Weed management in agroecosystems: Towards a ...

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Recent Res. Devel. Crop Sci., 1(2004): ISBN: 81-271-0046-3

Weed management in agroecosystems: Towards a holistic approach Anil Shrestha1, David R. Clements2 and Mahesh K. Upadhyaya3 1 University of California, Statewide IPM Program, Kearney Agricultural Center, 9240 South Riverbend Avenue, Parlier, CA 93648, USA 2Biology and Environmental Studies Trinity Western University, Langley, BC, V2Y 1Y1, Canada ; 3Faculty of Agricultural Sciences, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada

Abstract Weed science today is at a crossroads because the traditional approach to control weeds with herbicides is challenged by environmental, economic, and social concerns. Further, widespread development of herbicide-resistance has also been observed. We have gone through a “learning cycle” in our paradigms on weed management. A trend towards using a holistic approach to weed management has evolved at the end of this learning cycle. “Weed scientists of tomorrow” are being instilled with a more holistic rubric of weed management than their predecessors. This article reviews alternative approaches for a more holistic weed management strategy. These alternatives are Correspondence/Reprint request: Dr. Anil Shrestha, University of California, Statewide IPM Program, Kearney Agricultural Center, 9240 South Riverbend Avenue, Parlier, CA 93648, USA. E-mail: [email protected]

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Anil Shrestha et al.

classified under cropping system, economic/environmental, eco-physiological, technological innovation, chemical ecology, foodweb, and organic approaches. A “toolbox” of alternative weed management and three paths towards greater holism in weed science are presented.

Introduction It is clear that the driving force in weed science from World War II up until the 1970s was the development and use of effective herbicides (1). The historical means of controlling weeds manually, although still practiced in many developing countries, quickly gave way to herbicide control methods. Thus, weed science became the vanguard of herbicide-intensive approaches. Academics together with the herbicide industry sought to develop best herbicide-based solutions with minimal attention to environmentally-friendly options. However, a new phase in weed science began to emerge in the 1980s with a move towards ecological weed management. Zimdahl (2), has pointed that weed scientists are moving away from “what weed?” and “what herbicide?” to tackling “why?” questions and noted that contributions to Weed Biology and Ecology section of the Weed Science journal have significantly increased. Why is weed science at crossroads? Basically it is because the traditional approaches to the use of herbicide chemistry no longer function in conjunction with our agricultural systems. This is not to say that herbicides have been displaced as the primary tool for weed management; this is still the case in most industrial countries. Rather, there are serious impediments to the approach of continuously developing new chemicals and applying them prophylactically. These impediments include the widespread development of herbicide-resistance, economic difficulties affecting both agricultural producers and the herbicide industry, and the perceptions and realities of herbicide impacts on the environment and human health. There are now 526 documented cases of herbicide resistance (171 plant species) in 59 countries (3). Initially it was thought that, unlike insects, weeds would not develop resistance to control chemicals (4). However, development of herbicide-resistance in weed populations has restricted the choice of herbicides within certain chemical families. The thin profit margin for producers on most crops has made it difficult to afford herbicides. In fact, this has helped forge a new movement in the Canadian prairies called “Pesticide-free Production” which involves omission of herbicide applications for one year at a time (5). Thus, rather than eliminating pesticides as in the case of organic farming, the farmer reduces usage within a conventional system. The herbicide industry has downsized itself considerably in recent years. The golden years of prospecting for new chemistry seem to have passed. Many herbicides with long residual life have been discontinued due to environmental concerns, and government programs in Canada such as Ontario Food Systems 2002 that sought to reduce pesticide use by 50% have encouraged herbicides at low rates (6, 7). It was estimated that pesticide-use had decreased by 38.5% from 1983 to 1998 in Ontario (8). Despite these changes, there are still widespread concerns over herbicide use (9), and reports of contamination of groundwater in some areas (10). These concerns continue to fuel the development of more innovative “environmentally-friendly” weed management options. Weed scientists themselves have increasingly recognized the need for new paradigms. Many articles from the “My View” feature in the journal Weed Science,

Weed management in agroecosystems: Towards a holistic approach

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begun in 1997, attest to the need for a fundamental shift in the discipline of weed science. It has been said that, “Members of Weed Science Society of America (WSSA) face challenges, opportunities, and decisions that may be the most demanding in the discipline’s history” (11). “Many weed scientists are struggling to develop a broader range of weed control tools and tactics” (12) and, the “Public attitude associated with use of pesticides and other agricultural inputs has placed increased emphasis on development of ecologically based pest management (EBPM) (13). Hatzios (14) stated that, “Our biggest challenge will be to convince the presidents of land-grant universities, state governors and legislators, and the federal government that weed science is a mature discipline that can stand on its own and is vital to the success of American agriculture.” While there is clearly a need for improved weed management, poor commodity prices for agricultural products has hampered this effort. For example, recent poor economic returns in raisin-grape (Vitis sp.) production have led to abandonment of numerous vineyards or reduced weed control measures to save operation costs in the South Central Valley of California. This phenomenon may be responsible for the recent increase in horseweed (Conyza canadensis) in the South Central Valley (15). Although Integrated Weed Management (IWM) and other non-chemical options might work, herbicides offer the least expensive option. As mentioned previously, however, even herbicide costs are becoming prohibitive, especially in combination with other economic problems faced by producers. It is difficult to justify large expenditures on weed control. In some cases, considerable resources are being devoted to the development of herbicide-resistant crops, particularly in North America. Although various socio-economic and ecological factors are driving a trend towards greater holism in weed management, the development of herbicide-resistant crops may swing the pendulum back towards reductionist approaches (16). Some wonder whether weed science is making the same mistake in the science and promotion of herbicide-resistant crops as with herbicides themselves (17). Perhaps the unintended ecological consequences of transgenic crops need to be considered more thoroughly (16). It is not surprising that attempts to apply technology such as transgenic crops have met some skepticism. Even schemes that appear politically correct such as “conservation programs” present challenges to the development of sustainable weed management strategies. Large areas of land are being set aside for conservation in the US. Although these areas are not farmed, weed management and associated problems may continue to appear (18, 19). Thus, at every turn we find that weeds continue to present serious economic, environmental, and social challenges. It has been observed that, in some cases, better technology yielded other problems (e.g. soil erosion, chemical contamination problems or even the disintegration of rural society) (20). Radosevich (20) stated, “I imagine a better scenario, a different possibility in which people base their decisions on an understanding that they are part of a web in which every action causes a whole variety of reactions. In this scenario, human beings would be more humble and accountable to nature, adapting to what exists rather than the other way around.” In this article, we attempt to lay some of the groundwork for a “better scenario” by reviewing current research on alternative approaches for weed management.

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The learning cycle In our research on cropping systems (21) and agroecosystems (22), we have been developing an overarching model of human management and agriculture we term “the learning cycle.” In Figure 1, we adapt this learning cycle to weed management. The paradigm shifts in agriculture and weed management may be seen as a learning cycle that began with human intervention in natural ecosystems. As humans domesticated crops, agroecosystems constituted at first only slightly modified versions of natural ecosystems. At this point, agroecosystems began to develop weed problems as a function of the adaptations of pioneer species of weeds that favor disturbed sites (23, 24). Indeed, one fundamental attribute of agroecosystems that favors weeds in particular is the presence of bare ground. Thus began the process of human-weed co-evolution that has produced many weed ecotypes that are well-adapted to agricultural systems (25, 26, 27). The next objective was to maximize crop yields to feed a growing population; as fundamental an idea as this seems, it carries with it a myriad of unintended consequences when technology strives to reinvent an ecosystem. In the case of weed management through the 20th century and beyond, this has meant the development of a battery of strong tools (herbicides, cultivation, etc.) that consider weeds as entities that need to be eradicated. Of course, eradication has not happened by and large, although higher yields have been achieved. As the learning cycle indicates, the maximization of yields as a singular objective (via high input, mechanized cropping systems) eventually must be reconsidered in the light of environmental, social, and health issues such as off-site or non-target pesticide effects, reduced groundwater quality or soil degradation. Concerns may be raised by both the non-farming and farming population over environmental safety of cropping systems. The objectives of cropping systems thereafter included environmental safety, social acceptance, and economic viability.

Figure 1. The learning cycle in the development of agricultural systems, with special reference to weed management issues [Adapted from Shrestha and Clements (21)].


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In terms of weed management, this has resulted in a variety of strategies (Fig. 1), from herbicide-based, to IWM, through to organic systems. Aspects of the design of each of these systems, including herbicide-based, are derived from knowledge of the operation of natural ecosystems. This relates to the current rise in the interest by weed scientists in ecology, a discipline formerly dealing primarily with “natural” systems (28). An interesting twist on this is that government programs in many areas of the world, including the conservation reserve program in the US, are now completing the cycle by converting former agricultural areas into “natural” areas. Here again, weed science must strive to deal with the numerous weed problems that continue to arise and disrupt environmental values sought in these programs (18, 19). Lessons within the learning cycle clearly point to the need for a holistic approach that takes into account several factors that may be social, political, physical, biological, and economic in nature. However helpful this big picture is in appraising the overall trends, to understand the current situation, it is necessary to look at more specific obstacles to such holism and more specific tools that might provide bridges to help attain this vision. The word “tools” has particular applicability in this context, because the phrase coined by Liebman and Gallandt (29) describing IWM as the use of “many little hammers” has been widely quoted in recent years.

Obstacles to a holistic, ecological approach As discussed earlier, herbicides and the technology surrounding herbicide use have been the primary focus of weed management for more than 30 years (30, 31). The success of herbicides to control weeds inexpensively (32) and the amount of effort directed towards this technology can be cited as a major reason for lack of a holistic approach to weed management. Other obstacles include the projected success (33) of genetically modified herbicide tolerant crops (HTCs). Ecological aspects of inclusion of HTCs in agroecosystems are being investigated (16). However, such considerations are only a consequence of the development of HTCs in the first place. Therefore, inclusion of HTCs in cropping systems may direct substantial research towards issues surrounding this technology. Similarly, increase in farm size has been cited as an obstacle for ecological weed management (34, 35). It is argued that larger farms are less suited for ecological weed management and are more prone to using herbicides. Furthermore, the narrow disciplinary training and a reductionist approach in weed science is also considered as a hindrance to the development of an ecological approach (13). However, there are recent examples where weed scientists have begun to take alternative approaches. The following sections will discuss some of these approaches.

Alternative approaches to weed management A cropping systems approach Cropping systems research today is influenced by a broad range of agronomic, ecological, environmental, social, and economic issues. Any change in the cropping system to address these issues can have far reaching implications (21) to the agroecosystem including weed management. For example, limiting fertilizer nitrogen in order to address environmental pollution concerns may favor weed competition in crops and necessitate an increase in weed control costs and/or herbicide use (36). Similarly,

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incorporation of HTCs in a cropping system is challenged by several of the aforementioned issues. An integrated cropping systems approach is, therefore, necessary for ecological weed management (37). Tillage, soil quality improvement, cultivar selection, crop planting date, crop row spacing and population density, fertilization, irrigation, crop rotation, use of cover crops, and intercropping are major components of cropping systems and each of these have an effect on weed dynamics. Tillage Tillage has been an essential component of traditional agricultural systems. Although primarily done for seedbed preparation, tillage is also a major tool for weed management. Tillage affects weeds physically by uprooting, cutting, and burying them. Weed seeds are also often buried to depths too deep for emergence. Tillage also changes the soil environment (temperature and moisture) which in turn promotes or inhibits weed establishment. Concerns for negative effects of intensive tillage on soil structure and environmental quality (e.g., air pollution, soil erosion) led to the development of conservation tillage-based cropping systems (38). Weed species shifts occurring due to less tillage, however, have impeded the widespread adoption of these systems (39). Some common concerns in transition to conservation tillage include: i) the possibility of increase in weed populations because of recently produced seeds that are no longer being buried deep; ii) presence of surface residue under conservation tillage that may interfere with herbicides or other weed control practices; and iii) possibility of a species shifts towards annual grasses, perennials, and volunteer crops (40). However, evidence regarding these concerns, especially weed species shifts, has been contradictory. Studies showing greater abundance of some weeds with increasing levels of cultivation (41), similar distribution of broad-leaved weeds in conservation and conventional tillage systems (42), or no consistent response of weed populations to tillage have been reported (43). Long-term changes in weed flora may be driven more by interaction of tillage, environment (location, year), crop rotation, crop type and timing, and type of weed management practice (44, 45). Therefore, the role of tillage practices on weed communities continues to draw considerable interest as agriculture moves towards ‘environmentally-friendly conservation tillage’ systems. Cultivar selection Certain crop cultivars are known to be better competitors with weeds than others (46). For example, white bean (Phaseolus vulgaris L.) cultivars differ in their ability to compete with weeds (47). Certain tomato (Lycopersicon esculentum L.) cultivars have considerable tolerance to dodder (Cuscuta sp.), a severe parasitic weed in many parts of the world (48). Therefore, selection for crop competitiveness is a promising tool for weed management and it should be included in crop breeding programs (46). Crop planting date Crop planting dates can be adjusted to reduce yield losses from weed competition. Crops can be planted either earlier or later based on the knowledge of the prevalent weed species and their emergence pattern. If crops establish earlier than the weeds they may have a competitive advantage. On the other hand, crops can be planted after controlling


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early flush of weed emergence by chemical or mechanical means. For example, a twoweek delay in soybean planting date allowed effective mechanical weed control prior to planting and reduced weed densities in the crop substantially (49). Techniques such as stale seedbed management (50) and pre-irrigation have been used successfully to stimulate weed emergence and subsequent control prior to crop planting. Crop row spacing and population density Crop row spacing and population density can be used as a tool for weed management (51). For example, narrower row spacing and higher corn (Zea mays L.) density significantly reduced biomass of late emerging weeds (52). Similarly, increasing corn density from 40,000 to 100,000 plants ha-1 reduced weed biomass by 50% (53). However, the effect of row spacing and population density on weed biomass is inconsistent. Environmental conditions in a given year and the dominant weed species in the field may influence the outcome of such spatial manipulations in crop planting (54). Fertilization The effect of fertilizer (especially nitrogen) management on crop-weed competition has been demonstrated (55, 56). Fertilizer placement in proximity of a crop row can benefit crops more than weeds and thus reduce weed competition (57, 58). Further, the type (59, 60) and the time of fertilizer application (61) can reduce weed competition. However, lowering nitrogen rates in corn may increase competitiveness of weeds such as green foxtail (Setaria viridis L.) (36). Therefore, precautions should be taken while using manipulation of nutrients as a tool for weed suppression. Irrigation Type of irrigation system and its timing can influence weed populations and their management. Weed seed germination is influenced both by soil moisture and temperature (62). Therefore, a change in moisture regimes in the soil profile can affect the weed emergence patterns and population dynamics within a cropping system. For example, weed emergence and growth was suppressed under subsurface drip irrigation compared to sprinkler and furrow irrigation systems in semi-arid cropping systems because of a drier soil surface under the drip system (63). In orchards, weed management may need to be adjusted according to the irrigation and planting system. For example, if an orchard is planted in flat beds and the irrigation water is placed in the tree rows, weed management frequency may need to be increased because of high moisture and greater weed germination in the tree rows. Conversely, where trees are planted in berms (raised beds) the frequency of weed control measures may be reduced because of the drier soil surface in the tree rows (64). Crop rotation With the development of specialized monocultures, e.g. the US Corn Belt, an “industrial cropping system” emerged after World War II and crop rotation was marginalized as a component of cropping systems (65). Crop rotation is an essential tool for ecological weed management (29). It challenges weeds by providing a broad range of stress and mortality factors associated with different crops grown in the rotation (65).

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Negative effects of herbicides with different chemistries, tillage practices and planting dates during crop rotations on weeds populations have been demonstrated (66, 67). Cover crops Cover crops provide multiple benefits to agroecosystems. These include reduction in soil erosion, enhancement of soil quality, nutrient management, and pest management (68). Cover crops reduce weed populations by various mechanisms (69). The success of cover crops in reducing weed populations depends on the type of cover crop, environmental conditions, emergence rate, and biomass accumulation (70). Intercropping Intercropping is practiced in many production systems of the world. Differences in patterns of resource availability are created by interplanting of species with differing growth and development modes (71). These variations may provide opportunities for weed management because intercrops can deplete resources that would otherwise be available to weeds and hence suppress weed growth (67). There are various examples of suppression of weeds by intercrops (29, 72). The choice of the intercrop and its ability to suppress weeds may depend on the location, types of weed species, and time of emergence. Intercrops should be selected carefully because some intercrop species can compete and reduce the yield of the main crop (72).

An economic/environmental approach Thresholds The use of economic thresholds in pest management decisions has been around for almost half a century. The term “economic threshold” was first introduced in the 1950s (73) and it is now a well established concept in integrated pest management (IPM) programs especially for insects (74), pathogens (75) and nematodes (76). However, the use of economic thresholds in weed management decisions is relatively new and can only be traced back to the mid-1980s (77, 78). Various definitions have been provided for economic threshold (78, 79, 80). A broad definition of which is “the point at which losses equal cost of control” (79). Swanton et al. (81) concluded that use of thresholds is appropriate primarily in fields with low weed pressure and that crops with a high economic value may have a zero threshold. Norris (82) argues that it is not ecologically sound to adopt weed thresholds comparable to those developed for arthropods because of the risk of creating an unmanageable future problem if large weed seed banks are produced. However, more research is needed in variety of cropping systems to test predictions made by Norris (82). Critical periods of weed control Time of weed emergence relative to the crop is an important factor in determining the outcome of crop-weed competition (83). Weeds that emerge with or before the crop can cause greater crop yield losses than those that emerge after the crop. The critical period for weed control is a period in the crop growth cycle during which weeds must be controlled to prevent yield losses (84). Hence, season-long weed control may not be necessary to prevent crop yield losses.


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For the successful application of this approach, knowledge of weed emergence patterns is crucial. Therefore, studies have focused on developing parameters for weed seed germination and emergence patterns (62). Cropping systems (e.g., crop row spacing) and prevalent weed species can also influence critical periods of weed control (85). Herbicide dose-reduction Weed management is still an herbicide-dominated system in many parts of the world (37). However, research is being conducted to reduce the application rate of herbicides while achieving maximum efficacy. Such studies include the determination of “biologically effective dose” (BED), which is the minimal amount of herbicide required to result in an acceptable level (90%) of weed suppression (86). The premise behind such studies is that labeled herbicide rates are generic and determined to provide acceptable levels of weed control under the worst-case scenario (37). Weed species, growth stage, and environmental conditions can affect BEDs and these factors must be taken into consideration (88).

An eco-physiological approach Modeling An understanding of the phenological development of weeds is the basic premise of weed biology and ecology and seedling emergence is recognized as the most important phenological event influencing the success of an annual plant (89). Empirical and mechanistic models have been developed to predict crop-weed interaction. Models were first developed to describe static, empirical crop-weed interactions and such models generally described certain set of observations at a certain point in time (90). Several physiological parameters and variables, e.g. time of weed emergence, were later added to these models (89). Emergence of several weed species has been empirically modeled as a function of meteorological records (91). However, empirical models may not account for the effect of genetic variation and environmental factors on the development processes of crops and weeds (92, 93). In order to better understand these biological processes, physiologically based mechanistic models were developed (37). A mechanistic model is an ecophysiological approach to represent causality between the crop-weed competition system variables and provides a means to explain why observed results occurred (94). These models simulate seed dormancy, germination, and seedling elongation as functions of measured or estimated environmental variables (89). Weed seed dormancy is an important factor in predicting weed emergence (95). The factors that cause and terminate dormancy are soil temperature, moisture, light, nutrition, gaseous environment, and tillage. Several models that consider seed dormancy have been produced (89). Efforts have been made to develop parameters for weed seed germination in mechanistic models. Seed germination is influenced by the interaction of soil temperature and moisture. Hydrothermal time (interaction of soil temperature and moisture) was proposed as an approach to modeling seed germination (96). This approach was further expanded and seed germination was described with a single normalized curve at a range of temperatures and water potentials (97). Based on this principle, the cardinal (minimum, base, and optimal) temperatures and optimal water

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potential for weed species such redroot pigweed (Amaranthus retroflexus L.) (98), common lambsquarters (Chenopodium album L.) (62), and common ragweed (Ambrosia artemisiifolia L.) (99) germination were estimated and a hydrothermal time model was developed to explain their germination. Root and shoot elongation is the next essential step of seedling emergence. Temperature is the major factor influencing this process (62). Similar to parameters for seed germination, cardinal temperatures for root and shoot elongation have been determined for several weed species (62, 98, 99). Once the seedlings have emerged, the phenological development of a weed species is influenced by factors such as light, temperature, and moisture. The effects of photoperiod and temperature on several weed species have been determined and their phenological development quantified in terms of thermal time, i.e. heat units (e.g., 94). These parameters were developed for inclusion in mechanistic models for crop-weed competition.

A technological innovation approach Site-specific weed management Environmental and economic concerns of uniform broadcast of herbicides have led to site-specific weed management technology (100). Spatial variations in distribution of weed species in a field necessitate site-specific management because over-application of herbicides can cause environmental contamination and crop injury and under-application poor weed control (100, 101). Global positioning systems (GPS), geographic information systems (GIS), variablerate (VR) applicators, and weed sensors are the primary technologies currently used for site-specific weed management. Weed maps are created by GPS and GIS technology to identify the spatial and temporal distribution of weed populations in the field (100, 101). These maps can help in making VR herbicide applications depending on temporal and spatial distribution of weeds and soil organic matter, structure and pH (100). However, developing reliable maps is costly and may not be reliable because of uncertainties in seasonal weed emergence (102). Several innovative methods are being tested to reduce the cost of creating maps and increasing predictability. The spatial correlation between soil properties and weed presence has been examined (103). In addition to soil properties, elevation as a spatial variable has also been examined to predict weed presence in fields (104). Remote sensing technology is being explored as a way to reduce costs on weed scouting and mapping. This technology allows for automated weed mapping (105). However, ability to recognize weed species at the seedling stage is a challenge. Algorithms to distinguish weeds from soil, identify weed types and weed species have been developed (106, 107). The concept of using equipment that can sense their target (weeds) was envisioned when a plant sensor to apply herbicide on the undesired sugar beet (Beta vulgaris L.) plants in rows was developed (108). Over the years, the technology has been improved and better sensors developed. Integration of this technology with GPS technology has resulted in real-time sensors to differentiate weed and crop species and spray the weed with herbicides. Such techniques can reduce both pesticide use and potential drift (109).


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Whether site-specific weed management will become feasible and commercially viable in the near future remains to be seen (105). Several other examples of technological and mechanical innovations for weed control exist. These will be discussed under the section ‘an alternative toolbox”.

A chemical ecology approach Several plant species produce chemical compounds that affect the germination, growth, and development of another plant species. These chemicals are termed as allelochemicals and the process, allelopathy (110). Suppression of weeds can be achieved by using rotational or companion crops with allelopathic potential, mulching or soilincorporation of plant residue(s) with allelopathic activity, application of plant extracts, or incorporation of allelopathic potential in crops by breeding or genetic engineering (111). Both allelopathic influence(s) as well as physical effects of the plant cover (or residue) may influence weeds. The residue could also help conserve soil moisture, improve soil organic matter content, and reduce soil erosion. Many allelochemicals also exhibit herbicidal activity and could be of interest as templates for synthetic herbicides (112). Better understanding of allelopathic interaction in crop-weed systems, regulation of the production of allelochemicals and mechanism(s) of their action, genetics of allelopathy, flow of allelopathic genes in field populations, effect of allelochemicals on soil biology, factors affecting allelopathic interactions and the fate of allelochemicals in soil would enhance our ability to use this approach for weed management (111).

A foodweb approach Predation of weeds and weed seeds Seed predation by herbivores can reduce weed seeds in an ecosystem (113). Quantification of seed predation on weed population dynamics is difficult (114). Weed seeds are a part of the foodweb because they are an energy source for a number of vertebrates, insects, and mollusks (115). Practices (e.g., tillage) that encourage weed seed predation in agroecosystems have been studied (114). Some herbivores consume plant parts and pathogens infect weeds limiting weed populations, which forms the basis of biological control. Natural enemies of weeds (e.g. phytophagous insects, pathogens) have been used to control weeds in rangelands and non-agricultural systems. In classical biological control, these biotic agents are brought from their native range for weed control. Thus the natural balance, which was disrupted when the weed was introduced to a new area leaving its enemies behind, is reestablished. The idea here is not to eliminate the weed but to bring its abundance to an acceptable level. While several different types of organisms can be used for biological control, phytophagous insects have been most commonly exploited as biotic agents. This option is attractive because it may provide permanent control and therefore is cheap in the long run, does not significantly upset the environment, and does not pollute our ecosystem. Phytophagous insects can also flourish under conditions (e.g. steep slopes, rough terrain) where other control options (e.g. mechanical, cultivation) may be difficult or expensive. The potential for effective biological control using phytophagous insects in annual cropping systems has been limited because: i) several weeds are present in annual production systems and some are related to the crop grown, ii) the weed cover is generally spotty or discontinuous, iii) use of insecticides for pest control may be

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necessary, which may harm the biotic agent(s), iv) timely weed control (i.e. during the critical period of weed interference) is important for optimum profits, and v) there may be abrupt habitat changes because of crop rotation, harvesting and cultivation, to which the biotic agent(s) may not be able to adapt. Weeds can be effectively controlled by spraying endemic, host-specific pathogens in a massive dose at a susceptible stage of plant growth under suitable environmental conditions (116, 117). When the organism is a fungus, the weed control product is called a mycoherbicide. Spore inoculum is artificially grown and sprayed to control weeds. After the weed host is controlled, the population of the control organism returns to background levels and reapplication of the mycoherbicide may be necessary to control any new flush of weed growth. Mycoherbicides may not serve as alternatives to herbicides but may be complementary additions to an IWM program (117). This weed control tool is still in its early stages of development with exciting possibilities lying ahead. Research is continuing on identifying new mycoherbicides for economically important weeds, mycoherbicide production technology, improvement of virulence and specificity of organisms by various genetic manipulation techniques, development of superior formulations and application technology, factors affecting the efficacy of mycoherbicides, and compatibility of mycoherbicides with other pest management and crop production practices (116). Weeds as an essential component of agroecosystems While weeds in general are considered detrimental to crop yield and a nuisance in harvest operations, there are studies that show positive attributes (e.g., help in nutrient cycling, act as host for beneficial insects and pathogens, help in increasing biodiversity) of weeds in agroecosystems (118). Weeds in field margins also have positive contributions as they provide cover, habitat, and environmental heterogeneity in space and time (119). Complex interactions occur among weeds, arthropod pests, and their natural enemies in managed ecosystems (118, 119, 120). Weed control therefore must consider its effect on biodiversity.

An organic approach There is a trend towards greater acceptance of organic farming in certain areas of the world. In this farming system, weeds are the most serious threat to crop production (121). Weed management in organic crop production can be achieved by a combination of non-chemical approaches discussed earlier and/or organic herbicides. Some more options are discussed in a following section on “alternative weed management tool box.” Weed control in organic systems also stresses the need for increased knowledge of cropweed ecology and cropping system influences on weed population dynamics.

An alternative weed management tool box Based on the several approaches discussed earlier, this section describes some of the tools that can be used for alternative weed management.

Heat Several methods utilizing high temperature have been developed for weed control (122). These include burning (123), controlled flaming (124), solarization (125), hot


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water (126), and the use of microwave (127, 128), infrared (129, 130) and ultraviolet (UV) radiation (131). These options use heat to cause thermal injury to weed tissue. Heating ruptures cells due to protoplast expansion and denatures cellular proteins, which destroys cell integrity. Because many of these methods kill only the shoots, affected plants may regenerate making repeated heat applications necessary. Thermal heating methods are attractive because they provide rapid, non-chemical weed control, and unlike cultivation do not bring buried seeds to the soil surface. Dead plant biomass is generally left on the soil surface, which protects soil from erosion (123, 129). Heating may also kill some insect pests and pathogens. However, these methods are generally costly, may burn fossil fuels, provide no residual weed control, have a high risk for injury to control operator, and can start a fire. Adverse effects of thermal weed control on desirable insects and soil microorganisms are poorly understood and need further investigation. Thermal weed control methods are often promoted as “environmentally-friendly”. However, it must be emphasized that an option involving use of a large amount of energy per unit area may not always be environmentally sound. Although the final step that kills weeds may appear non-polluting, the overall procedure may involve steps that pollute the environment (e.g. smoke, gases). Controlled flaming Thermal heating of weed tissues for a very short duration by directing open flames towards shoots has been used to top-kill weeds (132). The foliage is heated enough to kill but not ignite. Though costly, it is an attractive option in organic farming. It is also useful for weed control in orchards, non-agricultural sites, cracks in asphalt and concrete, and gravel and brick paths, for top-killing in potato (Solanum tuberosum L.), and for insect pest and pathogen control. Unfortunately, it may also adversely influence some desirable insects and soil microbes. In developed countries, where human labor is costly, flaming may offer a viable alternative to manual weeding. Equipment of different designs and capacities are available for a variety of flaming applications. Propane, butane, a mixture of both, or liquid petroleum gas have been tested as fuels. Use of hydrogen as a clean burning, non-polluting gas has also been investigated. Burners may be shielded to protect the crop from heat injury. Shielding also keeps heat close to weeds for a longer duration maximizing thermal injury. For optimum weed control and cost, and reduced crop injury and risk of starting a fire, proper burner design, travel speed, burner height above ground, and flame intensity and angle are necessary. Timing of flaming is crucial for successful weed control. For example, small, tender weed seedlings are more susceptible to flaming and are less likely to regenerate following top-kill. Weeds in the crop row may be controlled by flaming and those in the inter-row space by cultivation to reduce the cost (132). Controlled flaming can be used for non-selective weed control prior to crop emergence. Selective weed control is possible after the crop has grown tall and sufficiently resistant to flaming or by directing flames towards weeds. Woody stems close to the ground and presence of underground food reserves may help a crop withstand flaming. Since this option generally kills only the shoots, single applications may not provide adequate control when weeds (e.g. perennial weeds) are able to regenerate from below-ground parts. Weeds vary in their susceptibility to flaming, with

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species having concealed growing points (e.g. grasses) being less susceptible compared to broadleaf weeds with more exposed meristems. Weeds with thinner leaves are more susceptible compared to those with thicker leaves. Leaf surface characteristics may also influence a species’ susceptibility to flaming (132). Controlled flaming is an attractive option because it uses only heat to kill weeds, unlike tillage it does not bring buried weeds seeds to the soil surface, it leaves dead shoot biomass on the soil surface which protects soil from wind and water erosion, and it can be used under moist soil conditions which would generally not allow cultivation for weed control. However, the high cost of fuel, gaseous and smoke pollution upon burning of fuel, lack of residual control, and risk of starting a fire in dry regions are drawbacks of this method. Heat from flaming may also release dormancy of some weed seeds present on the soil surface (132). Thermal weed control can also be achieved by infrared heaters. Here infrared radiation produced from a heated surface is used to kill weed foliage. Since plants are not exposed directly to an open flame, longer exposure durations compared to flaming are necessary. The initial price of the equipment used for infrared application is also high (129, 130). Soil solarization This tool involves laying a clear plastic film (0.03 to 0.15 mm thick) on a wellprepared soil surface (133). The film allows solar radiation to pass through, but does not allow the heat accumulated below to escape. This raises soil surface temperature (134), controlling weed seedlings, nematodes, soil-borne insects, and pathogens (135). Soil solarization can also kill some weed seeds (134). In addition to weed and pest control, solarization also facilitates release of nutrients (e.g. NO3, Ca++, Mg++) from soil organic matter, making them available for crop growth (125). Effectiveness of solarization depends on the intensity of solar radiation, day length, air temperature, duration of exposure, weed species, depth of seed burial in soil, soil characteristics and moisture content, seed bed preparation, cultivation following solarization, thickness and characteristics of the plastic film used, and air pockets under the film (122). A thin, antifogging, less reflective plastic film is more suitable for solarization because it allows greater transmission of sunlight. Thicker films, however, are less likely to tear. Films with additives to protect them from solar ultraviolet radiation are recommended (125, 133). Generally, soil solarization controls annual weeds more effectively compared to perennial weeds. Solarization is also more effective when the soil is moist. Soil moisture facilitates conduction of heat, and makes the imbibed weed seeds and other organisms more susceptible to high temperatures (136). Cost and availability of the film for solarization, and the problem of its disposal or recycling, however, may limit the use of solarization. Hot water Hot water kills weeds by damaging foliar plant cells. However, many weeds may regenerate since the roots may not be affected. Repeated applications of hot water therefore may be necessary. This tool requires large amounts of energy, which may be costly, and large volumes of water, which may not be available in dry regions. Hauling


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large volumes of water is also inconvenient and expensive. While this option may not be practical on a very large scale, it may be practical on small and environmentallysensitive areas, as a spot treatment, for weed control around poles, near fences, in cracks in concrete and asphalt, and on gravel. Hot water may also kill some desirable soilborne microorganisms and insects. On the positive side, it may control some soil-borne pathogens and nematodes (126). Steam While steam has been extensively used for soil disinfestation for over a century (137), its application for weed control has been realized only recently (122). This tool is environmentally safe and poses no health hazard. Furthermore, unlike non-specific burning and selective flaming, there is no danger of starting uncontrolled fires. Exposure to superheated steam (200º C) for two to four seconds has been shown to kill a variety of weed seedlings (122). The extent of injury depends on weed species, steam temperature, duration of exposure, and plant size. Plants, particularly perennial weeds, may regenerate and repeated exposures may be necessary. Fecundity of the weeds that survive may be significantly reduced. Exposures as brief as 0.1 to 0.2 second damage foliar epicuticular wax and cell membrane integrity. Steam (200ºC) also kills seeds, with the imbibed seeds generally being more susceptible. Seed coat and other seed coverings may offer some protection from steam injury. While steam is an effective and safe tool for weed control, the high cost of steam production, regeneration of weeds following top-kill, and unavailability of steaming equipment suitable for a variety of applications has limited its use (137). Any weed control operation using steam on agricultural land must consider any adverse effect on desirable insects and soil microorganisms. Ultraviolet radiation, microwave, and electrocution The potential for using UV-radiation for weed control has been demonstrated (131). Weeds were damaged due to heating of foliage caused by absorption of UV radiation. Exposure to UV-radiation (10 and 100 GJ/ha) under greenhouse conditions severely reduced fresh weight of shepherdspurse (Capsella bursa-pastoris (L.) Medicus), common groundsel (Senecio vulgaris L.), small nettle (Utrica urens L.), and annual bluegrass (Poa annua L.). The extent of UV-induced damage was influenced by weed species, stage of plant growth, and the height of UV lamp above the canopy (131). Before this control option can be implemented, its effectiveness under field conditions, influence on soil properties, and any other unwanted side-effects (e.g. mutation) must be investigated along with analysis of costs and risks (e.g. fire hazard). Exposure to radiofrequency energy (e.g. microwave) causes dielectric heating of plant tissues. This heating involves conversion of electromagnetic energy into heat. Investigation of relationship between the duration of exposure to microwave radiation, increase in soil temperature, and seed kill suggested the potential for use of this treatment for soils containing weed seeds (138). Microwave treatment that raised and maintained soil temperature above 80º C for 30 seconds was adequate for germination inhibition. While use of this option under field conditions to kill weed seeds and seedlings is far from becoming a reality, its use to kill seeds in small amounts of soil is a possibility. Effects of this treatment on soil chemistry and biology also need to be investigated.

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Electrocution has been tried for weed control (139, 140). Electric current is passed through the plant and the tissue is heated due to its electrical resistance and killed. The equipment consists of a generator, a transformer, and an electrode, and rolling coulters (140). Although electrocution was first tried for weed control more than 100 years ago, the method has not become popular because the weed control operation is slow, energyintensive, and costly (140). Furthermore, it uses fossil fuels.

Fertilizer spray Presence of leaf epicuticular wax in cole crops and the upright leaf orientation in crops such as onions (Allium cepa L.) repel foliar-applied solutions. Foliar application of fertilizer solution (e.g. ammonium nitrate) has been used to selectively kill foliage of weeds [e.g. redroot pigweed (Amaranthus retroflexus L.), shepherdspurse, and common chickweed (Stellaria media (L.) Cyrill.] (141, 142). The amount of nitrogen sprayed on the foliage is lower than the crops’ requirement for this nutrient. Side-dressing of additional nitrogen is therefore necessary for optimum crop growth. The fertilizer applied to foliage is eventually washed into the soil by irrigation or rain and becomes available for the crop growth. Weeds with abundant foliar epicuticular wax [e.g. common lambsquarters (Chenopodium album L.) and annual bluegrass] are not controlled (141, 142). The susceptibility of a species to foliar fertilizer spray depends on the amount of foliar epicuticular wax, which in turn is depends on climatic conditions (e.g. sunlight and ambient moisture status) (143, 144). Optimum selectivity is achieved when sunny and dry conditions prevail. Climatic dependence and corrosive effect of the fertilizer solution on application equipment have limited the usefulness of this control option.

Night-time cultivation Regulation of seed germination in some weedy species is phytochrome-mediated. Seed germination in these species is stimulated by red and inhibited by far-red light. Seeds buried in soil and those lying under a dense foliar canopy, that preferentially absorbs red light, do not germinate until their light requirement is satisfied by bringing the seeds to the soil surface (e.g. by cultivation) or removing the canopy cover (e.g. by grazing or mowing). Some of the buried seeds could be exposed to sunlight for a very short duration (a millisecond) during soil disturbance associated with tillage. It has been shown that such short exposures are sufficient to stimulate germination of light requiring seeds. Greater weed seedling emergence was found following daytime cultivation compared to cultivation during night. Seedling emergence decreased when the area subjected to daytime tillage was covered during cultivation to exclude the light. Seedling emergence increased when an area was artificially illuminated during nighttime cultivation (145). The seeds that were brought to the surface and left there would have germinated in both cases, but the seeds exposed for a millisecond to light only during the soil disturbance associated with tillage and then reburied would germinate only when the field was tilled during the daytime. These observations have interesting implications for weed management in that the light condition during cultivation could influence weed abundance and composition and seed bank depletion. It has been reported that the land tilled at night continuously for six years had significantly lower weed abundance compared to the adjoining strip cultivated during the daytime (146).


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However, the authors suggested that not all of the cultivation associated with seedbed preparation should be done at night. Most tillage operations could be done during the bright daylight so that the seedlings emerging in response to light-induced seed germination would be killed by subsequent cultivation operations. The last tillage operation before sowing, however, should be done in the night to prevent more seeds from germinating in the crop. Although this weed management option is rather attractive, further research is needed to answer some important questions. For example we need to know: i) would repeated night-time cultivation select in favor of weed species that do not require light for germination; ii) what is the fate the seeds that fail to germinate, would they eventually lose their light-sensitive dormancy and germinate anyway; iii) how much light could be tolerated during the night-time cultivation; iv) where cultivation machinery provides shade, would additional artificial light during cultivation enhance germination; v) whether it is possible to increase the effectiveness of night tillage by smoothing the disturbed surface (by using a roller) before dawn to minimize light penetration in the soil; vi) the economic feasibility and effectiveness of this option; and vii) the impact of this option on the reduction of herbicide use for weed control. In addition to the weed control tools discussed above, many other non-chemical alternatives are available in a weed scientist’s tool box. As public concern for health and environment grows, our reliance on herbicides for weed control should decline. This may enhance research on non-chemical weed control options making approaches considered too costly and impractical at one time the tools of choice in the future.

Current trends in the adoption of a more holistic approach It is interesting to see how a shift in the contents of the Weed Science journal from 1999 to the end of 2003 reflects the trend towards a holistic approach (Fig. 2). Weed Science divides the core articles (i.e. not including special topics and other miscellaneous papers) into 4 categories: 1) physiology, chemistry and biochemistry, 2) weed biology and ecology, 3) weed management, and 4) soil, air, and water. Of these, papers in category 1 and 2 are generally focused on herbicide issues, although category 4 deals with herbicides at the more holistic level of their impact on the environment. Category 3 also deals with herbicides to some extent, but usually as only one component of the management issues. Papers in category 2 frequently do not even mention herbicide issues, and many articles are concerned with development of a more in-depth understanding of weed biology and ecology in pursuit of a systems approach to weed management. Given the nature of these categories, it is interesting to note that although there are no clear trends through the five years, the “weed biology and ecology” category (category 2) always featured the highest number of articles, peaking in 2003 with 67 out of the 130 articles published in total. Category 1, which includes many articles dealing with physiological responses to herbicides, by contrast, was relatively low, ranging from 19-30% of the articles in a given year. It should be noted that the WSSA also publishes “Weed Technology” which tends to focus on issues similar to those in categories 1 or 3 in Weed Science, but fewer articles are published in Weed Technology than in Weed Science.

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80 70

Physiology, chemistry and biochemistry

Articles

60

Weed biology and ecology

50 40

Weed management

30 20

Soil, Air, and Water

10 0 1999

2000

2001

2002

2003

Year

Figure 2. Number of articles in the four main categories in the journal Weed Science from 19992003.

Shifts have also taken place in terms of institutional structures. The term “agroecology” (or similar) is replacing “agronomy” in the naming of college departments in the US and Canada, and along with it programs to deliver ecological approaches to agronomy (147). Basic weed science textbooks (e.g. 1) no longer focus on “how to kill a weed” but include many other facets of weed management. Furthermore, other texts are being developed to address the interface between ecology and weed science (e.g. 113). This means that the weed scientists of tomorrow are being instilled with a more holistic rubric of weed management than their predecessors. There are many signs of progress on the road to a more holistic approach. However, one must ask to what degree are the holistic designs of weed management systems developed by researchers being adopted by agricultural producers? While this issue is beyond the scope of this article, it is important to point out some key difficulties at the interface between researchers and practitioners. The ability to communicate and organize information is a major factor in this regard. Buhler et al. (148) suggested: “The progression from a focus on tools targeted at a single population in a single year to the adoption of a holistic approach to crop and pest management will require analysis, theory, and information to support implementation at the ecosystem level. Such systems must also consider the interactions of long term management practices, populations of crops, and populations of pests, beneficial, and innocuous organisms.” Less informationintense systems are easier to develop. It is relatively straight-forward for a herbicide company to develop a product, test it with the aid of academic researchers, then put it into the hands of producers along with a simple formula for its application, i.e. use rate and timing. It is difficult to compete with such approach by offering a whole IWM system, that involves varied components that must work synergistically, e.g. crop planting density, crop cultivar, modified methods of fertilization, a careful census of weed populations, banding the herbicide, timely cultivations, encouragement of beneficial insects, use of computer models, etc. For instance, two alternative no-tillage weed management systems involving cultivation and reduced use of herbicides were


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tested (149). Although the economic analysis revealed that these systems were comparable to a conventional system, the extensive input of information involved in the experimental study might preclude ready adoption by producers. One means that has been used to overcome the “information barrier” is the development of weed management decision models to attempt to provide access to relatively complex information on weed impacts on crop yield (150, 151, 152, 153). After a review of these models in 2002, one of the conclusions made was that although ostensibly designed as tools used in management, often the main purpose became more academic, i.e. to serve an education function (154). Despite the potential problems of information overload, it is becoming evident that the complex problems of agriculture require better integration of all available multidisciplinary knowledge. New approaches attempt to encompass aspects of agriculture not normally considered in field-plot research, such as the dynamics at the landscape level (155), the evolutionary ecology of weeds (25, 27), and the complex social ecology of pest management issues (156). The challenges that these three trends pose to weed scientists in the early 21st century are discussed below.

Three paths to greater holism in weed science Through much recent agricultural history, agriculture has been reduced to the boundaries of a single agricultural field. Within this reductionism, experimental plot approaches were thought to capture all that was necessary to know about the agricultural system. When a landowner works with a whole network of agricultural and/or nonagricultural fields, the isolated experimental plot no longer represents the reality accurately. It is argued that with a landscape approach, the inputs (e.g., herbicides) inevitably have impacts beyond a single field (155). One obvious target for further research in weed dynamics is the field margin. While some progress has been made in understanding how to optimize field margins for the sake of weed management in Europe (157, 158), there is a dearth of information from other areas of the world, including North America. Weed scientists are starting to think beyond the margins. The issue of invasive species has become a major public concern since the late 1990s (159, 160), and thus the need to track movements of weeds at a variety of scales beyond a single field is being recognized. In summary, greater spatial awareness needs to be cultivated, as the movements of components of weed management systems such as herbicides, herbicide residues, transgenic seeds, and weeds themselves are seen to go beyond the mythical isolated agricultural field. In addition to an extension of the spatial frame of reference, greater holism also requires extension of the temporal frame of reference. Standard weed science studies take place over two to three seasons to account for variability in climate. However, ecological and evolutionary timescales are longer, and we are slowly realizing the importance of processes at these longer timescales. For example, the timescale of weed studies has been expanded to include persistent weed seed banks to help manage weeds beyond the annual cycle (161). Successional dynamics represent an even longer ecological timescale, and weed scientists are likewise beginning to look at the role of successional dynamics in shaping weed communities (162, 163). There is also a need to recognize changes on an evolutionary scale; weeds are very adaptable plants that have been observed to undergo evolutionary change as they adapt to new environmental

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conditions that we have created. For example, in North America, physiological changes have been documented among some weed species that have expanded their ranges northward in response to northward expansion of certain crops (164). The development of a better understanding of weed evolution may include addressing conflicting selection pressures on weeds in agroecosystems, feed-back driven dynamics of human-weed coevolution, co-evolutionary mechanisms of weed adaptation in conjunction with other weed species or organisms, and the role of weed evolution in the restoration of agroecosystems (25). The third path to greater holism requires incorporating more than just the natural sciences into the equation of agroecosystem health, including weed management aspects (164). Weed scientists find themselves hard-pressed to apply weed management solutions if rural social systems are breaking down. Large-scale changes have been occurring in North America, as manifested in changing patterns of land tenure that stem from economic difficulties. Multidisciplinary approaches that attempt to include economic and social considerations into technological development have been advocated (21). Institutional systems in place to support agricultural research are in general oriented too narrowly to allow such innovative approaches (147). Thus, the way forward on this third path is difficult, but weed researchers must attempt to take a few steps forward in pursuit of greater holism in the face of the numerous challenges faced in weed management.

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