Resistance to herbicides occurs in weeds as the result of evolutionary adaptation (Jasieniuk et al. 1996). Basically, two types of mechanisms are involved in resistance (Beckie and Tardif 2012; Délye 2013). Target-site resistance (TSR) is caused by changes in the tridimensional structure of the herbicide target protein that decrease herbicide binding, or by increased activity (e.g., due to increased expression or increased intrinsic activity) of the target protein. Nontarget-site resistance (NTSR) is endowed by any mechanism not belonging to TSR, e.g., reduction in herbicide uptake or translocation in the plant, or enhanced herbicide detoxification (reviewed in Délye 2013; Yuan et al. 2007).

Glyphosate is regularly used to control annual bluegrass populations in dormant bermudagrass turf. A population of annual bluegrass not controlled by glyphosate at 840 g ha−1 (glyphosate resistant, GR) was identified on a golf course in Humboldt, TN in 2010. Mature tillers of GR plants were established in a greenhouse and treated with glyphosate at 0, 210, 420, 840, 1,680, 3,360, and 6,720 g ha−1. Mature tillers of a biotype known to be susceptible to glyphosate (SS) were also established in the greenhouse and subjected to the same treatments. At 14 d after treatment (DAT), glyphosate controlled the SS biotype > 95% at rates > 420 g ha−1. Comparatively, the GR biotype was only controlled 76% with glyphosate at 6,720 g ha−1. The rates required to provide 50% control (I50 values) for SS and GR biotypes were 236 and 2,812 g ha−1 respectively, resulting in a resistance factor of 12. Photochemical efficiency (Fv/Fm) values on SS plants treated with glyphosate at > 210 g ha−1 measured 0.000 at 14 DAT, whereas Fv/Fm values on GR plants were not significantly different from the untreated control with glyphosate rates ≤ 840 g ha−1 on the same date. In laboratory experiments, the SS biotype accumulated greater shikimate concentrations than the GR biotype 3 to 6 DAT. Future research should evaluate strategies for managing GR and SS annual bluegrass with alternative modes of action.

Johnsongrass is one of the most troublesome weeds of the world and is listed as a noxious weed in Arkansas. Reduced johnsongrass control with the recommended application rate of glyphosate (840 g ae ha−1) was reported in a continuous soybean field near West Memphis, AR, in the fall of 2007. A greenhouse study was conducted (1) to confirm and characterize glyphosate resistance in the johnsongrass biotype from West Memphis and (2) to determine whether resistant and susceptible biotypes have differential glyphosate absorption or translocation. Dose–response studies revealed that the resistant biotype was five- to seven-fold less sensitive to glyphosate than the susceptible biotype. Glyphosate absorption was similar in resistant and susceptible biotypes at 72 h after treatment (HAT). However, the treated leaf of the resistant biotype retained 28 percentage points more absorbed 14C glyphosate compared to the susceptible biotype at 72 HAT. Additionally, the resistant biotype had less 14C glyphosate translocated to the aboveground tissue below the treated leaf and to roots compared to the susceptible biotype at 24 and 72 HAT. Reduced translocation and increased retention of glyphosate in treated leaves is a probable mechanism of resistance in this glyphosate-resistant johnsongrass biotype.

Integrated weed management (IWM) for agronomic and vegetable production systems utilizes all available options to effectively manage weeds. Late-season weed control measures are often needed to improve crop harvest and stop additions to the weed seed bank. Eliminating the production of viable weed seeds is one of the key IWM practices. The objective of this research was to determine how termination method and timing influence viable weed seed production of late-season weed infestations. Research was conducted in Delaware, Michigan, and New York over a 2-yr period. The weeds studied included: common lambsquarters, common ragweed, giant foxtail, jimsonweed, and velvetleaf. Three termination methods were imposed: cutting at the plant base (simulating hand hoeing), chopping (simulating mowing), and applying glyphosate. The three termination timings were flowering, immature seeds present, and mature seeds present. Following termination, plants were stored in the field in mesh bags until mid-Fall when seeds were counted and tested for viability. Termination timing influenced viable seed development; however, termination method did not. Common ragweed and giant foxtail produced viable seeds when terminated at the time of flowering. All species produced some viable seed when immature seeds were present at the time of termination. The time of viable seed formation varied based on species and site-year, ranging from plants terminated the day of flowering to 1,337 growing degree d after flowering (base 10, 0 to 57 calendar d). Viable seed production was reduced by 64 to 100% when common lambsquarters, giant foxtail, jimsonweed, and velvetleaf were terminated with immature seeds present, compared to when plants were terminated with some mature seeds present. Our results suggest that terminating common lambsquarters, common ragweed, and giant foxtail prior to flowering, and velvetleaf and jimsonweed less than 2 and 3 wk after flowering, respectively, greatly reduces weed seed bank inputs.

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.

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?

The annual global economic loss caused by weeds has been estimated at more than $100 billion U.S. dollars (Appleby et al. 2000). Additionally, worldwide annual herbicide sales are in the range of U.S. $25 billion (Agrow 2003). In light of these large dollar figures, it becomes clear that a greater understanding of crop—weed interactions is essential in order to develop cost-effective and sustainable weed management practices.

Herbicides are small molecules that inhibit specific molecular target sites within plant biochemical pathways and/or physiological processes. Inhibition of these sites often has catastrophic consequences that are lethal to plants. The affinity of these compounds for their respective target sites makes them useful tools to study and dissect the intricacies of plant biochemical and physiological processes. For instance, elucidation of the photosynthetic electron transport chain was achieved in part by the use of herbicides, such as terbutryn and paraquat, which act on photosystem II and I, respectively, as physiological probes. Work stemming from the discovery of the binding site of PS II–inhibiting herbicides was ultimately awarded the Nobel Prize in 1988. Although not as prestigious as the seminal work on photosynthesis, our knowledge of many other plant processes expanded significantly through the ingenious use of inhibitors as molecular probes. Examples highlight the critical role played by herbicides in expanding our understanding of the fundamental aspects of the synthesis of porphyrins and the nonmevalonate pathway, the evolution of acetyl-coenzyme A carboxylase, cell wall physiology, the functions of microtubules and the cell cycle, the role of auxin and cyanide, the importance of subcellular protein targeting, and the development of selectable markers.

Although the problem of herbicide resistance is not new, the widespread evolution of glyphosate resistance in weed species such as Palmer amaranth (Amaranthus palmeri S. Wats.), common waterhemp (Amaranthus rudis Sauer), and kochia [Kochia scoparia (L.) Schrad.] raised awareness throughout the agricultural community of herbicide resistance as a problem. Glyphosate-resistant weeds resulted in the loss of a simple, single herbicide option to control a wide spectrum of weeds that gave efficacious and economical weed management in corn (Zea mays L.), soybean [Glycine max (L.) Merr.], and cotton (Gossypium hirsutum L.) crops engineered for tolerance to this herbicide and planted over widespread areas of the South and Midwest of the United States. Beyond these crops, glyphosate is used for vegetation management in other cropping systems and in noncrop areas across the United States, and resistance to this herbicide threatens its continued utility in all of these situations. This, combined with the development of multiple herbicide-resistant weeds and the lack of commercialization of herbicides with new mechanisms of action over the past years (Duke 2012), caused the weed science community to realize that stewardship of existing herbicide resources, extending their useful life as long as possible, is imperative. Further, while additional herbicide tolerance traits are being incorporated into crops, weed management in these crops will still be based upon using existing, old, herbicide chemistries.

Natural herbicides approved in organic agriculture are primarily nonselective burn-down essential oils applied POST. Multiple applications are often required due to their low efficacy. To address this problem, the in vivo herbicidal activity of manuka oil, the essential oil distilled from manuka tree (Leptospermum scoparium J.R. and G. Forst), was tested on selected broadleaf and grass weeds. While manuka oil exhibited good POST activity when applied in combination with a commercial lemongrass oil–based herbicide, it ultimately demonstrated interesting PRE activity, providing control of large crabgrass seedlings at a rate of 3 L ha−1. Manuka oil and its main active ingredient, leptospermone, were stable in soil for up to 7 d and had half-lives of 18 and 15 d, respectively. The systemic activity of manuka oil addresses many of the current limitations associated with natural herbicides. Additionally, its soil persistence opens up a multitude of new possibilities for the use of manuka oil as a tool for weed management and may be a potential bridge between traditional and organic agriculture.

Italian ryegrass populations investigated in this study were harvested in an alfalfa-based cropping system. In that system, the agronomic practices and chemical weed management, based on the use of aryloxyphenoxy-propionates herbicides (i.e., quizalofop ethyl ester), were optimized to obtain a dual seed–forage production. Five of seven populations tested were confirmed resistant to quizalofop ethyl ester with resistance indexes ranging from 4.5 to >209. Both target- and nontarget-site resistance mechanisms were most likely involved. Three allelic variants were detected (Ile-1781–Leu, Trp-2027–Cys, and Ile-2041–Asn) in four resistant populations, whereas no known mutations were found in one resistant population. The herbicide treatment on Italian ryegrass plants at different phenological stages suggested that to control regrowth, it is necessary to use two to fives times the herbicide dose suitable for younger plants. This situation is encountered in fields when Italian ryegrass plants need to be controlled to maximize the alfalfa seed production, and it is comparable to using a sublethal herbicide dose, leading to the selection of herbicide-resistant biotypes. In such a situation, the cropping system is not sustainable, and integrated weed management should be implemented to deplete the soil weed seed bank and prevent new weed seed production.

A large-plot field experiment was conducted at Keiser, AR, from fall of 2010 through fall of 2013 to understand to what extent soybean in-crop herbicide programs and postharvest fall management practices impact Palmer amaranth population density and seed production over three growing seasons. The effect of POST-only (glyphosate-only) or PRE followed by (fb) POST (glyphosate or glufosinate) + residual herbicide treatments were evaluated alone and in combination with postharvest management options of soybean residue spreading or soil incorporation, use of cover crops, windrowing with/without burning, and residue removal. Significant differences were observed between fall management practices on Palmer amaranth population density each fall. The use of cover crops and residue collection and removal fb the incorporation of crop residues into soil during the formation of beds were the most effective practices in reducing Palmer amaranth population. In contrast, the effects of fall management practices on Palmer amaranth seed production were inconsistent among years. The inclusion of a PRE herbicide application into the herbicide program significantly reduced Palmer amaranth population density and subsequent seed production each year when compared to the glyphosate-only program. Additionally, the glufosinate-containing residual program was superior to the glyphosate-containing residual program in reducing Palmer amaranth seed production. PRE fb POST herbicides resulted in significant decreases in the Palmer amaranth population density and seed production compared to POST application of glyphosate alone for all fall management practices, including the no-till practice. This study demonstrated that crop residue management such as chaff removal from the field, the use of cover crops, or seed incorporation during bed formation in combination with an effective PRE plus POST residual herbicide program is important for optimizing in-season management of Palmer amaranth and subsequently reducing the population density, which has a profound impact on lessening the risk for herbicide resistance and the consistency and effectiveness of future weed management efforts.

Smallflower umbrella sedge is a problematic weed in direct-seeded rice in the midsouthern United States. It recently has evolved resistance to the acetolactate synthase (ALS) –inhibiting herbicide halosulfuron in Arkansas rice. Studies were conducted (1) to determine if the resistant biotype is cross resistant to other ALS-inhibiting herbicides, (2) to evaluate alternative herbicide control options, and (3) to determine the mechanism of resistance. Whole-plant bioassay revealed that halosulfuron-resistant plants were not controlled by bispyribac–sodium, imazamox, and penoxsulam at the labeled field rate of each herbicide. The level of resistance to these herbicides, based on the lethal dose needed to kill 50% of plants (LD50) was ≥ 15-fold compared to a susceptible biotype. Both biotypes were controlled >96% with bentazon and propanil and ≤ 23% with quinclorac, thiobencarb, and 2,4-D. Hence, effective control measures exist; albeit, the number of herbicide options appear limited. Based on in vitro ALS enzyme assays, altered target site is the mechanism of resistance to halosulfuron and imazamox. Massively parallel sequencing with the use of the Illumina HiSeq detected an amino acid substitution of Pro197-to-His in the resistant biotype that is consistent with ALS-inhibiting herbicide resistance in other weed species.

Herbicides are the foundation of weed control in commercial crop-production systems. However, herbicide-resistant (HR) weed populations are evolving rapidly as a natural response to selection pressure imposed by modern agricultural management activities. Mitigating the evolution of herbicide resistance depends on reducing selection through diversification of weed control techniques, minimizing the spread of resistance genes and genotypes via pollen or propagule dispersal, and eliminating additions of weed seed to the soil seedbank. Effective deployment of such a multifaceted approach will require shifting from the current concept of basing weed management on single-year economic thresholds.

The use of eucalyptus leaves for weed control in maize-based cropping systems is proposed. Aqueous extracts of eucalyptus are known to exert phytotoxicity on many weeds and crops, but there is also experimental evidence of the relative tolerance of maize. Based on in vitro dose-response bioassays of leaf aqueous extracts, we conducted greenhouse pot experiments testing incorporated eucalyptus leaves as green manure. The phytotoxic effects were tested on the germination, establishment and growth of maize and some representative accompanying weeds, in comparison to the PRE herbicide metolachlor. Eucalyptus fresh leaves incorporated into the soil as green manure at 1 and 2% w/w reduced the emergence of the dicot weed species redroot pigweed and black nightshade. After one month of incorporation, both doses reduced aerial biomass >94% two monocot weed species (barnyardgrass + large crabgrass) with respect to the eucalyptus-free pots, and around 80% for the small seeded dicots. Although the aerial biomass of maize was reduced by 33%, the final relative yield of maize biomass with respect to the untreated control increased by 37%. On the assessment of the temporal phytotoxic effects, the reduction of aerial biomass in maize could be overcome by adopting a relay-planting of maize after 12 to 15 days from eucalyptus incorporation. Our results constitute evidence that the incorporation of E. globulus residues to soil could be a feasible practice to reduce the reliance on synthetic herbicides in maize-based cropping systems.

Development of herbicide-resistant crops has resulted in significant changes to agronomic practices, one of which is the adoption of effective, simple, low-risk, crop-production systems with less dependency on tillage and lower energy requirements. Overall, the changes have had a positive environmental effect by reducing soil erosion, the fuel use for tillage, and the number of herbicides with groundwater advisories as well as a slight reduction in the overall environmental impact quotient of herbicide use. However, herbicides exert a high selection pressure on weed populations, and density and diversity of weed communities change over time in response to herbicides and other control practices imposed on them. Repeated and intensive use of herbicides with the same mechanisms of action (MOA; the mechanism in the plant that the herbicide detrimentally affects so that the plant succumbs to the herbicide; e.g., inhibition of an enzyme that is vital to plant growth or the inability of a plant to metabolize the herbicide before it has done damage) can rapidly select for shifts to tolerant, difficult-to-control weeds and the evolution of herbicide-resistant weeds, especially in the absence of the concurrent use of herbicides with different mechanisms of action or the use of mechanical or cultural practices or both.

2,4-D, discovered independently in the United States and Europe in the mid-1940s, was one of the first synthetic herbicides to be used selectively for weed control (Cobb and Reade 2010). Since then, several herbicides belonging to different chemical classes and possessing diverse mechanisms of action have been synthesized and marketed globally. Herbicides have vastly contributed to increasing world food, fiber, fuel, and feed production in an efficient, economic, and environmentally sustainable manner. Before receiving regulatory approval, all herbicides (pesticides) undergo rigorous testing for their toxicological, residual, physicochemical, and biological properties. Additionally, herbicides are suitably formulated to reach their target site and maximize their efficacy on target weeds while being safe on crops.

Palmer amaranth, a dioecious summer annual forb, originating in Sonoran desert washes, compromises crop yields in much of the southern United States and its range is expanding northward. Appropriate tactics for managing this weed proactively in the Upper Midwest will depend on characterizing its damage niche, the geographic range in which it can reduce crop yields. We implemented a common garden study in 2011 and 2012, planting eight accessions of Palmer amaranth from the southern and midwestern United States, into soybean crops in southern, central, and northern Illinois, at a population density of 8 plants m−2 with a biocontainment protocol. Once Palmer amaranth plants initiated flowering, they were removed and burned. Weed survival, flowering, and weed biomass were measured, in addition to soybean yield and weather data. Analyses indicated that Palmer amaranth's damage niche in Illinois soybean was independent of weed genotype or maternal environment. Despite competing only briefly, Palmer amaranth reduced soybean yields in all site–years, indicating its damage niche in Illinois, and much of the Midwest, is limited primarily by seed immigration rate. These results highlight the urgent need for weed managers to learn Palmer amaranth identification, prevent seed introduction, and maintain a policy of zero seed return.