Experimental methods in plant competition research in crop-weed systems

The implications of recent studies on the inappropriatness of replacement series and additive methods in competition studies, and some possible alternatives, are discussed in the context of weed research, Replacement series are usually inadequate to assess competitive interactions and can be misleading. In particular they may be biased in favour of the larger species. Many of the criticisms of replacement series also apply to additive experiments. Response models relating yield per individual to the densities of the species in the mixture provide methodology for answering many questions about mixtures. This paper proposes a framework for using these models to: (i) measure the effect of weed species on yield per individual and yield per unit area for the crop and weed species; (ii) develop methods of biological control of weeds both within a seaons and over seasons, based on the interference between crop and weed species and the population biology of the weed species; (iii) establish a cost-benefit analysis of certain of the biological weed-control programmes. The inclusion of the effect of relative emergence time and management practices in response models is considered as well as experimental design for crop-weed experiments.     

Using plant population biology in weed research: a strategy to improve weed management

The aim of this paper is to show the need for developing plant population biology studies in order to improve weed management when traditional approaches in weed science have failed, as shown by recent reviews on herbicide resistance. It is also suggested that the usual weed definitions do not reflect the new aims of weed ecology. The results of studies on the genetic, demographic and spatial variability of weed populations and their regulation by pests and pathogens are reported, and are discussed in the specific case of clonal perennial species.     

The role of ecology in the development of weed management systems: an outlook

This paper discusses the extent to which a knowledge of weed biology and ecology can contribute to the development of weed management strategies. To date, such contributions have been modest and have been constrained by a number of factors that are discussed in this review. In contrast to other pest management disciplines, devising integrated weed management strategies that address a diversity of weed species with a diversity of life history traits is difficult. Because of this diversity, robust systems that require ecological insight beyond that of individual species are needed. Although the contributions have been modest, research findings have helped to shape weed management strategies in a number of important ways. Approaches directed at weed population management have revealed important insights into population equilibria, density-dependent mortality and life stages particularly important in regulating population size. Eco-physiological research has helped to guide the development of biologically effective herbicide dosage strategies, whereas mechanistic interplant competition modelling coupled with empirical field studies have aided in the identification of weed-suppressive crop phenotypes. Finally, much has been learned about the influence of control tactics and agronomic measures on the evolution of herbicide resistance and the development of integrated weed management strategies to address it. In this paper, examples are reviewed where research in ecology and biology has helped to shape the practice of integrated weed management. More importantly, characteristics of such research programmes are identified so that future efforts in the discipline will have a context in which the relevance of research questions and approaches can be considered.     

Weed species shifts in glyphosate-resistant crops

The adoption of glyphosate-based crop production systems has been one of the most important revolutions in the history of agriculture. Changes in weed communities owing to species that do not respond to current glyphosate-based management tactics are rapidly increasing. Clearly, glyphosate-resistant crops (GRCs) do not influence weeds any more than non-transgenic crops. For most crops, the trait itself is essentially benign in the environment. Rather, the weed control tactics imposed by growers create the ecological selection pressure that ultimately changes the weed communities. This is seen in the adoption of conservation tillage and weed management programs that focus on one herbicide mode of action and have hastened several important weed population shifts. Tillage (disturbance) is one of the primary factors that affect changes in weed communities. The intense selection pressure from herbicide use will result in the evolution of herbicide-resistant weed biotypes or shifts in the relative prominence of one weed species in the weed community. Changes in weed communities are inevitable and an intrinsic consequence of growing crops over time. The glyphosate-based weed management tactics used in GRCs impose the selection pressure that supports weed population shifts. Examples of weed population shifts in GRCs include common waterhemp [Amaranthus tuberculatus (Moq ex DC) JD Sauer], horseweed (Conyza canadensis L), giant ragweed (Ambrosia trifida L) and other relatively new weed problems. Growers have handled these weed population shifts with varying success depending on the crop.     

Possibilities and Limitations in Biological Control of Weeds

This contribution defines biological weed control as the attempt to reduce the competitivity of a weed and to stabilize it at low population levels with the aid of herbivorous organisms or phytopathogens. The main factors involved in the process of biological weed control are discussed. Biological weed control is compared to conventional control measures and its advantages as well as its limitations are studied. As a consequence of the interactions between a weed, its control agents and the environment, there are only relatively few weed species and weed situations in Europe which are promising targets for biological control programs. Some of these are discussed in the concluding chapter; they refer to Solidago, Ambrosia, Polygonum, Galinsoga, Ptertdium and Rumex spp,     

Introduction to Weed Problems 1

I have approached the very wide topic of this paper by outlining the modern concept of weediness as traced from the beginnings of agriculture and then giving examples of some important present day weed problems from various parts of the world. Weeds have no place in nature and are essentially a reflection of the environment and of man's activities and desires. When these change, the weed flora responds and weed control methods may have to be modified. In developed agricultural systems, weed control has been revolutionised by herbicides which have also allowed major advances in methods of crop production and land and water management. As a result some classical weed species are no longer important whilst hitherto unimportant species have emerged as major problems. In many parts of the world there are obstacles to the introduction of chemical weed control techniques, and weed problems and control methods are much the same as they have always been. With the increasing world population and the need to provide more food, intensification of agriculture is inevitable. If weed control is not to be a limiting factor, more recognition and support must be given to weed science as an essential discipline of agricultural research.     

Predicting the risk of weed infestation in winter oilseed rape crops

Chemical weed control before crop and weed emergence is a systematic practice in winter oilseed rape crops in France. It would be profitable both for farmers and the environment to predict the level of weed infestation early on in the growing season and to control weeds only when necessary using post‐emergence weed control. The objective of this paper was to develop and evaluate simple models to predict weed biomass in oilseed rape crops. The model input variables were related to weed population characteristics and farmers’ practices. The models can be used to classify oilseed rape plots into two categories: plots with a level of weed infestation above a threshold or those with level of weed infestation below a threshold. A data set including 3 years of experiments, conducted across several regions in France, was used to estimate the parameters and to evaluate the models. High values of sensitivity and specificity were obtained when weed biomass was predicted as a function of sowing date, type of soil tillage, soil mineral nitrogen, crop density, weed density at emergence, and main characteristics of the most abundant weed species. Model performance strongly decreased when input variables related to the weed population were not taken into account. The best models correctly classified 90% of the plots with high weed infestation and 64% of the plots with low weed infestation.     

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.     

春麦田除草剂的应用与杂草群落演替

作者采用定点定位试验,对春麦田除草剂应用与杂草群落演替趋势进行研究。结果表明:在杂草群落多样性麦田,小麦连作并分别连续5年施用同一种除草剂,原杂草群落中占优势的靶标杂草得以控制,而非靶标杂草和抗、耐药性杂草因失去竞争和制约对象而猖獗发展,发生量较原来增加几倍至几十倍,并形成优势种群,对小麦造成新的更严重的危害。一种除草剂在同田块连续施用4年,由于杂草群落演替,抗、耐药杂草兴起,除草效果显著下降而失去其使用意义。作者提出除草剂配套使用、轮用、混用等措施,并配合以合理的轮作制度,以减轻杂草群落长期受到单一的定向选择性压力。     

Non-chemical weed management in organic farming systems

Concern about potential increases in weed populations without the use of herbicides has limited the uptake of organic farming. However, as both public demands for organic produce and the profile of organic farming have increased in recent years, so too has the range of weed control options. Progress in cultural methods of weed control has included the use of novel weed-suppressing cover crops, and the identification of specific crop traits for weed suppression. Direct weed control has also seen developments, with new implements appearing on the market that could benefit in the future from sophisticated machine guidance and weed detection technology. Advances in novel techniques such as steaming have also been made. Many weed control operations in organic systems present the grower with conflicts, and both these and many of the most recent developments in organic weed control are reviewed. An increase in our understanding of weed biology and population dynamics underpins long-term improvements in sustainable weed control. The outcome of these studies will benefit conventional and organic growers alike. Emphasis is given to the need for flexibility and a combination of weed biology knowledge, cultural methods and direct weed control to maintain weed populations at manageable levels.     

Chemical weed control in rainfed peanuts

Field studies at Bangalore, India, demonstrated that benfiuralin, fluchloralin and profluralin were effective for the selective control of many weed species in field experiments with peanuts (Arachis hypogaea L.‘BH-8-18′). Trifluralin and dinitramine were effective against many weed species but reduced crop stand by about 10%. These two herbicides also reduced the incidence of leaf spot disease (Cercospora arachidicola) and peanut pod yields were similar to clean weeded treatments. Bentazon was ineffective on most weed species and metribuzin was lethal to the crop. Alachlor, nitrofen and chloramben were only partially effective and would require supplemental hand weeding or mechanical tillage to obtain satisfactory weed control.     

我国小麦田禾本科杂草对精噁唑禾草灵的抗药性研究进展

小麦是我国主要的粮食作物, 麦田草害的发生与危害严重影响小麦的产量与品质。我国小麦田的禾本科杂草主要包括日本看麦娘、看麦娘、菵草和多花黑麦草等, 田间杂草防除主要依靠化学除草剂。精噁唑禾草灵自20世纪80年代进入我国市场以来, 长期用于小麦田禾本科杂草防除, 已经报道麦田多种禾本科杂草对精噁唑禾草灵产生了高水平抗药性。为更加科学合理地控制小麦田杂草的发生与危害, 一些学者就麦田禾本科杂草对精噁唑禾草灵的抗药性水平监测和抗性机理进行了系统研究。本文系统总结了我国小麦田抗精噁唑禾草灵杂草的发生种群、抗性水平、靶标酶抗性机理与非靶标抗性机理, 还梳理了抗性杂草的交互抗性与多抗性发生情况, 分析了我国在抗性杂草治理方面的经验以及面临的问题, 为农田杂草防除提供指导。     

Introduced weeds pollinated by introduced bees: Cause or effect?

In the present review we attempt to synthesize evidence for a causative relationship between the presence of non-native bee pollinators and the successful establishment and spread of introduced weed species. Using data drawn from the literature and from our own survey conducted in New Zealand, we show that introduced bees favor foraging on introduced plant species, and that in some cases they depend totally on these plants as sources of nectar and pollen. It is also apparent that the flowers of many introduced plants are visited, exclusively or predominantly, by introduced bees. Accepting that visitation does not necessarily imply pollination, and not all plants require pollination to reproduce, we review relationships between seed set of exotic weeds and visitation by introduced pollinators. Although few studies have been carried out, we show that those reported so far all point to increased seed set when introduced plants are visited by non-native bee species. There is clear evidence for a positive link between the spread of weeds and the presence of introduced bees. Nevertheless, ecologists have neglected this aspect of weed population biology and remarkably few studies have been conducted on the importance of pollinator availability in the management of weed species. We suggest several avenues along which future research can be conducted and highlight how the management of present and future weed species may be influenced by this research.     

A Comparison of Various Methods for Competition Studies Between Crop Plants and Weeds1

The assessment of economic thresholds of weeds is difficult because there are so many interactions between the various species and the crop. In addition, a precise forecasting of the weather is needed when pre-emergence treatments are performed since the conditions prevailing at that moment will be decisive for the abundance of weeds. Still, there are various possibilities to assess the crop losses resulting from weed competition and attention is drawn to the following two approaches: 1) Long-term assessment in relation to the total weed population in a region, and 2) Competition studies between individual weed species and crop plants in pots or in field experiments. The advantages and disadvantages of the various methods are discussed and the data obtained critically evaluated. It appears that the application of average values to a special situation remains for the moment highly uncertain.     

Co‐operative versus non‐co‐operative farmers' weed control decisions in an agricultural landscape

This study presents a simple landscape model of the influence of seed dispersal on weed population dynamics between fields. In the model, three fields are interconnected through weed seed dispersal, where seed might move with field equipment, in irrigation water or may be wind‐dispersed. The model is intended to characterise the impact of field‐level weed management decisions on landscape‐level weed population dynamics. Two simple scenarios were studied. The first simulates farmers adopting common effective methods of control on each of the three fields. In the second scenario, farmers manage the weed population independently on each of the three fields. In the first scenario, weed populations were driven to extinction as might be expected with uniformly high levels of weed suppression in each of the three fields. In the second scenario, when the two nearest fields in the sequence experienced control, the weed population was driven to extinction in the second field but not in the first where weed populations were able to survive in spite of high levels of suppression. The results suggest that control measures within a field may not adequately reflect their impact on weed population dynamics when between field seed movement occurs. Another important result is the importance of proximity and spatial arrangement of fields and the resulting influence on weed population dynamics within a field.     

Evaluation of the allelopathic activity of five Oxalidaceae cover plants and the demonstration of potent weed suppression by Oxalis species

Laboratory and field experiments were conducted to evaluate the usefulness of Oxalis spp. as allelopathic ground-cover plants for weed management. Some Oxalis spp. have previously been reported to possess strong allelopathic activities but few studies have been conducted on their activities in fields. This study aimed to investigate allelopathic activities and the possibility of weed suppression in five species of common Oxalis : shamrock oxalis ( Oxalis articulata Savigny), Bowie's woodsorrel ( Oxalis bowiei Lindl.), trefoil ( Oxalis brasiliensis Lodd. ex Knowl. et West.), lucky clover ( Oxalis deppei Lodd. ex Sweet) and Oxalis hirta L. The effects of the leachates from dry leaves and the exudates from living roots of these plant species were tested in laboratory experiments. The leachates from O. articulata , O. bowiei , O. deppei and O. hirta and the exudates from O. deppei caused > 84% inhibition of the radicle elongation of lettuce seedlings, but no effect was observed on the seed germination of lettuce. In the field experiment, O. deppei significantly reduced the weed population in July. A significant relationship was observed between the weed population and the percentage ground coverage of Oxalis spp. In contrast to the weed population, a significant relationship was observed between the weed above-ground biomass and the allelopathic activity of exudates from Oxalis spp.     

超高茬麦套稻田杂草发生特点、成因及化除策略

1998～2001年对江苏省5个不同生态区125个麦套稻田示范点的田间杂草发生情况进行了调查分析,初步掌握了麦套稻田杂草的发生特点,主要表现为种类多、发生早、群体大、危害重。免耕方式、前茬余草和前期田间管理粗放是造成杂草发生严重的原因。通过试验提出了"前封后杀"的化除策略,并研究出多项配套化学除草技术,促进了超高茬麦套稻田技术的大面积推广应用。     

Role of sorption and degradation in the herbicidal function of isoxaflutole

BACKGROUND: The fate of isoxaflutole (IFT) in soil is closely related to soil sorption. Sorption and transformation of IFT were investigated in laboratory incubations with four soils, and these results were used to interpret greenhouse studies using IFT to control several weed species. RESULTS: Degradation proceeded by previously observed pathways to form diketonitrile (DKN) and benzoic acid (BA) derivatives, as well as traces of unidentified products. Over the course of the incubation, DKN was the dominant active form of the herbicide present in the experimental system, and was thus critical to the soil activity of the herbicide for weed control. CONCLUSION: Control of most weed species appeared to be a function of both sorption and biodegradation of DKN, with greatest weed control being observed in soils in which a significant portion of the DKN that was formed persisted and remained bioavailable over the course of the incubation. Copyright © 2009 Society of Chemical Industry     

Greenhouse and field evaluations of commonly occurring weed species for their host suitability to Meloidogyne species

Since weeds serve as hosts for nematode pests, the host status of 20 weed species (commonly occurring in fields of developing farmers) to Meloidogyne incognita and M. javanica, respectively, were investigated. Greenhouse studies showed that seven weed species had Rf values >1 for both nematode species indicating susceptibility, while 13 had Rf values ≤ 1 indicating resistance. Greenhouse results showed that Hibiscus trionum and Amaranthus tricolor were identified as the most susceptible and Chenopodium carinatum and Datura ferox the poorest hosts for M. incognita and M. javanica. For field experiments at Kuruman, Solanum retroflexum was the most susceptible weed to a M. javanica population, while the same was evident for H. trionum at Nelspruit, where a mixed population of M. incognita and M. javanica occurred and at Potchefstroom, where a population of M. incognita was present. Results from this study indicated that certain weed species are highly susceptible to root-knot nematodes and should be removed timely and effectively to prevent population level increases of root-knot nematode pests in the fields of farmers.     

湖南省烟田杂草种群调查

为了明确湖南省烟田杂草种群与分布,采用对角线五点取样法在全省烟田进行了系统调查。结果表明,烟田杂草共有30科77种,其中禾本科杂草占比14.29%,阔叶杂草占比81.82%,莎草科杂草占比3.89%;一年生杂草占比75.32%,多年生杂草占比24.68%。相对多度大于3%的杂草有铁苋菜、酸模叶蓼、稗、碎米荠、马唐、繁缕、喜旱莲子草、看麦娘、通泉草、鸭跖草、水苦荬等11种,其中铁苋菜、酸模叶蓼、稗为湖南省烟田优势杂草种群,碎米荠、马唐、繁缕、喜旱莲子草、看麦娘、通泉草、鸭跖草、水苦荬为湖南省烟田区域性优势杂草种群。湖南省烟田杂草防除主要采用苗前土壤处理和苗后茎叶处理相结合的化学防除方法,主要使用的化学除草剂是精异丙甲草胺和砜嘧磺隆。本研究结果表明,湖南省烟田杂草存在区域性差异,应因地制宜制定合适的化学防治方案。