Weed Fitness Research Articles

Light partitioning strategies impact relative fitness of weeds and cover crops when drill-interseeding in corn

AbstractDrill-interseeding cover crops into corn (Zea mays L.) is an emerging establishment method in northern U.S. production regions. However, cover crop performance in interseeded systems remains variable, and creating environments that are conducive to cover crop but not weed growth is challenging. Cultural practices that partition resources between corn and interseeded cover crops have potential to improve performance if weeds are adequately managed. This study evaluated interactions among corn hybrids differing in leaf architecture (upright, pendulum), corn row spacing (76 cm, 152 cm), and interseeding timing (V3, V6) on light transmittance, relative fitness of cover crop species (cereal rye [Secale cereale L.], annual ryegrass (Lolium multiflorum Lam), red clover [Trifolium pratense L.]) and weeds, and corn grain yield at three U.S. Northeast locations. Results showed that light transmittance through the corn canopy was greater in 152-cm row spacing compared with 76-cm row spacing at the V6 growth stage, with the magnitude of difference increasing at the V10 corn growth stage. Corn hybrids had a marginal effect on light transmittance. The effect of row spacing and interseeding timing on fall cover crop biomass varied across cover crop species and locations. In 76-cm rows, interseeding earlier (V3) increased cover crop biomass production. The relative fitness of cover crops was greater than that of weeds in each combination of cultural practices that included narrow spacing (76 cm), whereas the relative fitness of weeds was greater than that of cover crops when interseeding in wide rows (152 cm). The effect of row spacing on corn yield varied among locations, with higher yields observed in 76-cm row spacing compared with 152-cm at two of three locations. Our results show that interseeding early (V3) on 76-cm row spacing can balance cover crop and corn production management goals, while placing cover crops at a relative fitness advantage over weeds.

Intraspecific functional trait variation in weeds: A strategy in response to competition with crop and weed plants

AbstractWeeds are a key component of agroecosystems and compete with crop plants. Crop‐weed competition has been widely investigated over the past decades, although how functional traits mediate these competitive interactions has rarely been explored. In this study, we measured functional traits in 4400 individual plants belonging to 32 weed species, growing among four crops differing in functional traits. We assessed the intraspecific variation in weed functional traits in response to competition with crops but also with the other weeds. We found that the most abundant weeds have a lower intraspecific trait variability than other weed species for leaf dry matter content (LDMC), but this pattern was not observed for canopy height (CH) nor for specific leaf area (SLA). We found that most of the weed species tended to grow taller and have higher LDMC values when crop CH and LDMC increased. We propose that these species display a ‘crop matching’ strategy, tending to match crop functional trait values in response to competition. We also found that weed‐weed competition affected weed functional traits, sometimes more strongly than crop‐weed competition. This study highlights that intraspecific trait variation is important to consider when investigating the processes underlying weed species community assembly, especially resource competition. Further studies are required to identify the source of intraspecific trait variation in weeds and the consequences for weed fitness.

A perspective of herbicide-resistant weeds and management options: A Review

In crop lands around the globe, various interventions for weed suppression are used and among them are chemicals which are widely recommended for weed control. This paper will try to bring forth ideas that can be integrated into the development of herbicide resistance. Chemical herbicides have been known to affect weed fitness, ecosystem and the diversity of their community changes over a period of time in response to both herbicides and other intervention strategies imposed on them. Regular application of the herbicides with same active ingredients and site of action repeatedly and intensively has the potential to swiftly result in population that is more tolerant, and difficult to suppress, this will ultimately result in weed community that is herbicide resistant, particularly in absence of using herbicides with different modes of action. Appropriate herbicide resistance management practices should be adopted to avoid resistance emerging to the new active ingredients as well. Generally, farmers prefer to use one active ingredient that still provides proper weed control on susceptible species while adding a second active ingredient to suppress weeds that develop resistant instead of taking a more economical strategy. Therefore, there is need for concerted efforts and more work to be done by both weed experts and evolutionary biologists towards an improvement and broader knowledge with regard to resistant development in plants. This collaboration is cardinal in offering innovative and tangible solutions to the herbicide resistance challenges being faced by the world.

A dicamba resistance‐endowing IAA16 mutation leads to significant vegetative growth defects and impaired competitiveness in kochia (Bassia scoparia)†

BACKGROUNDPrecise quantification of the fitness cost of synthetic auxin resistance has been impeded by lack of knowledge about the genetic basis of resistance in weeds. Recent elucidation of a resistance‐endowing IAA16 mutation (G73N) in the key weed species kochia (Bassia scoparia), allows detailed characterization of the contribution of resistance alleles to weed fitness, both in the presence and absence of herbicides. Different G73N genotypes from a segregating resistant parental line (9425) were characterized for cross‐resistance to dicamba, 2,4‐d and fluroxypyr, and changes on stem/leaf morphology and plant architecture. Plant competitiveness and dominance of the fitness effects was quantified through measuring biomass and seed production of three F2 lines in two runs of glasshouse replacement series studies.RESULTS G73N confers robust resistance to dicamba but only moderate to weak resistance to 2,4‐d and fluroxypyr. G73N mutant plants displayed significant vegetative growth defects: (i) they were 30–50% shorter, with a more tumbling style plant architecture, and (ii) they had thicker and more ovate (versus lanceolate and linear) leaf blades with lower photosynthesis efficiency, and 40–60% smaller stems with less‐developed vascular bundle systems. F2 mutant plants had impaired plant competitiveness, which can lead to 80‐90% less biomass and seed production in the replacement series study. The pleiotropic effects of G73N were mostly semidominant (0.5) and fluctuated with the environments and traits measured.CONCLUSION G73N is associated with significant vegetative growth defects and reduced competitiveness in synthetic auxin‐resistant kochia. Management practices should target resistant kochia's high vulnerability to competition in order to effectively contain the spread of resistance.

The role of population and quantitative genetics and modern sequencing technologies to understand evolved herbicide resistance and weed fitness.

Evolution of resistance to multiple herbicides with different sites of action and of nontarget site resistance (NTSR) often involves multiple genes. Thus, single‐gene analyses, typical in studies of target site resistance, are not sufficient for understanding the genetic architecture and dynamics of NTSR and multiple resistance. The genetics of weed adaptation to varied agricultural environments is also generally expected to be polygenic. Recent advances in whole‐genome sequencing as well as bioinformatic and statistical tools have made it possible to use population and quantitative genetics methods to expand our understanding of how resistance and other traits important for weed adaptation are genetically controlled at the individual and population levels, and to predict responses to selection pressure by herbicides and other environmental factors. The use of tools such as quantitative trait loci mapping, genome‐wide association studies, and genomic prediction will allow pest management scientists to better explain how pests adapt to control tools and how specific genotypes thrive and spread across agroecosystems and other human‐disturbed systems. The challenge will be to use this knowledge in developing integrated weed management systems that inhibit broad resistance to current and future weed‐control methods. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Global Status of Herbicide Resistance Development: Challenges and Management Approaches

Chemicals are widely recommended for the suppression of weed in crop land. This paper attempts to a greater integration of ideas into the development of herbicide resistance. This may lead researchers to focus less on simply defining herbicide resistance and more towards comprehensive investigations of the resistance development. Weed expert in collaboration with plant biologists can work in synergy to come up with better approach and innovation aimed to curtain herbicides resistance challenges. Chemical herbicides exert undue pressure on weed fitness and the diversity of weed community’s changes over time in response to both herbicides and other strategies imposed on them. Repeatedly and intensively, the regular application of herbicides with similar effect may swiftly result in population shifts to tolerant, difficult to suppress and ultimately result to weed community that is herbicide resistant, particularly in absence of using herbicides with different modes of action. Weed expert and evolutionary biologists have to work in synergy toward an improve and broader knowledge of plant resistant development. This collaboration is likely to proffer innovative solutions to the herbicide resistance challenges.

Fitness correlates of crop transgene flow into weedy populations: a case study of weedy rice in China and other examples

Whether transgene flow from crops to cross‐compatible weedy relatives will result in negative environmental consequences has been the topic of discussion for decades. An important component of environmental risk assessment depends on whether an introgressed transgene is associated with a fitness change in weedy populations. Several crop‐weed pairs have received experimental attention. Perhaps, the most worrisome example is transgene flow from genetically engineered cultivated rice, a staple for billions globally, to its conspecific weed, weedy rice. China's cultivated/weedy rice system is one of the best experimentally studied systems under field conditions for assessing how the presence of transgenes alters the weed's fitness and the likely impacts of that fitness change. Here, we present the cultivated/weedy rice system as a case study on the consequences of introgressed transgenes in unmanaged populations. The experimental work on this system reveals considerable variation in fitness outcomes ‐ increased, decreased, and none ‐ based on the transgenic trait, its introgressed genomic background, and the environment. A review of similar research from a sample of other crop‐wild pairs suggests such variation is the rule. We conclude such variation in fitness correlates supports the case‐by‐case method of biosafety regulation is sound.

Reintroduction of a rare arable weed: Competition effects on weed fitness and crop yield

Arable weeds have severely declined due to intensification of farming during the past decades, and some of them are close to extinction in large parts of western and central Europe. Recent conservation strategies suggest reintroduction of rare arable weeds to suitable types of land use, for example organic fields. To test the feasibility of this conservation option we investigated the effects of spring wheat and a common weed (Stellaria media) on growth and reproduction of a rare weed (Legousia speculum-veneris) in a semi-open glasshouse experiment. Additionally, the impact of the weeds on crop yield was assessed. Aboveground biomass and seed production of L. speculum-veneris were significantly reduced under competition with the crop and the common arable weed. With increasing competition L. speculum-veneris allocated disproportionally more resources to seeds. Wheat yield was not affected by the arable weeds. We conclude that L. speculum-veneris is a subordinate species that is negatively affected by more competitive plants. In the presence of wheat the addition of other weeds does not further harm the species. Enhanced allocation to reproduction might be a specific strategy of the species to maintain populations under competition. Reduced crop densities would help reintroducing the species, while yield losses due to reintroduction of that rare arable weed are unlikely.

The weed fitness in herbicide resistance researchFitnes chwastów w badaniach odporności na herbicydy

The aim of the studies was review of literature date on weed fitness. However the expression-weed fitness has not been commonly used by the Polish weed scientists so far, some research dealt with the above phenomenon were done in Poland. In general, weed fitness can be defined as weed reproductive capabilities, and adjustment of singe genes from the gene pool to new conditions being a result of herbicide applications. Therefore, weed survival and reproduction are regarded as two main criteria determining weed fitness. In each weed population there are individual plants characterized by a different level of sensitivity to applied herbicides. Sensitive weed plants of the same species are effectively controlled by herbicides. As a result of intraspecific competitiveness, resistant weed plants or plants with acquired resistance survive and become dominant in agrophytocenosis. Studies on weed fitness are mainly concerned with the effects of long term application of herbicides. Among other things, morphophysiological differences between resistance and susceptible weed species biotypes are determined, biotype reproduction capabilities as well as quantitative relations depending on agro-ecological conditions.

Transgenes for insect resistance reduce herbivory and enhance fecundity in advanced generations of crop-weed hybrids of rice.

Gene flow from transgenic crops allows novel traits to spread to sexually compatible weeds. Traits such as resistance to insects may enhance the fitness of weeds, but few studies have tested for these effects under natural field conditions. We created F2 and F3 crop–weed hybrid lineages of genetically engineered rice (Oryza sativa) using lines with two transgene constructs, cowpea trypsin inhibitor (CpTI) and a Bt transgene linked to CpTI (Bt/CpTI). Experiments conducted in Fuzhou, China, demonstrated that CpTI alone did not significantly affect fecundity, although it reduced herbivory. In contrast, under certain conditions, Bt/CpTI conferred up to 79% less insect damage and 47% greater fecundity relative to nontransgenic controls, and a 44% increase in fecundity relative to the weedy parent. A small fitness cost was detected in F3 progeny with Bt/CpTI when grown under low insect pressure and direct competition with transgene-negative controls. We conclude that Bt/CpTI transgenes may introgress into co-occurring weedy rice populations and contribute to greater seed production when target insects are abundant. However, the net fitness benefits that are associated with Bt/CpTI could be ephemeral if insect pressure is lacking, for example, because of widespread planting of Bt cultivars that suppress target insect populations.

Principal Canopy Factors of Sweet Corn and Relationships to Competitive Ability with Wild-Proso Millet (Panicum miliaceum)

Univariate analyses fail to account for covariance among phenomorphological traits implicated in crop competitive ability. A more complete analysis of cultivar–weed interactions would reduce a number of important traits to a few underlying principal factors responsible for sweet corn competitiveness. Twenty-three commercial sweet corn hybrids from nine seed companies were grown in the presence and absence of wild-proso millet to (1) quantify the extent to which phenomorphological traits vary in sweet corn, (2) identify underlying principal factors that describe variation in crop canopy development, and (3) determine functional relationships between crop canopy factors and competitive ability. A principal component factor analysis revealed that 7 of the 18 weed-free crop traits measured at silking loaded highly (0.65 to 0.90) into the first factor, including plant height, shoot biomass, per plant leaf area, leaf area index, and intercepted light, as well as thermal time from emergence to silking and emergence to maturity. All seven traits were highly correlated (0.38 to 0.93) and were interpreted as a “late canopy and maturity” factor. Another five traits formed two additional principal factors that were interpreted as an early “seedling quality” factor (e.g., kernel mass, seedling vigor, and height at two-leaf stage) and a mid-season “canopy closure” factor (e.g., leaf area index and intercepted photosynthetically active radiation at six-leaf stage). Relationships between principal factors and competitive abilities were quantified using least-squares linear regression. Cultivars with greater loadings in the late canopy and maturity and canopy closure factors were more competitive with wild-proso millet. In contrast, crop competitive ability declined with cultivars that loaded highly into the seedling quality factor. The analyses showed that sweet corn's ability to endure weed interference and suppress weed fitness relates uniquely to three underlying principal factors that capture crop canopy development around emergence and near canopy closure and during the reproductive phase.

Wild Proso Millet (Panicum miliaceum) Suppressive Ability among Three Sweet Corn Hybrids

Abstract Due to known variation in canopy properties among sweet corn hybrids, weed suppressive ability (WSA), the crop's ability to reduce weed fitness, may not be uniform among hybrids. This hypothesis was tested using a range of wild proso millet densities subjected to four canopy treatments (three hybrids + weedy monoculture) under irrigated conditions in Washington and primarily rainfed conditions in Illinois. Parameter estimates for responses of weed growth and seed rain to wild proso millet density were used to quantify variation in WSA among hybrids. The same parameter estimates were used in a correlation analysis to identify associations between weed response and sweet corn canopy properties. Weed suppressive ability, as measured by wild proso millet shoot biomass and seed rain, varied among canopy treatments. Hybrid GH2547 was 25 to 31% more suppressive of wild proso millet than hybrid Spirit in Washington and 70 to 91% more suppressive in Illinois. Weed fitness was negatively correlated with le...

Influence of Herbivory and Competition on Invasive Weed Fitness: Observed Effects of <I>Cyphocleonus achates</I> (Coleoptera: Curculionidae) and Grass-Seeding Treatments on Spotted Knapweed Performance

The root-feeding weevil Cyphocleonus achates (Fahraeus) is a promising biological control agent for managing the exotic, invasive weed spotted knapweed. The objective of this study was to compare the relative and potentially interactive effects of competition and specialized herbivory on spotted knapweed fitness. Competition was assessed through three grass seeding treatments: a nonseeded control and seeding to long-lived wheatgrasses originating either from North America, Pseudoroegneria spicata (Pursh) A. Love ssp. spicata, or Europe, Thinopyrum intermedium (Host) Barkworth and D.R. Dewey. Comparisons were made of above-ground biomass, stem length, number of flowers, and life stage densities for spotted knapweed plants either infested or free of C. achates and growing under one of the grass seeding treatments. Infested adult spotted knapweed plants had lower shoot weight, stem length, and flowers/plant. The density of adult spotted knapweed plants was lower in grass-seeded plots than in nonseeded control plots. Results indicate C. achates herbivory is correlated with a reduction in measures of fitness and reproductive potential in field populations of spotted knapweed in North America. Furthermore, these results suggest that this agent might play a significant role in reducing spotted knapweed populations when competitive grasses are also present.

Influence of Herbivory and Competition on Invasive Weed Fitness: Observed Effects of Cyphocleonus achates (Coleoptera: Curculionidae) and Grass-Seeding Treatments on Spotted Knapweed Performance

Influence of Herbivory and Competition on Invasive Weed Fitness: Observed Effects of Cyphocleonus achates (Coleoptera: Curculionidae) and Grass-Seeding Treatments on Spotted Knapweed Performance

Integrating arthropod herbivory and reduced herbicide use for weed management

Few studies have examined the combined effect of herbicide-induced stress and arthropod herbivory to reduce weed fitness. The purpose of this study was to quantify the effect of arthropod herbivory on the herbicide dose–response of a perennial weed. Fluroxypyr dose–response bioassays using volunteer potato were conducted in the presence and absence of Colorado potato beetle (CPB) herbivory. Logistic model parameter estimates for leaf area, shoot biomass, tuber number, and tuber biomass were often lower with herbivory, compared with no herbivory. Greater variance of parameter estimates within herbivory plots was attributed largely to differential feeding because CPB density was not manipulated in the field. Results from short-season field studies (1,000 growing degree days [GDD] after postemergence [POST] herbicide application) indicated that herbivory had the most effect on potato during a period that coincided with high CPB density and optimal temperatures for CPB development. Season-long bioassays (> 3,100 GDD after POST) revealed that addition of herbivory reduced herbicide use 65 to > 85%, compared with the dose needed to achieve the same reduction in tuber production in the absence of herbivory. Integrated weed management systems targeting volunteer potato are more effective when fluroxypyr applications are made before periods of high herbivory. Moreover, this article describes an experimental approach contributing to optimization of combined effects of arthropod herbivory and reduced herbicide doses.

Within-field soil heterogeneity effects on herbicide-mediated crop injury and weed biomass

Soil organic carbon (OC), clay content, water content, and pH often influence the bioactivity of soil-applied herbicides, and these soil properties can vary greatly within fields. The purpose of this work was to determine the influence of within-field soil heterogeneity on the efficacy of RPA-201772 where corn, shattercane, and velvetleaf were seeded as bioassays. An experimental approach was developed to quantify RPA-201772 dose–response across a range of soil conditions in an agricultural field. Based on a logistic model, crop injury was quantified with the I20 parameter, the dose eliciting 20% greenness reduction, using a series of photographic standards. Weed biomass was quantified with the I80 parameter, the dose eliciting 80% biomass reduction, relative to the untreated control. Crop and weed responses varied by two orders of magnitude. Significant correlation, as high as 0.76, was observed between measures of plant response and soil properties, namely particle size and OC. Furthermore, native velvetleaf spatial distribution at the study site was heterogeneous, and seedlings were observed in plots where seeded velvetleaf biomass was high. Spatial heterogeneity of soil affinity for herbicide results in differential weed fitness and contributes to weed “patchiness.”

The Evolution and Genetics of Herbicide Resistance in Weeds

The importance of various factors influencing the evolution of herbicide resistance in weeds is critically examined using population genetic models. The factors include gene mutation, initial frequency of resistance alleles, inheritance, weed fitness in the presence and absence of herbicide, mating system, and gene flow. Where weed infestations are heavy, the probability of selecting for resistance can be high even when the rate of mutation is low. Subsequent to the occurrence of a resistant mutant, repeated treatments with herbicides having the same mode of action can lead to the rapid evolution of a predominantly resistant population. At a given herbicide selection intensity, the initial frequency of resistance alleles determines the number of generations required to reach a specific frequency of resistant plants. The initial frequency of resistance alleles has a greater influence on the evolutionary process when herbicides impose weak selection, as opposed to very strong selection. Under selection, dominant resistance alleles increase in frequency more rapidly than recessive alleles in random mating or highly outcrossing weed populations. In highly self-fertilizing species, dominant and recessive resistance alleles increase in frequency at approximately the same rate. Gene flow through pollen or seed movement from resistant weed populations can provide a source of resistance alleles in previously susceptible populations. Because rates of gene flow are generally higher than rates of mutation, the time required to reach a high level of resistance in such situations is greatly reduced. Contrary to common misconception, gene flow from a susceptible population to a population undergoing resistance evolution is unlikely to slow the evolutionary process significantly. Accurate measurements of many factors that influence resistance evolution are difficult, if not impossible, to obtain experimentally. Thus, the use of models to predict times to resistance in specific situations is markedly limited. However, with appropriate assumptions, they can be invaluable in assessing the relative effectiveness of various management practices to avoid, or delay, the occurrence of herbicide resistance in weed populations.

Weeds in a Changing Climate

Current and projected increases in the concentrations of CO2and other radiatively-active gases in the Earth's atmosphere lead to concern over possible impacts on agricultural pests. All pests would be affected by the global warming and consequent changes in precipitation, wind patterns, and frequencies of extreme weather events which may accompany the “greenhouse effect.” However, only weeds are likely to respond directly to the increasing CO2concentration. Higher CO2will stimulate photosynthesis and growth in C3weeds and reduce stomatal aperture and increase water use efficiency in both C3and C4weeds. Respiration, and photosynthate composition, concentration, and translocation may be affected. Perennial weeds may become more difficult to control, if increased photosynthesis stimulates greater production of rhizomes and other storage organs. Changes in leaf surface characteristics and excess starch accumulation in the leaves of C3weeds may interfere with herbicidal control. Global warming and other climatic changes will affect the growth, phenology, and geographical distribution of weeds. Aggressive species of tropical and subtropical origins, currently restricted to the southern U.S., may expand northward. Any direct or indirect consequences of the CO2increase that differentially affect the growth or fitness of weeds and crops will alter weed-crop competitive interactions, sometimes to the detriment of the crop and sometimes to its benefit.