Kochia [Bassia scoparia (L.) A. J. Scott] is a problematic annual broadleaf weed species in the North American Great Plains. Bassia scoparia inherits unique biological characteristics that contribute to its propensity to evolve herbicide resistance. Evolution of glyphosate resistance in B. scoparia has become a serious threat to the major cropping systems and soil conservation practices in the region. Bassia scoparia populations with resistance to four different herbicide sites of action are a concern for growers. The widespread occurrence of multiple herbicide–resistant (HR) B. scoparia across the North American Great Plains has renewed research efforts to devise integrated weed management strategies beyond herbicide use. In this review, we aim to compile and document the growing body of literature on HR B. scoparia with emphasis on herbicide-resistance evolutionary dynamics, distribution, mechanisms of evolved resistance, agronomic impacts, and current/future weed management technologies. We focused on ecologically based, non-herbicidal strategies such as diverse crop rotations comprising winter cereals and perennial forages, enhanced crop competition, cover crops, harvest weed seed control (HWSC), and tillage to manage HR B. scoparia seedbanks. Remote sensing using hyperspectral imaging and other sensor-based technologies would be valuable for early detection and rapid response and site-specific herbicide resistance management. We propose research priorities based on an improved understanding of the biology, genetic diversity, and plasticity of this weed that will aid in preserving existing herbicide resources and designing sustainable, integrated HR B. scoparia mitigation plans.

Kochia [Bassia scoparia (L.) A. J. Scott] is one of the most troublesome weeds throughout the North American Great Plains. Herbicides such as glyphosate and dicamba have been used widely to control B. scoparia for decades. However, many B. scoparia populations have evolved resistance to these herbicides due to selection. Especially, dicamba-resistant B. scoparia populations are often also found to be glyphosate-resistant. The objective of this research was to determine whether these two herbicide resistances are linked in B. scoparia. Reciprocal crosses were performed between glyphosate- and dicamba-resistant (GDR) and glyphosate- and dicamba-susceptible (GDS) B. scoparia to produce F1 and F2 progeny. Two F1 and seven F2 progeny families were screened with various doses of dicamba or glyphosate. All the F1 progeny survived both dicamba and glyphosate treatments. Chi-square analyses of F2 progeny suggest (1) glyphosate and dicamba resistances in B. scoparia are inherited via single, dominant nuclear genes; and (2) glyphosate- and dicamba-resistant genes are not linked. Thus, the dicamba and glyphosate resistances appear to have evolved independently due to intense selection but do not seem to spread together.

The level of glyphosate resistance in kochia [Bassia scoparia (L.) A. J. Scott] was reported to be due to an increase in 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene copy number. A field study was conducted near Manhattan, KS, in 2014 and 2015 to evaluate the relationship between EPSPS gene copy number and growth and fecundity variables of B. scoparia individuals within suspected glyphosate-resistant (GR) populations from western Kansas. Initial assays of EPSPS gene copy and in vivo shikimate accumulation showed that B. scoparia populations from Finney (FN-R), Scott (SC-R), and Thomas (TH-R) counties were segregating for glyphosate resistance, with some individuals still being glyphosate susceptible (GS). A target-neighborhood competition approach was used to evaluate the competitive response of individual target plants with relatively low (classified as GS) and high (classified as GR) EPSPS gene copy number within the populations. There was no relationship observed between EPSPS gene copy number and vegetative or fecundity variables. There was no differential competitive response of target plant biomass to increasing neighbor density between individuals with low and high EPSPS gene copy number within each population. Lack of associated vegetative growth and fecundity cost to the increased EPSPS gene copy in the GR B. scoparia plants suggests that the plants are likely to persist in field populations, except when effective weed management strategies are adopted that would prevent their growth and seed production.

Soluble phosphate availability is a major limiting factor for plant growth, development, and yield. To assure a constant phosphorous supply, plants employ both high- and low-affinity phosphate acquisition mechanisms. Glyphosate is an herbicide widely used throughout the world, and previous studies have suggested that it can be transported across the plasma membrane via phosphate transporters in herbaceous species. The effects of phosphate status on glyphosate uptake were investigated in the tree grand eucalyptus (Eucalyptus grandis W. Hill ex. Maid.). Eucalyptus grandis’s putative phosphate transporters showed differential gene expression in leaves and roots in response to phosphate limitation. Overall, the expression of high-affinity phosphate transporters increased in phosphate-limiting treatments, particularly in roots. More [14C]glyphosate was absorbed and translocated in phosphate-limiting plants compared with control plants grown in phosphate-replete treatments. In leaf mesophyll protoplast assays, rapid uptake of [14C]glyphosate into protoplasts was observed, and addition of phosphate to the assays competed with [14C]glyphosate uptake. These data indicate that phosphate transporters represent one mechanism of glyphosate uptake in E. grandis. These results have implications for best management practices for weed control and glyphosate application under phosphate application regimes.

Poverty brome (Bromus sterilis L.) [sterile or barren brome, syn. Anisantha sterilis (L.) Nevski] is a problematic UK arable weed. There are currently no confirmed cases of glyphosate resistance in any weed species in the United Kingdom or in B. sterilis worldwide. However, there are reports of poor control by glyphosate in this species. Here, we report experiments to confirm the suspected on-farm resistance of B. sterilis populations to glyphosate. Glyphosate screening and dose–response experiments established that glyphosate sensitivity of three UK B. sterilis populations exhibiting poor field control is outside the normal range of sensitivity of 30 sensitive populations and adjacent unexposed populations. Control of sensitive populations ranged from 49% to 82% and for suspected resistant populations from 21% to 30%. Dose–response ED50 values of sensitive populations ranged between 241 and 313 g ai ha−1; corresponding values of suspected resistant populations ranged between 420 and 810 g ha−1, and resistance indices ranged from 1.55 to 4.5. Suspected resistant populations were incompletely controlled at the recommended field rate of glyphosate (540 g ha−1), while adjacent unexposed populations were completely controlled. We conclude that some UK populations of B. sterilis have reduced glyphosate sensitivity and are in the process of evolving resistance. This is the first reported case of reduced glyphosate sensitivity in any UK weed species and in B. sterilis worldwide. This, coupled with increasing glyphosate use, highlights the need for increased vigilance and monitoring for glyphosate resistance in the United Kingdom.

Glyphosate-resistant (GR) goosegrass [Eleusine indica (L.) Gaertn.] was recently identified in Brazil, but its resistance mechanism was unknown. This study elucidated the resistance mechanism in this species and developed a molecular marker for rapid detection of this target-site resistance trait. The resistance factor for the resistant biotype was 4.4-fold compared with the glyphosate-susceptible (GS) in greenhouse dose–response experiments. This was accompanied by a similar (4-fold) difference in the levels of in vitro and in planta shikimate accumulation in these biotypes. However, there was no difference in uptake, translocation, or metabolism of glyphosate between the GS and GR biotypes. Moreover, both biotypes showed similar values for 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) copy number and transcription. Sequencing of a 330-bp fragment of the EPSPS gene identified a single-nucleotide polymorphism that led to a Pro-106-Ser amino acid substitution in the enzyme from the GR biotype. This mutation imparted a 3.8-fold increase in the amount of glyphosate required to inhibit 50% of EPSPS activity, confirming the role of this amino acid substitution in resistance to glyphosate. A quantitative PCR–based genotyping assay was developed for the rapid detection of resistant plants containing this Pro-106-Ser mutation.

Kochia [Bassia scoparia (L.) A. J. Scott] is a problematic weed species across the Great Plains, as it is spreading fast and has developed herbicide-resistant biotypes. It is imperative to understand key life-history stages that promote population expansion of B. scoparia and control strategies that would provide effective control of these key stages, thereby reducing population growth. Diversifying weed control strategies has been widely recommended for the management of herbicide-resistant weeds. Therefore, the objectives of this study were to develop a simulation model to assess the population dynamics of B. scoparia and to evaluate the effectiveness of diverse weed control strategies on long-term growth rates of B. scoparia populations. The model assumed the existence of a glyphosate-resistant (GR) biotype in the B. scoparia population, but at a very low proportion in a crop rotation that included glyphosate-tolerant corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. The parameter estimates used in the model were obtained from various ecological and management studies on B. scoparia. Model simulations indicated that seedling recruitment and survival to seed production were more important than seedbank persistence for B. scoparia population growth rate. Results showed that a diversified management program, including glyphosate, could provide excellent control of B. scoparia populations and potentially eliminate already evolved GR B. scoparia biotypes within a given location. The most successful scenario was a diverse control strategy that included one or two preplant tillage operations followed by preplant or PRE application of herbicides with residual activities and POST application of glyphosate; this strategy reduced seedling recruitment, survival, and seed production during the growing season, with tremendous negative impacts on long-term population growth and resistance risk in B. scoparia.

Sesbania [Sesbania cannabina (Retz.) Pers.] is a problematic emerging weed species in Australian cotton-farming systems. However, globally, no information is available regarding its seed germination biology, and better understanding will help in devising superior management strategies to prevent further infestations. Laboratory and glasshouse studies were conducted to evaluate the impact of various environmental factors such as light, temperature, salt, osmotic and pH stress, and burial depth on germination and emergence of two Australian biotypes of S. cannabina. Freshly harvested seeds of both biotypes possessed physical dormancy. A boiling-water scarification treatment (100±2 C) of 5-min duration was the optimum treatment to overcome this dormancy. Once dormancy was broken, the Dalby biotype exhibited a greater germination (93%) compared with the St George biotype (87%). The nondormant seeds of both biotypes showed a neutral photoblastic response to light and dark conditions, with germination marginally improved (6%) under illumination. Maximum germination of both biotypes occurred under an alternating temperature regime of 30/20 and 35/25 C and under constant temperatures of 32 or 35 C, with no germination at 8 or 11 C. Seed germination of both biotypes decreased linearly from 87% to 14% with an increase in moisture stress from 0.0 to −0.8 MPa, with no germination possible at −1.0 MPa. There was a gradual decline in germination for both biotypes when imbibed in a range of salt solutions of 25 to 250 mM, with a 50% reduction in germination occurring at 150 mM. Both biotypes germinated well under a wide range of pH values (4.0 to 10.0), with maximum germination (94%) at pH 9.0. The greatest emergence rate of the Dalby (87%) and St George (78%) biotypes was recorded at a burial depth of 1.0 cm, with no emergence at 16.0 cm. Deep tillage seems to be the best management strategy to stop S. cannabina’s emergence and further infestation of cotton (Gossypium hirsutum L.) fields. The findings of this study will be helpful to cotton agronomists in devising effective, sustainable, and efficient integrated weed management strategies for the control of S. cannabina in cotton cropping lands.

Prairie groundcherry [Physalis hederifolia (A. Gray) var. fendleri (A. Gray) Cronquist] is an invasive perennial weed with the potential to become a significant summer weed across 409 million hectares in Australia. Current management practices do not provide effective control of established populations. A better understanding of the seed biology is needed to effectively manage this weed. A series of field and laboratory studies were conducted to determine plant fecundity, soil seedbank longevity, and the factors that affect seed germination. Physalis hederifolia has the capacity to produce 66 to 86 berries plant−1, 51 to 74 seeds berry−1, and approximately 4,500 seeds plant−1, with the seeds potentially able to persist in the soil seedbank for 20 yr if buried in an intact dry berry pod. The bare-seed component of the soil seedbank can be virtually exhausted within 3 yr if cultivation is minimized to avoid burial of seed. Optimal temperature for germination is diurnal fluctuations of 15 C within the temperature range of 10 and 30 C. Increasing osmotic stress levels reduced the germination under all temperature regimes, with less than 6% germination occurring at −0.96 MPa. Physalis hederifolia seed germination was not significantly affected by substrate pH 4 to 10 or salt levels less than 160 mM, while the germination was significantly reduced at NaCl concentrations above 160 mM. These results suggest that P. hederifolia can adapt to a range of substrate conditions. Stopping seed set, avoiding grazing plants with viable seeds, and minimizing seed burial in the soil are some effective strategies to control this weed.

Lentil (Lens culinaris Medik.) is an important and expanding crop in southern Australia and a significant crop in western Canada. Currently, production in both countries is limited by an inability to effectively control weeds, due in part to a lack of registered safe and effective herbicides. Metribuzin is a broad-spectrum herbicide providing an alternative weed control option to the imidazolinones, but it has low crop safety in lentil. Two methods, germplasm screening using a hydroponic sand assay and field screening of a large mutated population of the Australian cultivar ‘PBA Flash’ were initially used to identify lines with putative metribuzin tolerance over current cultivars. Dose–response experiments showed the germplasm line SP1333 had GR50 (the rate required to reduce dry weight 50%) values up to four times higher than PBA Flash. However, the mutation selections M043 and M009 had GR50 values more than 25 times higher than PBA Flash. A field study in Canada, under conditions of induced shade and no shade 72 h before POST application of metribuzin, confirmed the intermediate level of tolerance in SP1333 and the high level in the two mutant lines compared with 20 Canadian and Australian genotypes. This relative increase in metribuzin tolerance of the two mutant lines over the parent cultivar is higher than all previous reports in a range of crop species. The development of large mutant populations combined with large M2 field screens was a successful method for developing high levels of metribuzin tolerance in lentil. The estimated mutation rate of the mutant lines was 9.4×10−8. All three lines are currently being used as parents in lentil breeding programs.

Weed management in tobacco (Nicotiana tabacum L.) is accomplished primarily with soil-residual herbicides, cultivation, and hand removal. Management practices that reduce weed emergence, like reduced tillage and cover crop mulches, may improve weed management efficacy. Depending on cover-cropping goals, growers face trade-offs in species selection and management priorities—producing weed-suppressive mulches may lead to transplanting difficulties and soil-residual herbicide interception. Managing more complex cover crop mixtures may result in different challenges. We established on-farm trials across 4 site-years to study impacts of cover crop composition [wheat (Triticum aestivum L.) monoculture or mixture], termination treatment (early or late chemical termination or removing aboveground biomass), and soil-residual herbicides on weed density and biomass. The cover crop mixture contained cereal rye (Secale cereale L.), crimson clover (Trifolium incarnatum L.), and hairy vetch (Vicia villosa Roth.), with canola (Brassica napus L.) at 1 site-year. The mixture typically produced more biomass than monoculture wheat, although composition had few impacts on weed density or biomass. With residual herbicides, termination treatment had few impacts on weed density, suggesting that residues did not adversely affect herbicide efficacy. Without residual herbicides, early-season weed density was often higher following the late-terminated cover crop compared with other termination treatments, though midseason weed density was typically lower. When termination treatment affected final weed biomass, it was lower following late termination, with one exception—crop establishment was reduced at 1 site-year, leading to reduced weed–crop competition and greater weed biomass. Our results suggest that growers can use mixtures and, if well-timed to a rainfall event for incorporation, still effectively use soil-residual herbicides to maintain adequate weed control in tobacco regardless of how the cover crop is managed. Later termination, resulting in more residue, may lead to less weed biomass accumulation in the absence of herbicide use.

In a series of seed burial studies, we tested the hypothesis that reduced tillage and cereal rye (Secale cereale L.) cover cropping influence seed persistence and that these effects are mediated by differences in fungal pathogens and exposure to light. Seeds of Powell amaranth (Amaranthus powellii S. Watson) and large crabgrass [Digitaria sanguinalis (L). Scop.] were buried in mesh bags in a long-term experiment with two levels of tillage (full-width tillage [FWT] or strip tillage [ST]) and two levels of cover cropping (none or cereal rye). In Experiment 1, seeds were exhumed each spring for 3 yr and tested for viability. In Experiment 2, untreated and fungicide-treated seeds were buried, exhumed at shorter intervals, and tested for viability. In addition, a subset of seeds in FWT treatments were exhumed and stored in either light or darkness during tillage operations and evaluated for persistence at 8.5 mo after burial (MAB). In Experiment 1, the persistence of D. sanguinalis seeds declined by 80% at 7 MAB regardless of cover crop or tillage treatment. The persistence of A. powellii seeds at 19 MAB declined by 95% in FWT compared with only 50% in ST. In Experiment 2, seed persistence of both species was greater in ST compared with FWT treatments, for seeds that had been exposed to light, but not for those that were maintained in darkness. Rye cover cropping resulted in a 2-fold increase in overwinter persistence of seeds of D. sanguinalis regardless of fungicide treatment. These results demonstrate that increased persistence under ST was primarily due to reductions in light-induced fatal germination and that increased overwinter persistence of D. sanguinalis in rye cover crop treatments could not be explained by differences in decay due to fungal pathogens controlled by the seed treatment.

Strip-intercropping of functionally diverse cover crop mixtures including cereal rye (Secale cereale L.) and hairy vetch (Vicia villosa Roth) is one mechanism by which nitrogen (N) banding can be applied to an organic, strip-tilled system to increase crop competitiveness over weeds. We hypothesized that by targeting hairy vetch, a low C:N legume, to the tilled strip directly in row with future crop establishment, and cereal rye, a high C:N grass, to the untilled strip directly between future crop rows, that N would be preferentially available to the crop. We conducted a field study between 2011 to 2013 in southwest Michigan to examine the effects of rye–vetch mixture spatial arrangement (strip intercropping vs. full-width mixture) on (1) soil inorganic N; (2) weed biomass; and (3) sweet corn (Zea mays L.) biomass, yield, and competitiveness against weeds. We found that as the proportion of vetch biomass in the crop row (in-row, IR) increased, we also saw increasing levels of IR soil inorganic N and greater early sweet corn N uptake and growth relative to weeds. However, sweet corn yield and final biomass were more responsive to vetch biomass across the whole plot (WP) and did not respond to rye and vetch segregation into strips. Increasing vetch WP biomass increased sweet corn final biomass across both years, but only increased corn competitiveness against weeds in 1 out of 2 years and decreased sweet corn competitiveness in the other year. Strip-intercropping of cereal rye and hairy vetch has potential to increase soil N availability to the crop, thereby increasing early crop competitiveness, which may lower weed management costs.

Information about weed biology and weed population dynamics is critical for the development of efficient weed management programs. A field experiment was conducted in Fayetteville, AR, during 2014 and 2015 to examine the effects of Palmer amaranth (Amaranthus palmeri S. Watson) establishment time in relation to soybean [Glycine max (L.) Merr.] emergence and the effects of A. palmeri distance from the soybean row on the weed’s height, biomass, seed production, and flowering time and on soybean yield. The establishment time factor, in weeks after crop emergence (WAE), was composed of six treatment levels (0, 1, 2, 4, 6, and 8 WAE), whereas the distance from the crop consisted of three treatment levels (0, 24, and 48 cm). Differences in A. palmeri biomass and seed production averaged across distance from the crop were found at 0 and 1 WAE in both years. Establishment time had a significant effect on A. palmeri seed production through greater biomass production and height increases at earlier dates. Amaranthus palmeri that was established with the crop (0 WAE) overtopped soybean at about 7 and 10 WAE in 2014 and 2015, respectively. Distance from the crop affected A. palmeri height, biomass, and seed production. The greater the distance from the crop, the higher A. palmeri height, biomass, and seed production at 0 and 1 WAE compared with other dates (i.e., 2, 4, 6, and 8 WAE). Amaranthus palmeri establishment time had a significant impact on soybean yield, but distance from the crop did not. The earlier A. palmeri interfered with soybean (0 and 1 WAE), the greater the crop yield reduction; after that period no significant yield reductions were recorded compared with the rest of the weed establishment times. Knowledge of A. palmeri response, especially at early stages of its life cycle, is important for designing efficient weed management strategies and cropping systems that can enhance crop competitiveness. Control of A. palmeri within the first week after crop emergence or reduced distance between crop and weed are important factors for an effective implementation of weed management measures against A. palmeri and reduced soybean yield losses due to weed interference.