Prevalence of herbicide‐resistant weed species in Malaysian rice fields: A review

The management of weeds in Malaysian rice fields is very much herbicide‐based. The heavy reliance on herbicide for weed control by many rice‐growers arguably eventually has led to the development and evolution of herbicide‐resistant biotypes in Malaysian rice fields over the years. The continuous use of synthetic auxin (phenoxy group) herbicides and acetohydroxyacid synthase‐inhibiting herbicides to control rice weeds was consequential in leading to the emergence and prevalence of resistant weed biotypes. This review discusses the history and confirmed cases and incidence of herbicide‐resistant weeds in Malaysian rice fields. It also reviews the Clearfield Production System and its impact on the evolution of herbicide resistance among rice weed species and biotypes. This review also emphasizes the strategies and management options for herbicide‐resistant rice field weeds within the framework of herbicide‐based integrated weed management. These include the use of optimum tillage practices, certified clean seeds, increased crop competition through high seeding rates, crop rotation, the application of multiple modes of action of herbicides in annual rotations, tank mixtures and sequential applications to enable a broad spectrum of weed control, increase the selective control of noxious weed species in a field and help to delay the resistance evolution by reducing the selection pressure that is forced on those weed populations by a specific herbicidal mode of action.     

Dealing with transgene flow of crop protection traits from crops to their relatives

Genes regularly move within species, to/from crops, as well as to their con‐ specific progenitors, feral and weedy forms (‘vertical’ gene flow). Genes occasionally move to/from crops and their distantly related, hardly sexually interbreeding relatives, within a genus or among closely related genera (diagonal gene flow). Regulators have singled out transgene flow as an issue, yet non‐transgenic herbicide resistance traits pose equal problems, which cannot be mitigated. The risks are quite different from genes flowing to natural (wild) ecosystems versus ruderal and agroecosystems. Transgenic herbicide resistance poses a major risk if introgressed into weedy relatives; disease and insect resistance less so. Technologies have been proposed to contain genes within crops (chloroplast transformation, male sterility) that imperfectly prevent gene flow by pollen to the wild. Containment does not prevent related weeds from pollinating crops. Repeated backcrossing with weeds as pollen parents results in gene establishment in the weeds. Transgenic mitigation relies on coupling crop protection traits in a tandem construct with traits that lower the fitness of the related weeds. Mitigation traits can be morphological (dwarfing, no seed shatter) or chemical (sensitivity to a chemical used later in a rotation). Tandem mitigation traits are genetically linked and will move together. Mitigation traits can also be spread by inserting them in multicopy transposons which disperse faster than the crop protection genes in related weeds. Thus, there are gene flow risks mainly to weeds from some crop protection traits; risks that can and should be dealt with. © 2014 Society of Chemical Industry     

Establishment of Lolium species resistant to acetolactate synthase‐inhibiting herbicide in and around grain‐importation ports in Japan

Compared with natural seed dispersal, human‐mediated seed dispersal could spread herbicide resistance genes on a much larger scale. Herbicide‐resistant weed seeds have been reported as contaminants in commercial grain. We investigated the contamination of seeds of Lolium species with target‐site mutations conferring resistance to acetolactate synthase (ALS)‐inhibiting herbicides in wheat imported from the USA, Canada and Australia into Japan. We also investigated the establishment of ALS‐inhibiting herbicide‐resistant Lolium species in 12 seaports in Japan that are major entry points for international commodities. We found herbicide‐resistant Lolium spp. seeds from all classes of wheat samples. Resistant individuals became established at six of eight ports where more than 50 kt of imported wheat is unloaded every year. The establishment of resistant Lolium spp. individuals was common at major grain landing ports. Monitoring over 3 years at one port revealed that the frequency of resistant individuals did not fluctuate between years. Many resistant individuals were distributed in front of the entrance of a fodder company, but a few resistant individuals were found in areas 2 km away from the port. The results indicate that gene flow is rare through pollen or seed movement from resistant plants to peripheral populations. Further extensive and long‐term monitoring is necessary to perform a comprehensive risk assessment of herbicide‐resistant plants entering Japan through major commercial ports.     

Simple sequence repeat analysis of genetic diversity among Acetyl‐CoA carboxylase inhibitor‐resistant and ‐susceptible Echinochloa crus‐galli and E. oryzicola populations in Korea

Echinochloa species are amongst the most problematic weeds in rice fields of Korea. The steady reliance on the Acetyl‐CoA carboxylase (ACCase) and acetolactate synthase inhibiting herbicides for control of these weeds has led to resistance to these herbicides. The objective of this study was to assess the genetic diversity among populations of ACCase inhibitor‐resistant and ‐susceptible Echinochloa crus‐galli and E. oryzicola in Korea, to better understand their population structure and possible origins of resistance. Seven simple sequence repeat markers were applied to assess the genetic diversity between resistant and susceptible E. crus‐galli and E. oryzicola from 12 populations in Korea. Genetic diversity was slightly higher in the resistant group. The Unweighted Pair Group Method using Arithmetic algorithm (UPGMA) dendrogram generated two distinct clades. One clade consisted of Echinochloa spp. from three populations, i.e. Anmyeondo, Gimje 4 and Gongju, which are resistant and differentiated from the susceptible populations, and the other clade contained the rest of the populations. Structure modelling supported two clades of UPGMA clustering. Based on these data, we can infer that some resistant populations are greatly differentiated, whereas other resistant biotypes are still building up resistance in rice fields in Korea. Resistance traits will be fixed and continue to spread over time without proper control measures.     

Herbicide-resistant genetically modified crop: assessment and management of gene flow

Genetically modified (GM) crops have become a reality in our cropping system. The experiences with GM oilseed rape have shown that gene flow from a GM crop causes genetic contamination of non‐GM crops and natural flora. This review summarizes technically available methods for gene flow assessment and proposes possible management methods. Methods for direct monitoring of gene flow include direct bioassay of plants and detection of phenotypic and molecular genetic markers contained in GM crops. A recent green fluorescent protein (GFP) marker technique can be powerful in monitoring gene flow as GFP inserted into a plant can be observed macroscopically under UV light. Appropriate analysis of data from direct assessment may give more useful information to mitigate gene flow. Observation with direct method provides real‐time data and mathematical‐statistical approaches may enable the long‐term consequence to be predictable. Although an estimated gene flow is less than an acceptable level, gene flow must be maintained as low as possible with a systematic management. The management should be conducted stepwise; selection of gene flow‐proof GM crops in the stage of development, risk assessment and regulation in the registration stage, cultural management, produce handling/transportation and a long‐term monitoring in cropping stage. Promising methods for developing gene flow‐proof GM crops include conferring cleistogamy and chloroplast transformation to mitigate pollen flow, and breeding non‐ or minimum shedding cultivars to mitigate seed dispersal. We strongly suggest that very high expression of a transgene or stacking multiple transgenes in the chloroplast could disturb the function of normal physiology, hence decreased performance of the GM crop. Before the approval of GM crops, proposed GM crops must go through the risk assessment. If the estimated risk of a GM crop exceeds an acceptable level, approval must be suspended. Once a GM crop is allowed for commercial release, additional efforts must follow, such as a continued long‐term monitoring of the impact of GM crop cultivation, crop and herbicide rotations, GM crop‐suited cultural practices, ‘right‐time’ harvest, and all necessary gene flow‐preventive practices. Such a systematic management incorporating various methods for the stages of GM crop utilization will minimize the risk of gene flow.     

A strategy to provide long‐term control of weedy rice while mitigating herbicide resistance transgene flow,and its potential use for other crops with related weeds

Transgenic herbicide‐resistant rice is needed to control weeds that have evolved herbicide resistance, as well as for the weedy (feral, red) rice problem, which has been exacerbated by shifting to direct seeding throughout the world—firstly in Europe and the Americas, and now in Asia, as well as in parts of Africa. Transplanting had been the major method of weedy rice control. Experience with imidazolinone‐resistant rice shows that gene flow to weedy rice is rapid, negating the utility of the technology. Transgenic technologies are available that can contain herbicide resistance within the crop (cleistogamy, male sterility, targeting to chloroplast genome, etc.), but such technologies are leaky. Mitigation technologies tandemly couple (genetically link) the gene of choice (herbicide resistance) with mitigation genes that are neutral or good for the crop, but render hybrids with weedy rice and their offspring unfit to compete. Mitigation genes confer traits such as non‐shattering, dwarfism, no secondary dormancy and herbicide sensitivity. It is proposed to use glyphosate and glufosinate resistances separately as genes of choice, and glufosinate, glyphosate and bentazone susceptibilities as mitigating genes, with a six‐season rotation where each stage kills transgenic crop volunteers and transgenic crop × weed hybrids from the previous season. Copyright © 2009 Society of Chemical Industry     

Development of herbicide resistance in weeds in a crop rotation with acetolactate synthase‐tolerant sugar beets under varying selection pressure

The development of acetolactate synthase (ALS) tolerant sugar beet provides new opportunities for weed control in sugar beet cultivation. The system consists of an ALS?inhibiting herbicide (foramsulfuron + thiencarbazone‐methyl) and a herbicide‐tolerant sugar beet variety. Previously, the use of ALS‐inhibitors in sugar beet was limited due to the susceptibility of the crop to active ingredients from this mode of action. The postulated benefits of cultivation of the ALS‐tolerant sugar beet are associated with potential risks. Up to now, with no relevant proportion of herbicide‐tolerant crops in Germany, ALS‐inhibitors are used in many different crops. An additional use in sugar beet cultivation could increase the selection pressure for ALS‐resistant weeds. To evaluate the impact of varying intensity of ALS‐inhibitor use on two weed species (Alopecurus myosuroides and Tripleurospermum perforatum) in a crop rotation, field trials were conducted in Germany in two locations from 2014 to 2017. Weed densities, genetic resistance background and crop yields were annually assessed. The results indicate that it is possible to control ALS‐resistant weeds with an adapted herbicide strategy in a crop rotation including herbicide‐tolerant sugar beet. According to the weed density and species, the herbicide strategy must be extended to graminicide treatment in sugar beet, and a residual herbicide must be used in winter wheat. The spread of resistant biotypes in our experiments could not be attributed to the integration of herbicide‐tolerant cultivars, although the application of ALS‐inhibitors promoted the development of resistant weed populations. Annual use of ALS‐inhibitors resulted in significant high weed densities and caused seriously yield losses. Genetic analysis of surviving weed plants confirmed the selection of ALS‐resistant biotypes.     

First glyphosate‐resistant Lolium spp. biotypes found in a European annual arable cropping system also affected by ACCase and ALS resistance

In Europe, glyphosate‐resistant weeds have so far only been reported in perennial crops. Following farmers' complaints of poor herbicide efficacy, resistance to glyphosate as well as to ACCase and ALS inhibitors was investigated in 11 populations of Lolium spp. collected from annual arable cropping systems in central Italy. Field histories highlighted that farmers had relied heavily on glyphosate, often at low rates, as well as in a non‐registered crop. The research aimed at elucidating the resistance status, including multiple resistance, of Lolium spp. populations through glasshouse screenings and an outdoor dose–response experiment. Target‐site resistance mechanism was also investigated for the substitutions already reported for EPSPs, ALS and ACCase genes. Three different resistant patterns were identified: glyphosate resistant only, multiple resistant to glyphosate and ACCase inhibitors and multiple resistant to glyphosate and ALS inhibitors. Amino acid substitutions were found at position 106 of the EPSPs gene, at position 1781, 2088 and 2096 of the ACCase gene and at position 197 and 574 of the ALS gene. Not all populations displayed amino acid substitutions, suggesting the presence of non‐target‐site‐mediated resistance mechanisms. After 39 years of commercial availability of glyphosate, this is the first report of multiple resistance involving glyphosate selected in annual arable crops in Europe. Management implications and options are discussed.     

Identifying the problem weeds of rice‐based systems along the inland‐valley catena in the southern Guinea Savanna,Africa

While weeds generally are considered as the most important overarching production constraints in inland‐valley cropping systems in West Africa, little is known about species' associations with environmental and crop management factors. Weed species' associations with seasonal and environmental factors, such as their position on the catena, soils and cropping systems, were studied during two seasons (dry and wet) in 45 arable fields of three inland valleys in south‐western Benin, Africa. The three most dominant weed species were Dactyloctenium aegyptium, Commelina benghalensis and Digitaria horizontalis on the inland‐valley crests (uplands), Ludwigia hyssopifolia, Corchorus aestuans and Ludwigia octovalvis on the sloping hydromorphic fringes and Leersia hexandra, Ipomoea aquatica and Fimbristylis ferruginea in the valley bottoms (lowlands). Echinochloa colona, Cleome viscosa and Talinum triangulare were the three most dominant species in the dry‐season crops (maize or vegetables) and Leer. hexandra, I. aquatica and Sphenoclea zeylanica were the three most dominant species in the wet‐season crop (rice). Ageratum conyzoides, Synedrella nodiflora and D. horizontalis were observed throughout the catena. Problem weeds in inland‐valley agro‐ecosystems are those that combine a high frequency with a high submergence tolerance and ecological plasticity, C₄ grasses, perennial C₃ species with persistent root structures and broad‐leaved species with high propagation rates. Weed management strategies that are aimed at increasing the resilience of rice‐based cropping systems in the inland valleys of the southern Guinea Savanna of Africa should address the categories of problem species that were identified in this study. This can be done best by following an integrated approach, including the use of more weed‐competitive cultivars and rotation crops.     

Genetic diversity of sulfonylurea-resistant and -susceptible Monochoria vaginalis populations in Japan

Monochoria vaginalis is one of the most serious weeds of rice fields in Asia. The species is predominantly selfing. To reveal the potential for multiple mutational events, outcrossing and gene flow in the sulfonylurea‐resistant (SU‐R) M. vaginalis populations, we investigated (i) if each SU‐R population was a single SU‐R biotype or a mixture of several SU‐R biotypes using restriction analysis or direct sequencing of acetolacatate synthase (ALS) genes and (ii) genetic diversity of SU‐R and ‐susceptible (S) populations using amplified fragment length polymorphism (AFLP) analysis. Nineteen or 20 individuals were sampled from four SU‐R and five SU‐S populations respectively. Amino acid substitutions conferring resistance in the SU‐R populations were Pro₁₉₇Ser in the ALS1 or ALS3, or Asp₃₇₆Glu in the ALS1 and each SU‐R population was composed of a single SU‐R biotype. In cluster analysis each SU‐R individual formed a cluster, whereas the individuals from a SU‐S population belonged to different clusters. Some SU‐R populations showed polymorphic AFLP loci. The results indicated that these SU‐R biotypes emerged from a single mutational event and any gene flow of SU‐R genes from adjacent populations did not occur. A low level of outcrossing and recombinations of SU‐R genes occurred within some SU‐R populations of M. vaginalis.     

Development of chemical weed control and integrated weed management in China

More than 200 species of weeds are infesting main crop fields in China, among which approximately 30 species are major weeds causing great crop yield losses. About 35.8 million hectares of crop fields are heavily infested by weeds and the annual reduction of crop yields is 12.3–16.5% (weighted average). Along with rural economic development, approximately 50% of the main crop fields undergo herbicide application. Chemical weed control has changed cultural practices to save weeding labor in rice, wheat, maize, soybeans and cotton. At the same time, continuous use of the same herbicides has caused weed shift problems and weed resistance to herbicides. Consequently, integrated weed management in main crops is being developed.     

Perspectives on transgenic,herbicide‐resistant crops in the United States almost 20 years after introduction

Herbicide‐resistant crops have had a profound impact on weed management. Most of the impact has been by glyphosate‐resistant maize, cotton, soybean and canola. Significant economic savings, yield increases and more efficacious and simplified weed management have resulted in widespread adoption of the technology. Initially, glyphosate‐resistant crops enabled significantly reduced tillage and reduced the environmental impact of weed management. Continuous use of glyphosate with glyphosate‐resistant crops over broad areas facilitated the evolution of glyphosate‐resistant weeds, which have resulted in increases in the use of tillage and other herbicides with glyphosate, reducing some of the initial environmental benefits of glyphosate‐resistant crops. Transgenic crops with resistance to auxinic herbicides, as well as to herbicides that inhibit acetolactate synthase, acetyl‐CoA carboxylase and hydroxyphenylpyruvate dioxygenase, stacked with glyphosate and/or glufosinate resistance, will become available in the next few years. These technologies will provide additional weed management options for farmers, but will not have all of the positive effects (reduced cost, simplified weed management, lowered environmental impact and reduced tillage) that glyphosate‐resistant crops had initially. In the more distant future, other herbicide‐resistant crops (including non‐transgenic ones), herbicides with new modes of action and technologies that are currently in their infancy (e.g. bioherbicides, sprayable herbicidal RNAi and/or robotic weeding) may affect the role of transgenic, herbicide‐resistant crops in weed management. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.     

Control by glyphosate and its alternatives of glyphosate‐susceptible and glyphosate‐resistant Echinochloa colona in the fallow phase of crop rotations in subtropical Australia

Echinochloa colona is the most common grass weed of summer fallows in the grain‐cropping systems of the subtropical region of Australia. Glyphosate is the most commonly used herbicide for summer grass control in fallows in this region. The world's first population of glyphosate‐resistant E. colona was confirmed in Australia in 2007 and, since then, >70 populations have been confirmed to be resistant in the subtropical region. The efficacy of alternative herbicides on glyphosate‐susceptible populations was evaluated in three field experiments and on both glyphosate‐susceptible and glyphosate‐resistant populations in two pot experiments. The treatments were knockdown and pre‐emergence herbicides that were applied as a single application (alone or in a mixture) or as part of a sequential application to weeds at different growth stages. Glyphosate at 720 g ai ha^?1 provided good control of small glyphosate‐susceptible plants (pre‐ to early tillering), but was not always effective on larger susceptible plants. Paraquat was effective and the most reliable when applied at 500 g ai ha^?1 on small plants, irrespective of the glyphosate resistance status. The sequential application of glyphosate followed by paraquat provided 96–100% control across all experiments, irrespective of the growth stage, and the addition of metolachlor and metolachlor + atrazine to glyphosate or paraquat significantly reduced subsequent emergence. Herbicide treatments have been identified that provide excellent control of small E. colona plants, irrespective of their glyphosate resistance status. These tactics of knockdown herbicides, sequential applications and pre‐emergence herbicides should be incorporated into an integrated weed management strategy in order to greatly improve E. colona control, reduce seed production by the sprayed survivors and to minimize the risk of the further development of glyphosate resistance.     

Herbicide‐resistant weeds in the Philippines: Status and resistance mechanisms

Two major weeds in rice in the Philippines, Sphenochlea zeylanica Gaertn. and Echinochloa crus‐galli (L.) Beauv., are controlled with chemical and cultural methods. In the 1980s, after >10 years of continuous use of 2,4‐D, S. zeylanica evolved resistance to the chemical in those rice fields that had been treated with 2,4‐D once or twice every cropping season. In the 1990s, E. crus‐galli evolved resistance to butachlor and propanil in rice monocrop areas where both herbicides were used continuously for 7–9 years. Rice farmers continue to use 2,4‐D, butachlor and propanil extensively and are often unaware of herbicide resistance or the potential for cross‐resistance, its causes or its implications. In order to control herbicide‐resistant E. crus‐galli, farmers are shifting to locally available herbicides with different modes of action, such as bispyribac, an acetolactate synthase inhibitor, and cyhalofop, an acetyl coenzyme A carboxylase inhibitor. Follow‐up manual weeding or rotary weeding after herbicide spraying, a common farmers’ practice, removes the susceptible and resistant biotypes and could help to delay or prevent the evolution of resistance. Although the resistance mechanisms of both weeds are not determined yet, they could be related to enhanced degradation that is similar to the mechanisms that are shown by the resistant biotypes in other countries.     

Integrated pest management and weed management in the United States and Canada

There is interest in more diverse weed management tactics because of evolved herbicide resistance in important weeds in many US and Canadian crop systems. While herbicide resistance in weeds is not new, the issue has become critical because of the adoption of simple, convenient and inexpensive crop systems based on genetically engineered glyphosate‐tolerant crop cultivars. Importantly, genetic engineering has not been a factor in rice and wheat, two globally important food crops. There are many tactics that help to mitigate herbicide resistance in weeds and should be widely adopted. Evolved herbicide resistance in key weeds has influenced a limited number of growers to include a more diverse suite of tactics to supplement existing herbicidal tactics. Most growers still emphasize herbicides, often to the exclusion of alternative tactics. Application of integrated pest management for weeds is better characterized as integrated weed management, and more typically integrated herbicide management. However, adoption of diverse weed management tactics is limited. Modifying herbicide use will not solve herbicide resistance in weeds, and the relief provided by different herbicide use practices is generally short‐lived at best. More diversity of tactics for weed management must be incorporated in crop systems. © 2014 Society of Chemical Industry     

Mechanisms of resistance to acetyl‐coenzyme A carboxylase‐inhibiting herbicides in a Hordeum leporinum population

A failure of acetyl‐coenzyme A carboxylase (ACCase)‐inhibiting herbicides to control a population of Hordeum leporinum Link (barleygrass) occurred following eight applications of these herbicides in both crops and pastures. This population was 7.6‐fold resistant to fluazifop‐P‐butyl compared with standard susceptible populations. The population was between 3.6‐ and 3.8‐fold resistant to other ACCase‐inhibiting herbicides, except butroxydim to which it was susceptible. ACCase extracted from resistant plants and assayed in the presence of herbicides in vitro was susceptible to fluazifop acid and other aryloxyphenoxypropanoate herbicides, but was 4‐fold less sensitive to sethoxydim compared with ACCase from susceptible plants. Resistant plants metabolised fluazifop acid about 1.3‐fold more rapidly compared with susceptible plants; however, sethoxydim was metabolised equally in both populations. Resistance to fluazifop‐P‐butyl and other aryloxyphenoxypropanoate herbicides may be the result of increased herbicide detoxification, whereas resistance to sethoxydim appears to be due to a modified target enzyme. Herbicide resistance in this population is unusual in that different mechanisms appear to confer resistance to the aryloxyphenoxypropanoate and cyclohexanedione herbicides. © 2000 Society of Chemical Industry     

Penoxsulam‐resistant barnyardgrass (Echinochloa crus‐galli) in rice fields in China

Barnyardgrass (Echinochloa crus‐galli) proliferation seriously threatens rice production worldwide. Whole‐plant bioassays were conducted in order to test the sensitivity to penoxsulam of 52 barnyardgrass populations and the resistance of six penoxsulam‐resistant populations to 12 other herbicides that are commonly used in rice fields. Among the 48 populations that had escaped penoxsulam control in the rice fields, 8.3% showed a very high level of resistance, 58.3% showed a high level of resistance and 10.4% showed a moderate level of resistance. Multiple resistance was confirmed in all six penoxsulam‐resistant populations that were tested further. They exhibited at least a moderate level of resistance; that is, to 6–10 of the total of 13 herbicides that was tested. Most of the six penoxsulam‐resistant populations showed at least a moderate level of resistance to bispyribac‐sodium, quinclorac, metamifop, cyhalofop‐butyl and oxadiazon, three populations held at least a moderate level of resistance to oxyfluorfen and pretilachlor, two populations also held at least a moderate level of resistance to pyrazosulfuron‐ethyl, pyribenzoxim and fenoxaprop‐P‐ethyl, but the resistance indices of the six populations to pendimethalin were all low. This study has confirmed resistance to pretilachlor and oxadiazon in weeds for the first time.     

抗药性杂草与治理

抗药性杂草对农田杂草治理和农业生产构成严重威胁,成为备受全球关注的严重问题。随着长期、大量使用相对有限的化学除草剂,全球抗药性杂草发展迅猛,目前已有217种杂草对化学除草剂产生了抗药性,我国抗药性杂草发展也十分迅猛。本文在介绍杂草抗药性基本概念、抗药性杂草发展过程、抗药性杂草现状的基础上,重点描述了抗药性杂草治理策略,以期为我国抗药性杂草研究和治理提供参考。     

Contamination of internationally traded wheat by herbicide‐resistant Lolium rigidum

As herbicide‐resistant weeds have spread in the agricultural fields of grain‐exporting countries, their seeds could be introduced into other countries as contaminants in imported grain. The spread of resistance genes through seed and pollen can cause significant economic loss. In order to assess the extent of the problem, we investigated the contamination by herbicide‐resistant annual ryegrass (Lolium rigidum) of wheat imported from Western Australia into Japan. Annual ryegrass seeds were recovered from wheat shipments and seed bioassays were conducted to identify resistance to the herbicides that are commonly used in Australia: diclofop‐methyl, sethoxydim, chlorsulfuron, and glyphosate. Nearly 4500 ryegrass seeds were detected in 20 kg of wheat that was imported in both 2006 and 2007. About 35% and 15% of the seeds were resistant to diclofop‐methyl, 5% and 6% were resistant to sethoxydim, and 56% and 60% were resistant to chlorsulfuron in 2006 and 2007, respectively. None was resistant to glyphosate in either year. As the contamination of crops by herbicide‐resistant weeds is probably a common phenomenon, the monitoring of incoming grain shipments is necessary to stem the further spread of herbicide‐resistant weeds into importing countries.