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Photosystem II (PS II)-inhibitor herbicide resistance in Ontario waterhemp [Amaranthus tuberculatus (Moq.) Sauer] populations is conferred via target-site resistance (TSR) and non–target site resistance (NTSR) mechanisms. Metribuzin-resistant (MR) A. tuberculatus is due to TSR. Conversely, in other populations of PS II–inhibitor resistant A. tuberculatus, plants are resistant to atrazine but metribuzin sensitive (MS). The objective of this study was to determine the biologically effective dose of metribuzin applied preemergence and postemergence for the control of MS and MR A. tuberculatus. Ten field experiments were conducted in 2019 and 2020 to determine the effective doses of metribuzin for 50%, 80%, and 95% control of MS and MR A. tuberculatus. Metribuzin applied preemergence at the calculated doses of 133, 350, and 1,070 g ai ha−1 controlled MS A. tuberculatus 50%, 80%, and 95%, respectively, whereas the calculated doses of 7,868 and 17,533 g ai ha−1 controlled MR A. tuberculatus 50% and 80%, respectively, at 12 wk after application (WAA). Metribuzin applied postemergence at the calculated doses of 245 and 1,480 g ai ha−1 controlled MS A. tuberculatus 50% and 80%, respectively; the calculated dose for 50% MR A. tuberculatus control was greater than the highest dose (17,920 g ai ha−1) included in this study. Metribuzin at 560 and 1,120 g ha−1 and pyroxasulfone/flumioxazin (240 g ai ha−1) applied preemergence controlled MS A. tuberculatus 88%, 95%, and 98%, respectively, at 12 WAA, whereas the same treatments only controlled MR A. tuberculatus 0%, 4%, and 93%, respectively, at 12 WAA. Metribuzin at 560 and 1,120 g ha−1 and fomesafen (240 g ai ha−1) applied postemergence controlled MS A. tuberculatus 65%, 70%, and 78%, and MR A. tuberculatus 0%, 1%, and 49%, respectively, at 12 WAA. Based on these results, PS II–inhibitor resistant A. tuberculatus with NTSR (enhanced metabolism) is controlled with metribuzin applied preemergence and postemergence; in contrast, PS II–inhibitor resistant A. tuberculatus with TSR (glycine-264-serine altered target site) is not controlled with metribuzin.

A goosegrass [Eleusine indica (L.) Gaertn.] population uncontrolled by paraquat (R) in a vegetable production field in St. Clair County, AL, was collected in summer 2019. Research was conducted to assess the level of resistance of the suspected resistant population compared with three populations with no suspected paraquat resistance (S1, S2, and S3). Visual injury at all rating dates and biomass reduction at 28 d after treatment (DAT) of S populations occurred exponentially to increasing paraquat rates. S biotypes were injured more than R at 3 DAT, with biomass recovery at 28 DAT only occurring at rates −1. Plant death or biomass reduction did not occur for any rate at any date for R. Paraquat rates that induced 50% or 90% injury or reduced biomass 50% or 90% compared with the non-treated (I50 or I90, respectively) ranged from 10 to 124 times higher I50 for R compared with S and 54 to 116 times higher I90 for R compared with S biotypes. These data confirm a paraquat-resistant E. indica biotype in Alabama, providing additional germplasm for study of resistance to photosystem I electron-diverting (PSI-ED) resistance mechanisms.

Populations of rigid ryegrass (Lolium rigidum Gaudin) from southern Australia have evolved resistance to the thiocarbamate herbicide prosulfocarb. The inheritance of prosulfocarb resistance was explored by crossing resistant (R) and susceptible (S) individuals. In all families within each cross, except 16.2, the response of the F1 was intermediate between the parents, suggesting that resistance is inherited as a single, partially dominant trait. For 16.2, the response of the F1 was more similar to the S parent, suggesting resistance may be a recessive trait in this population. Segregation at the discriminating dose of 1,200 g ai ha−1 prosulfocarb in population 375-14 fit the ratio (15:1) consistent with two independent dominant alleles; in population 198-15, it fit a ratio (13:3) for two independent alleles, one dominant and one recessive; and in population EP162, it fit a ratio (9:7) for two additive dominant alleles. In contrast, segregation of population 16.2 fit a ratio (7:9) consistent with two independent recessive alleles contributing to prosulfocarb resistance. Four different patterns of resistance to prosulfocarb were identified in different R populations, with inheritance as a dominant allele, dominant and recessive, additive dominant and as an independent recessive allele. This suggests there are several different mechanisms of prosulfocarb resistance present in L. rigidum.

Since its initial introduction in the late 1950s, chemical control has dominated weed management practices in China. Not surprisingly, the development of herbicide resistance has become the biggest threat to long-term, sustainable weed management in China. Given that China has followed the same laissez-faire approach toward resistance management that has been practiced in developed countries such as the United States, herbicide resistance has evolved rapidly and increased steadily over the years. Previously, we carried out a systematic review to quantitatively assess herbicide-resistance issues in China. In this review, our main objective is to focus on mechanistic studies and management practices to document the (1) history of herbicide application in China; (2) resistance mechanisms governing the eight most resistance-prone herbicide groups, including acetolactate synthase inhibitors, acetyl-CoA carboxylase inhibitors, synthetic auxin herbicides, 5-enolpyruvylshikimate-3-phosphate synthase inhibitors, protoporphyrinogen oxidase inhibitors, photosystem I electron diverters, photosystem II inhibitors, and long-chain fatty-acid inhibitors; and (3) herbicide-resistance management strategies commonly used in China, including chemical, cultural, biological, physical, and integrated approaches. At the end, perspectives and future research are discussed to address the pressing need for the development of integrated herbicide-resistance management in China.

Wild poinsettia (Euphorbia heterophylla L.) is a troublesome broadleaf weed in grain production areas in South America. Herbicide resistance to multiple sites of action has been documented in this species, including protoporphyrinogen oxidase (PPO) inhibitors. We investigated the physiological and molecular bases for PPO-inhibitor resistance in a E. heterophylla population (RPPO) from Southern Brazil. Whole-plant dose–response experiments revealed a cross-resistance profile to three different chemical groups of PPO inhibitors. Based on dose–response parameters, RPPO was resistant to lactofen (47.7-fold), saflufenacil (8.6-fold), and pyraflufen-ethyl (3.5-fold). Twenty-four hours after lactofen treatment (120 g ha−1) POST, RPPO accumulated 27 times less protoporphyrin than the susceptible population (SPPO). In addition, RPPO generated 5 and 4.5 times less hydrogen peroxide and superoxide than SPPO, respectively. The chloroplast PPO (PPO1) sequences were identical between the two populations, whereas 35 single-nucleotide polymorphisms were found for the mitochondrial PPO (PPO2). Based on protein homology modeling, the Arg-128-Leu (homologous to Arg-98-Leu in common ragweed [Ambrosia artemisiifolia L.] was the only one located near the catalytic site, also in a conserved region of PPO2. The cytochrome P450 monooxygenase inhibitor malathion did not reverse resistance to lactofen in RPPO, and both populations showed similar levels of PPO1 and PPO2 expression, suggesting that metabolic resistance and PPO overexpression are unlikely. This is the first report of an Arg-128-Leu mutation in PPO2 conferring cross-resistance to PPO inhibitors in E. heterophylla.

Annual bluegrass (Poa annua L.) is a problematic annual weed in established turf where the intensive use of herbicides has resulted in the evolution of herbicide resistance. In 2017, 31 populations of P. annua suspected to be resistant to herbicides commonly used to control this weed in turf were collected from golf courses across southeastern Australia to check the resistance status to different herbicide groups. All populations were found to be resistant to multiple turf herbicides. Dose–response experiments confirmed resistance to propyzamide, simazine, rimsulfuron, foramsulfuron, endothall, and pinoxaden. Levels of resistance to rimsulfuron (>56-fold), foramsulfuron (>19-fold), endothall (>7-fold), and pinoxaden (>4.3-fold) compared with the susceptible population were high, but levels of resistance to propyzamide (>2-fold) and simazine (>2-fold) were lower. Considerable variation in resistance to endothall and pinoxaden was observed among the populations of P. annua. Target-site resistance was confirmed for acetolactate synthase and acetyl-CoA carboxylase inhibitors, but not for photosystem II and microtubule assembly inhibitors. This study documented the extensive resistance to herbicides in P. annua from turf in Australia. Three of the populations investigated exhibited multiple resistance to herbicides from five mechanisms of action. The identification of multiple-resistant P. annua on several golf courses is a serious concern for turf managers.

Harvest weed seed control (HWSC) technology, such as impact mills that destroy weed seeds in seed-bearing chaff material during grain crop harvest, has been highly effective in Australian cropping systems. However, the impact mill has never been tested in soybeans [Glycine max (L.) Merr.] and weeds common to soybean production systems in the midwestern and mid-Atlantic United States. We conducted stationary testing of Harrington Seed Destructor (HSD) impact mill and winter burial studies during 2015 to 2016 and 2017 to 2018 to determine (1) the efficacy of the impact mill to target weed seeds of seven common weeds in midwestern and five in the mid-Atlantic United States, and (2) the fate of impact mill–processed weed seeds after winter burial. The impact mill was highly effective in destroying seeds of all the species tested, with 93.5% to 99.8% weed seed destruction in 2015 and 85.6% to 100% in 2017. The weak relationships (positive or negative) between seed size and seed destruction by impact mill and the high percentage of weed seed destruction by impact mill across all seed sizes indicate that the biological or practical effect of seed size is limited. The impact mill–processed weed seeds that retained at least 50% of their original size, labeled as potentially viable seed (PVS), were buried for 90 d overwinter to determine the fate of weed seeds after winter burial. At 90 d after burial, the impact mill–processed PVS were significantly less viable than unprocessed control seeds, indicating that impact mill processing physically damaged the PVS and promoted seed mortality overwinter. A very small fraction (

Southern crabgrass [Digitaria ciliaris (Retz.) Koeler] is an annual grass weed that commonly infests turfgrass, roadsides, wastelands, and cropping systems throughout the southeastern United States. Two biotypes of D. ciliaris (R1 and R2) with known resistance to cyclohexanediones (DIMs) and aryloxyphenoxypropionates (FOPs) previously collected from sod production fields in Georgia were compared with a separate susceptible biotype (S) collected from Alabama for the responses to pinoxaden and to explore the possible mechanisms of resistance. Increasing rates of pinoxaden (0.1 to 23.5 kg ha−1) were evaluated for control of R1, R2, and S. The resistant biotypes, R1 and R2, were resistant to pinoxaden relative to S. The S biotype was completely controlled at rates of 11.8 and 23.5 kg ha−1, resulting in no aboveground biomass at 14 d after treatment. Pinoxaden rates at which tiller length and aboveground biomass would be reduced 50% (I50) and 90% (I90) for R1, R2, and S ranged from 7.2 to 13.2 kg ha−1, 6.9 to 8.6 kg ha−1, and 0.7 to 2.1 kg ha−1, respectively, for tiller length, and 7.7 to 10.2 kg ha−1, 7.2 to 7.9 kg ha−1, and 1.6 to 2.3 kg ha−1, respectively, for aboveground biomass. Prior selection pressure from DIM and FOP herbicides could result in the evolution of D. ciliaris cross-resistance to pinoxaden herbicides. Amplification of the carboxyl-transferase domain of the plastidic ACCase by standard PCR identified a point mutation resulting in an Ile-1781-Leu amino acid substitution only for the resistant biotype, R1. Further cloning of PCR product surrounding the 1781 region yielded two distinct ACCase gene sequences, Ile-1781 and Leu-1781. The amino acid substitution, Ile-1781-Leu in both resistant biotypes (R1 and R2), however, was revealed by next-generation sequencing of RNA using Illumina platform. A point mutation in the Ile-1781 codon leading to herbicide insensitivity in the ACCase enzyme has been previously reported in other grass species. Our research confirms that the Ile-1781-Leu substitution is present in pinoxaden-resistant D. ciliaris.

In recent years, herbicide resistance has attracted much attention as an increasingly urgent problem worldwide. Unfortunately, most of that effort was focused on confirmation of resistance and characterization of the mechanisms of resistance. For management purposes, knowledge about biology and ecology of the resistant weed phenotypes is critical. This includes fitness of the resistant biotypes compared with the corresponding wild biotypes. Accordingly, fitness has been the subject of many studies; however, lack of consensus on the concept of fitness resulted in poor experimental designs and misinterpretation of the ensuing data. In recent years, methodological protocols for conducting proper fitness studies have been proposed; however, we think these methods should be reconsidered from a herbicide-resistance management viewpoint. In addition, a discussion of the inherent challenges associated with fitness cost studies is pertinent. We believe that the methodological requirements for fitness studies of herbicide-resistant weed biotypes might differ from those applied in other scientific disciplines such as evolutionary ecology and genetics. Moreover, another important question is to what extent controlling genetic background is necessary when the aim of a fitness study is developing management practices for resistant biotypes. Among the methods available to control genetic background, we suggest two approaches (single population and pedigreed lines) as the most appropriate methods to detect differences between resistant (R) and susceptible (S) populations and to derive herbicide-resistant weed management programs. Based on these two methods, we suggest two new approaches that we named the “recurrent single population” and “recurrent pedigreed lines” methods. Importantly, whenever the aim of a fitness study is to develop optimal resistance management, we suggest selecting R and S plants within a single population and evaluating all fitness components from seed to seed instead of measuring changes in the frequency of R and S alleles through multigenerational fitness studies.

The parasitic purple witchweed [Striga hermonthica(Del.) Benth.] is a serious constraint to maize production in sub-Saharan Africa, especially in poor soils. VariousStrigaspp. control measures have been developed, but these have not been assessed in an integrated system. This study was conducted to evaluate a set of promising technologies forS. hermonthicamanagement in western Kenya. We evaluated three maize genotypes either intercropped with peanut (Arachis hypogaeaL.), soybean [Glycine max(L.) Merr.], or silverleaf desmodium [Desmodium uncinatum(Jacq.) DC] or as a sole crop at two locations under artificialS. hermonthicainfestation and at three locations under naturalS. hermonthicainfestation between 2011 and 2013. Combined ANOVA showed significant (P−1) under artificial infestation and inD. uncinatumand peanut cropping systems (3,203 kg ha−1and 3,193 kg ha−1) under natural infestation. Grain yield was highest for theStrigaspp.-resistant hybrid under both methods of infestation. A lower number of emergedS. hermonthicaplants per square meter were recorded at 10 and 12 wk after planting on maize grown underD. uncinatumin the artificialS. hermonthicainfestation. A combination of herbicide-resistant maize varieties intercropped with legumes was a more effective method forS. hermonthicacontrol than individual-component technologies. Herbicide-resistant andStrigaspp.-resistant maize integrated with legumes would help reduce theStrigaspp. seedbank in the soil. Farmers should be encouraged to adopt an integrated approach to controlStrigaspp. for better maize yields.

Harvest weed seed control (HWSC) techniques have been implemented in Australian cropping systems to target and reduce the number of weed seeds entering the seedbank and thereby reduce the number of problematic weeds emerging in subsequent years to infest subsequent crops. However, the influence of HWSC on ameliorating herbicide-resistance (HR) evolution has not been investigated. This research used integrated spatial modeling to examine how the frequency and efficacy of HWSC affected the evolution of resistance to initially effective herbicides. Herbicides were, in all cases, better protected from future resistance evolution when their use was combined with annual HWSC. Outbreaks of multiple HR were very unlikely to occur and were nearly always eliminated by adding annual, efficient HWSC. The efficacy of the HWSC was important, with greater reductions in the number of resistance genes achieved with higher-efficacy HWSC. Annual HWSC was necessary to protect sequences of lower-efficacy herbicides, but HWSC could still protect herbicides if it was used less often than once per year, when the HWSC and the herbicides were highly effective. Our results highlight the potential benefits of combining HWSC with effective herbicides for controlling weed populations and reducing the future evolution of HR.

Weed species develop resistance to herbicides through the repeated use of the same herbicide mechanism of action (MOA). Farmers often resort to different MOAs once a weed population has become resistant to the MOA that resulted in a resistant weed population. Delaying herbicide resistance is of great importance to growers due to the limited number of commercially available MOAs. Resistance may occur through monogenic or polygenic traits, and various academic and industrial modeling tools have been developed to help infer cause–effect from multiple interacting factors that may not be intuitive. This work explores various best management practices in delaying weed resistance, and we give details for monogenic and quantitative polygenic resistance models and investigate combinations of management strategies that lead to maximizing the product life span for a herbicide. Management practices under parametric uncertainty are provided to showcase how various practices can be used to extend lifetime product performance before resistance is manifest. Penalty functions associated with choosing a unique management strategy, based upon grower constraints, are the subject of a companion manuscript.

Research was conducted to determine whether resistance to glyphosate among Palmer amaranth (Amaranthus palmeriS. Watson) populations within the U.S. state of Arkansas was due solely to increasedEPSPSgene copy number and whether gene copy number is correlated with resistance level to glyphosate. One hundred and fifteenA. palmeriaccessions were treated with 840 g ae ha−1glyphosate. Twenty of these accessions, selected to represent a broad range of responses to glyphosate, underwent further testing. Seven of the accessions were controlled with this dose; the rest were resistant. The effective dose to cause 50% injury (ED50) for susceptible accessions ranged from 28 to 207 g ha−1. The glyphosate-resistant (GR) accessions had ED50values ranging from 494 to 1,355 g ha−1, a 3- to 48-fold resistance level compared with the susceptible standard (SS). The 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene relative copy number was determined for 20 accessions, 4 plants accession−1. Resistant plants from five GR accessions (38% of resistant plants tested) did not have increasedEPSPSgene copies. Resistant plants from the remaining eight GR accessions (62% of resistant plants tested) had 19 to 224 moreEPSPSgene copies than the SS. Among the accessions tested, injury declined 4% with every additionalEPSPScopy. ED50values were directly correlated withEPSPScopy number. The highly resistant accession MIS11-B had an ED50of 1,355 g ha−1and 150 gene copies. Partial sequences ofEPSPSfrom GR accessions withoutEPSPSamplification did not contain any of the known resistance-conferring mutations. Nearly 40% of GR accessions putatively harbor non–target site resistance mechanisms. Therefore, elevatedEPSPSgene copy number is associated with glyphosate resistance amongA. palmerifrom Arkansas.

Genetically engineered (GE) herbicide-resistant crops have been widely adopted by farmers in the United States and other countries around the world, and these crops have caused significant changes in herbicide use patterns. GE crops have been blamed for increased problems with herbicide-resistant weeds (colloquially called by the misnomer “superweeds”); however, there has been no rigorous analysis of herbicide use or herbicide-resistant weed evolution to quantify the impact of GE crops on herbicide resistance. Here, I analyze data from the International Survey of Herbicide Resistant Weeds and the USDA and demonstrate that adoption of GE corn varieties did not reduce herbicide diversity, and therefore likely did not increase selection pressure for herbicideresistant weeds in that crop. Adoption of GE herbicide-resistant varieties substantially reduced herbicide diversity in cotton and soybean. Increased glyphosate use in cotton and soybean largely displaced herbicides that are more likely to select for herbicide-resistant weeds, which at least partially mitigated the impact of reduced herbicide diversity. The overall rate of newly confirmed herbicideresistant weed species to all herbicide sites of action (SOAs) has slowed in the United States since 2005. Although the number of glyphosate-resistant weeds has increased since 1998, the evolution of new glyphosate-resistant weed species as a function of area sprayed has remained relatively low compared with several other commonly used herbicide SOAs.Nomenclature: Glyphosate; corn, Zea mays L.; cotton, Gossypium hirsutum L.; soybean, Glycine max (L.) Merr.

Concern over the development of herbicide-resistant weeds has led to interest in integrated weed management systems that reduce selection pressure by utilizing mechanical and cultural weed control practices in addition to herbicides. Increasing crop seeding rate increases crop competitive ability and thus can enhance herbicide efficacy. However, it is unknown how increasing the seeding rate affects an herbicide’s efficacy. The objective of this study was to examine the interaction between increasing seeding rate and herbicide dose to control weeds. To meet this objective, the herbicide fluthiacet-methyl was applied to field-grown lentil, with Indian mustard, a proxy for wild mustard, used as a model weed. The experiment was a factorial design with four lentil seeding rates and seven herbicide rates. Overall the herbicide dose response was altered by changing lentil seeding rate. Increasing lentil seeding rate decreased the weed biomass production when herbicides were not applied. In two of the four site-years, increasing lentil seeding rate lowered the herbicide ED50, the dose required to result in a 50% reduction in weed biomass. Increasing the crop seeding rate altered the dose response to provide greater weed control at lower herbicide rates compared with normal crop seeding rates. Increased seeding rates also resulted in higher and more stable crop seed yields across a wider range of herbicide dosages. These results suggest that dose–response models can be used to evaluate the efficacy of other weed management practices that can interact with herbicide performance.

There is great value in quantifying and reporting weed seed production as a component of herbicide efficacy evaluations for two reasons. First, visual weed control ratings and associated measurements such as weed density and biomass are not sufficient indicators of fecundity. Second, knowledge of fecundity associated with herbicide treatments can guide the development of effective management programs that impact long-term weed population dynamics and reduce the risk of herbicide resistance.

Control of glyphosate-resistant (GR) horseweed in soybean with glyphosate (900 g ai ha−1) plus saflufenacil (25 gaiha−1) has been variable. The objective of this research was to determine the effect of GR horseweed height, density, and time of day (TOD) at application on saflufenacil plus glyphosate efficacy in soybean. All experiments were completed six times during a 2 yr period (2014, 2015) in fields previously confirmed with GR horseweed. Applications from 0900 to 2100 hours provided optimal control of GR horseweed 8 WAA. Soybean yield paralleled GR horseweed control with the highest yield of 3000kgha−1at 1500 hours, and the lowest yield of 2400kgha−1at 0600 hours. The height and density of GR horseweed at application had minimal effect on saflufenacil efficacy. Saflufenacil provided>99% control of GR horseweed when applied to small plants and low densities; however, control decreased to 95% when>25 cm tall, and to 96% in densities>800 plants m−2at 6 WAA due to some plant regrowth. TOD of application had a greater influence on GR horseweed control with saflufenacil than height or density. To optimize control of GR horseweed, saflufenacil should be applied during daytime hours to small plants at low densities. Optimizing GR horseweed control minimizes weed seed return and weed interference.

The management of glyphosate-resistant Palmer amaranth has been a challenge in southern United States cropping systems. Registration of dicamba-resistant crops will provide an alternative management option to control herbicide-resistant Palmer amaranth populations, particularly those having resistance to herbicide Groups 2, 3, 5, 9, 14, and 27. However, repeated use of sublethal doses of dicamba may lead to rapid evolution of herbicide resistance, especially in Palmer amaranth—a species with a strong tendency to evolve resistance. Therefore, selection experiments with dicamba were conducted on Palmer amaranth using sublethal doses. In the greenhouse, a known susceptible Palmer amaranth population was subjected to sublethal dicamba doses for three generations (P1–P3). Susceptibility of the individuals to dicamba was evaluated, and its susceptibility to 2,4-D was characterized. Based on the greenhouse study, following three generations of dicamba selection, the dose required to cause 50% mortality increased from 111 g ae ha−1for parental individuals (P0) to 309 g ae ha−1for the P3. Furthermore, reduced susceptibility of the P3to 2,4-D was also evident. This research presents the first evidence that recurrent use of sublethal dicamba doses can lead to reduced susceptibility of Palmer amaranth to dicamba as well as 2,4-D. Here, we show that selection from sublethal dicamba doses has an important role in rapid evolution of Palmer amaranth with reduced susceptibility to auxin-type herbicides.

The control of glyphosate-resistant (GR) horseweed (Conyza canadensis) in soybean has been variable with glyphosate plus saflufenacil. The objective of this research was to determine the biologically effective rate (BER) of saflufenacil, saflufenacil mixed with glyphosate, and metribuzin mixed with saflufenacil and glyphosate applied preplant (PP) for the control of GR horseweed in no-till soybean; a study was conducted to determine each of the three treatments. For each study, seven field sites infested with GR horseweed were used over a 2-yr period (2014, 2015). Saflufenacil alone at 25 and 36 g ai ha–1 provided 90 and 95% control of GR Horseweed 8 wk after application, while the BER to achieve 98% control was outside of the treatment range tested. The saflufenacil plus glyphosate (900 g ai ha–1) BER experiment found less saflufenacil was required as 25, 34, and 47 g ha–1 provided 90, 95, and 98% control of GR horseweed respectively. The metribuzin BER experiment found 61, 261, and 572 g ha–1 was required to provide 90, 95 and 98% control of GR horseweed, respectively, mixed with saflufenacil (25 g ha–1) and glyphosate (900 g ha–1). The addition of metribuzin with the recommended rate of saflufenacil (25 g ha–1) plus glyphosate improved control and a second effective herbicide mode of action for the control of GR horseweed. The use of a threeway herbicide mixture can be an effective weed management strategy to control GR horseweed in soybean.

Continuous use of herbicides has triggered a phenomenon called herbicide resistance. Nowadays, herbicide resistance is a worldwide problem that threatens sustainable agriculture. A study of over a decade on herbicides in Iran has revealed that herbicide resistance has been occurring since 2004 in some weed species. Almost all the results of these studies have been published in national scientific journals and in conference proceedings on the subject. In the current review, studies on herbicide resistance in Iran were included to provide a perspective of developing weed resistance to herbicides for international scientists. More than 70% of arable land in Iran is given over to cultivation of wheat, barley, and rice; wheat alone covers nearly 52%. Within the past 40 years, 108 herbicides from different groups of modes of action have been registered in Iran, of which 28 are for the selective control of weeds in wheat and barley. Major resistance to ACCase-inhibiting herbicides has been shown in some weed species, such as winter wild oat, wild oat, littleseed canarygrass, hood canarygrass, and rigid ryegrass. With respect to the broad area of wheat crop production and continuous use of herbicides with the sole mechanism of action of ACCase inhibition, the provinces of West Azerbaijan, Tehran, Khorasan, Isfahan, Markazi, and Semnan are at risk of resistance development. In addition, because of continuous long-term use of tribenuron-methyl, resistance in broadleaf species is also being developed. Evidence has recently shown resistance of turnipweed and wild mustard populations to this herbicide. Stable monitoring of fields in doubtful areas and providing good education and training for technicians and farmers to practice integrated methods would help to prevent or delay the development of resistance to herbicides.

An overreliance on herbicides in several important grain- and cotton-producing regions of the world has led to the widespread evolution of herbicide-resistant weed populations. Of particular concern are weed populations that exhibit simultaneous resistance to multiple herbicides (MHR). Too often, herbicides are the only tool used for weed control. We use the term herbicide-only syndrome (HOS) for this quasi-addiction to herbicides. Growers and their advisers focus on herbicide technology, unaware of or ignoring basic evolutionary principles or the necessary diversity provided by other methods of weed control. Diversity in weed control practices disrupts resistance evolution. Significant challenges exist to implementing diversity, including how to address information so that producers choose to alter existing behaviors (HOS) and take calculated risks by attempting new and more complex strategies. Herbicide resistance management in the long term will require creativity in many sectors, including roles for growers, industry, researchers, consultants, retailers, and regulators. There can be creativity in herbicide registration and regulation, as exemplified by the recent U.S. Environmental Protection Agency program that encourages herbicide registrants to register products in minor crops. We propose one idea for a regulatory incentive to enable herbicide registrants in jurisdictions such as the United States to receive an extended data exclusivity period in exchange for not developing one new herbicide in multiple crops used together in rotation, or for implementing stewardship practices such as robust mixtures or limitations on application frequency. This incentive would provide a mechanism to register herbicides in ways that help to ensure herbicide longevity. Approaches based only on market or financial incentives have contributed to the current situation of widespread MHR. Our suggestion for regulatory creativity is one way to provide both financial and biological benefits to the registering company and to the overall stakeholder community by incentivizing good resistance management.

Herbicides have been the principal means of weed control in developed countries for approximately 50 yr because they are the most cost-effective method. Such general use of herbicides has resulted in weed resistance to herbicides, which continues to be a growing problem. Within the past decade, the evolution of resistance to the once-dominant herbicide glyphosate has resulted in major concerns about the future ability to control weeds in many crop systems. Moreover, many weed species have evolved resistance to multiple mechanisms of herbicide action. Given the dearth of new herbicides with novel mechanisms of action, it appears inevitable that weed management programs will need to be supplemented by the use of tactics other than herbicides. However, the inclusion of more diversity for weed management also introduces complexity, cost, and time constraints to current crop production systems. This paper describes broadly the considerations, opportunities, and constraints of diverse weed management tactics to address the burgeoning problems with herbicide resistance.

Sociologists define a wicked problem as one without clear causes or solutions, and thus difficult or impossible to solve. Herbicide resistance is the epitome of a wicked problem: the causes are convoluted by myriad biological and technological factors, and are fundamentally driven by the vagaries of human decision-making. Weed scientists for decades have conducted research and developed educational programs to prevent or mitigate evolution of herbicide resistance, yet resistance is more prevalent today than ever before. If we expect to achieve success in herbicide resistance management, different approaches will be essential. The second Herbicide Resistance Summit focused on “doing something different,” bringing in rural sociologists, agricultural economists, weed scientists, and crop consultants to discuss the decision-making process itself, community-based approaches to resistance management, economics of resistance management, potential regulatory and incentive programs, new approaches to educational programs, diversification of weed management, and a call to action for everyone involved in the decision-making process.

Effective outreach is critical to achieving success in managing herbicide-resistant weeds. Interdisciplinary collaboration is needed to adapt information delivery and to engage communities to address the herbicide-resistance problem. Weed scientists must partner with the production community to adapt herbicide-resistance practices for local needs, to work collaboratively with state and regional stakeholders to create effective resistance-management practices, and to provide an overarching national message as to the causes of, and solutions to, resistance.

The evolution of herbicide-resistant weeds is a major concern in the corn- and soybean-producing Pampas region of Argentina, where growers predominantly plant glyphosate-resistant crop varieties and depend heavily on glyphosate for weed control. Currently, 16 weed species in Argentina are resistant to one or more of three different herbicide mechanisms of action, and resistant weed populations continue to increase, posing a serious threat to agricultural production. Implementation of integrated weed management to address herbicide resistance faces significant barriers in Argentina, especially current land ownership and rental patterns in the Pampas. More than 60% of Pampas cropland is rented to tenants for periods that rarely exceed 1 yr, resulting in crop rotation being largely abandoned, and crop export taxes and quotas have further discouraged wheat and corn production in favor of continuous soybean production. In this paper we discuss ways to facilitate new approaches to weed management in Argentina, including legal and economic reforms and the formation of a national committee of stakeholders from public and private agricultural sectors.

Managing weed resistance has become a major challenge for many agricultural producers. Resistance is growing in terms of the number of weeds exhibiting resistance and the number of herbicides to which weeds are becoming resistant. The susceptibility of weeds to herbicides in many regions is a diminishing common pool resource affected by local producer weed control actions and natural conditions. Given the growing number of weeds exhibiting resistance, and the recognition that weed resistance is not a private property issue, we argue that managing resistance must be viewed as a wicked problem with no standard template across regions. Finding farm management approaches that help farmers successfully address weed resistance requires a shared perspective that incorporates an improved understanding of the human dimensions of weed management. Through an analysis of wicked problem characteristics, we argue that a people-centered approach to weed management is necessary. We offer principles learned from tackling other wicked agriculture and resource conservation issues to guide such approaches. Education, technical assistance, incentive schemes and regulatory efforts, and other strategies can play roles in constructing management approaches for herbicide resistance, but will have to vary from current efforts to unravel the mysteries of more effective weed management. Building a more inclusive approach, in terms of stakeholders and disciplines, will be key to achieving progress.

Herbicide-resistant weeds are the result of evolutionary processes that make it easy to think about the problem from a purely biological perspective. Yet, the act of weed management, guided by human production of food and fiber, drives this biological process. Thus, the problem is socioeconomic as well as biological. The purpose of this article is to explain how well-known socioeconomic phenomena create barriers to herbicide-resistance management and highlight important considerations for knocking down these barriers. The key message is that the multidimensional problem requires a multifaceted approach that recognizes differences among farmers; engages the regulatory, academic, extension, seed and chemical suppliers, and farmer communities; and aligns the diverse interests of the members of these communities with a common goal that benefits all—more sustainable weed management. It also requires an adaptive approach that transitions from moreuniform and costly standards and incentives, which can be effective in the near-term but are unsustainable, to more-targeted and less-costly approaches that are sustainable in the long term.

A bioeconomic model is used to estimate how managing glyphosate resistance to horseweed affects short- and long-run profits in corn, soybean, and corn-soybean rotation systems. Model results found that resistance management reduces profits in the first year of implementation, but increases profits in the second and subsequent 18 yr. In all three systems, long run gains begin to outweigh short-run costs (and resistance management “pays for itself”) by the second year. Over a 20-yr horizon, the estimated annual average profit advantage from managing resistance exceeded $158 ha–1($64/acre) for corn, $137 ha–1($55/acre) for corn-soybean, and $55 ha–1($22/acre) for soybean. Seed immigration from a neighbor's field can reduce these gains, but this reduction is negligible if the neighbor also practices resistance management. If the neighbor did not manage resistance, however, the grower's estimated annual profit advantage fell to roughly $123 ha–1($50/acre) for corn, $60 ha–1($24/acre) for the corn-soybean rotation, and virtually zero for soybean. Methods applied in this study identify corn and corn-soybean rotations as cases where resistance management “pays for itself' quickly and significantly, even if the neighbor does not manage resistance. Continuous soybean presents a more challenging case that may require additional programs and incentives to encourage collective resistance management among growers. Even for continuous soybean, however, joint grower–neighbor resistance management can pay for itself within 2 yr.

Wild oat is a problematic weed species that requires new management techniques in the face of herbicide resistance; harvest weed-seed control (HWSC) may be an option. Wild oat demographic information was collected in long-term, rotational field studies in Lacombe, AB, Canada, in 2006 and 2007, and a periodic matrix model was parameterized using management extremes (no IPM, no herbicide to high IPM, and full herbicide). Population growth rates were calculated for each treatment and year. Prospective (elasticity) and retrospective (LTRE) analyses were conducted alongside a rearrangement of the model equation in which population growth rates were designated and the required proportion of newly shed seed survival that gives that growth rate was solved for. All populations had λ > 1 or increasing populations. Elasticity analyses indicated that λ was most-highly elastic to the overwinter seedbank (Esw = 1), followed by seedling survival, fecundity, and survival of newly shed seed (0.63 to 0.86 across treatments). The latter may be the most-accessible vital rate for management of herbicide resistant populations. LTRE exposed the stochasticity of wild oat population growth rates between years and their ability to take advantage of lapses in control. Decreasing the proportion of newly shed seeds (snew) that survives was the most-effective and available control strategy until reduced to 0.1 to 0.3 when the summer seedbank becomes more critical. When averaged across treatments, > 80% of newly shed seed must be eliminated to stop the population from growing, resulting in a stable population, but not a decline. Because of preharvest shattering, HWSC will likely not be effective enough alone to cause wild oat populations to decline. New management techniques for wild oat control that can be used in combination with HWSC and integrated weed management strategies are needed.

Giant ragweed has been increasing as a major weed of row crops in the last 30 yr, but quantitative data regarding its pattern and mechanisms of spread in crop fields are lacking. To address this gap, we conducted a Web-based survey of certified crop advisors in the U.S. Corn Belt and Ontario, Canada. Participants were asked questions regarding giant ragweed and crop production practices for the county of their choice. Responses were mapped and correlation analyses were conducted among the responses to determine factors associated with giant ragweed populations. Respondents rated giant ragweed as the most or one of the most difficult weeds to manage in 45% of 421 U.S. counties responding, and 57% of responding counties reported giant ragweed populations with herbicide resistance to acetolactate synthase inhibitors, glyphosate, or both herbicides. Results suggest that giant ragweed is increasing in crop fields outward from the east-central U.S. Corn Belt in most directions. Crop production practices associated with giant ragweed populations included minimum tillage, continuous soybean, and multiple-application herbicide programs; ecological factors included giant ragweed presence in noncrop edge habitats, early and prolonged emergence, and presence of the seed-burying common earthworm in crop fields. Managing giant ragweed in noncrop areas could reduce giant ragweed migration from noncrop habitats into crop fields and slow its spread. Where giant ragweed is already established in crop fields, including a more diverse combination of crop species, tillage practices, and herbicide sites of action will be critical to reduce populations, disrupt emergence patterns, and select against herbicide-resistant giant ragweed genotypes. Incorporation of a cereal grain into the crop rotation may help suppress early giant ragweed emergence and provide chemical or mechanical control options for late-emerging giant ragweed.

Resistance to glyphosate in hairy fleabane and horseweed is a problem in orchards and vineyards in California. Population genetic analyses suggest that glyphosate resistance evolved multiple times in both species, but it is unknown if resistance to other herbicides is also present. Two approaches of research were undertaken to further evaluate herbicide resistance in Conyza sp. in the perennial crop systems of California. In the initial study, the distribution of Conyza sp. in the Central Valley, using a semistructured field survey, was coupled with evaluation of the presence and level of glyphosate resistance in plants grown from field-collected seed. In a subsequent study, single-seed descendants representing distinct genetic groups were self-pollinated in the greenhouse and these accessions were evaluated for response to glyphosate and paraquat. Conyza sp. were commonly found throughout the Central Valley and glyphosate-resistant individuals were confirmed in all field collections of both species. The level of glyphosate resistance among field collections varied from 5- to 21-fold compared with 50% glyphosate resistance (GR50) of the susceptible, with exception of one region with a GR50 similar to the susceptible. When self-pollinated accessions from different genetic groups were screened, the level of glyphosate resistance, on the basis of GR50 values, ranged from 1.7- to 42.5-fold in hairy fleabane, and 5.9- to 40.3-fold in horseweed. Three accessions of hairy fleabane from different genetic groups were also resistant to paraquat (40.1- to 352.5-fold). One glyphosate-resistant horseweed accession was resistant to paraquat (322.8-fold), which is the first confirmed case in California. All paraquat-resistant accessions of Conyza sp. identified so far have also been resistant to glyphosate, probably because glyphosate resistance is already widespread in the state. Because glyphosate and paraquat resistances are found across a wide geographical range and in accessions from distinct genetic groups, multiple resistant Conyza sp. likely developed independently several times in California.

Weed Science continues to be one of the premier journals in the discipline and in the agronomic sciences. The Editorial Board and the staff strive to improve the efficiency of the publication process and improve the scope of the journal.Weed Science seeks to publish more reviews and symposium articles. This year,Weed Science will publish a special issue ofWeed Science from presenters at the WSSA Herbicide Resistance Summit II. For the 12 mo preceding September 1, 2015, 211 manuscripts were submitted to Weed Science, compared to 208 in the previous 12-mo period. Of the manuscripts accepted, 44.3% were accepted, compared to 40.8% the previous year. The time from submission to first decision was 44 d this past year compared to 45 d for the previous year. Table 1 shows the Impact Factor and Article Influence Score for Weed Science and other related journals for 2014. The impact factor forWeed Science increased from 1.687 to 1.870 this past reporting cycle, whereas the 5-yr impact factor increased from to 1.928 from 1.835. I believe the latest impact factor and the fiveyear impact factor are the best the journal has ever had. For the year preceding September 1, 2015, the journal depended on 283 unique reviewers from 34 countries for peer review. The excellent reputation of the journal would not be possible without the cutting-edge research submitted by authors and the diligent work of the editorial board and the hundreds of referred reviews that are submitted annually. I wish to thank the members of the editorial board and reviewers for 2015. Editorial Board 2015

In a recent essay, Harker and coauthors stated that considering herbicide resistance as a wicked problem “without clear causes or solutions” ignores what weed scientists know about the biology and management of herbicide-resistant weeds. In this response, we argue that this misrepresents what is meant by “wicked” and that the wicked problem concept is valuable in understanding the multifaceted nature of herbicide resistance as a human-caused phenomenon.

Managing the risk of herbicide resistance (HR) is strategically important to leading herbicide technology providers and is the focus of the Global Herbicide Resistance Action Committee (HRAC), an organization with representation from 8 major companies, working as a part of CropLife International (Brussels, Belgium). Early detection of HR, understanding the extent of HR in a defined area, and mitigation of resistance through efforts to limit its spread are important aspects of managing HR. Monitoring for HR populations has been employed by weed scientists for both early detection and to define the extent of resistance (Baumgartner et al. 1999; Beckie et al. 2013; Davis et al. 2008; Falk et al. 2005; Owen et al. 2007). Methods used to monitor for resistance have included random or nonrandom seed sample collection from fields, followed by resistance bioassay (field surveys); market research surveys of farmers and weed management experts; and tracking farmer performance inquiries, followed by appropriate field evaluation and testing. Each method has advantages and disadvantages in costs, accuracy, and effectiveness to address HR management goals. Before embarking on a resistance-monitoring program, the objectives, probability of achieving those objectives, and the associated resource needs should be carefully considered. Based on a review of existing monitoring studies and company experiences, HRAC offers the following considerations for herbicide resistance monitoring

Reports of kochia control failure with glyphosate in western Kansas increased dramatically in the years following confirmed presence of glyphosate-resistant (GR) populations in 2007. In this study, progeny from 8 of 16 geographically dispersed kochia populations in western Kansas (seed collected in 2010) were confirmed to be resistant to glyphosate by conducting whole-plant dose-response (in greenhouse and/or outdoor environments) and shikimate-accumulation assays. Additionally, the relationship between 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene copy number and glyphosate resistance levels was investigated. A known glyphosate-susceptible (GS) kochia population from Ellis County, Kansas was used for comparison in all studies. Based on the herbicide rate that caused 50% reduction in biomass compared to untreated control (GR50) values, the 8 GR kochia populations were 4 to 11 times more resistant to glyphosate compared to the GS population. The GR50values of kochia populations were 1.58 to 1.85 times higher under an outdoor environment compared to when grown in the greenhouse. Glyphosate-treated leaf discs of the GS kochia plants accumulated consistently higher amounts of shikimate than those of the GR plants. Additionally, the GR plants with higher levels of resistance to glyphosate had higher EPSPS : acetolactate synthase (ALS) relative gene copy number compared to those with low levels of resistance.

The extensive use of foliar-applied protoporphyrinogen oxidase (PPO)-inhibiting herbicides indisputably contributes to the continued selection of waterhemp resistant to PPO-inhibiting herbicides (PPO-R). However, the role of soil-residual applications of PPO-inhibiting herbicides in the selection of PPO-R has been clouded by the efficacy that these herbicides have on PPO-R waterhemp. The aim of the present study was to understand if soil-residual PPO-inhibiting herbicides have the potential to influence the proportion of resistant waterhemp in emerging plants as herbicide concentrations diminish in the soil. Greenhouse and field experiments were conducted in a PRE dose-response experiment testing fomesafen or fomesafen pluss-metolachlor in the presence of mixed seed populations of PPO-R and -susceptible (S) waterhemp. The first 20 (greenhouse) or 10 (field) waterhemp plants that survived the residual herbicide treatment were sampled for genotypic analysis to detect the presence of the allele responsible for PPO-R in waterhemp (ΔG210). Relative to the nontreated control, the highest rate of fomesafen increased the frequency of resistance (FOR) by 70% in the greenhouse experiments and 20% in the field experiments. The addition ofs-metolachlor did not reduce the fomesafen-induced increase in the FOR in the surviving plants from field or greenhouse experiments. However, the additional herbicide from an alternate site of action (s-metolachlor) substantially improved soil-residual herbicide efficacy over fomesafen alone, which limited the number of waterhemp plants surviving the herbicide application. Thus, the selection for resistance can be delayed with the addition of a herbicide from an alternative site of action by postponing and/or reducing waterhemp emergence. These data strongly suggest that soil-residual PPO-inhibiting herbicides can influence the FOR in a field population, placing even greater importance on the implementation of best management practices such as full herbicide use rates and using herbicides from multiple sites of action to mitigate the risk of selecting for herbicide-resistant weed biotypes.

Waterhemp resistance to foliar applications of protoporphyrinogen oxidase (PPO)–inhibiting herbicides has become increasingly disconcerting given the widespread distribution of glyphosate resistance. Fortunately, soil-residual PPO-inhibiting herbicides remain efficacious in waterhemp populations resistant to PPO-inhibiting herbicides; however, these herbicides should theoretically select for the resistant biotype as herbicide concentrations diminish in the soil. Accordingly, the objectives of this research were twofold: (1) evaluate the efficacy of three PPO-inhibiting herbicides, foliar- and soil-applied, on PPO-resistant (PPO-R) and PPO-susceptible (PPO-S) waterhemp, and (2) investigate the differential effects of PPO-inhibiting herbicides on an R biotype and an S biotype during several discrete developmental events relevant to soil–residual herbicide activity (i.e., radicle protrusion, radicle elongation, and waterhemp emergence). Greenhouse and growth chamber experiments indicated that the R biotype was least sensitive to the diphenylether herbicide fomesafen, followed by sulfentrazone and flumioxazin; however, fomesafen pluss-metolachlor improved soil-residual efficacy over fomesafen alone. Growth stage considerably influenced the R : S ratio, decreasing from 38× to 3.4×, when comparing ratios generated from foliar applications and soil-residual applications measuring radicle protrusion, respectively. Overall, this research supports the use of full soil-residual herbicide rates, reinforcing the importance of best management practices to manage the spread of herbicide resistance.

A survey of soybean fields containing waterhemp was conducted just prior to harvest in 2012 to determine the scope and extent of herbicide resistance and multiple herbicide resistances among a sample of Missouri waterhemp populations. Resistance was confirmed to glyphosate and to acetolactate synthase (ALS), protoporphyrinogen oxidase (PPO), photosystem II (PSII), and 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors, but not to 2,4-D. Of the 187 populations tested, 186 exhibited resistance to chlorimuron. The proportions of populations with atrazine or glyphosate resistance were similar, with 30 and 29% of the populations surviving the 3× rates. Lactofen resistance was observed in 5% of the populations, whereas mesotrione resistance was only found in 1.6% of the populations. All populations tested were susceptible to 2,4-D at the 3× rate. At least 52% of the waterhemp populations tested exhibited resistance to herbicides from two mechanism of action. Resistance to atrazine plus chlorimuron as well as glyphosate plus chlorimuron was present in 29% of the populations. Three-way resistance, primarily comprised of resistance to atrazine plus chlorimuron plus glyphosate, was present in 11% of the populations. Resistance to herbicides from four mechanisms of action was found in 2% of the populations, and one population exhibited resistance to herbicides from five mechanisms of action. DNA analysis of a subsample of plants revealed that previously documented mechanisms of resistance in waterhemp, including the ΔG210 deletion conferring PPO-inhibitor resistance, the Trp574Leu amino acid substitution conferring ALS-inhibitor resistance, and elevated 5-enolypyruvyl-shikimate-3-phosphate synthase copy number and the Pro106Ser amino acid substitution resulting in glyphosate resistance, explained survival in many, but not all, instances. Atrazine resistance was not explained by the Ser264Gly D1 protein substitution. Overall, results from these experiments indicate that Missouri soybean fields contain waterhemp populations with resistance to glyphosate, ALS-, PPO-, PSII-, and HPPD-inhibiting herbicides, which are some of the most common mechanisms of action currently utilized for the control of this species in corn and soybean production systems. Additionally, these results indicate that slightly more than half of the populations tested exhibit resistance to more than one herbicide mechanisms of action. Managing the current resistance levels in existing populations is of utmost importance. The use of multiple, effective herbicide modes of action, both preemergence and postemergence, and the integration of optimum cultural and mechanical control practices will be vital in the management of Missouri waterhemp populations in the future.

Herbicide Resistance Genes andPlant FitnessSince the beginning of agriculture, crops havebeen exposed to recurrent invasion by weeds thatcan impose severe reductions in crop quality andyield. There have been continuing efforts to reducethe impacts of weeds on production. More than40 yr ago, overreliance on herbicide technology toreduce weed infestations resulted in the selection ofadaptive traits that enabled weed survival andreproduction under herbicide treatments (De´lye etal. 2007; Powles and Yu 2010; Vila-Aiub et al.2008). As a result, herbicide resistance in . 200weed species has evolved worldwide (Heap 2013;Powles 2008).Resistant weeds are able to withstand the toxicityof herbicides because of the presence of resistancealleles originating from random DNA mutations(Powles and Yu 2010). These resistance allelesregulate a number of highly efficient, constitutivedefense mechanisms that prevent herbicides frominhibiting key metabolic pathways. A set of defensemechanisms are involved in a reduction in herbicidethat reaches the herbicide target protein (nontarget-site resistance) (Powles and Yu 2010). Mechanismsthat (1) impair herbicide leaf uptake or transloca-tion within plants via vacuolar sequestration orreduced cellular uptake, or (2) change the chemicalproperties of herbicides via herbicide-enhancedmetabolism (detoxification) are included in thisgroup (Ge et al. 2012; Nandula et al. 2008; Prestonand Wakelin 2008; Preston et al. 1996; Sammonset al. 2010; Vila-Aiub et al. 2012; Wakelin et al.2004).Another type of defense mechanism againstherbicides involves a structural modification, viachanges in the amino acid sequence, of the herbicidetarget protein, which minimizes herbicide binding(target-site resistance) (Powles and Yu 2010). Geneoverexpression, resulting in the synthesis of excessiveherbicide sensitive target protein because of pro-moter changes or gene amplification, is alsoregarded a target-site resistance defense mechanism(Dinelli et al. 2006; Gaines et al. 2010).Target and nontarget-site herbicide resistancealleles protect plants from fatal damage caused byherbicides. In other words, the presence of resistancealleles minimizes the plant fitness reduction expect-ed from herbicide activity. Fitness can be defined asthe average success in producing offspring contrib-uting to the next generation by a particularphenotype relative to another phenotype (Crawley1997; Primack and Hyesoon 1989; Scott et al.2006). A simple way to represent plant fitness (W)is given in Futuyma (2013):W~P

Herbicides inhibit biochemical and physiological processes or both with lethal consequences. The target sites of these small molecules are usually enzymes involved in primary metabolic pathways or proteins carrying out essential physiological functions. Herbicides tend to be highly specific for their respective target sites and have served as tools to study these physiological and biochemical processes in plants (Dayan et al. 2010b).

Increased soybean seed cost has generated recent interest in reducing seeding rates to improve economic returns. However, low seeding rates result in reduced established plant stands with slower canopy development, and canopy development is an important element of integrated weed management (IWM). Field studies were conducted in 2012 and 2013 in Wisconsin to determine the trade-off between reduced seeding rates and PRE residual herbicide use for POST herbicide exposure. Soybean was planted in mid May in 38-cm-wide rows at five seeding rates ranging from 148,200 to 469,300 seeds ha−1. A PRE application of metolachlor plus fomesafen was made to half of the plots. One of two POST herbicide programs were sprayed at the V4 soybean growth stage to determine whether blending herbicide-resistant (HR) and non-HR soybean cultivars could be a practical alternative to reduce soybean seed expenses while maintaining the potential benefit of weed suppression before the POST herbicide application. An increase in seeding rate did not reduce the density or size of weeds exposed to the POST herbicide, and furthermore, end-of-season weed density and biomass were not influenced. In contrast, the use of a PRE herbicide reduced total weed density and biomass before POST application by 93 and 95%, respectively, in both years. In 2012, the season was dry early and harvest stands of 161,100 and 264,100 plants ha−1 produced 95% of the maximum yield for the PRE and no-PRE treatments, respectively. The difference was not repeated in 2013 with adequate early season rainfall. In conclusion, PRE herbicide use produced maximum yield with fewer plants per hectare by limiting early season weed competition and reduced weeds exposed to POST herbicide application thus contributing to HR management (HRM). In contrast, higher plant densities generated within the seeding rate range of this study did little to improve IWM or HRM.

Weed Science continues to be one of the premier journals in the discipline and in the agronomic sciences. The Editorial Board and the staff strive to improve the efficiency of the publication process and improve the scope of the journal. Weed Science is seeking to publish more reviews and symposium articles. This year, Weed Science will publish a special issue of Weed Science that will serve as a research methods book, and another special issue from the WSSA Herbicide Resistance Summit II is being planned. For the 12 mo preceding September 1, 2014, 207 manuscripts were submitted to Weed Science compared to 211 in the previous 12-mo period. Of the manuscripts accepted, 66% were accepted, compared to 63% the previous year. The time from submission to first decision was 45 d this past year compared to 47 d for the previous year. Table 1 shows the impact factor and article influence score for Weed Science and other related journals for 2012. The impact factor for Weed Science declined slightly from 1.759 to 1.684 from 2011 to 2012, while the 5-yr impact factor increased from 1.770 to 1.835. For the year preceding September 1, 2014, the journal depended on 264 unique reviewers from 30 countries for peer review. The excellent reputation of the journal would not be possible without the cutting edge research submitted by authors, the diligent work of the editorial board, and the hundreds of referred reviews that are submitted annually. I wish to thank the members of the editorial board and reviewers for 2014. Editorial Board 2014

Glyphosate-resistant (GR) weeds, including giant ragweed, are among the most challenging weeds for growers to control in cotton. A field study was conducted in 2011 and 2012 to determine the competitiveness of giant ragweed with densities of 0, 0.1, 0.2, 0.4, 0.8, or 1.6 plants m−1of row. Early in the growing season, giant ragweed competition with densities of at least 0.8 plants m−1row reduced cotton height compared with the weed-free control. Based on node above white flower (NAWF) and node above cracked boll (NACB) data, a delay in cotton maturity was observed for treatments with giant ragweed present at a density of 1.6 m−1of cotton row for NAWF and 0.8 m−1or 1.6 m−1of row for NACB. Lint yield losses of 50% were estimated for cotton with rows growing along side of giant ragweed at a density of 0.26 plants m−1row. Cotton in rows located 140 cm away from giant ragweed required an estimated 1.85 plants m−1row to reduce yield by 50%. These data suggest that giant ragweed sphere of influence was at least 1 m wide. Cotton fiber quality was not affected by giant ragweed at any density. Giant ragweed is a highly competitive weed in cotton, even at low densities, and efforts should be implemented to control giant ragweed early in the season to prevent cotton yield loss.

Reduced control of some glyphosate-resistant hairy fleabane populations with paraquat has raised concerns about evolved multiple resistance to both glyphosate and paraquat. The objective of this study was to confirm the presence of multiple-resistant (glyphosate and paraquat) hairy fleabane populations in California. A series of dose-response experiments was conducted to evaluate the effect of glyphosate and paraquat in a known susceptible (S) and putative multiple-resistant (R) population of hairy fleabane. The greenhouse experiments were conducted during summer, fall, and winter under controlled temperature and natural light conditions. Multiple-resistant hairy fleabane was identified; however, the level of resistance to glyphosate varied substantially among seasons. During the summer, the glyphosate rate required to reduce growth by 50% (GR50) for the R population was 0.94 kg ae ha−1, 5.2-fold more than for the S population. In the fall and winter experiments, however, the R population response to glyphosate was similar to the S population with a GR50of 0.22 kg ae ha−1or less. Multiple-resistant plants were controlled in the fall and winter at rates that did not control the same population during summer. GR50of paraquat varied among seasons (0.94, 0.24, and 0.07 kg ai ha−1during summer, fall, and winter, respectively); however, plant mortality was more consistent. This is the first reported case of glyphosate–paraquat resistance in hairy fleabane and the multiple-resistant population could pose a significant challenge to annual no-till and perennial cropping systems in California. Further research on the mechanisms of resistance and the physiological factors underlying the seasonally variable response to glyphosate is needed.

Studies were conducted to determine the growth, fecundity, and competitive ability of an acetyl-CoA carboxylase (ACCase)–inhibitor resistant (R) sterile wild oat biotype compared with a susceptible (S) biotype. Seed germination studies indicated that there were no differences in seed germination and seedling vigor between R and S biotypes at any temperature regime. R and S biotypes were grown under noncompetitive and competitive arrangement in the greenhouse. Under noncompetitive greenhouse conditions, growth of the R biotype was similar to that of the S biotype on the basis of plant height, canopy area, and plant biomass. Seed production and weight of R and S plants were also at the same levels. Furthermore, relative competitiveness among the R and S sterile wild oat biotypes was investigated by means of replacement series experiments. The R and S biotypes were compared under seven mixture proportions (6 : 0, 5 : 1, 4 : 2, 3 : 3, 2 : 4, 1 : 5, and 0 : 6). No significant differences in competitive ability were observed between R and S biotypes on the basis of plant height, canopy area, or plant biomass. In most cases, relative crowding coefficient (RCC) values at 20, 60, and 100 d after transplanting (DAT) were close to one, indicating equal competitiveness between the R and S biotypes of wild oat used in this competitive study. However, in some cases, the RCC value was 1.31 for plant height, evident of a slight competitive advantage for the R biotype at 100 DAT. In general, ACCase-inhibitor R and S sterile wild oat biotypes were equally competitive, clearly without any growth penalty for R plants in either noncompetitive or competitive conditions.

Glyphosate-resistant Palmer amaranth is a serious problem in southern cropping systems. Much phenotypic variation is observed in Palmer amaranth populations with respect to plant growth and development and susceptibility to herbicides. This may be related to levels of genetic diversity existing in populations. Knowledge of genetic diversity in populations of Palmer amaranth may be useful in understanding distribution and development of herbicide resistance. Research was conducted to assess genetic diversity among and within eight Palmer amaranth populations collected from North Carolina and Georgia using amplified fragment length polymorphism (AFLP) markers. Pair-wise genetic similarity (GS) values were found to be relatively low, averaging 0.34. The highest and the lowest GS between populations were 0.49 and 0.24, respectively, while the highest and the lowest GS within populations were 0.56 and 0.36, respectively. Cluster and principal coordinate (PCO) analyses grouped individuals mostly by population (localized geographic region) irrespective of response to glyphosate or gender of individuals. Analysis of molecular variance (AMOVA) results when populations were nested within states revealed significant variation among and within populations within states while variation among states was not significant. Variation among and within populations within state accounted for 19 and 77% of the total variation, respectively, while variation among states accounted for only 3% of the total variation. The within population contribution towards total variation was always higher than among states and among populations within states irrespective of response to glyphosate or gender of individuals. These results are significant in terms of efficacy of similar management approaches both in terms of chemical and biological control in different areas infested with Palmer amaranth.

A waterhemp population from a native-grass seed production field in Nebraska was no longer effectively controlled by 2,4-D. Seed was collected from the site, and dose-response studies were conducted to determine if this population was herbicide resistant. In the greenhouse, plants from the putative resistant and a susceptible waterhemp population were treated with 0, 18, 35, 70, 140, 280, 560, 1,120, or 2,240 g ae ha−12,4-D. Visual injury estimates (I) were made 28 d after treatment (DAT), and plants were harvested and dry weights (GR) measured. The putative resistant population was approximately 10-fold more resistant to 2,4-D (R:S ratio) than the susceptible population based on both I50(50% visual injury) and GR50(50% reduction in dry weight) values. The R:S ratio increased to 19 and 111 as the data were extrapolated to I90and GR90estimates, respectively. GR50doses of 995 g ha−1for the resistant and 109 g ha−1for the susceptible populations were estimated. A field dose-response study was conducted at the suspected resistant site with 2,4-D doses of 0, 140, 280, 560, 1,120, 2,240, 4,480, 8,960, 17,920, and 35,840 g ha−1. At 28 DAT, visual injury estimates were 44% in plots treated with 35,840 g ha−1. Some plants treated with the highest rate recovered and produced seed. Plants from the resistant and susceptible populations were also treated with 0, 9, 18, 35, 70, 140, 280, 560, or 1,120 g ae ha−1dicamba in greenhouse bioassays. The 2,4-D resistant population was threefold less sensitive to dicamba based on I50estimates but less than twofold less sensitive based on GR50estimates. The synthetic auxins are the sixth mechanism-of-action herbicide group to which waterhemp has evolved resistance.

Strigais a major constraint to food production in Africa. Most technologies developed for the eradication ofStriga asiaticafrom the United States are not adaptable to Africa. Imazapyr and pyrithiobac coated imidazolinone-resistant (IR)-resistant maize seed prior to planting at rates of 30 to 45 g ha−1provide near season long control ofStrigaand can increase maize yields three- to fourfold if supplied with fertilizer. Slow release seed coatings reduce maize injury when post-planting rains are sparse and improveStrigacontrol when there is excessive rainfall early in the season. Models suggest that herbicide resistance may not be a significant threat in short season maize, but vigilance in removing floweringStrigaplants that are not controlled is recommended due to the known risk of evolution of resistance to these herbicides. Stacking the IR gene with glyphosate resistance and using imazapyr treated seed and applying glyphosate mid-season would provide season longStrigacontrol and delay the evolution of resistance to both herbicides. To date, adoption of this technology has been limited by a number of factors. However, it should be included as one component of a multi-factor approach to increasing maize productivity in areas of Africa whereStrigais problematic.