Response and effect traits of arable weeds in agro‐ecosystems: a review of current knowledge

Integrating principles of ecological intensification into weed management strategies requires an understanding of the many relationships among weeds, crops and other organisms of agro‐ecosystems in a changing context. Extensively used during the last two decades in weed science, trait‐based approaches have provided general insights into weed community response to agricultural practices, and recently to understanding the effect of weeds on agro‐ecosystem functioning. In this review, we provide a holistic synthesis of the current knowledge on weed response and effect functional traits. Based on the literature and recent advances in weed science, we review current knowledge on (i) weed functional groups and ecological strategies, (ii) weed functional response traits to cropping systems and (iii) weed functional effect traits affecting agro‐ecosystem functioning. For each functional trait, we explicitly present the assumptions and evidence on the linkage between trait values and ecological functions, in response to either management practices, for example tillage, sowing and herbicides, or biotic interactions, for example crop–weed competition and pollination. Finally, we address and discuss major research avenues that may significantly improve the use of traits and the knowledge of functional diversity in weed science for the future, especially to design and implement more environmentally sustainable weed management strategies.     

A new hypothesis for the functional role of diversity in mediating resource pools and weed–crop competition in agroecosystems

We develop a new conceptual model we call the Resource Pool Diversity Hypothesis (RPDH) aimed at explaining how soil resource pool diversity may mediate competition for soil resources between weeds and crops. The primary tenets of the RPDH are that (i) in plant communities, the intensity of inter-specific competition can depend upon the degree to which niche differentiation and resource partitioning occur among species, (ii) agricultural systems are unique in that management practices, such as crop rotation, source of fertility and weed management, result in inputs to the soil and (iii) these inputs directly or indirectly become soil resource pools from which crops and weeds may partition resources. The RPDH leads to the novel prediction that along a gradient of increasing cropping system diversity, yield loss due to weed–crop competition (i.e. the impact on yield per unit weed density) for soil resources should decrease. Similarly, the degree to which crops and weeds overlap in soil resource niche breadth (which is determined by species-specific functional traits for resource acquisition), will determine the extent to which weed–crop competition weakens as resource pool diversity increases. While there have been no direct tests of the RPDH, we highlight evidence from the agricultural literature that provides strong support for components of the hypothesis. Validation of the RPDH would have important implications across a broad range of cropping systems for the development of management strategies that aim to reduce yield loss impact per unit weed plant density and the fundamental principles of integrated weed management, such as the concepts of weed thresholds and critical periods.     

The role of weeds in supporting biological diversity within crop fields*

Summary Weeds are major constraints on crop production, yet as part of the primary producers within farming systems, they may be important components of the agroecosystem. Using published literature, the role of weeds in arable systems for other above‐ground trophic levels are examined. In the UK, there is evidence that weed flora have changed over the past century, with some species declining in abundance, whereas others have increased. There is also some evidence for a decline in the size of arable weed seedbanks. Some of these changes reflect improved agricultural efficiency, changes to more winter‐sown crops in arable rotations and the use of more broad‐spectrum herbicide combinations. Interrogation of a database of records of phytophagous insects associated with plant species in the UK reveals that many arable weed species support a high diversity of insect species. Reductions in abundances of host plants may affect associated insects and other taxa. A number of insect groups and farmland birds have shown marked population declines over the past 30 years. Correlational studies indicate that many of these declines are associated with changes in agricultural practices. Certainly reductions in food availability in winter and for nestling birds in spring are implicated in the declines of several bird species, notably the grey partridge, Perdix perdix. Thus weeds have a role within agroecosystems in supporting biodiversity more generally. An understanding of weed competitivity and the importance of weeds for insects and birds may allow the identification of the most important weed species. This may form the first step in balancing the needs for weed control with the requirements for biodiversity and more sustainable production methods.     

The role of models for multicriteria evaluation and multiobjective design of cropping systems for managing weeds

Weeds are both harmful for crop production and important for biodiversity, while herbicides can pollute the environment. We thus need new cropping systems optimising all cultural techniques, reconciling agricultural production, herbicide reduction and biodiversity conservation. Here, we show how to (i) develop models quantifying the effects of cropping systems on weed dynamics, (ii) integrate interactions between weeds and other organisms, (iii) predict the impact on production and biodiversity and (iv) use the model for multicriteria evaluation and multiobjective design of cropping systems. Among the existing weed dynamics models, we chose the one closest to our requirements to illustrate these different steps, that is, FlorSys which predicts multispecific weed dynamics as a function of cultural techniques and pedoclimate. We have illustrated the development of interaction submodels with the example of a crop pathogen whose propagation is increased when infecting grass weeds. To evaluate the weed flora impact, predicted weed densities were translated into indicators of harmfulness (crop yield loss, technical harvest problems, harvest pollution, field infestation, crop disease increase) and biodiversity (weed species richness and equitability, trophic resources for birds, insects and pollinators). Simulations were run over several years and with different weather scenarios (i) to optimise cultural techniques to control harmful weeds, (ii) to analyse the impact of changing agricultural practices (e.g. simplified tillage and rotations, no‐till, temporary crops) on weed density, species and trait composition and (iii) to evaluate cropping systems for their ability to reconcile agricultural production and biodiversity, thus identifying levers for designing sustainable cropping systems.     

Managing arable weeds for biodiversity

As a result of the recent intensification of crop production, the abundance and diversity of UK arable weeds adapted to cultivated land have declined, with an associated reduction in farmland birds. A number of questions need to be addressed when considering how these declines can be reversed. Firstly, can the delivery of crop production and biodiversity be reconciled by spatially separating cropping from designated wildlife areas? A number of subsidised environmental schemes in the UK take this approach and are focused on establishing vegetation cover on uncropped land. However, because of the lack of regular disturbance in these habitats, they are dominated by perennials and they therefore have limited potential for promoting the recovery of annual weed populations. A number of farmland bird species also rely on the provision of resources in field centres, and it is therefore likely that the recovery of their populations will rely on weed management options targeted at the cropped areas of the field. This raises two further questions. Firstly, is it possible to identify beneficial weed species that are relatively poor competitors with the crop and also have biodiversity value? Secondly, are the tools available to manage these species at acceptable levels while controlling pernicious weeds? A number of approaches are being employed to answer these questions, including predicting yield loss from weed competition models and exploiting herbicide selectivity. The further development of these tools is crucial if farmer opposition to managing weeds in crops is to be overcome. Copyright © 2007 Society of Chemical Industry     

Competition between annual weeds and vining peas grown at a range of population densities: effects on the crop

Competition between annual weeds and vining peas (Pisum sativum L.) at five target population densities between 11 and 194 plants/m² was examined by means of periodic destructive sampling of weedy and weed-free plots. A further area of each plot was cut and vined to assess yields. Weeds impaired vegetative development, particularly by reducing tillering in low density crops. This resulted in weedy plots having fewer pods per plant at harvest but a lower proportion of flat pods than weed-free plots. Weeds had no effect on numbers or weights of peas in full pods nor on tenderometer values of samples of vined peas. Adverse effects of weeds on the growth of individual crop plants decreased with increasing crop density. However, at lower crop densities many of the additional pods on weed-free plots contributed little to vined yield, while at higher densities, direct or indirect effects of weeds increased the problem of maintaining sufficient photosynthetic area during pod swelling to prevent pod abscission and poor ovule development. Regression analysis of yield on crop density and of yield on numbers of pods per plant showed that vined yield per hectare was reduced by weeds by a constant amount across the range of densities and numbers of pods examined. Vining throughput was also reduced in weedy as compared with weed-free crops, even on high density plots where little or no weed vegetation remained at harvest. In general, weed presence had effects similar to those caused by increasing crop density, but without the additional contribution to yield made by extra plants. Higher density crops suppressed weeds very effectively but were no less vulnerable to yield loss than those of lower density; they therefore merit just as much attention to effective weed control as crops suffering more visibly from competition by weeds.     

发展化学除草重视综合治理

我国农田杂草有２５０多种，全国农田受草害面积４３００多万ｈｍ２，平均受草害减产１３．４％，每年减产粮食１７５０万ｔ，皮棉２５．５万ｔ和大豆５０万ｔ。传统农业生产采用机械作业及人力等除草。随着农村经济的发展，化学除草面积迅速扩大，全国农田化学除草面积从１９７５年的１７０万ｈｍ２增加到１９９５年的４１３３万ｈｍ２。但是，长期化学除草也带来了除草剂土壤残留对后茬作物药害、农田杂草种群更替和产生抗药性等新问题。必须重视农田杂草综合治理，通过采用各种有效的农业技术措施，为农作物保持良好的生态条件，结合化学除草才是最有效的防除杂草方法     

The role of weeds in the epidemiology of pospiviroids

Many weeds that are closely associated with horticultural activities are known as natural reservoirs of plant viruses. However, whether these weeds can also serve as hosts of pospiviroids is not well known. Pospiviroids are naked, non‐coding RNA pathogens that cause severe economic damage in many solanaceous crops. In this study, we examined the overall risk of pospiviroid spreading from weeds to economically important crops, by combining the results from previous inoculation studies with new results coming from a survey, a contact experiment and an inoculation experiment. A survey of commercial ornamental glasshouses revealed that ornamental plants mainly belonging to the Solanaceae harbour pospiviroids, in contrast to weed species sampled in the same places. No new weed hosts could be identified after testing weeds that grew in contact with Tomato apical stunt viroid (TASVd)‐infected plants of tomato (Solanum lycopersicum) and jasmine nightshade (Solanum jasminoides) in an experimental glasshouse. Finally, in mechanical inoculation experiments with TASVd, none of the six tested weed species were determined to be a host at 6 weeks after inoculation. Commonly occurring weed species therefore do not appear to play a significant role as reservoir hosts for pospiviroids. This does not rule out other potential weed hosts that have not yet been tested. Inoculation studies should include rigorous experimental protocols with a sufficient number of replicated as well as adequate positive controls. The information gained through this study may prove useful in future risk assessments for the pospiviroid group.     

Quantification of regulating ecosystem services provided by weeds in annual cropping systems using a systematic map approach

Ecosystem services have received increasing attention in life sciences, but only a limited amount of quantitative data are available concerning the ability of weeds to provide these services. Following an expert focus group on this topic, a systematic search for articles displaying evidence of weeds providing regulating ecosystem services was performed, resulting in 129 articles. The most common service found was pest control and the prevailing mechanism was that weeds provide a suitable habitat for natural enemies. Other articles showed that weeds improved soil nutrient content, soil physical properties and crop pollinator abundance. Weeds were found to provide some important ecosystem services for agriculture, but only a small number of studies presented data on crop yield. Experimental approaches are proposed that can: (i) disentangle the benefits obtained from ecosystem services provisioning from the costs due to weed competition and (ii) quantify the contribution of diverse weed communities in reducing crop competition and in providing ecosystem services. Existing vegetation databases can be used to select weed species with functional traits facilitating ecosystem service provisioning while having a lower competitive capacity. However, for services such as pest control, there are hardly any specific plant traits that have been identified and more fundamental research is needed.     

近年我国农田杂草防控中的突出问题与治理对策

我国田园杂草有1 400多种,严重危害的130余种,恶性杂草37种。我国杂草发生面积约9 246.7万hm2次,防治面积1.04亿hm2次,挽回粮食损失2 699万t,每年主粮作物仍有近300万t产量损失。杂草防控中的突出问题是:杂草群落演替,难治杂草种群增加;除草剂单一使用,杂草抗药性发展迅速;除草剂对作物药害频发,影响种植结构调整;新除草剂创制能力不足,难以满足不同作物田除草需求;农村劳动力短缺,杂草防控更依赖于化学防治。解决上述问题,应实施以下对策:加强杂草发生危害的监测预警,科学轮换使用除草剂,推广除草剂减量与替代技术,加快新除草剂研制及推广应用,加速耐除草剂作物商业化进程,推进统防统治及农民培训。     

Use of perennial cover crops to suppress weeds in Nicaraguan coffee orchards

Both uncontrolled weed growth and vegetation‐free orchard floors have been shown to affect coffee (Coflea arabica L.) negatively, but using cover crops as a solution has yielded conflicting results in different studies. In this study we tested the establishment success of three cover crop species under different management intensities and planting densities, as well as their long term weed‐controlling abilities and effects on weed community composition. Monthly manual weedings during the first 12 weeks after planting resulted in more rapid and extensive cover crop development compared with less intensive management. Transplanted Commelina diffusa Burm. f. grew most rapidly and controlled weeds by limiting light availability, but disappeared during the dry season and failed to establish at all on one of the farms. Arachis pintoi established and persisted for over 2 years, providing excellent weed control by outcompeting weeds for water and/or nutrient resources. Desmodium ovalifolium Wall required the longest time to establish and controlled weeds by an undetermined competitive mechanism. The sowing method of Desmodium led to intense intraspecific competition which probably decreased its effectiveness. Both Arachis and Desmodium led to lower relative abundances of grassy weeds and more perennial forbs, but total weed biomass was so low that these differences have no practical implications.     

Modelling the effects of weeds on crop production

In most quantitative studies on interplant competition, static regression models are used to describe experimental data. However, the generality of these models is limited. More mechanistic models for interplant competition, which simulate growth and production of species in mixtures on the basis of the underlying physiological processes, have been developed in the past decade. Recently, simulation models for competition between species for light and water were improved and a detailed version was developed for sugarbeet and fat hen (Chenopodium album L.). The model was validated with data sets of five field experiments, in which the effect of fat hen on sugarbeet production was analysed. About 98% of the variation in yield loss between the experiments (which ranged from –6 to 96%) could be explained with the model. Further analysis with the model showed that the period between crop and weed emergence was the main factor causing differences in yield loss between the experiments. Sensitivity analysis showed a strong interaction between the effect of the variables weed density and the period between crop and weed emergence on yield reduction. Different quantitative approaches to crop-weed competition are discussed in view of their practical applicability. Simulations of experiments, where both the weed density and the period between crop and weed emergence were varied over a wide range, showed a close relation between relative leaf cover of the weeds shortly after crop emergence and yield loss. This relation indicates that relative leaf cover of the weeds accounts for both the effect of weed density and the period between crop and weed emergence. This relation has the potential to be developed into a powerful tool for weed-control advisory systems.     

Increased crop diversity reduces the functional space available for weeds

There is a drive to improve the sustainability of agricultural systems including the biodiversity component. Cultivar mixtures offer yield benefits from the same land area, but the mechanisms behind this overyielding have not been completely worked out. One potential mechanism is improved competition with weeds. We use an experimental approach of varying barley (Hordeum vulgare L.) genotypic and phenotypic diversity to test the hypothesis that increases in diversity have an impact on weed growth strategies and community assembly, or if responses are driven by barley performance. There was no effect of increasing barley mixture diversity on weed traits, either in terms of species means or the community-weighted mean. However, Functional Richness of the weed community decreased with increasing barley mixture diversity mainly as a result of reduced specific leaf area Functional Richness. This pattern was driven by a reduction in species richness of the weed community rather than by reduced variation within species. Whilst barley phenotype had different impacts on weed traits, there were no specific mixtures of phenotypes or genotypes that had consistent effects on community assembly or weed species responses. The competitive exclusion of weed species could have agronomic and environmental benefits, through better targeting or less frequent use of herbicides. Growing crop mixtures is one of many strategies available to improve agricultural sustainability and resilience, and one that has clear benefits.     

Herbicide-resistant crops and weed resistance to herbicides

The adoption of genetically modified (GM) crops has increased dramatically during the last 3 years, and currently over 52 million hectares of GM crops are planted world-wide. Approximately 41 million hectares of GM crops planted are herbicide-resistant crops, which includes an estimated 33.3 million hectares of herbicide-resistant soybean. Herbicide-resistant maize, canola, cotton and soybean accounted for 77% of the GM crop hectares in 2001. However, sugarbeet, wheat, and as many as 14 other crops have transgenic herbicide-resistant cultivars that may be commercially available in the near future. There are many risks associated with the production of GM and herbicide-resistant crops, including problems with grain contamination, segregation and introgression of herbicide-resistant traits, marketplace acceptance and an increased reliance on herbicides for weed control. The latter issue is represented in the occurrence of weed population shifts, the evolution of herbicide-resistant weed populations and herbicide-resistant crops becoming volunteer weeds. Another issue is the ecological impact that simple weed management programs based on herbicide-resistant crops have on weed communities. Asiatic dayflower (Commelina cumminus L) common lambsquarters (Chenopodium album L) and wild buckwheat (Polygonum convolvulus L) are reported to be increasing in prominence in some agroecosystems due to the simple and significant selection pressure brought to bear by herbicide-resistant crops and the concomitant use of the herbicide. Finally, evolution of herbicide-resistant weed populations attributable to the herbicide-resistant crop/herbicide program has been observed. Examples of herbicide-resistant weeds include populations of horseweed (Conyza canadensis (L) Cronq) resistant to N-(phosphonomethyl)glycine (glyphosate). An important question is whether or not these problems represent significant economic issues for future agriculture.     

Competitive interactions between chick-pea genotypes and weeds

Growing chick‐pea in sustainable systems requires the use and development of more competitive genotypes which can complement the effects of reduced input weed control. A 2‐year study assessed the competitive ability of 13 genotypes grown in either the presence or absence of weeds, in a split‐plot design including the weeds in pure stands. Crop and weed density, phenology, relative biomass of crop (RB_c) and weeds (RB_w), crop yield characters, crop biometric traits in the absence of weeds, relative biomass total of mixtures (RBT) and crop competitive ability (C_b = ln RB_c/RB_w) were recorded. Lines C136, C120, C101 and C106, and cultivars Pascià, Visir and Sultano gave the best seed yield in the absence of weeds (1.8–2.0 t ha^?1 DM). Weeds reduced yield by 75% and 83% in C136 and C133 and by 87–97% in the other genotypes. Weed biomass in mixture (mainly Chenopodium album) averaged 4.42 t ha^?1 DM. Chick‐pea genotypes C136 and C133 were the most competitive, but weeds were more competitive than any of the chick‐peas. C_b was correlated directly to the height of first fertile pod (r² = 0.84) and inversely to the insertion angle of primary branches to the vertical (r² = 0.77). Intergenotypic variation for competitive ability could be exploited in integrated weed control using more competitive genotypes, or used in breeding programmes aimed to develop highly competitive cultivars on the basis of easily screenable characters.     

The taxonomy and evolution of weeds

Taxonomy describes and classifies (i.e. maps) the diversity of nature. The most useful classifications are those that permit the delimitation, and subsequent identification, of species in such a way that they exhibit genetic differences and ecological preferences. However, all classifications have limitations for weed scientists. Genetic variation within species may be just as important as differences between species to the development of weed control practices. Because traditional concepts, such as species, variety and ecotype, cannot fully reflect the products of evolutionary processes, alternative taxonomic or para-taxonomic representations of variation are considered. The dependence of plant nomenclature on these inherently limited taxonomic concepts is also discussed, and ways are suggested of minimizing the resultant degree of instability in the scientific names of weeds. The evolutionary responses of weeds to changing conditions suggest that although selection within populations is important, changes in the species composition of a weed flora may be a more common response to selection. Recombination through hybridization can be particularly significant, especially in the evolution of weeds which are genetically close to the crop species.     

Asymptomatic weeds are frequently colonised by pathogenic species of Fusarium in cereal‐based crop rotations

Several Fusarium species cause harmful cereal diseases, such as fusarium head blight and crown rot, which, during pathogenesis, may result in significant grain yield and quality losses. Several species of agricultural weed are believed to be alternative and reservoir hosts for Fusarium spp.; however, studies have not comprehensively evaluated those weed species in cropping systems that may harbour these fungi. The objective of this study was to determine weed species in cereal‐based crop rotations that are asymptomatically colonised by Fusarium spp. We sampled all species of weed present in fields that were managed under six different crop sequences in 2015 and 2016. The study yielded 2326 single‐spore isolates of Fusarium spp. derived from various organs of asymptomatic weeds. Isolates were identified morphologically and then confirmed using PCR with species‐specific primers and/or sequencing of tef1α gene fragments. Isolates of nine Fusarium spp. were obtained from 689 of the 744 individuals collected that represented 56 weed species. Each weed species harboured at least one species of Fusarium, and >80% were colonised by 3–9 Fusarium spp. In total, we identified 27 dicotyledonous weed species that were previously undocumented as Fusarium hosts and 251 new weed × Fusarium species combinations were revealed. Consequently, there is a greater risk of negative Fusarium impacts on cereal crops than was previously thought. We suggest effective weed management and inversion soil tillage may help mitigate these impacts.     

United States Department of Agriculture-Agricultural Research Service research on pest biology: weeds

Over 125 permanent full-time scientists conduct research within the USDA Agricultural Research Service (ARS) on issues related to weeds. The research emphasis of most of these scientists involves ecology and management or biological control of weeds. Many scientists perform research on weed biology as components of their primary projects on weed control and integrated crop and soil management. Describing all ARS projects involved with weed biology is impossible, and consequently only research that falls within the following arbitrarily chosen topics is highlighted in this article: dormancy mechanisms; cell division; diversity of rangeland weeds; soil resources and rangeland weeds; poisonous rangeland plants; horticultural weeds; weed traits limiting chemical control; aquatic and semi-aquatic weeds; weed/transgenic wheat hybrids; seedbanks, seedling emergence and seedling populations; and weed seed production. Within these topics, and others not highlighted, the desire of ARS is that good information on weed biology currently translates or eventually will translate into practical advice for those who must manage weeds.     

Plant classification for weed detection using hyperspectral imaging with wavelet analysis

The goal of this study is to develop a new weed detection method that can be applied for automatic mechanical weed control. For successful weed detection, plants must be classified into crops and weeds according to their species. In this study, we employed a portable hyperspectral imaging system. The hyperspectral camera can capture landscape images that include crops, weeds, and the soil surface, and can provide more extensive information than conventional red, green, and blue (RGB) images. Although RGB images consist of red, green, and blue wavebands, the obtained hyperspectral images consist of 240 wavebands of spectral information. Hyperspectral imaging is expected to provide powerful technology for agricultural sensing. In the initial step of this study, the image pixels of the plants (crop or weeds) were segmented from the background soil surface using Euclidean distance as the discriminant function. In the next step, the image pixels of the crop (sugarbeet) and weeds (four species) were classified using the difference in the spectral characteristics of the plant species. In this process, classification variables were generated using wavelet transformation for data compression, noise reduction, and feature extraction, and then stepwise linear discriminant analysis was applied. The validation results indicate that the developed classification method has potential for practical use.     

Dealing with transgene flow of crop protection traits from crops to their relatives

Genes regularly move within species, to/from crops, as well as to their con‐ specific progenitors, feral and weedy forms (‘vertical’ gene flow). Genes occasionally move to/from crops and their distantly related, hardly sexually interbreeding relatives, within a genus or among closely related genera (diagonal gene flow). Regulators have singled out transgene flow as an issue, yet non‐transgenic herbicide resistance traits pose equal problems, which cannot be mitigated. The risks are quite different from genes flowing to natural (wild) ecosystems versus ruderal and agroecosystems. Transgenic herbicide resistance poses a major risk if introgressed into weedy relatives; disease and insect resistance less so. Technologies have been proposed to contain genes within crops (chloroplast transformation, male sterility) that imperfectly prevent gene flow by pollen to the wild. Containment does not prevent related weeds from pollinating crops. Repeated backcrossing with weeds as pollen parents results in gene establishment in the weeds. Transgenic mitigation relies on coupling crop protection traits in a tandem construct with traits that lower the fitness of the related weeds. Mitigation traits can be morphological (dwarfing, no seed shatter) or chemical (sensitivity to a chemical used later in a rotation). Tandem mitigation traits are genetically linked and will move together. Mitigation traits can also be spread by inserting them in multicopy transposons which disperse faster than the crop protection genes in related weeds. Thus, there are gene flow risks mainly to weeds from some crop protection traits; risks that can and should be dealt with. © 2014 Society of Chemical Industry