The discipline of weed science is at a critical juncture. Decades of efficient chemical weed control have led to a rise in the number of herbicide-resistant weed populations, with few new herbicides with unique modes of action to counter this trend and often no economical alternatives to herbicides in large-acreage crops. At the same time, the world population is swelling, necessitating increased food production to feed an anticipated 9 billion people by the year 2050. Here, we consider these challenges along with emerging trends in technology and innovation that offer hope of providing sustainable weed management into the future. The emergence of natural product leads in discovery of new herbicides and biopesticides suggests that new modes of action can be discovered, while genetic engineering provides additional options for manipulating herbicide selectivity and creating entirely novel approaches to weed management. Advances in understanding plant pathogen interactions will contribute to developing new biological control agents, and insights into plant-plant interactions suggest that crops can be improved by manipulating their response to competition. Revolutions in computing power and automation have led to a nascent industry built on using machine vision and global positioning system information to distinguish weeds from crops and deliver precision weed control. These technologies open multiple possibilities for efficient weed management, whether through chemical or mechanical mechanisms. Information is also needed by growers to make good decisions, and will be delivered with unprecedented efficiency and specificity, potentially revolutionizing aspects of extension work. We consider that meeting the weed management needs of agriculture by 2050 and beyond is a challenge that requires commitment by funding agencies, researchers, and students to translate new technologies into durable weed management solutions. Integrating old and new weed management technologies into more diverse weed management systems based on a better understanding of weed biology and ecology can provide integrated weed management and resistance management strategies that will be more sustainable than the technologies that are now failing.

Itchgrass [Rottboellia cochinchinensis (Lour.) Clayton] is recognized as one of the most noxious and troublesome annual weeds in tropical and subtropical regions. Acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicides have been frequently used for managing R. cochinchinensis POST in a variety of crops, resulting in evolved resistance to these herbicides. Recently, resistance to fluazifop-P-butyl has been demonstrated for this weed, as the result of a G-to-C single-nucleotide polymorphism (SNP) that leads to the Trp-2027-Cys substitution in the ACCase enzyme. This study was conducted to develop a high-resolution melting analysis (HRMA) for the detection of the mutation underlying the Trp-2027-Cys substitution. The HRMA assay allowed differentiating between fluazifop-P-butyl–resistant (C mutant) and susceptible (G wild type) R. cochinchinensis plants. HRMA accuracy was confirmed with DNA sequencing of the target-site mutation, and no false positives or negatives were observed. Our results illustrated how HRMA is effective detecting the Trp-2027-Cys substitution in an R. cochinchinensis resistance, and how this technique can be of great value for developing high-throughput programs for monitoring evolution and dispersion of target site–based herbicide resistance at large scales.

Research was conducted to determine whether resistance to glyphosate among Palmer amaranth (Amaranthus palmeri S. Watson) populations within the U.S. state of Arkansas was due solely to increased EPSPS gene copy number and whether gene copy number is correlated with resistance level to glyphosate. One hundred and fifteen A. palmeri accessions were treated with 840 g ae ha^-1 glyphosate. 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 ED₅₀ values ranging from 494 to 1,355g 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 increased EPSPS gene copies. Resistant plants from the remaining eight GR accessions (62% of resistant plants tested) had 19 to 224 more EPSPS gene copies than the SS. Among the accessions tested, injury declined 4% with every additional EPSPS copy. ED₅₀ values were directly correlated with EPSPS copy number. The highly resistant accession MIS11-B had an ED₅₀ of 1,355 g ha^-1 and 150 gene copies. Partial sequences of EPSPS from GR accessions without EPSPS amplification did not contain any of the known resistance-conferring mutations. Nearly 40% of GR accessions putatively harbor non-target site resistance mechanisms. Therefore, elevated EPSPS gene copy number is associated with glyphosate resistance among A. palmeri from Arkansas.

In the rapid response (RR) biotype of glyphosate-resistant (GR) giant ragweed (Ambrosia trifida L.), exposure to glyphosate elicits H₂O₂ production in mature leaves, resulting in foliage loss and reduced glyphosate translocation. When glyphosate is applied with POST herbicides intended to improve control of A. trifida, the RR to glyphosate has the propensity to antagonize these herbicide combinations. This research documents how transient changes in air temperature, soil moisture, and light intensity during a 6-d period surrounding herbicide application regulate induction of the RR and the effect on POST herbicide interactions with glyphosate. Air temperature had the greatest influence on H₂O₂ accumulation in leaf disks following treatment with glyphosate, as plants at 30 C produced more than twice the amount of H₂O₂ at 2.5 h after treatment compared with 10 C. Plants under field capacity conditions accumulated nearly 50% more H₂O₂ than those at one-third field capacity, while those under no shade had only 18% more H₂O₂ compared with those in a shaded environment. Despite these initial results, dry weight reduction at 21 d after treatment never differed by more than 8% between levels of environmental factors, thus indicating a negligible influence on glyphosate efficacy. The magnitude of glyphosate-induced antagonism was generally greater at 30 C (12% to 21% less than expected control) versus 10 C (11% to 16%) on atrazine, cloransulam, dicamba, and topramezone and was greater at field capacity (20% to 24%) versus one-third field capacity (11% to 15%) on cloransulam and topramezone. These results indicate air temperatures and soil moisture levels conducive to optimal plant growth accelerate the RR to glyphosate, thereby increasing the likelihood of glyphosateinduced antagonism on several translocated herbicides.

Downy brome (Bromus tectorum L.) is a widely distributed invasive winter annual grass across western North America. Bromus tectorum phenology can vary considerably among populations, and those differences are considered adaptively significant. A consensus hypothesis in the literature attributes the majority of observed differences in B. tectorum phenology to differing vernalization requirements among populations. A series of greenhouse experiments were conducted to identify differences in B. tectorum vernalization requirements and link vernalization to expression of annual false-brome [Brachypodium distachyon (L.) P. Beauv.]-derived vernalization gene homolog (BdVRN1). Results from this study indicate that variation in time to flowering is partially governed by differing vernalization requirements and that flowering is linked to the expression of BdVRN1.

Vinasse, a liquid fermentation residual of bio-ethanol production that also contains solid particles in suspension, is commonly used as a soil amendment. Previous studies reported vinasse reduced seed germination and seedling establishment, suggesting herbicidal activity. Laboratory experiments were conducted to determine whether vinasse herbicidal activity is present in the liquid or solid phase, and whether it affects plants during seed early germination (i.e., imbibition), late germination (i.e., embryo growth and radicle protrusion), or seedling growth. Most of the herbicidal activity was associated with the liquid phase, and for most species, seed viability was predominantly affected after the imbibition phase. Susceptibility to vinasse was species dependent. Lettuce (Lactuca sativa L.) germination was <8% when seeds were imbibed and germinated in vinasse solutions or imbibed in water and germinated in vinasse. Conversely, imbibing lettuce seeds in vinasse solutions and germinating them in water did not change their germination in comparison with seeds imbibed and germinated in water (>80% germination). Wheat (Triticum aestivum L.) and sicklepod [Senna obtusifolia (L.) H. S. Irwin & Barneby] germination decreased 10% and 35% when seeds were imbibed and germinated in vinasse, respectively, while Palmer amaranth (Amaranthus palmeri S. Watson) and southern crabgrass [Digitaria ciliaris (Retz.) Koeler] germination decreased >90%. All evaluated species reduced radicle growth as vinasse concentration increased. Filtered liquid vinasse with reduced concentration of salt and ionic compounds inhibited radicle growth similarly to unfiltered vinasse, indicating that the herbicidal activity was not due to osmotic effects and was likely present in the organic liquid phase. Amaranthus palmeri, S. obtusifolia, and D. ciliaris increased the proportion of dormant seed more than 2-fold when they were imbibed or imbibed and germinated in vinasse solutions. Vinasse might be useful for weed management to reduce germinable weed seedbanks by increasing seedling mortality and seed dormancy either by properly timing of its application as a soil amendment or by purifying herbicidal compounds and using them directly for weed control.

This study provides the first phytochemical characterization of the morphologically identified natural hybrid Solidago × niederederi Khek compared with the native Solidago virgaurea and two invasive species, Canada goldenrod (Solidago canadensis L.) and giant goldenrod (Solidago gigantea Aiton). The phenolic compounds, namely, chlorogenic acid, rutin, isoquercitrin, hyperoside, and quercitrin, were detected in leaves and inflorescences by the high-performance liquid chromatography–photodiode array detector/ultraviolet (PAD/UV) method. All analyzed Solidago species contained all of the phenolic compounds investigated. The quantitative phytochemical differentiation among Solidago taxa was shown by principal component analysis. The results indicated that S. gigantea plants were characterized by significantly different quantities of phenolic compounds compared with three other Solidago taxa, which formed a separate cluster in the space of the principal component model, indicating the high similarity of their profiles. An additional multivariate analysis of the three species studied revealed a chemical gradient from S. canadensis to S. virgaurea with a slightly overlapping zone on the score plots presented by S. × niederederi and S. virgaurea accessions. The results showed that S. × niederederi was closely related to S. virgaurea. This result is suggestive of a hybrid origin with significant contributions from the native species. However, S. × niederederi was significantly different from its parental species with respect to chlorogenic acid and quercitrin in leaves and rutin with isoquercitrin in inflorescences. Conversely, samples indicating intermediate chemical composition between native S. virgaurea and invasive S. gigantea were not distinguished. The comparison of phenolic compound accumulation in Solidago plants supported the additional identification of the origin of S. × niederederi.

Seed dormancy allows weedy rice (Oryza sp.) to persist in rice production systems. Weedy and wild relatives of rice (Oryza sativa L.) exhibit different levels of dormancy, which allows them to escape weed management tactics, increasing the potential for flowering synchronization, and therefore gene flow, between weedy Oryza sp. and cultivated rice. In this study, we determined the genetic diversity and divergence of representative dormant and nondormant weedy Oryza sp. groups from Arkansas. Twenty-five simple sequence repeat markers closely associated with seed dormancy were used. Four populations were included: dormant blackhull, dormant strawhull, nondormant blackhull, and nondormant strawhull. The overall gene diversity was 0.355, indicating considerable genetic variation among populations in these dormancy-related loci. Gene diversity among blackhull populations (0.398) was higher than among strawhull populations (0.245). Higher genetic diversity was also observed within and among dormant populations than in nondormant populations. Cluster analysis of 16 accessions, based on Nei's genetic distance, showed four clusters. Clusters I, III, and IV consisted of only blackhull accessions, whereas Cluster II comprised only strawhull accessions. These four clusters did not separate cleanly into dormant and nondormant populations, indicating that not all markers were tightly linked to dormancy. The strawhull groups were most distant from blackhull weedy Oryza sp. groups. These data indicate complex genetic control of the dormancy trait, as dormant individuals exhibited higher genetic diversity than nondormant individuals. Seed-dormancy trait contributes to population structure of weedy Oryza sp., but this influence is less than that of hull color. Markers unique to the dormant populations are good candidates for follow-up studies on the control of seed dormancy in weedy Oryza sp.

Much of the agricultural area in California's southwestern San Joaquin Valley (SJV) is prone to moisture stress and high soil-salinity conditions. Increased prevalence of glyphosate-resistant (GR) biotypes of junglerice [Echinochloa colona (L.) Link] in these environments and their ecological implications need to be further explored. Studies were conducted on GR and glyphosate-susceptible (GS) biotypes of E. colona to compare the effects of moisture and salinity stress on seed germination and salinity stress alone on growth and seed production. Intraspecific competition between the GR and GS plants was also assessed in a replacement series design experiment. With respect to germination, both biotypes were tolerant to moisture and salinity stress at germination; however, the GR biotype was more tolerant than the GS biotype. Water potential and electrical conductivity (EC) levels that reduced germination by 50% were estimated as –1.5 and –2.3 MPa and 8.5 and 12 dS m^-1 for the GS and GR biotypes, respectively. The EC levels that reduced aboveground biomass by 50% were estimated as 9 and 11.5 dS m^-1 for the GS and GR biotypes, respectively. Seed production was generally greater in the GR than the GS plants below 10 dS m^-1. All plants produced up to 140 seeds plant^-1, even at 20 dS m^-1. The GR plants were more competitive and produced more aboveground dry biomass and seeds than the GS plants when grown together or alone. In conclusion, differences between these particular GR and GS biotypes to environmental stresses and intraspecific competition were noted that could have ecological implications for their prevalence in the southwestern SJV. The results also suggested that there could be high genetic variability and phenotypic plasticity in E. colona populations in the SJV and further population shifts could occur due to selection pressure from glyphosate.

Herbicide resistance, and in particular multiple-herbicide resistance, poses an ever-increasing threat to food security. A biotype of junglerice [Echinochloa colona (L.) Link] with resistance to four herbicides, imazamox, fenoxaprop-P-ethyl, quinclorac, and propanil, each representing a different mechanism of action, was identified in Sunflower County, MS. Dose responses were performed on the resistant biotype and a biotype sensitive to all four herbicides to determine the level of resistance. Application of a cytochrome P450 inhibitor, malathion, with the herbicides imazamox and quinclorac resulted in increased susceptibility in the resistant biotype. Differential gene expression analysis of resistant and sensitive plants revealed that 170 transcripts were upregulated in resistant plants relative to sensitive plants and 160 transcripts were upregulated in sensitive plants. In addition, 507 transcripts were only expressed in resistant plants and 562 only in sensitive plants. A subset of these transcripts were investigated further using quantitative PCR (qPCR) to compare gene expression in resistant plants with expression in additional sensitive biotypes. The qPCR analysis identified two transcripts, a kinase and a glutathione S-transferase that were significantly upregulated in resistant plants compared with the sensitive plants. A third transcript, encoding an F-box protein, was downregulated in the resistant plants relative to the sensitive plants. As no cytochrome P450s were differentially expressed between the resistant and sensitive plants, a single-nucleotide polymorphism analysis was performed, revealing several nonsynonymous point mutations of interest. These candidate genes will require further study to elucidate the resistance mechanisms present in the resistant biotype.

Leafy spurge (Euphorbia esula L.) is an invasive perennial weed infesting range and recreational lands of North America. Previous research and omics projects with E. esula have helped develop it as a model for studying many aspects of perennial plant development and response to abiotic stress. However, the lack of an assembled genome for E. esula has limited the power of previous transcriptomics studies to identify functional promoter elements and transcription factor binding sites. An assembled genome for E. esula would enhance our understanding of signaling processes controlling plant development and responses to environmental stress and provide a better understanding of genetic factors impacting weediness traits, evolution, and herbicide resistance. A comprehensive transcriptome database would also assist in analyzing future RNA-seq studies and is needed to annotate and assess genomic sequence assemblies. Here, we assembled and annotated 56,234 unigenes from an assembly of 589,235 RNA-seq-derived contigs and a previously published Sanger-sequenced expressed sequence tag collection. The resulting data indicate that we now have sequence for >90% of the expressed E. esula proteincoding genes. We also assembled the gene space of E. esula by using a limited coverage (18X) genomic sequence database. In this study, the programs Velvet and Trinity produced the best gene-space assemblies based on representation of expressed and conserved eukaryotic genes. The results indicate that E. esula contains as much as 23% repetitive sequences, of which 11% are unique. Our sequence data were also sufficient for assembling a full chloroplast and partial mitochondrial genome. Further, marker analysis identified more than 150,000 high-quality variants in our E. esula L-RNA–scaffolded, whole-genome, Trinity-assembled genome. Based on these results, E. esula appears to have limited heterozygosity. This study provides a blueprint for low-cost genomic assemblies in weed species and new resources for identifying conserved and novel promoter regions among coordinately expressed genes of E. esula.

The semiarid Espinal in central Argentina, being recently transformed from natural semiarid grasslands into agriculture, represents an interesting scenario to understand the early stages of weed community assembly and its relationship with crop identity and management. Our aim was to characterize the weed communities in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.], the main crops of the Espinal region, under the dominant rainfed conditions. Weed surveys were carried out in 53 fields, and farmers were interviewed to collect information about crop management. Floristic composition was compared within and between crops by calculating the additive partition of the abundance-based Bray-Curtis dissimilarity. We compared the frequency and mean cover of functional groups between crops through generalized linear models. Finally, canonical correspondence analysis was carried out to analyze the associations between floristic composition and agronomic variables. Mean alpha and gamma diversity was greater in corn (10.0 and 80 species, respectively) than in soybean (7.6 and 46 species, respectively). Furthermore, species composition of weed communities was more similar among soybean fields than among either cornfields or fields of both crops. Hence, floristic differences between crops are potentially the result of different microenvironmental heterogeneity above- and belowground, with corn likely to be more permissive to weed establishment compared with soybean. The higher frequency of annual, dicotyledonous, and native species, and the high proportion of rare species, mostly native, suggest a strong legacy of the original vegetation that thrived in these recently cultivated systems. The functional composition was also affected by agronomic management, with sulfur, nitrogen, and grass herbicide application being the most important factors related to the floristic composition of weed communities. This early description can be used as a starting point for studies concerning trajectories, mechanisms, and processes of weed communities related to environment and management.

Incorporation of cover crop residue into the soil has been suggested as a means for reducing weed seedbanks. To explore this hypothesis, we buried mesh bags of seeds mixed with sand at 15-cm depth in late fall in plots that had been planted with rye (Secale cereale L.) or hairy vetch (Vicia villosa Roth.) or left unplanted. Separate bags contained either velvetleaf (Abutilon theophrasti Medik.), giant foxtail (Setaria faberi Herrm.), Powell amaranth (Amaranthus powellii S. Watson), or common lambsquarters (Chenopodium album L.). The experiment used a randomized complete block design with five replications, and enough bags were buried to allow a final recovery in each of the following three springs. Each spring, bags were exhumed, and seeds were either counted and tested for viability or mixed with chopped cover crop material or simply stirred for control bags, and the material was reburied. The experiment was completed twice with initial burials in fall of 2011 and 2013. Rye had no consistent effect on persistence of seeds of any of the species. For two observation intervals, rye increased persistence of a species; for another two intervals, it decreased persistence relative to the control; but mostly rye did not affect persistence. Hairy vetch decreased persistence of C. album and A. powellii in both runs of the experiment but had no effect on persistence of A. theophrasti and S. faberi. Germination of the first two species is promoted by nitrate, whereas A. theophrasti germination is not sensitive to nitrate, and S. faberi is only rarely nitrate sensitive. We suggest that nitrate released during decomposition of hairy vetch may have promoted fatal germination of C. album and A. powellii. Incorporation of legume cover crops like hairy vetch may provide a means for decreasing the seedbanks of the many weed species whose germination is promoted by nitrate. The lack of any reduction of A. theophrasti and S. faberi seed persistence in response to hairy vetch and the inconsistent and mostly negligible effect of rye indicate that a general increase in readily decomposable organic matter through incorporation of cover crops may be ineffective at reducing weed seedbanks.

Research was conducted from 2015 to 2017 to investigate the potential for 2,4-D and multiple herbicide resistance in a waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] population from Missouri (designated MO-Ren). In the field, visual control of the MO-Ren population with 0.56 to 4.48 kg 2,4-D ha^-1 ranged from 26% to 77% in 2015 and from 15% to 55% in 2016. The MO-Ren population was highly resistant to chlorimuron, with visual control never exceeding 7% either year. Estimates of the 2,4-D dose required to provide 50% visual control (I₅₀) of the MO-Ren population were 1.44 kg ha^-1 compared with only 0.47 kg 2,4-D ha^-1 for the susceptible population. Based on comparisons to a susceptible population in dose–response experiments, the MO-Ren population was approximately 3-fold resistant to 2,4-D, and 7-, 7-, 22-, and 14-fold resistant to atrazine, fomesafen, glyphosate, and mesotrione, respectively. Dicamba and glufosinate were the only two herbicides that provided effective control of the MO-Ren population in these experiments. Examinations of multiple herbicide resistance at the individual plant level revealed that 16% of the plants of the MO-Ren population contained genes stacked for six-way herbicide resistance, and only 1% of plants were classified as resistant to a single herbicide (glyphosate). Results from these experiments confirm that the MO-Ren A. tuberculatus population is resistant to 2,4-D, atrazine, chlorimuron, fomesafen, glyphosate, and mesotrione, making this population the third 2,4-D-resistant A. tuberculatus population identified in the United States, and the first population resistant to six different herbicidal modes of action.

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.

Five bio-ionic liquids (BILs) with choline cations and fatty acid anions derived from pelargonic acid, glycerol tristearate, glycerol trioleate, canola oil, and coconut oil were synthesized and applied as spray adjuvants with three sulfonylurea herbicides: metsulfuron-methyl, iodosulfuron-methyl-sodium, and tribenuron-methyl. Physicochemical properties, including thermal stability, solubility, and surface activity, were determined, and the influence of these BILs on herbicidal efficacy was studied in greenhouse tests using four target weed species: common lambsquarters (Chenopodium album L.), cornflower (Centaurea cyanus L.), corn poppy (Papaver rhoeas L.), and oilseed rape (Brassica napus L.). BILs, particularly those with the oleic anion and anions derived from canola oil and coconut oil, greatly improved herbicidal activity. Addition of BILs to the spray solution significantly reduced the surface tension and contact angle of spray droplets and increased the area of herbicide deposit on the leaf surface.