Rigid ryegrass, an important annual weed species in cropping regions ofsouthern Australia, has evolved resistance to 11 major groups of herbicides.Dose–response studies were conducted to determine response of threeclethodim-resistant populations and one clethodim-susceptible population ofrigid ryegrass to three different frost treatments (−2 C).Clethodim-resistant and -susceptible plants were exposed to frost in a frostchamber from 4:00 P.M. to 8:00 A.M. for three nights before or afterclethodim application and were compared with plants not exposed to frost. Areduction in the level of clethodim efficacy was observed in resistantpopulations when plants were exposed to frost for three nights before orafter clethodim application. In the highly resistant populations, thesurvival percentage and LD50 were higher when plants were exposedto frost before clethodim application compared with frost after clethodimapplication. However, frost treatment did not influence clethodim efficacyof the susceptible population. Sequencing of the acetyl coenzyme Acarboxylase (ACCase) gene of the three resistant populations identifiedthree known mutations at positions 1781, 2041, and 2078. However, mostindividuals in the highly resistant populations did not contain any knownmutation in ACCase, suggesting the resistance mechanism was a nontargetsite. The effect of frost on clethodim efficacy in resistant plants may bean outcome of the interaction between frost and the clethodim resistancemechanism(s) present.

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.

Herbicide resistance has become a major topic of international research in weed science, with a steady stream of presentations and papers on various aspects of the biology of herbicide resistant weeds. Diverse sources—including WSSA—have generated an equally steady stream of recommendations for preventing and managing herbicide resistance. The situation in the field, however, continues to deteriorate, with new cases of herbicide resistant weeds being reported at an increasing rate. We know what to do, but why aren't we doing it?

Doveweed is a summer annual that is difficult to control in turfgrass.Photosystem II inhibitors have the potential to control doveweed, butresearch is limited on the efficacy of these herbicides. The objectives ofthis research were to evaluate (1) the differential tolerance levels ofdoveweed to atrazine and simazine, (2) the influence of applicationplacement and rate on herbicide efficacy, and (3) uptake and metabolism ofthese herbicides in doveweed. In greenhouse experiments, the time requiredto injure doveweed 50% was three to five times faster for atrazine thansimazine. Simazine soil or foliar + soil application reduced doveweedbiomass 77% from the nontreated, but foliar-only treatments reduced biomass51%. Application placements for atrazine equally reduced shoot biomass 96%from the nontreated. In a dose–response experiment, atrazine and simazinerequired ≤ 1.8 kg ha−1 and ≥ 5.1 kg ha−1 to injuredoveweed 50% from 8 to 16 d after treatment (DAT), respectively. Doveweedrequired 79% less atrazine to reduce biomass 50% from the nontreatedcompared with simazine. In laboratory experiments, doveweed had similar rootabsorption levels of 14C-atrazine and 14C-simazine.Metabolism of both herbicides linearly increased from 1 to 7 DAT, but parentherbicide levels averaged 39 and 25% of the extracted radioactivity from 14C-atrazine and 14C-simazine, respectively.Doveweed metabolized 14C-simazine to three major metabolites,including hydroxysimazine, that each ranged from 24 to 29% of the extractedradioactivity. Hydroxyatrazine was the only major metabolite (> 10% oftotal 14C extracted) of 14C-atrazine. Overall,doveweed has slower metabolism of atrazine compared with simazine and is thebasis for differential tolerance levels to these herbicides.

A Powell amaranth population suspected to be resistant (R) to linuron wasdiscovered in a carrot field in Keswick, Ontario, Canada, in 1999.Dose–response analysis with different herbicides and DNA sequencing of the psbA gene encoding the D1 protein of photosystem II weredone to confirm the resistance and identify its basis. A calculatedresistance factor indicated a 12-fold increased resistance when linuron wasapplied to an R population compared with a susceptible (S) population.Moreover, the R population showed 6.4- and 3.1-fold greater resistance totwo other phenylurea herbicides (diuron and monolinuron), 1.8- and 1.4-foldgreater resistance to two triazine herbicides (metribuzin and prometryn),and 2.6-fold greater resistance to the triazinone metribuzin. R populationwas also cross-resistant to bentazon and bromoxynil when compared with Spopulation, with a calculated resistance factor of 1.4 and 2.2,respectively. The partial nucleotide sequence of the psbAgene of R populations differed at two locations when compared with Spopulations. The first mutation coded for a Val219Ilesubstitution in the deduced amino acid sequence of the D1 protein, and thesecond mutation was silent and encoded for a proline at position 279 in bothR and S populations. The Val219Ile substitution in the psbA gene is most likely the cause of this Powellamaranth population resistance to linuron and other PSII inhibitors. This isthe first recorded instance of a Val219Ile substitution in an Amaranthus species.

Metsulfuron is used for POST control of spotted spurge in many warm-seasonturfgrasses. A suspected resistant (R) biotype of spotted spurge wascollected from turfgrass in Georgia with a history of exclusive metsulfuronuse. Research was conducted to evaluate the resistance level of this biotypeto metsulfuron, efficacy of other mechanisms of action for control, and themolecular basis for resistance. Compared with a susceptible (S) biotype, theR biotype required >90 and >135 times greater metsulfuron rates toreach 50% injury and reduce biomass 50% from the nontreated, respectively.The R biotype was also resistant to trifloxysulfuron but was injuredequivalent to the S biotype from dicamba, glyphosate, and triclopyr. Genesequencing of the R biotype revealed a Trp574 to Leu substitutionthat has conferred resistance to acetolactate synthase (ALS) inhibitors inprevious research. This is the first report of ALS resistance in spottedspurge. More importantly, this is the first report of a herbicide-resistantbroadleaf weed from a turfgrass system in the United States.

Determining the mechanisms of herbicide resistance in weeds allows for the development and implementation of applied management practices aimed to control and to prevent further spread of herbicide-resistant populations in crop fields. This research was conducted to determine propanil resistance and cross-resistance to other photosystem II (PSII) inhibitors in ricefield bulrush biotypes and to elucidate the mechanism of propanil resistance. To this end, propanil-resistant (R) and propanil-susceptible (S) biotypes were selected from field-collected populations after propanil spraying at the field rate, and whole-plant, dose–response experiments were conducted to evaluate cross-resistance to PSII inhibitors and interactions between propanil and the insecticides malathion and carbaryl. In addition, the psbA gene from R and S biotypes was sequenced for amino acid alterations following polymerase chain reaction (PCR) amplification. Plant survival data indicated the R biotype displayed a 14-fold increase in propanil resistance relative to the susceptible (S) biotype. In addition, the propanil-R biotype also had increased resistance to the PSII-inhibitors bromoxynil, diuron, and metribuzin but was more susceptible to bentazon than were propanil-S plants. Synergism between propanil and the insecticides carbaryl and malathion was greater in the S biotype than it was in the R biotype, indicating that, unlike propanil resistance in weedy grasses, enhanced degradation of the herbicide molecule is not a mechanism of resistance for propanil in ricefield bulrush. A Val219 to Ile substitution in the propanil-R chloroplast D1 protein was identified following sequencing of the psbA gene. This research suggests a single-point mutation at the target site causes resistance to propanil, diuron, metribuzin, and bromoxynil but increasing susceptibility to bentazon in propanil-R ricefield bulrush, a novel Val219–Ile feature. To our knowledge, this is the first instance of propanil resistance in weeds because of a mechanism other than enhanced herbicide metabolism. Tank-mixing bentazon and propanil, where permitted, can control both propanil-R and propanil-S biotypes.

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.

A yellow nutsedge biotype (Res) from an Arkansas rice fieldhas evolved resistance to acetolactate synthase (ALS)-inhibiting herbicides.The Res biotype previously exhibited cross-resistance toALS inhibitors from four chemical families (imidazolinone, pyrimidinylbenzoate, sulfonylurea, and triazolopyrimidine). Experiments were conductedto evaluate alternative herbicides (i.e., glyphosate, bentazon, propanil,quinclorac, and 2,4-D) currently labeled in Arkansas rice–soybean productionsystems. Based on the percentage of aboveground dry weight reduction,control of the yellow nutsedge biotypes with the labeled rate of bentazon,propanil, quinclorac, and 2,4-D was < 44%. Glyphosate (867 g ae ha−1) resulted in 68 and > 94% control of the Res and susceptible yellow nutsedge biotypes,respectively, at 28 d after treatment. Dose-response studies were conductedto estimate the efficacy of glyphosate on the Res biotype,three susceptible yellow nutsedge biotypes, and purple nutsedge. Based onthe dry weights, the Res biotype was ≥ 5- and ≥ 1.3-foldless responsive to glyphosate compared to the susceptible biotypes andpurple nutsedge, respectively. Differences in absorption and translocationof radiolabeled glyphosate were observed among the yellow nutsedge biotypesand purple nutsedge. The susceptible biotype had less14C-glyphosate radioactivity in the tissues above the treatedleaf and greater radioactivity in tissues below the treated leaf compared tothe Res biotype and purple nutsedge. Reduced translocationof glyphosate in tissues below the treated leaf of the Resbiotype could be a reason for the lower glyphosate efficacy in the Res biotype. No amino acid substitution that wouldcorrespond to glyphosate resistance was found in the5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene of the Res biotype. However, an amino acid (serine) additionwas detected in the EPSPS gene of the Res biotype; albeit,it is not believed that this addition contributes to lower efficacy ofglyphosate in this biotype.

Bermudagrass and goosegrass are problematic weeds with limited herbicidesavailable for POST control in creeping bentgrass. Metamifop effectivelycontrols these weeds with greater selectivity in cool-season grasses thanother ACCase inhibitors. The objectives of this research were to determinethe physiological basis for metamifop selectivity in turfgrasses. Ingreenhouse experiments, metamifop rate required to reduce shoot biomass 50%from the nontreated (GR50) at 4 wk after treatment was >6,400, 2,166, and 53 g ai ha−1 for creeping bentgrass, Kentuckybluegrass, and goosegrass, respectively. The GR50 forbermudagrass treated with diclofop-methyl or metamifop was 2,850 and 60 g ha−1, respectively. In laboratory experiments, peak absorptionof 14C-metamifop was reached at 48, 72, and 96 h after treatment(HAT) for goosegrass, creeping bentgrass and Kentucky bluegrass,respectively. Grasses translocated < 10% of the absorbed radioactivityout of the treated leaf at 96 HAT, but creeping bentgrass translocated threetimes more radioactivity than goosegrass and Kentucky bluegrass. Creepingbentgrass, Kentucky bluegrass, and goosegrass metabolized 16, 14, and 25% of 14C-metamifop after 96 h, respectively. Goosegrass had aroundtwo times greater levels of a metabolite at retention factor 0.45 thancreeping bentgrass and Kentucky bluegrass. The concentration of metamifoprequired to inhibit isolated ACCase enzymes 50% from the nontreated (I50) measured > 100, > 100, and 38 μM for creepingbentgrass, Kentucky bluegrass, and goosegrass, respectively. In otherexperiments, foliar absorption of 14C-metamifop in bermudagrasswas similar to 14C-diclofop-methyl. Bermudagrass metabolized 23and 60% of the absorbed 14C-diclofop-methyl to diclofop acid anda polar conjugate after 96 h, respectively, but only 14% of14C-metamifop was metabolized. Isolated ACCase was equallysusceptible to inhibition by diclofop acid and metamifop (I50 =0.7 μM), suggesting degradation rate is associated with bermudagrasstolerance levels to these herbicides. Overall, the physiological basis formetamifop selectivity in turfgrass is differential levels of target siteinhibition.

The objective of this study was to evaluate the response of hairy beggarticks plants to different doses of glufosinate ammonium and the range in sensitivities of the plants and their progenies to the herbicide. Three studies were conducted, all in a greenhouse and repeated at different times. In the first study, two experiments were conducted to examine the dose–response curve, and the treatments were seven different doses of the herbicide glufosinate ammonium (0, 50, 100, 200, 400, 800, and 1,600 g ai ha−1), with four replications each. In the second study, which examined the range in sensitivity of hairy beggarticks to glufosinate ammonium, 44 plants were sprayed with a dose of 200 g ai ha−1 of the herbicide. Finally, in the third study, the range in sensitivity of the progeny of hairy beggarticks to glufosinate ammonium was investigated; in this experiment, the progenies of seven of the previous plants were sprayed with 200 g ai ha−1 of herbicide. The ammonium contents in the tissues were measured and percent injury wase visually assessed. Ammoniun content in hairy beggarticks leaves was increased more than seven times by glufosinate application and the maximum ammonium content was observed for the highest dose of the herbicide. Variability existed in the ammonium content among the individuals of the population of hairy beggarticks; however, the behavior was not replicated in the same way in the progenies. The survival of the plants after application of the herbicide allows the production of progenies with wide variability in their sensitivity to the product, independent of the behavior for the progenitor plants.

A goosegrass biotype with suspected resistance to acetyl-CoA carboxylase(ACCase) inhibitors was identified in Georgia. The objectives of thisresearch were to evaluate the resistance level of this biotype to ACCaseinhibitors, efficacy of various herbicide mechanisms of action for control,and the physiological and molecular basis of resistance. In greenhouseexperiments, the rate of diclofop-methyl that reduced dry shoot biomass 50% (SR50) from the nontreated for the resistant (R) andsusceptible (S) biotypes measured 4,100 and 221 g ai ha−1,respectively. The SR50 for sethoxydim measured 615 and 143 g ai ha−1 for the R and S biotype, respectively. The R biotype wascross resistant to clethodim, fenoxaprop, and fluazifop. The R and Sbiotypes were equally susceptible to foramsulfuron, glyphosate, monosodiummethylarsenate (MSMA), and topramezone. In laboratory experiments, the twobiotypes had similar foliar absorption of 14C-diclofop-methyl.Both biotypes metabolized 14C-diclofop-methyl to diclofop acidand a polar conjugate, but the R biotype averaged ∼2 times greatermetabolism than the S biotype. Gene sequencing revealed an Asp-2078-Glysubstitution in the ACCase of the R biotype that has previously conferredresistance to ACCase inhibitors. A second mutation was identified in the Rbiotype that yielded a Thr-1805-Ser substitution that has been previouslyreported, but is not associated with ACCase resistance in other species.Thus, the Asp-2078-Gly substitution is the basis for resistance to ACCaseinhibitors for the R biotype. This is the first report of ACCase-inhibitorresistance in goosegrass from the United States and from a turfgrasssystem.

Secondary plant metabolites may influence plant–plant interactions and plantinvasions. Distinguishing such chemicals requires integrating varyingchemical ecology approaches, information on the amounts and persistence ofspecific chemicals in nature, and measures of effects (e.g., phytotoxicityassays) to judge the importance of the chemicals (e.g., allelochemicals).The invasive plant croftonweed has caused substantial ecological andeconomic losses in China. We examined contents and degradation ofcroftonweed chemicals in the soil and their potential phytotoxic effects onconspecific and five allospecific plant species. Soils in which croftonweedwas grown had four phytotoxins: DEHP, DBP, DTD, and HHO. All chemicals weredetected in croftonweed-invaded soil, with contents ranging from 0.013 (forDEHP) to 0.353 (for DTD) µg g−1 of soil. All four compounds weredegraded rapidly in 1 wk. Combinations of the chemicals inhibited seedgermination or seedling growth of four of the six plants, includingcroftonweed itself, at mean contents found in the soil. The putativeallelochemicals degraded rapidly in the soil, and the low levels ofallelochemicals observed in the soil may be sufficient to affect seedgermination and plant growth.

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.

Henbit is a facultative broadleaf winter annual in the Lamiaceae family.Acetolactate synthase (ALS) inhibitors are primarily used to control a broadspectrum of weeds, including henbit. During 2012 to 2013, field applicationsof ALS-inhibiting herbicides were ineffective in controlling a henbitpopulation from Marion County, KS (MCK). To confirm field-evolved resistanceto ALS inhibitors, response of MCK henbit and a known susceptible henbitpopulation from Kansas (DPS) to varying doses of three different ALSinhibitors were examined: chlorsulfuron, imazamox, and propoxycarbazone.Results of the dose–response experiments suggest that the MCK population ishighly resistant to chlorsulfuron (resistance index [R/S] > 1,000) andpropoxycarbazone (R/S = 331) but is susceptible to imazamox. A full-length ALS gene sequence obtained using the 5′- and 3′- rapidamplification of complementary DNA ends approach revealed a Pro197 to Arg point mutation (a common mutation that confersresistance to sulfonylurea herbicides, e.g., chlorsulfuron) in the MCKhenbit. No other known resistance-conferring mutations were found in thestudy. Evolved resistance to major classes of ALS inhibitors in the MCKhenbit will reduce herbicide options for its control. To our knowledge, thisis the first case of evolution of herbicide resistance in henbit.

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.

Absorption and translocation of aminocyclopyrachlor (AMCP) was investigatedin black walnut. Radiolabeled AMCP was applied foliarly as a free acid, andbasally as a free-acid-formulated oil-soluble liquid and as a methyl esteremusifiable concentrate. Maximum absorption was 68% for the methyl ester,47% for the free-acid oil-soluble liquid, and 8% for the free acid whenevaluated as percent applied. When recovered AMCP was presented as Bq g−1, no difference was observed in AMCP absorption ortranslocation of 14C-labeled material out of treated plant parts.Recovery of applied radiolabeled material in the roots from basalapplications was 5 to 11% 72 h after treatment (HAT), compared with 0.8%recovery from a foliar application. Regardless of chemical form orformulation type, comparable injury was observed within 72 HAT despite lowlevels of translocation.

The study of colonizing and of dominant grass species is essential forprairie conservation efforts. We sought to answer how naturalized Kentuckybluegrass in the northern Great Plains has become successful in the last 20yr despite its long history in the northern Great Plains. We tested forevidence of geographical differentiation using flow cytometry andmicrosatellite markers to ascertain the population genetics of Kentuckybluegrass. Across all tested wild populations, high levels of geneticdiversity were detected along with moderate levels of structure. Manteltests of geographical patterns were not significant. Using clonalassignment, we found two major clones that made up the majority of thetested wild populations. When we compared the wild individuals to pedigreecultivars, we found virtually no genetic overlap across all tests, which didnot support our hypothesis of developed cultivars contributing to highgenetic diversity in natural populations. Furthermore, DNA content testsindicated a narrow range in ploidy in wild populations compared with lawncultivars, further supporting a hypothesis of divergence between wild andpedigree cultivars. These results indicate the recent invasion of Kentuckybluegrass in the northern Great Plains is not because of adaptation orpropagule pressure, but rather likely an environmental or land useshift.

The susceptibility to glyphosate and genetic diversity based on intersimple sequence repeat markers were characterized for 17 tropical sprangletop populations collected from two separate regions mainly in Persian lime groves in Veracruz, Mexico. The whole-plant dose response together with shikimic acid assays indicated different levels of glyphosate resistance in those populations. Genetic diversity values (h) estimated using POPGENE ranged from 0.119 to 0.198 and 0.117 to 0.214 within susceptible and resistant populations, respectively. The average genetic diversity (H S) within the susceptible populations was 0.157, and the total genetic diversity (H T) was 0.218. The H S of the resistant populations was 0.144, and the H T was 0.186. The analysis of molecular variance based on the response to glyphosate indicated that most of the genetic variation was found within groups of susceptible and resistant populations (90% of the genetic variation), whereas 10% or less was among groups. The high level of genetic diversity between glyphosate-resistant tropical sprangletop populations from distant and adjacent locations is likely due to both short- and long-distance seed dispersal and independent evolutionary events in tropical sprangletop populations among Persian lime groves in Veracruz.