Herbicides state of the art. II. Achievements …...2014/08/07 · Isoxaflutole (Figure 3) was also developed in the late 1990s for pre-emergence use in corn (Luscombe et al., 1995)

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Herbicides state of the art. II. Achievements

Hansjoerg Kraehmer

Kantstrasse 20, D-65719 Hofheim Germany

[email protected]

+49 6192 296560

Plant Physiology Preview. Published on August 7, 2014, as DOI:10.1104/pp.114.241992

Copyright 2014 by the American Society of Plant Biologists

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Herbicides as weed control agents – state of the art. II. Recent achievements

Hansjoerg Kraehmer*, Andreas van Almsick, Roland Beffa ,Hansjoerg Dietrich, Peter Eckes, Erwin Hacker, Ruediger Hain, Harry John Strek, Hermann Stuebler, Lothar Willms

Bayer CropScience AG, Industriepark Hoechst, Building H 872, D-65926 Frankfurt am Main

Herbicide discovery has faced significant challenges over the past few decades and weed control innovations are urgently required.



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ABSTRACT

In response to changing market dynamics, the discovery of new herbicides has significantly declined over the past few decades and has only seen a modest upsurge in recent years. Nevertheless the few introductions have proven to be interesting and have brought useful innovation to the market. In addition, HT (herbicide tolerant) or HR (herbicide resistant) crop technologies have also allowed the use of existing non-selective herbicides to be extended into crops. An increasing and now major challenge is being posed by the inexorable increase in biotypes of weeds that are resistant to herbicides. This problem is now at a level that threatens future agricultural productivity and needs to be better understood. If herbicides are to remain sustainable then it is a must that we adopt diversity in crop rotations and herbicide use as well as increase the use of non-chemical measures to control weeds. Nevertheless, despite the difficulties posed by resistant weeds and increased regulatory hurdles, new screening tools promise to provide an upsurge of promising herbicide leads. Our industry urgently needs to supply agriculture with new, effective resistance breaking herbicides along with strategies to sustain their utilities.



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Introduction

Only a few companies are significantly pursuing herbicide discovery in the 21st century. Most of these have combined

seed and traits businesses since fees for traits constitute a considerable part of the income of agrochemical companies

today. In concert with the review of the historical perspectives of herbicide research (Kraehmer, et al., 2014) we

provide here a short description of the current major research activities within the remaining 21st century agrochemical

companies. After an overview of the chemicals that have entered the market in the 21st century, we provide a brief

summary of the current nature of the problem of weed resistance to herbicides. We then go on to summarise breeding-

assisted and transgenic approaches towards the improvement of crop selectivity through the delivery of so called HT

(herbicide tolerant) or HR (herbicide resistant) crops and conclude with a discussion of the new herbicide discovery

screening tools that have been employed since the year 2000 and prospects for the future.

Major chemical trends after 2000

Abbreviations for Modes of Action and Inhibitors

ACCase: acetyl-CoA carboxylase

ALS: acetolactate synthase

CBI: cellulose biosynthesis inhibitors

HPPD: 4-hyddroxphenylpyruvate dioxygenase

PPO: protoporphyrinogen IX oxidase

VLCFA: very long chain fatty acid biosynthesis

Several new compounds have entered the herbicide market in recent years. Although not representing new modes of

action, they have increased the number of tools available for farmers to control weeds. Even in known and older

herbicidal classes, new, interesting and marketable molecules have been discovered. For example, and perhaps

surprising given the relative age of the class of herbicides, new (after 2000) ALS inhibitors have provided solutions for

farmers that can be regarded as real innovations.. One of them is mesosulfuron-methyl (Figure 1), a sulfonylurea

herbicide which, when combined with iodosulfuron-methyl sodium, has broad-spectrum post-emergence grass weed

control at dose rates of 4.5-15 g a.i. ha-1 (Safferling, 2005).

Another very successful new ALS herbicide is thiencarbazone-methyl (TCM; Figure 2), a compound of the

sulfonylaminocarbonyl-triazolinone subgroup. TCM is a broad-spectrum herbicide with a maximum seasonal use rate

of 45 g a.i. ha-1 that is able to control a wide range of grasses and broadleaf weeds. Due to its lack of inherent

selectivity, utility in a crop is only possible when combined with safeners such as mefenpyr-diethyl for cereals (Veness

et al., 2008) or the new safener cyprosulfamide for corn (Santel, 2012). Different safeners thus make TCM a product

for different crops. It can be used flexibly for pre- and post-emergence weed control and is a good example of a

herbicide that can be used in multiple situations given the right mixture partners. Although TCM controls a wide



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spectrum of weeds, it has gaps that require mixture partners. For example, IFT, (Figure 3), a 4-

hydroxyphenylpyruvate dioxygenase (4-HPPD)-inhibitor which is used together with the safener cyprosulfamide,

provides a good mixture partner for TCM in the pre-emergence control of weeds in corn.A suitable mixture partner for

post-emergence applications is another 4-HPPD-inhibitor, tembotrione (Figure 3), that is marketed together with the

safener isoxadifen-ethyl. One key selling point of this mixture is that it complements the efficacy of glyphosate and

glufosinate in HT-corn and provides resistance management options especially against glyphosate resistant weeds

(Müller et al., 2007).

A third ALS inhibitor that entered the market after the year 2000 is pyroxsulam (Figure 4; Wells, 2008). The

compound belongs to the ALS subgroup triazolopyrimidine sulfonamides and controls a broad spectrum of annual

grass and broadleaf weeds with an application rate of 9-15 g a.i. ha-1. Crop selectivity is achieved in wheat, rye and

triticale varieties, in combination with the safener cloquintocet-mexyl. To complete the weed spectrum pyroxsulam is

mixed with other products, e.g., with florasulam (Figure 4). It is also sold in a mixture with pendimethalin in Europe.

Since the first registration of pyroxsulam in Chile in 2007, the compound has taken significant market share and it has

become one of the most important herbicides for cereals in Europe. It is surprising that without exception, the latest

innovative ALS solutions having a significant market impact all depend on safeners for crop selectivity.

The 4-HPPD-inhibitor herbicides have included some remarkably successful introductions over recent years especially

in corn, but also in other crops (Ahrens et al., 2013). The first 4-HPPD products pyrazolynate, pyrazoxyfen and

benzofenap (Figure 5), were introduced to the market in the 1980s, and were used in rice production in Japan with very

high application rates of up to 4 kg a.i. ha-1 (van Almsick, 2012). The first HPPD-inhibitor for corn was sulcotrione

(Figure 6), a triketone with a somewhat lower but still relatively high application rate of 300 – 450 g a.i. ha-1 for post-

emergence control of mainly broadleaf weeds. The real market success began, however, with introduction of the

second generation of the triketone HPPD-inhibitors. Mesotrione (Figure 6) represented a significant innovation not

only because it could be applied at much lower rates than the previous generation but because it could be applied

either pre- or post-emergence. Rates of only 70-150 g a.i. ha-1 in post-emergence treatments and somewhat higher

rates of 100-225 g a.i. ha-1 in pre-emergence treatments are sufficient to achieve good control of targeted weeds

(Edmunds et al., 2012). To complete the spectrum it is always mixed with other compounds, for e.g., S-metolachlor

and atrazine or alternatively with terbuthylazine in countries where atrazine is no longer registered. Since its

introduction into the USA in 2001, mesotrione has been a major success. Sales of mesotrione-based products have

been steadily increasing, such that in 2007 it was already among the five best-selling herbicides worldwide (Cheung et

al., 2008).

Isoxaflutole (Figure 3) was also developed in the late 1990s for pre-emergence use in corn (Luscombe et al., 1995).

Even though the herbicide gives excellent control of selected broadleaf and grass weeds, the necessary application

rates to achieve such a broad spectrum were near 100 g a.i. ha-1. Unfortunately such rates led to problems in crop

selectivity from time to time. With lower application rates of 75 g a.i. ha-1 the crop injury problems could be solved,

but at that rate significant weed control was lost in certain grass weeds and there was a risk of ending up with only a

broadleaf herbicide. Once again the addition of a safener made the difference, allowing the use of the higher rate. An

additional triketone HPPD-inhibitor for post-emergence control in corn, tembotrione, has recently followed sulcotrione

and mesotrione into the market (Figure 3). It offers a broader weed spectrum than the older compounds and also has

outstanding selectivity in combination with the safener isoxadifen-ethyl (van Almsick et al., 2009).. With application

rates of 75–100 g a.i. ha-1, tembotrione is able to control common grass weeds, such as foxtails (Setaria spp.) and



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woolly cupgrass (Eriochloa villosa), but also a large number of broadleaved species. This includes a few glyphosate-,

ALS-, or dicamba-resistant weeds for example. Tembotrione is not persistent in soil and therefore does not limit crop

rotation opportunities for other crops in the following seasons. Another new representative of the 4-HPPD inhibitors

is pyrasulfotole (Figure 7), a pyrazolone compound related to the above mentioned rice HPPD-inhibitors in Figure 5

(Schmitt et al., 2008). With a weed control spectrum limited to broadleaves, the compound (in mixture with

bromoxynil or MCPA) is the first, and still only, HPPD-inhibitor herbicide in the cereal market. A new mode of action

for a crop often offers the chance to control weeds that have developed herbicide resistance. This is the case of for

ALS resistant Kochia scoparia for example. Once again this compound (pyrasulfotole) needed a safener, even though

its herbicidal activity on grass weeds is very limited, and thus theoretically possesses sufficient tolerance to monocot

crops. In combination with mefenpyr-diethyl a safe post-emergence use is possible in all varieties of wheat, barley and

triticale. A further interesting aspect is the synergism of HPPD inhibitors with a photosystem II inhibitor such as

bromoxynil that helps to limit the application rate of pyrasulfotole to 25 – 50 g a.i. ha-1 and serves to broaden the

broadleaf control spectrum. Mixture partners for additional grass weed control such as fenoxaprop-P-ethyl are

required. In this case, the safener mefenpyr-diethyl works for both compounds, fenoxaprop-P-ethyl and pyrasulfotole,

even though the mode of action of both is completely different (ACCase-inhibitor and HPPD-inhibitor).

There are additional compounds which complete the newest generation of HPPD-inhibitors (Figure 8) such as

topramezone for corn or tefuryltrione for rice. Other molecules in this class such as bicyclopyrone for corn and

fenquinotrione for rice are currently in development.

Completely nonselective herbicides are rarely found and developed. One relatively new compound that entered the

nonselective market in 2010 is indaziflam (Ahrens, 2011; Figure 9). The herbicide belongs to the so called alkylazine

class and is a cellulose biosynthesis inhibitor (CBI), representing a new mode of action for this market. From the

chemical point of view the compound represents a high degree of innovation in manufacturing because of the need to

synthesize this complicated chiral compound in relatively large quantities. Indaziflam controls weeds in established

permanent crops such as tree plantations, perennial crops such as sugar cane and in turf grasses. With application rates

of 73-95 g a.i. ha-1, indaziflam provides control of weeds up to 90 days or longer after treatment. When weeds are

present at application the addition of a foliar herbicide such as glyphosate or glufosinate-ammonium is useful due to its

limited post-emergent activity. To expand the spectrum of weed control indaziflam can be mixed with a range of other

herbicides such as metribuzin and isoxaflutole.

With the success of HT-crops and the ease of post-emergence applications combined with relatively low herbicide

costs, and combined with the perceived advantages of applying a herbicide only when weed growth was observed, the

end of residual herbicides was prophesied to have arrived a while ago. The situation has recently changed with the

appearance of glyphosate-resistant weeds such as Amaranthus tuberculatus and A.palmeri. The demonstrated

advantages of using pre-emergent herbicides to reduce the population of these highly competitive weeds that germinate

over an extremely long period (Hager et al., 2002) prompted companies to develop new residual products such as

saflufenacil (Figure 10), a PPO-inhibitor (Anon, 2008). This compound can be used alone or, more important, mixed

with glyphosate and applied pre-plant for burndown applications. Saflufenacil is therefore a useful addition to a very

important segment of glyphosate-tolerant crops where glyphosate use predominates (Knezevic et al., 2009). The

compound controls primarily dicot weeds, but it controls more than 80 of them including key driver weeds resistant to

glyphosate and ALS herbicides. It can be used as a pre-emergence treatment in corn and sorghum to control major

dicot or broadleaf weeds without triazine herbicides. It can also be used as a pre-plant burn down product for other

crops.



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Another new trend in weed control is the renaissance of auxinic herbicides (Figure 11), the class that provided the first

modern herbicides (Peterson, 1967). Compounds such as aminopyralid (Masters et al., 2012, aminocyclopyrachlor

(Claus et al., 2012) and halauxifen methyl are new representatives of this long- established mode of action (Schmitzer

et al., 2013). The latter of this group is supposed to enter the market in 2014 (http://newsroom.dowagro.com/press-

release/dow-agrosciences-announces-arylex-active-global-commercial-brand-name-new-herbicide). Aminopyralid

controls primarily broadleaf weeds including noxious, poisonous and invasive plants in rangeland, pasture and

industrial vegetation management sites. It was discovered and registered in the US for non-crop and turfgrass uses for

the control of annual and perennial broadleaves and brush weeds. Halauxifen methyl is also potentially useful as a

broadleaf herbicide in selected row crops and a potential mixture partner for cereal portfolios.

The inhibition of acetyl-CoA carboxylase (ACCase) is, like ALS, one of the most commercially important modes of

action for weed management, even though ACCase inhibitors are active only on grass weeds. The

aryloxyphenoxypropionates (fops) and cyclohexanediones (dims) have been present in the marketplace for more than

30 years. The only commercially available phenylpyrazoline ACCase-inhibitor with selectivity in cereals is pinoxaden

(Figure 12; Hofer et al., 2006). The herbicide is a post-emergence graminicide for a wide range of key annual grass

species in cereals at rates of 30-60 g a.i. ha-1. It is mixed with the safener cloquintocet-mexyl as previously mentioned.

Pinoxaden shows also some activity against several ACCase-inhibitor resistant biotypes but is not active against all of

them.

A new chemical entry within a well-known mode of action is pyroxasulfone (Figure 16), which belongs to a new class

of isoxazoline herbicides and is an inhibitor of the synthesis of very long-chain fatty acids (Nakatani et al., 2012). This

new herbicide demonstrates excellent efficacy against a broad range of grass and broadleaf weeds with both pre- and

post-emergence activities. It is selective for use on corn, soybean, cereals and cotton at application rates between 50

and 250 g a.i. ha-1. More importantly, the herbicide provides effective control of trifluralin-, ALS- and ACCase-

resistant Lolium rigidum in Australia and of glyphosate-resistant Amaranthus rudis in the US. In addition, it has a

favorable soil residual profile which allows its application to be extended from the very early pre-plant stage through

post-emergence stages without consequences to following crops. The compound was discovered by Kumiai Chemical

Industry Co, Ltd and is being developed by several companies for different crops. A second compound of the same

class is fenoxasulfone (Figure 13) which is currently undergoing development as a selective rice herbicide in Japan.

Table 1 summarizes the above mentioned new herbicides of the 21st century.

Herbicide resistance

Herbicide resistance has been defined in numerous ways (HRAC, 2014; WSSA, 1998; Heap and LeBaron, 2001), but

ultimately the definitions agree that a resistant weed is one that survives and reproduces following an herbicide

treatment that would normally kill it. The selection of survivors with existing traits that are present in a population at a

relatively low frequency is generally considered to be the antecedent to resistance and is set in motion through the

intensity of selection pressure (Holt & LeBaron, 1990; Neve et al., 2009; Powles & Yu, 2010; Délye et al., 2013).

Survival of an herbicide treatment results in selection of individual plants with the enabling resistance trait or traits,

giving them an opportunity to pass these on to future generations. Several factors including the biology and genetics

of the weed species, herbicide chemistry and its mode of action (MoA), as well as key agro-ecosystem characteristics

and herbicide application and handling influence the development of herbicide resistance, which follows evolutionary

processes (Darmency, 1994; Jasienuk et al., 1996; Christoffers, 1999; Powles and Yu, 2010).



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The possibility of herbicide resistance was first predicted over 50 years ago (Harper, 1956), just over a decade after the

introduction of the first modern commercial herbicide 2,4-D in 1945 (Peterson, 1967). The first occurrences of

resistance were reported just one year after Harper’s prophetic publication in two disparate cases (Heap, 2014). We

have been living with resistance to an increasing extent ever since. After a relatively quiet period in the 1950s and

1960s, the first big wave of resistance hit the PSII inhibitors which include the triazines (HRAC Group C1), in the

1970s, which was followed a decade later by the next wave of resistance to ALS inhibitors and ACCase inhibitors

beginning in the mid-1980s (Fig. 14). It is often enlightening to revisit discussions of the past, where fears of

resistance predominantly to products with longer soil residual activity were characterized as the major issue (Anon.,

1990). This has now been eclipsed by fears of resistance to products with little to no soil residual, to products that are

primarily applied to foliage. A few years after the introduction of crops resistant to glyphosate to the North American

market in 1996, the first case of resistance development by a weed Conyza canadensis in a row crop (soybean) was

reported (VanGessel, 2001). Glyphosate resistance in weeds was, however, already detected in a population of Lolium

rigidum in Australia as early as 1995 (Pratley et al., 1996). Since then the number of weed species resistant to

glyphosate has been growing steadily (Fig. 14) and this situation has been extensively reviewed (Powles, 2008; Duke

& Powles, 2009; Nandula, 2010; Vencill et al., 2012). Much has been said therein about what is driving this

phenomenon. Despite awareness of the problem and recognition of the danger of continuing the use of glyphosate as

the sole weed control measure, many farmers have been reluctant to change (Prince et al., 2012), even if studies show

long-term benefits to proactive resistance management (Norsworthy et al., 2012). One important contributing factor in

the overall resistance predicament is that over 75% of the global herbicide market is served by herbicides from only 6

MoAs as shown in Figure 19. The situation is similar for resistance to fungicides and insecticides, where

approximately 75% of the global market is served by 6 and 4 MoAs, respectively (Casida, 2009).

The rapid adoption of glyphosate as the single weed control measure in major American row crops, particularly

soybean and cotton had a profound effect on not only farmers, but also on the agrochemical industry. It led to a loss of

overall herbicide market value, reduction in the use of other herbicides and thus directly and indirectly contributed to

significant reductions in investment into herbicide discovery (Duke, 2005; Duke, 2012). These factors and the loss of

intellectual capacity (chemists and biologists) that followed the protracted consolidation of the industry (Rüegg et al.,

2007, Duke, 2012) are partially responsible for the lack of introductions of herbicides with new modes of action over

the last two decades. The industry is still recovering from this downturn.

Resistance confirmation

The first indication of resistance to an herbicide in a field is often a report of non-performance. Many cases of

reported resistance are actually weed control failures due to other causes, attributed usually to either agronomic or

climatic factors (Bayer CropScience, unpublished results, 2014). Thus proper testing methods are extremely important

to correctly assess whether the lack of expected efficacy was due to an agronomic issue, or truly due to resistance.

There is a well-accepted approach on how to respond to a field complaint where resistance is suspected. The first step

is to record detailed field observations including the herbicide treatment history, the next step is to properly sample

seeds, and then to test them in the greenhouse using (preferably) whole pot assay techniques, and the last, but

extremely important step, is interpreting the results in the proper context (HRAC, 1999; Beckie et al., 2000; Burgos et

al., 2013). It is better to test using more than just one discriminating rate and to generate dose-response curves using

several rates in order to determine the resistance factor or index correctly (HRAC, 1999). This is particularly

important with populations that have non-target site mechanisms of resistance, especially enhanced metabolism,

because these types of resistance impart variable levels of tolerance to herbicides (Beffa et al., 2012). Fig. 15 shows



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an illustration of the variability of enhanced metabolism within populations that exhibit various degrees of enhanced

metabolism. The radiochromatograms of four individual Alopecurus myosuroides plants from five populations, two

sensitive and three resistant to mesosulfuron methyl, demonstrated relatively low levels of metabolite formation in the

sensitive plants. In a few of the sensitive plants some metabolism had occurred as shown by the presence of

metabolites. The more rapid degradation of mesosulfuron methyl in these individual plants results in a higher degree

of resistance, which is represented by the presence of greener plants among the dying, yellow plants in the pot.

However, in most of the resistant plants a large number of metabolite peaks are observed with a corresponding

decrease in the mesosulfuron methyl peak. In some of the plants the relative size of the intact mesosulfuron methyl

peak is very low compared with some of the metabolite peaks, indicating that it has been extensively metabolized and

is no longer present at a concentration high enough to injure the weed. In one plant from one of the resistant

populations, hardly any metabolism has occurred (top row middle radiochromatogram), indicating that this plant is

most likely sensitive. The accompanying photograph illustrates that plants exhibiting enhance metabolism as a

resistance mechanism can show a high degree of variability within a population, partly due to the complex polygenic

control and accumulation of resistance alleles over several selection cycles (Busi et al., 2012; Délye, 2012).

Resistance Mechanisms

Weeds have evolved numerous mechanisms of resistance that can be classified broadly into two main types, target site

and non-target site (Powles & Yu, 2010; Beckie & Tardif, 2012). Mutations to the target site that confer resistance

have been well studied whereas non-target resistance mechanisms remain less clear (Powles & Yu, 2010). The first

group of resistance mechanisms, collectively known as Target Site Resistance (TSR), includes all modifications of

proteins targeted by herbicides including gene coding sequence mutations, gene over-expression, and gene duplication

(Powles & Yu, 2010; Délye et al., 2013). TSR generally confers a relatively narrow and generally high level of

resistance to weeds within a single MoA, but digressions from this do occur (Powles & Yu; 2010). Alteration of the

target site through mutations that modify herbicide binding and thus herbicidal efficacy can usually be effected by a

single nucleotide substitution, hence making it relatively easy to select for this type of resistance (Yu & Powles, 2014).

There can be differences in resistance expression to a particular target site mutation between subgroups (chemical

classes) within a single MoA, as for example between the aryloxyphenoxyproprionate, cyclohexadione and

phenylpyrazoline classes within the ACCase inhibitors (Yu et al., 2007; Délye et al., 2008). Other types of TSR, for

e.g., enhanced enzyme expression or increased gene copy number can increase the number of active enzymes and thus

sufficiently dilute the effective relative concentration of an herbicide, conferring resistance (Gaines et al., 2010; Délye,

2012). The second group of resistance mechanisms, known collectively as Non Target Site Resistance (NTSR), where

processes not directly involving the targeted proteins such as the modification of the herbicide penetration into the

plant, decreased rate of herbicide translocation, increased rate of herbicide sequestration, or metabolism confer

resistance (Powles & Yu, 2010; Délye, 2012). NTSR, especially enhanced metabolic resistance (EMR), can confer

resistance to a much broader range of herbicides (Powles & Yu, 2010; Délye, 2012). They are surmised to develop

through an accumulation of different mechanisms and are likely polygenetic (Délye, 2012), thus theoretically more

difficult to evolve. The use of lower than full herbicide rates has been implicated in the selection of NTSR through

cycles of selection of individuals with slightly enhanced metabolism and can evolve quite rapidly (Neve & Powles,

2005; Busi et al., 2012). Especially threatening for the future are herbicide-degrading cytochrome P450 (CytP450),

glutathione-S-transferases (GST) and other enzymes potentially able to detoxify current, relatively new, and future

herbicides, even herbicides from new structural classes yet to be discovered (Powles & Yu, 2010). Despite extensive

studies and reviews of herbicide resistance, genetic issues associated with resistance evolution have not yet been

extensively investigated (Powles and Shaner, 2001, Gressel, 2002, Busi et al., 2013). Modern molecular biology



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methods, and in particular new generation sequencing, are now being implemented. Initial results have identified GST

and CytP450 genes associated with EMR (Gaines at al 2014, in press). This approach contributes not only to helping

increase our basic knowledge of this kind of resistance but also has the potential to allow development of better

diagnostic tools. As resistance becomes more complex, accurate and sensitive resistance diagnostics tools can

contribute to making the best possible weed management decisions. An integrated approach to relieve selection

pressure on herbicides is critical to preserve their usefulness.

Organizations

The Herbicide Resistance Action Committee (HRAC) is one of the organizations concerned with herbicide resistance.

It is comprised of representatives of the agrichemical industry (Table 2) whose aim is to manage resistance by

fostering a responsible attitude to herbicide use, support and promote research, understanding causes of herbicide

resistance, communication of effective resistance management strategies and collaboration with public and private

researchers (HRAC, 2014). It is supported financially by member companies and CropLife International and though

without a set structure, its members meet regularly at global and regional levels to facilitate communication between

industry members. It supports the International Survey of Herbicide-Resistant Weeds (Heap, 2014), a survey of

confirmed resistance cases and is a good resource for the current state of resistance. One of the most recognized

projects is the classification of herbicide modes of action and embodied in the “World of Herbicides” poster available

online (http://www.hracglobal.com/Portals/5/moaposter.pdf). It is revised periodically to reflect new discoveries.

HRAC supports and participates in local, regional and global research into resistance to understand its causes and

effects as well as outreach programs to bring the best and latest knowledge to management programs. Local HRAC

organizations tailor their activities to specific issues within each area.

Other organizations such as the Weed Science Society of America, the European Weed Research Society, Asian-

Pacific Weed Science Society, and la Asociación Latinoamericana de Malezas (Latin American Weed Association) are

mentioned here as examples of regional institutions sponsoring research, organizing regular conferences, meetings and

workshops on weed resistance.

Management strategies

Recommendations for best management strategies begin with understanding the biology of the targeted weeds,

understanding the situation in the particular field, using a diversified approach including pre-emergent and post-

emergent herbicides with multiple MoAs at labelled rates in sequences and mixtures, and inclusion of non-chemical

practices including cultivation “where appropriate” (Norsworthy et al., 2012; Walsh & Powles, 2014). The inclusion

of non-chemical control methods and diversified cropping systems greatly aids consistency in weed control and slows

the evolution of resistance (Beckie, 2006; Walsh & Powles, 2014). More research needs to be done in combining

chemical and non-chemical methods in order to protect the continued utility of all herbicides. In response to the

worsening resistance situation, we must reexamine our thinking about herbicides as the sole weed control technology

to be implemented simply out of convenience. We are facing the loss of many more chemical tools through resistance

if we continue to rely exclusively on them. This loss would make weed management in many crops much more

difficult, and perhaps, impossible. We must become better at implementing integrated approaches.

Future of resistance

Acknowledgment of the current status of resistance as a threat to the production of some crops and its continued

development in intensity and complexity has led to calls for new herbicide options or a new “paradigm” in weed



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control (Tranel et al., 2011). We need to understand it better. In previous years much of the work by academia and

industry focused on the goal of preventing herbicide resistance. Given the economic and other factors driving weed

control decisions on the farm, the situation is changing to resignation that resistance is inevitable, and the best result

that one can achieve is to delay the onset of resistance (Neve et al., 2011). The solution to resistance has been stated

simply – getting farmers to add diversity in their weed control programs to reduce selection pressure from any one

means of control while at the same time keeping populations sufficiently controlled (Beckie, 2006). The key is

making this argument compelling to farmers and offering effective, integrated management tools at an affordable cost.

Until industry is successful at delivering new weed control products, we must continue to protect the remaining

chemical tools and increase the integration of non-chemical tools. Once a new herbicide is discovered and introduced

into the market, all efforts should be made to protect it from the beginning of its market introduction.

Herbicide tolerant crops

Safener technologies have allowed the introduction of novel weed control solutions in a number of crops such as

cereals, rice and corn. Safeners for dicot crops such as soybeans, oilseed rape and sugarbeet, however, could not be

found despite immense screening efforts by many companies. In the past broad spectrum, one-shot weed control was

only possible with mixtures. The rapid development of breeding and molecular engineering tools at the end of the last

century led some agrochemical companies to a completely new approach: the development of herbicide tolerant crops

(HTs). The first HT crop worldwide was a glyphosate tolerant soybean from Monsanto, which was deregulated and

approved from use by growers in 1994 in the US and commercialized in 1995 (see list of other HT crops in Table 3).

The first commercial example of herbicide tolerance in crop plants in Europe, bromoxynil tolerance based upon

expression of a bacterial nitrilase gene was deregulated in 1994 (one month after the US approval for Monsantos’s

glyphosate tolerant soybeans) and entered the market in 1995 (MacKenzie, 1994). It was developed by the French

company SEITA. In 1995 several other HT crops received commercial approval in the US (e.g., Calgene: Bromoxynil

tolerance in cotton, Monsanto: Glyphosate tolerance in cotton) and Canada (e.g., AgrEvo/PGS: Glufosinate tolerance

in canola, Monsanto: Glyphosate tolerance in canola). At that time DuPont also was working on a transgenic ALS

herbicide tolerance system (James and Krattiger, 1996). From the HT traits mentioned previously, only glyphosate

and glufosinate tolerance have gained a significant market share, with glyphosate tolerance being far ahead. The

adoption of herbicide tolerance traits took place at an unprecedented speed. In 2012 herbicide tolerant soybeans and

cotton had gained a market share of 81% worldwide. In 2013 93% of soybean acreage, 85% of corn acreage and 82%

of cotton acreage in the US has been planted with HT crops (USDA, 2013). Monsanto, together along with the seed

company KWS, introduced the glyphosate tolerant sugarbeet line H7-1 in the US in 2007. Two years later, in 2009,

approximately 96% of the US sugarbeet area was planted with this genetically modified sugarbeet line (Nehls et al,

2010). The ease of use and its efficacy against a broad range of weeds made glyphosate the by far most widely used

herbicide (Powles, 2008). In the US, the lack of rotation to other HT crops and limited use of herbicides other than

glyphosate can clearly be identified as the major factors contributing to the development of glyphosate resistance.

Canadian farmers, on the other hand, have tended to rotate different HT-systems such as glyphosate-, glufosinate- and

to a much lesser degree imidazolinine-tolerant canola with, as yet, only seven cases of weeds resistant to gyphosate

documented in Canada (as of 20 June 2014) (Heap I, 2014). It is becoming clearer meanwhile, that weed management

should not rely on a single herbicide, but that it is imperative to rotate and to use mixtures of herbicides with different

modes of action. All major agrichemical companies still participating in herbicide research have reacted to this need.

In their continuous efforts to control weeds, and especially glyphosate resistant weeds, one common response from

companies has been in the area of HT crops. Many companies have now have started to transfer multiple genes



13

conferring tolerance to several classes of herbicides with different modes of action into crops in order to provide

farmers with more options within a season. Corn plants tolerant to both glyphosate and glufosinate have been

developed recently. Glyphosate and glufosinate tolerant cotton plants have already been commercialized. Another

platform for corn and soybeans combines glyphosate tolerance with tolerance to 2,4-D. This particular technology is

awaiting its approval for commercialization in the US and is anticipated to enter the market in 2015. The same is true

for a platform that combines glyphosate tolerance with tolerance to dicamba, another herbicide from the auxin class is

also anticipated to enter the market in 2015. A further approach is the combination of tolerance to glyphosate with

tolerance to the sulfonylurea class of herbicides (ALS inhibitors). Finally, varieties tolerating HPPD-inhibitors plus

glyphosate and glufosinate will enter the market in a few years from now.

Mutation breeding of herbicide tolerant crops

Since the discovery of naturally occurring HT mutant plants and those not involving gene transfers, herbicide-tolerant

crops have also been conventionally bred. For instance, various herbicide-resistant canola culture systems are

currently available. Imidazolinone-tolerant Clearfield® canola was achieved through microspore mutagenesis and

selection with imazethapyr and conventional breeding. HT crops created by induced mutation and breeding are

classified as non - genetically modified crops (non-GM crops). Unlike genetically modified HT crops no heterologous

gene transfer is involved. The endogenous target gene is modified/mutated at the natural location in the plant’s

genome, thus position effects can be excluded.

Crop mutants can be created by different means (for review see Meksem and Kahl 2010). In brief the following

methods are used for HT mutant selection:

- Selection of spontaneous mutants with the herbicide

- Chemical mutagenesis and selection with the herbicide

- Chemical mutagenesis and target sequencing (Tilling*)

- Physically induced mutation with Ion beams

- Molecular breeding technologies

(*Targeting induced local lesions in genomes)

In vitro HT mutant selection: As a starting point for the selection of HT crop mutants, different plant material can be

used in plant cell and tissue culture. Depending on the crop and its properties in tissue culture, the starting materials

can be leaves, calluses, suspension cultures, protoplasts, microspore derived embryos, and immature embryo derived

cultures. However, the prerequisite for successful mutant selection experiments is that the plant material must be

rapidly growing with rapidly dividing cells that can be regenerated to fertile plants. The tissue culture approach is

advisable when spontaneous mutations are required in the target gene for herbicide tolerance.

In vivo HT mutant selection: As a starting point for the selection of HT crop mutants, plant materials can, for example,

be regenerable plant cells in tissue culture. Depending on the crop and its properties in tissue culture, the starting

materials can be leaves, calluses, suspension cultures, protoplasts, microspore derived embryos, and immature embryo

derived cultures. However, the prerequisite for successful mutant selection experiments is that the plant material must

be rapidly growing with rapidly dividing cells that can be regenerated to fertile plants. The tissue culture approach may

be preferable when spontaneous mutations are required in the target gene for herbicide tolerance. Nature can itself be a

source of non-GM HT crops with the selection of naturally occurring mutant plants. Another very successful method

to isolate HT mutants is the chemical mutagenesis of seeds e. g. with EMS, subsequently growing the seeds into the



14

M1 and the M2 generation followed by selection of HT mutants through herbicide applications or identifying

mutations/mutants in the target gene through sequencing (TILLING).

ALS-Tolerant Herbicide Systems

The Clearfield® system confers tolerance to crops otherwise susceptible to imidaziloninone (ALS) herbicides. It

consists of two elements: non-GM imidazolinone tolerant crops (Tan et. al., 2005) and the respective imidazolinone

herbicides which can be used selectively in the now tolerant crop. Since 1992 the Clearfield® technology has been

consequently introduced in several crops and launched to the market as shown in Table 3. The Clearfield® system of

BASF is currently marketed as a win-win situation for the farmer and the industry. The advantages for the farmer are

more weed control options. As a result, the advantage for the company is that the herbicide active ingredients from this

class will be utilized on a much broader scale (Pfenning, 2013).

Novel weed control in non-GM sugarbeet

In Europe and countries in other regions that do not accept GM crops there is a strong demand for effective one-pass

solutions in all dicot crops due to the lack of selective herbicide innovation (e.g., for sugarbeet). Phenmedipham-based

products have contributed to reliable weed control in sugarbeet for more than 40 years. However, no fundamentally

new herbicide active ingredients in sugarbeet have come onto the market for many years, unlike in other crops like

wheat or corn. Thus, a project to select sugarbeet mutants tolerant to ALS herbicides was started in 2001 by Bayer

CropScience. The technology is based on the breeding of sugarbeet varieties that are tolerant to herbicides in the ALS-

inhibitor-class with broad-spectrum weed control (Hain et al. 2012a,b). A mutant having a naturally occurring amino

acid substitution at position 574 in the ALS enzyme, which is involved in the biosynthesis of essential branched chain

amino acids, was selected and used in further breeding. It was very important that these varieties are not a product of

transfer to the crop genome from another organism so that they could be registered in Europe as a non-GM crop. In

spring 2012 Bayer CropScience and KWS SAAT AG signed an agreement to jointly develop and commercialize this

system for weed control in sugarbeet for the global market.

The novel herbicide tolerance trait was selected in Frankfurt approximately 10 years ago using sugarbeet cell culture

techniques. Out of about 1.5 billon cells tested one herbicide tolerant cell was selected and regenerated to produce a

sugarbeet plant labelled FM12-1, forming the basis for the development of the new weed control system. The number

of cells selected is equivalent to selecting one single sugarbeet plant out of 15,000 ha of the crop. Subsequently the

HT trait has been introduced into the elite sugarbeet germplasm of KWS by marker-assisted breeding.

Non-GM SU tolerance in soybean

At DuPont Pioneer a soybean line was developed through seed mutagenesis and rounds of selection through

application of a sulfonylurea herbicide normally not tolerated by soybean (Sebastian, et al., 1989). The mutant line

displays a high degree of ALS-based resistance to both post-emergence and pre-emergence applications of a variety of

SU herbicides (Kay et al., 2014, in press).

New discovery approaches

In parallel with the development of new herbicide tolerant crops, new screening tools have been employed for the

selection of chemicals with new herbicidal modes of action. With the discovery of new highly potent low-dose

herbicide classes, specifically the sulfonylureas in the 1980s, requirements for compound quantities for herbicide



15

testing decreased significantly, from grams down to the low milligram scale. Subsequently, in the mid-1990s

combinatorial chemistry, a novel synthesis tool able to produce hundreds of thousands of new chemical entities in

relatively short time, became possible (Smith, 2003; Lindell and Scherkenbeck, 2005; Scherkenbeck and Lindell,

2005; Lindell et al., 2009). As a result, plant pot-based primary screening for lead identification was replaced with

novel screening systems able to cope with high numbers of chemicals in a cost-effective way (Ridley et al., 1998). In

response to this, all major agrochemical companies introduced high-throughput screening technologies: both in-vitro

High-Through-Put-Biochemical (HTBS)- and High-Through-Put-In-Vivo (HTVS), and consequently the number of

compounds screened reached new heights (Figure 17). However, soon after the new screening technology was

adopted, it was realized that this enormous increase in screening input did not lead to the expected higher number of

strong hits and subsequent development projects (Kraehmer, 2012). As a result, screening inputs were lowered again

in favor of smaller and more diverse and targeted compound libraries (Figure 17).

From a purely statistical point of view, today an average of 140000 chemical compounds per indication need to be

screened in order to bring one new crop protection product to the market (Phillips McDougall, 2010). This translates

into 420000 compounds needed on average for a continuous product flow in all indications: fungicides, insecticides or

herbicides, including safeners.

Successful agrochemical research requires a constant input of novel chemistry to the screening cascade because, once a

chemical compound has proven to be inactive against the tested species, there is usually no reason to retest it again.

The primary objective is to find herbicidal activity. The next objective is to characterize this activity and potential for

crop selectivity. The bar at the screening level is usually set low enough to ensure that activity is found at reasonable

use rates. Sources of chemical innovations for herbicide research arise from in-house chemistry research, other

indications, life science compound pools, commercial providers, academia, natural products and others. The huge

numbers of chemical compounds being processed require large-scale automated storage and retrieval systems for

sample management, together with powerful logistics, all serving the individual indications in an efficient way.

Screening is defined as the stepwise assessment of the biological activity of a compound leading to strong candidates

for field development testing. The basic principles of compound screening in agrochemical research have been

described in several review articles (Giles, 1989; Copping, 2002; Cobb and Reade, 2010). This process can be broken

down into two main consecutive steps: Lead Finding and Lead Optimization. Usually, although sometimes named

differently, the test procedure normally consists of a primary and secondary screening, followed by field trials (Giles,

1989). As a result of the strongly increased input numbers observed at the end of the last century, high-throughput

screening systems (HTS) were introduced as initial screening tools (Figure 18). HTS in agrochemical discovery has

been reviewed recently (Tietjen et al., 2005; Drewes et al., 2012). Another approach to discover new herbicidal

precepts, consists of the systematic analysis of plant gene functions (Lein et al., 2004). This approach is expected to

aid the development of new herbicidal target assays. The study of small-molecule metabolite profiles, generally

referred to as metabolomics (Kamp et al., 2012), and gene expression profiling (GEP) (Eckes and Busch, 2012) are

valuable tools in this context. Alternatively, new herbicidal leads may also arise from the combination of whole plant

screening with physiological investigations, recently defined as physionomics (Grossmann et al., 2012). All these

approaches are covered under the general concept of systems biology, which is a more holistic approach to biological

research (Kitano, 2002).

One big advantage of agrochemical screening over pharmaceutical screening is that agrochemicals can be directly

applied to the living target organisms in early screening stages. There is no intrinsic need to start with a model system.

Relevant properties like compound uptake, speed of action, metabolism are directly covered within whole organism



16

screening. However, hit identification in HTVS is limited to metabolically stable and bioavailable compounds. Any

active ingredient with marginal stability or poor bioavailability can rarely be identified with whole-organism

screening. In consequence, both techniques, HTBS and HTVS, due to their complementarity, are required in

agrochemical discovery.

In HTVS, the 96-well micro-titer plate (MTP) format is used extensively across the agrochemical industry. Substance

requirements to provide information on the herbicidal potential against selected target plants are generally low (Tietjen

et al., 2005). There are several criteria to consider concerning the plant species being used in HTVS, e.g., the size of

the seeds, germination potential, ease of visual assessment, representation of the species in downstream screening

levels, or the relative importance of the test plant with respect to the target markets.

Despite being extensively employed, use of automated 96-well MTP assessment techniques remains a challenge when

screening using HTVS. Innovations in small-scale whole plant imaging technologies remain very limited compared to

other recent developments, e.g., high content screening (HCS) or other areas of image analysis, that have taken place

in pharmaceutical research (Haney, 2008). Plant growth assessment using image analysis ideally provides a three

dimensional view, but this makes it very challenging to fulfil all requirements for fully automated systems applied to

continually growing plants. Today the numbers of sensors for plant phenotyping are numerous: RGB, Fluorescence or

NIR are standard technologies which are used for many assays with image acquisition restricted to a two-dimensional

perspective when considering the top view for micro-titer plates. The high density of plants grown in a small area

restricts the use of available standard technologies. On the other hand, there is good progress in image analysis

software. Powerful hardware and commercial imaging software enables trained users to evaluate herbicide screening

trials (also restricted to 2D) in totally different ways. The combination of new sensors, time-lapse imaging and ultra-

high quality images provide by far much deeper insights than any standard visual assessment technologies or

techniques. Software tools like, Methamorph® by Molecular Devices or the Lemnatec image analysis platforms are

well established tools to support the screener in plant phenotyping to extract useful information out of images.

With the introduction of HTS, an enormous increase in test data followed, exceeding 100,000 data points per day on

every single technology platform. These experimental raw data need to be stored in appropriate databases and

processed for the development of structure-activity-relationships (SAR). Research at Bayer CropScience for example

applies ActivityBase® from ID Business Solutions as a data management tool and Spotfire® DecisionSite for

visualization of screening results (Tietjen et al., 2005). This, together with tailored in-house information technology

solutions, permits a rapid correlation of biological results for the high numbers of chemical structures over all

screening levels. In this context, it has to be stressed that a close and effective interaction between the individual

research departments, specifically Chemistry, Biology, and Biochemistry is crucial, given the fact that the discovery of

a development candidate is primarily based on iterative cycles of syntheses and screening, thus optimizing initial lead

compounds, rather than just filtering ‘the right compound’ from a big substance pool. Finding a product like this

(merely by filtering), in fact, is a very rare event.

Abbreviations of screening terms

2D 2-Dimensional BCS Bayer CropScience HCS High-Content Screening HTS High-Throughput Screening HTBS High-Throughput Biochemical Screening HTVS High-Throughput in-Vivo Screening MTP micro-titer plate



17

NIR Near Infrared RGB Red-Green-Blue SAR Structure-Activity Relationship

Outlook

The world is likely facing its biggest challenge ever in our ability to feed our global population. According to the

FAO, agricultural production must increase by 70% until 2050 to supply 9 billion people with sufficient food (FAO,

2009). Current yield trends suggest that our efforts to raise production are insufficient. A turnaround for yield

increases in broad acre crops as the basis for world food security is urgently required. Furthermore, by 2050 more than

6 billion people (67 %) will live in urban areas, ranging from an urbanization level of 86 % in developed countries to

64 % in less developed regions (United Nations, 2012). In view of limited possibilities to expand the area of arable

land, high-yielding but sustainable agriculture is the only plausible alternative. Preservation of soil, maintenance of

soil fertility and high water use efficiency, as well as maintaining high levels of local biodiversity, are of key

importance for sustainably managing the global bio-economy.

To fully exploit the maximum yield potential in crop production, weed control with management programs based upon

effective herbicides is of utmost importance for sustainable land use. Along with preventing potential yield losses,

other measures to help improve soil fertility, including minimizing wind and water erosion as well as enabling an

increase of organic matter are necessary. Today’s modern crop production has all too often abandoned diverse crop

rotations and tillage, trading them for monoculture crops and no till/low till technologies relying on an effective

herbicide technology. However, after several decades of herbicide use, weed resistance to major chemical classes

continues to spread further. Currently more than 60 % of the global herbicide market (value) is represented by

products from only 4 modes of action, all of which actually have serious resistance issues (Figure 19). With an

additional 3 modes of action, together they cover approximately 80% of the global market in value. Due to the slower

growth of weed control markets during the past few decades, increased costs for discovering and developing new

active ingredients, as well as the impact of glyphosate crop tolerance adoption in the immense North and South

American corn and soybean markets led to significantly reduced efforts in global herbicide discovery. As a

consequence of these factors the herbicide industry has undergone a protracted consolidation process, which continues

to impact agriculture.

The analysis of published patent applications for new active ingredient indicates a striking decrease of the numbers of

applications since 1990 (Fig. 20). It is obvious that only a very small group of companies remain actively engaged in

discovery of novel herbicide candidates. A review of the known development pipeline shows the same tendency. It is

estimated that from now until 2020 only 5–10 new herbicides will enter the market. Another indication of the effects

of the market forces on herbicide discovery is the number of products introduced by decade. Whereas approximately

50 new active ingredients were introduced per decade in the eighties and nineties of the last century only circa 20

herbicides were launched within the last decade (Fig. 21).

Due to an increasing lack of effective herbicide solutions and an increase in multiple resistant populations, weed

control has become more complex in order to combat resistant weed species in major broad acre crops. The soaring

agro-economy as well as greater inputs into weed management has induced further growth of the herbicide market,

which will exceed 20 bn€ per year by 2020 (BASF News Release 10 Mar 2011, Crop Protection pipeline value jumps

to € 2.4 billion. http://www.agro.basf.com/agr/AP-



18

Internet/en/function/conversions:/publish/content/news_room/news/downloads/11-03-10-press-release-basf-crop-

protection-pipeline-value-jumps-to-2.4-billion-eur.pdf). The demand for new resistance management solutions is

rewarding the renewed focus on herbicide discovery. However, the regulatory requirements to develop and register

new herbicides are ever increasing, especially in Europe. Consequently, the total cost for discovery and development

of one new herbicidal active ingredient is approaching 200 million € (Phillips Mc Dougall, 2012). These costs could

continue to increase further.

To achieve a sufficient return on investment for those rising R & D cost the industry requires increasing business

opportunities per development candidate. The development of single new herbicides exclusively for smaller or niche

crops is economically unfavorable. The threshold for entry of new players in herbicide discovery is extremely high,

thus no new herbicide research oriented companies are expected to enter the market. However various research

institutes in China are increasingly engaged in herbicide discovery, but thus far are lacking in internationally focused,

integrated weed research entities.

Due to recent extraordinary market dynamics some companies are once again increasing their engagement in herbicide

discovery. Their main strategies are:

- Mode of action & target identification of herbicidal leads followed by high throughput screening and/or

structure based design

- Design of new herbicidal structural scaffolds with validated targets

- Agrophore synthesis strategies

To fully exploit the discovery potential, a fully integrated approach employing all state of the art technologies and

scientific approaches is essential. However, based on analysis and assessment of financial analyst publications as well

as patent applications (Figure 20), a market introduction of significant new herbicide classes having new resistance-

breaking modes of actions can probably only be expected after 2025.

Therefore in the near future all stakeholders must contribute to the sustainability of current herbicides by adopting

diversity - in crop rotations, herbicide combinations and sequences as well as other non-chemical measures. The

industry has to take responsibility to redouble its herbicide discovery efforts and supply agriculture with new, effective

resistance-breaking herbicides.



19

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25

FIGURE LEGENDS

Figure 1: The ALS-inhibitors mesosulfuron-methyl and iodosulfuron-methyl-sodium

Figure 2: The ALS-inhibitor thiencarbazone-methyl

Figure 3: The 4-HPPD-inhibitors isoxaflutole and tembotrione

Figure 4: The ALS-inhibitors pyroxsulam and florasulam

Figure 5: The first HPPD-inhibitors for rice: Pyrazolynate, pyrazoxyfen and benzofenap

Figure 6: The HPPD-inhibitors sulcotrione and mesotrione

Figure 7: The HPPD-inhibitor pyrasulfotole for cereals

Figure 8: Further HPPD-inhibitors: topramezone, tefuryltrione, bicyclopyrone and fenquinotrione

Figure 9: The CBI indaziflam

Figure 10: The PPO-inhibitor saflufenacil

Figure 11: The auxins aminopyralid, aminocyclopyrachlor and halauxifen methyl

Figure 12: The ACCase-inhibitor pinoxaden

Figure 13: Pyroxasulfone and fenoxasulfone

Figure 14. Number of resistant species for herbicides with selected sites of action (HRAC codes). Note: PSII

Inibititors Combined. With permission of Dr. Ian Heap, WeedScience.org 2014.

Figure 15. Radiochromatograms of sensitive (S) and resistant (R) Alopecurus myosuroides plants incubated with 14C-

labelled mesosulfuron methyl. The symbol ▲ indicates the position of the chromatographic peak of the intact active

substance (mesosulfuron methyl) and peaks to the left are those of inactive metabolites. From Beffa et al., 2012.

Figure 16: Selection of non-GM HT-mutants

Figure 17. Evolution of compound input to herbicide screening over time, highlighting the impact of combinatorial

chemistry (dotted circle).

Figure 18. Modern herbicide screening process based on screening technologies with numbers of compounds and

active ingredien requirements.

Figure 19: Herbicide Market 2012 per mode of action (without trait fees). Shaded background indicates MoAs with

significant resistance problems.

Figure 20: Active herbicide ingredient patent applications from 1990 until 2012

Figure 21: Active herbicide ingredient introductions by decade from 1945 until 2015



26

Table 1 Summary of some new compounds developed for weed control after 2000

(MoA = mode of action, post = post-emergence application, pre = pre-emergence application)

MoA, target site Examples Use Launch date Auxins Aminopyralid Post, rangeland,

industrial sites, dicots

2005

Aminocyclopyrachlor Post, non-crop, brush control

2010

Halauxifen-methyl Post, dicots in cereals

expected 2014

Cellulose biosynthesis

Indaziflam Plantations, turf 2010

AHAS- or ALS- Inhibitors

Mesosulfuron-methyl Post, cereals, grasses

2001

Thiencarbazone-methyl

Post, cereals and corn

2008

Pyroxsulam Post, cereals, grasses

2008

HPPD-inhibitors Topramezone Post, corn 2006 Tembotrione Post, corn 2007 Pyrasulfotole Post, cereals,

dicots 2008

Tefuryltrione Post, rice 2010 Bicyclopyrone Post, corn and

sugarcane unknown

Fenquinotrione Not specified unknown

Protoporphyrinogen oxidase

Saflufenacil Pre and post, various crops

2010

ACCase Pinoxaden Post, cereals, grasses

2006

Very long chain fatty acid biosynthesis

Pyroxasulfone Pre + post; various crops, monocots and dicots

2012

Fenoxasulfone Pre + post; various crops, monocots and dicots

unknown



27

Table 2. HRAC Global Member Companies

BASF

Bayer Crop Science

Dow AgrowSciences

DuPont Crop Protection

FMC

Makhteshim Agan

Monsanto

Syngenta Crop Protection

Sumitomo



28

Table 3. Year of Commercial Introduction of Herbicide Tolerant Crops

Year Crop Type

1992 corn Non-transgenic

1995 canola, cotton & soybeans Transgenic

2001 wheat Non-transgenic

2002 rice Non-transgenic

2003 sunflower Non-transgenic

2006 lentils Non-transgenic

2011 winter oilseed rape Non-transgenic



29

SNH

NH

N

N

OO O

CO2Me

OMe

OMe

NHSO2Me S

N NH

N N

N

OO O

I

CO2Me

OMe

Na+

mesosulfuron-methyl iodosulfuron-methyl-sodium

Figure 1: The ALS-inhibitors mesosulfuron-methyl and iodosulfuron-methyl-sodium



30

SNH

NN

N

OO

S

O O

CO2MeOMe

Figure 2: The ALS-inhibitor thiencarbazone-methyl



31

O

O

N

SO2Me

CF3

Cl

O

O

OH

O

SO2Me

CF3

isoxaflutole tembotrione

Figure 3: The 4-HPPD-inhibitors isoxaflutole and tembotrione



32

N

N N

NNH

S

NO

O

OMe

OMe

OMe

CF3 N N N

NS

NH

F

F

OO

F

OMe

pyroxsulam florasulam

Figure 4: The ALS-inhibitors pyroxsulam and florasulam



33

Cl

Cl

O

N

NO

SO

O

Cl

Cl

O

N

NO

O

Cl

Cl

O

N

NO

O

pyrazolynate pyrazoxyfen benzofenap

Figure 5: The first HPPD-inhibitors for rice: Pyrazolynate, pyrazoxyfen and benzofenap



34

ClO

OH

O

SO2Me

O

OH

O NO2

SO2Me

sulcotrione mesotrione

Figure 6: The HPPD-inhibitors sulcotrione and mesotrione



35

O

N

NOH

SO2Me

CF3

Figure 7: The HPPD-inhibitor pyrasulfotole for cereals



36

ONO

N

NOH SO2Me

Cl

O

O

O

OH

O

SO2Me

N

OO

OH

O OMe

CF3

N

N

O

Cl

O

OH

OOMe

topramezone tefuryltrione

bicyclopyrone fenquinotrione

Figure 8: Further HPPD-inhibitors: topramezone, tefuryltrione, bicyclopyrone and fenquinotrione



37

N

N

N

F

NH2NH

Chiral

SR

Figure 9: The CBI indaziflam



38

N

N O

NH

SN

O OO

ClFO

CF3

Figure 10: The PPO-inhibitor saflufenacil



39

N

NH2

Cl

Cl CO2H

N

N

NH2

Cl

CO2H

N

NH2

Cl

FCl

CO2Me

OMe

aminopyralid aminocyclopyrachlor halauxifen methyl

Figure 11: The auxins aminopyralid, aminocyclopyrachlor and halauxifen methyl



40

Figure 12: The ACCase-inhibitor pinoxaden



41

pyroxasulfone fenoxasulfone

Figure 13: Pyroxasulfone and fenoxasulfone



42

Figure 14. Number of resistant species for herbicides with selected sites of action (HRAC codes). Note: PSII

Inibititors Combined. With permission of Dr. Ian Heap, WeedScience.org 2014.



43

Figure 15. Radiochromatograms of sensitive (S) and resistant (R) Alopecurus myosuroides plants incubated with 14C-

labelled mesosulfuron methyl. The symbol ▲ indicates the position of the chromatographic peak of the intact active

substance (mesosulfuron methyl) and peaks to the left are those of inactive metabolites. From Beffa et al., 2012.



44

Figure 16: Selection of non-GM HT-mutants



45

Figure 17. Evolution of compound input to herbicide screening over time, highlighting the impact of combinatorial

chemistry (dotted circle).



46

Figure 18. Modern herbicide screening process based on screening technologies with numbers of compounds and

active ingredient requirements.



47

Figure 19: Herbicide Market 2012 per mode of action (without trait fees). Shaded background indicates MoAs with

significant resistance problems.

EPSPS

ALS

AuxinsACCase

PSII

VLCFA

HPPD

Others



48

Figure 20: Active ingredient patent applications from 1990 until 2012



49

Figure 21: Active ingredient introductions by decade from 1945 until 2015



Documents

Herbicides state of the art. II. Achievements …...2014/08/07 · Isoxaflutole (Figure 3) was also developed in the late 1990s for pre-emergence use in corn (Luscombe et al., 1995)