herbicide modes of action, and to recommend diverse cultural and herbicidal methods. However, growers are successfully adjusting their weed control programs with existing technology, controlling weeds, and maintaining crop yields.

Glyphosate is not the herbicide it used to be: A North American perspective

Glyphosate is a special chemical, with the combination of low mammalian toxicity, favorable environmental profile, and broad spectrum control imparting a uniqueness to this herbicide. However, it was the development of glyphosate-tolerant crops that resulted in wide-scale adoption of glyphosate as a foundational herbicide over hundreds of millions of hectares in the
Americas. For several years in the late 1990s, weed control was highly effective, economical, and simple. However, with this massive selection pressure, the evolution of weeds no longer controlled by glyphosate was inevitable, and this has occurred in numerous locations in several genera. The major agronomic examples of these weeds include Conyza canadensis, a winter annual that is problematic in no-tillage production systems, and Amaranthus palmeri, a tremendously competitive summer annual that is a growing problem in the southern United States. Incomplete control from glyphosate has also been reported on species from the SorghumAmbrosiaConyza, and Amaranthus genus. This talk introduces the important topic of herbicide resistance in today's American agriculture. Issues discussed include comprehensive surveys of the occurence of herbicide resistance, how farmers are responding to this change, and future opportunities and responses to manage this problem.

Surveys of herbicide resistances in Missouri waterhemp populations with a focus on correlations between field management practices and glyphosate resistance

Herbicide-resistant weed populations have evolved rapidly in response to the selection pressures imposed upon them in current agricultural production systems. Specifically, in the Midwest the number of acres affected by glyphosate-resistant (GR) weeds has increased dramatically in recent years due to the rapid adoption of GR crops and the extensive use of glyphosate. One of the GR weed species that is currently infesting millions of corn and soybean acres in the Midwest is waterhemp (Amaranthus rudis Sauer). In Missouri, we have conducted a number of field surveys of waterhemp populations 1) to determine the frequency and distribution of glyphosate resistance in waterhemp, 2) to determine the percentage of waterhemp populations with multiple herbicide resistances, and 3) to determine if there are any field management factors that can serve as indicators of GR in future waterhemp populations. In a 2008-2009 field survey, glyphosate resistance was confirmed in 99 out of 144, or 69% of the total waterhemp populations sampled. These resistant populations occurred across 41 counties of Missouri. More recently, the incidence of multiple herbicide resistances to glyphosate, to 2,4-D, and to HPPD-, ALS-, and PPO-inhibiting herbicides is being determined in approximately 180 different Missouri waterhemp populations collected in 2012. Based on the data collected from each sampling site in the 2008-2009 survey, soybean fields confirmed with GR waterhemp were more likely to be free of other weed species, were more likely to occur where soybeans were continuously cropped, were more likely to occur where glyphosate was the only herbicide applied for several seasons consecutively, and were more likely to show obvious signs of surviving herbicide treatment.

Iowa survey of herbicide resistances in common waterhemp including glyphosate and other herbicide groups

Evolved herbicide resistance in common waterhemp (Amaranthus tuberculatus) has been a serious issue in Iowa agriculture when this weed first began to appear as a major production problem in soybeans in the 1980's. At that time, ALS inhibitor herbicides (Group 2) were applied to a majority of the corn and soybean acres across the Midwest and common waterhemp populations quickly evolved resistance. In the 1990's, concerns focused on the inevitability of evolved resistance to glyphosate (Group 9). The first glyphosate-resistant common waterhemp populations were identified coincidentally in Badger and Everly, Iowa in 1997. However, glyphosate resistance in common waterhemp was generally scattered in fields and had not become a major concern to agriculture. This changed as the adoption of glyphosate-based corn and soybean production systems became the dominate practice. It was clear that common waterhemp had become the major concern in Iowa soybean production and ultimately in corn production. The more recent identification of common waterhemp populations with evolved resistance to auxinic herbicides (Group 4) and the HPPD inhibitor herbicides (Group 27) as well as the historic resistance to the PSII inhibitor herbicides (Group 5) and PPO inhibitor herbicides (Group 14) leaves few herbicide options for the control of common waterhemp, particularly for postemergence strategies. In order to assess the extent of herbicide-resistant common waterhemp populations in Iowa, a project was initiated in 2011 with support from the Iowa Soybean Association. This project supplements a study conducted in 2008 where resistance to glyphosate was detected in 16% of approximately 220 Iowa common waterhemp populations collected arbitrarily.

Weed resistance in Arkansas: One crisis averted another begins?

Since 2002, six weeds (horseweed, common ragweed, giant ragweed, Palmer pigweed, Johnsongrass, and Italian ryegrass) have been confirmed to be resistant to glyphosate in Arkansas. Of these, horseweed and pigweed have by far had the greatest impact on farming practices. In addition to glyphosate resistance, pigweed populations also exist that are resistant to the ALS and dinitroanaline families of chemistry. Ryegrass populations exist that not only tolerate glyphosate but also ALS and certain ACCase chemistries, causing difficulties in not only spring burndown, but also wheat ryegrass control. Glyphosate resistant crops were grown on approximately 100 percent of the corn, cotton, and soybean acres in Arkansas by the year 2008. In 2010, it was estimated that most all cotton acres and at least 50% of the soybean acres had become infested with glyphosate resistant Palmer amaranth. Due to resistance management practices that were implemented statewide from 2009-2012, a true crisis in weed control has been averted in the state. Although pigweed is still both the most common and troublesome weed in both soybean and cotton, it is relatively under control. Many acres of cotton still require hand weeding for complete control, and farmers do still struggle in soybean with added input costs and time management. Pigweed control options and current management strategies will be discussed. In addition to pigweed in row crops, a similar resistance situation is evolving in rice with barnyardgrass. This speaks to an overall greater concern with the lack of discovery of new herbicide modes of action and the bottlenecks that it creates for certain cropping systems, where only very specific herbicide recommendations will actually control weeds, which can inevitably lead to more resistance.

Glyphosate-resistant weeds: Lessons learned in Tennessee

There are six confirmed glyphosate-resistant (GR) weeds in Tennessee. Horseweed (marestail) and Palmer amaranth (pigweed) are now an issue on virtually every row crop acre in Tennessee. Horseweed was the first of these to show glyphosate resistance in 2001. Just four years later, Palmer amaranth was confirmed to be resistant to glyphosate and has in that short time become the biggest weed problem in the state. The effect GR horseweed had on the state of Tennessee was to increase producer costs and reduce conservation tillage acres. Management of this weed cost Tennessee producers an additional $20/acre and reduced conservation tillage acres 20% by 2004. These conservation tillage acres were mostly regained as soybean and cotton producers became comfortable with alternative burndown programs in recent years. Though GR horseweed was a problem, GR Palmer amaranth is proving to be the game changer with respect to weed control in Tennessee. Palmer amaranth is one of the most competitive plants in the world as it can grow 5 cm per day and in Tennessee has overwhelmed soybean fields to the point that they cannot be harvested effectively. Soybean producers have found that if they cannot control GR Palmer amaranth prior to it reaching a height of 7 cm, they cannot control it with any herbicide. As a result, in fields where a pre applied herbicide has failed, even with a good crop stand, the field is often tilled and replanted. With the unprecedented selection pressure now on glufosinate and PPO herbicides in controlling Palmer amaranth, the concern shifts to how long before resistance to these herbicides develops. The introduction of the new auxin tolerant traits in soybeans and cotton is slated for 2015. These new technologies should help alleviate some of this selection pressure on glufosinate and the PPO herbicides.

HPPD-inhibitor herbicide resistance in the USA: A Syngenta perspective

Over the past 3 years, tall waterhemp (Amaranthus tuberculatus Moq. Sauer) resistance to 4-hydroxyphenyl-pyruvate dioxygenase (HPPD) inhibiting herbicides has been confirmed in Illinois, Iowa, and Nebraska seed corn production fields. Resistance to HPPD-inhibiting herbicides in Palmer amaranth (A. palmeri S. Wats.) has also been reported in Kansas. These populations are also resistant to other herbicides with different mechanisms of action (e.g., atrazine), and can therefore, be considered multiple-herbicide resistant populations. The existence of these HPPD-resistant populations illustrates the propensity of different species within this genus, to evolve resistance to HPPD-inhibitors, in addition to other mechanisms of action. The evolution of HPPD-inhibitor resistance in these seed corn production fields has been attributed to the limited diversity of cultural practices and of mechanisms of action of herbicides used, necessitated by the sensitivity of inbred lines to herbicides that are more widely available for use in field corn production. Nevertheless, this development is an early warning of potentially more widespread occurrences. In common with resistance in other herbicides, the risk of HPPD-resistance development is high if there is reliance on the exclusive use of HPPD-inhibiting herbicides for weed control, particularly in species with high genetic diversity and which are obligate out-crossers, such as A. tuberculatus or A. palmeri, in which resistance to other herbicide mechanisms of action has already evolved. Considerable resources have been employed in research to understand the mechanism of resistance and to assess the probability of evolution of further resistance to HPPD-inhibitors in waterhemp. In addition, research continues to explore the options for managing this issue from both proactive and solution-based approaches, in corn and soybean production systems. A diverse approach, which includes the use of multiple mechanisms of action and other methods of weed management, is critical to the sustainable use of HPPD-inhibiting herbicides.


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