PART I:  INTRODUCTION

Weeds are undesirable plants that interfere with the quantity and quality of products and services provided by ecosystems.  Traditionally, weed management has focused primarily on interventions with various control tactics such as herbicides, tillage, and biological control agents with the intent of eliminating the weedy species while minimizing damage to desirable species.  Reliance on any single tactic often results in selection for and development of weed populations that escape control and threaten ecosystem productivity.  A systems approach using multiple tactics can permit a more comprehensive program that is more effective in the short term and more consistent in the long term.

In addition to tactics to eliminate current weed infestations, an important component of weed management systems is development of preventive management strategies.  Various preventive strategies can be designed into the ecosystem to minimize opportunities for weedy species to invade, establish, and multiply.  This would include use of components of the system itself such as crop rotations, cover crops, competitive crop cultivars, soil fertility management, and rangeland grazing intensity that could be manipulated to deny weed populations a niche in the ecosystem.  The goal is development of an environment that negatively impacts any or all stages of the weed life cycle including seed survival, establishment, growth, and reproduction.  Knowledge of weed biology and population dynamics and their response to crop and soil management will provide important inputs for developing optimum long-term weed control strategies. 

Development and assessment of effective weed management systems require a broad base of knowledge relating to plant biology and ecology, soil physical and biological processes, environmental resource management, economics, and rural sociology.  Establishment of multidisciplinary collaborations is essential to provide this knowledge base and facilitate the broad vision that is required for systems research.  Partnerships with members of the local agricultural community will be essential to provide relevance to research goals and facilitate implementation and adoption of new management systems.  Through effective collaborations, a complete assessment of management objectives will include not only an assessment of weed populations but also factors such as productivity, profitability, soil resource maintenance, environmental protection, and influences on local community welfare.  Estimation of tradeoffs will be essential to balancing the benefits and liabilities of alternative systems.  Primary importance will be given to the long-term stability and sustainability of systems.

PART II.  PROBLEMS TO BE ADDRESSED

A.  Cultural and Mechanical Control

Problem Statement
Rationale.  Before herbicides, weeds were managed primarily with cultural and mechanical practices.  The successful development of herbicide technology has reduced the use and knowledge of cultural and mechanical weed control practices in recent decades.  Although herbicides will continue to be the dominant technology in weed management programs, several problems have arisen from reliance on herbicides including herbicide movement to nontarget areas, environmental contamination, and development of herbicide-resistant weeds.  Also, the potential environmental and economic benefits of organic farming have renewed interest in weed management systems based on nonchemical tactics.  Accordingly, there is a need to broaden our potential weed control alternatives by developing and refining cultural and mechanical control practices.

What is known.  Research on individual cultural or mechanical practices usually demonstrates some level of weed suppression but not consistent or complete weed control when used alone.  Cultural or mechanical practices often must be integrated with other control practices to develop effective weed management systems.  For centuries, tillage has been one of the most widely used practices to prepare land for planting.  Although effective for clearing fields of existing vegetation and preparing a seedbed, tillage also predisposes many weed species to germination, disperses weed seed and aquatic weed propagules, and can lead to sediment losses.  Developing a proper balance and integration of tillage with other practices continues to be a goal of weed scientists.

Gaps.  Because of the perceived lack of efficacy of many cultural and mechanical practices, they rarely have been the primary focus of research and often are relegated to the role of a supporting factor.  Research is needed to define the degree of weed control that specific practices afford, the weed species that are most susceptible, and the conditions that affect efficacy.  Through a better understanding of the processes by which cultural/mechanical practices disrupt weed populations, these practices can be manipulated more effectively.  Effective practices need to be integrated into weed management systems such that their strengths contribute to the system and the system compensates for their weaknesses.  Potential negative impacts from cultural and mechanical practices need to be clarified and mitigation processes established.  These options will be particularly important for organic growers who cannot rely on chemical technology.

Goals

• Determine the efficacy and liabilities of cultural practices for controlling weeds including crop rotation, cover and companion crops, crop population and planting arrangement, crop competitive traits, soil amendments and fertility management, and livestock grazing intensity and frequency.
• Determine the efficacy and liabilities of mechanical methods for destroying weeds including tilling, mowing, dredging, and burning.
• Use knowledge of weed biology and population dynamics and their response to cultural and mechanical practices to develop short-term and optimum long-term weed management strategies that reduce weed populations.

Approach
Conduct field experiments to assess the efficacy of cultural and mechanical weed management practices.  Long-term studies will be needed to determine crop rotation sequences and soil management practices that best disrupt weed populations and minimize population levels.  Research will emphasize separating the effects of tillage from effects of cropping sequences in assessing long-term impacts of cropping systems on weed populations.  Evaluate nonchemical practices for weed management in organic production systems.  Investigate processes that define the mechanisms by which cultural and mechanical approaches suppress weeds.  Assess cultural and mechanical practices with and without chemical control for efficacy, profitability, and environmental impacts to determine potential tradeoffs between alternative weed management strategies.

Outcomes

• Effective, practical cultural and mechanical weed control methods that reduce or eliminate the need for chemicals
• Cropping systems and rotations that minimize weed population levels
• Mechanical practices that provide effective weed control, minimize negative impacts on soil quality and erosion, and reduce the dispersal of weed seeds and other propagules

Impact
Improved weed management systems that incorporate effective, profitable cultural and mechanical weed control practices

Linkages to Other ARS National Programs
Crop Production (305)
Integrated Agricultural Systems (207)
Rangeland, Pasture, and Forages (205)

B.   Integrated Weed Management in Cropland

Problem Statement
Rationale.  Weeds compete with crops for plant resources and interfere in many indirect ways with crop production.  Weed science research has successfully developed numerous tactics that control weeds and reduce crop losses on an annual basis.  However, many tactics that are relied upon as single control measures, such as herbicides or tillage, can lead to adverse consequences including development of herbicide-resistant weeds, contamination of water, and soil erosion.  The dynamic nature of weed populations can result in development of problem weed species that are adapted to any given control tactic.  A systems approach is needed to address these weed population shifts, minimize reliance on any single tactic, and reduce overall negative impacts on the environment.

What is known.  Weed management in cropping systems traditionally has centered on individual weed control tactics.  Extensive research has been conducted on herbicide technology, biological control, and cultural practices for controlling weeds, often in isolation of each other.  Various nonchemical approaches such as cover crops, mechanical cultivation, competitive cultivars, and biological control agents have been found to provide various levels of weed suppression but often are inadequate to provide acceptable and consistent weed control by themselves.  Bioeconomic models have been developed that incorporate information on weed biology, weed-crop competition, and various management and environmental parameters that allow selection of appropriate herbicides; however, these models are limited in the scope of potential integration and the region of applicability.

Gaps.  New weed management systems are needed that can be integrated with and address national concerns such as ecosystem health, water quality, and sustainability of the soil resource base.  In particular, there is a need to focus on integrating system components and understanding how systems function.  There is a need for research on linking weed management with soil management, nutrient management, cultivar selection (including transgenic crops), and other cultural practices that can influence weed populations.  Long-term effects of crop management systems on weed population dynamics,  in both space and time, need to be assessed.  New weed management systems ultimately must address multiple national priorities relating to production, time/labor constraints, economics, and environmental concerns, not just traditional weed control alone.  New partnerships are needed with other scientific disciplines, as well as agricultural community leaders to facilitate development of effective weed management systems.

Goals

• Determine interactions among weed control tactics (e.g., herbicides, tillage, crop populations and patterns, biologically-based practices, transgenic crops) and identify synergistic combinations that optimize weed control.
• Integrate crop rotations and cultural practices with appropriate weed control practices to maximize disruption of weed populations and minimize the need for interventions by herbicides and/or mechanical practices.
• Develop analytical approaches for understanding the processes that influence system behavior and build models/decisions aids to select and implement the most appropriate management approach for given weed populations and production goals.
• Develop technologies and systems to identify and manage the spatial variability of weed populations.

Approach
Conduct field experiments to determine synergistic combinations of weed management practices.  Long-term and on-farm cropping systems trials will be conducted to determine long-term weed population responses to crop (including transgenic crops) and soil management practices.  Participation in interdisciplinary teams will be critical to understand how crop management systems influence important biotic, soil, and climatic determinants of the abundance and composition of weed populations.  Utilize basic biological and ecological information to develop models that predict short and long-term consequences of alternative management approaches.  New technologies such as remote sensing, GPS, GIS, and spatial statistical methods will be adapted to describe the spatial patterns of weed populations and understand how spatial dynamics contribute to temporal weed population dynamics.

Outcomes

• Improved efficiency of inputs and resources, including time and labor, used for managing weeds
• Minimization of the number and cost of operations specifically required to control weeds
• Reduced weed population levels and minimal risk of long-term population fluctuations
• Models and decision tools to manage weed populations and crop production

Impact
Crop management systems that are agronomically productive, economically viable, environmentally safe, and socially acceptable

Linkages to Other ARS National Programs
Crop Production (305)
Integrated Agricultural Systems (207)
Soil Resource Management (202)
Water Quality and Management (201)

C.  Integrated Weed Management in Noncropland

Problem Statement
Rationale.  Noncrop weed management encompasses a variety of sites and uses ranging from right-of-ways to natural aquatic and riparian habitats to rangelands.  In these sites, weeds interfere with the direct uses such as desirable range species, water storage and conveyance, access to roads, utilities, commercial and recreational uses of waterways, and flood control.  In addition, the incursion and spread of exotic invasive weeds impair critical habitat for range and native plants and may significantly reduce species diversity.  This impact is at the ecosystem level and usually is shaped by biogeographical limitations rather than  political or economic boundaries (i.e., farms, counties, townships, or states).  In contrast to croplands, there is usually far less ability in noncropland to control the important resources that drive growth, reproduction, dispersal, and longevity of weeds, as well as those conditions that affect competition between weedy species and desirable vegetation.  Also, unlike cropland, the goals of weed control in noncropland may not be related to the production, harvest, and marketing of commodities.  Therefore, integrated approaches must be based firmly on an understanding of both the intended site uses and ecosystem-level functions.

What is known.  Weed management approaches in noncrop systems have relied on the independent use of three basic direct approaches:  herbicides, mechanical removal, or biological control.  Indirect approaches to weed control also have been used, such as grazing management.  Although many successes have been achieved, noncrop ecosystems usually respond by shifting from one problem weed species to another as each species is sequentially controlled.  Invasion by exotic weeds can result from disturbances by natural perturbations or weed control activities, often with lag-phases of several years for reasons that are not always clear.  Because of the activities of people and animals (e.g., waterfowl), dispersal of weeds into noncrop sites is widespread and mostly unregulated.  Unlike seeds for crops, which often must be certified “weed free,” there are no such constraints on sale, establishment, or production of most plants used in noncrop settings. 

Gaps.  New weed management systems are needed that utilize and account for ecosystem functions.  Specifically, existing methods (biological, herbicides, mechanical, habitat manipulation) need to be applied based upon better information about plant-plant interactions, dispersal mechanisms, physiological controls of germination and dormancy, and characteristics of seed and vegetative propagule banks.  Influences of long-term, large-scale physical drivers such as temperature, day-length, soil moisture, aquatic sediment influences, and light quality need to be clarified.  Interactions between biological agents, target plant characteristics, and ecosystem processes need to be understood, particularly pertaining to survival and reproductive success of selective agents.  New, efficient GIS-based survey and detection methods need to be developed for large noncropland areas, which are complicated by difficulties pertaining to on-ground detection, surveying, and monitoring plant populations.  Since weed impacts in these types of sites are not easily quantified, better methodologies are needed to assess the influences of weeds on native populations, socioeconomic values, or other measures that can be used to determine when weed management objectives are achieved.

Goals

• Develop improved GIS-GPS-linked methods to detect  new infestations, quantify spread and dispersal of weeds, and assess management success.
• Integrate multiple weed control strategies to incorporate ecosystem-scale functions and appropriate spatial and temporal scales.
• Develop methodologies to assess the impacts of integrated approaches on the target ecosystems, such as transformation and movement of herbicides, fate of newly open habitat, influences on further spread and dispersal of the target weed or other likely invading species.
• Quantify the impacts of noncropland weeds on natural biota, including measures of species diversity, ecosystem stability, and economic losses and gains.
• Develop ecosystem-level methods to assess impacts of biological control agents and, conversely, the influence of ecosystem components on the establishment and efficacy of biological control agents.
• Enhance ecosystem stability by providing conditions that discourage incursion by nonnative or undesirable plants.

Approach
Employ a multiplicity of tools, including habitat manipulation (e.g., water level or flow), biological control, herbicides, introduction of beneficial plants, and controlling human impacts (e.g., introduction of exotic plants) to develop fully integrated weed management.  Identify potential new “signals” and new detection systems for invasive plants and their propagules (e.g., visual and UV, IR spectrum, acoustical and other signals).  These signals will be linked in real-time to GPS/GIS systems.  Determine the interactions of various specific weed control methods and optimize their combined uses to meet management objectives.  Establish large-scale (areawide, region-wide) management programs to identify impacts of weeds on ecosystem functions.  Determine optimal  methods, appropriate species, and long-term practices as part of revegetation programs to prevent reinfestations by nonnative plants.  Form partnerships to balance the typically multiple goals of landowners, government managers, recreational users, and others that represent interests in these ecosystems.  Coordinate biological control release programs among ARS laboratories through teams of weed scientists, entomologists, and fish and wildlife scientists within ARS and in other resource agencies so that ecosystem effects can be evaluated thoroughly.  Establish teams of economists, sociologists, weed scientists and state and federal regulators to develop optimal environmental stewardship practices that will minimize introductions of nonnative plants.

 Outcomes

• Long-term reduction of impacts by noncropland weeds
• Reduced reliance on single-approach strategies, such as biological control, herbicides, or destructive mechanical approaches
• Improved detection and survey methodologies
• Heightened clientele and public awareness of appropriate practices to ensure sustainable noncropland ecosystems

Impact
Viable and sustainable noncropland ecosystems that support multiple uses and that can be managed economically with minimal environmental damage

Linkages to Other ARS National Programs
Rangeland, Pasture, and Forages (205)
Integrated Agricultural Systems (207)
Animal Health (103)
Water Quality and Management (201)

D.  Rehabilitation, Revegetation, and Restoration

Problem Statement
Rationale.  Once an invasive weed has been controlled in a parcel of land, the gap created in the biological community may be usurped quickly by other species.  In many instances, these gaps are filled by the same or equally pernicious species, with the end result being a community with as much cover of invasive species as the original, precontrol community.  Thus, true rehabilitation of a community previously infested with one or more invasive species must include revegetation with other plant species and possibly restoration to the community’s natural vegetation.

What is known.  A good deal of information is available on the response of weed species to herbicide and mechanical treatments and on some aspects of the basic biology of weedy and desirable species.  Numerous revegetation trials have been conducted over the past 50 years.  However, much of the revegetation work has not been mechanistic in nature, and thus reasons for success or failure have not been well documented.

Gaps.  Research is needed to identify timing, depth, and extent of soil resource use by individual weedy and desirable species.  There is also a need to better understand the physiology and life history of key weedy and desirable species.  Since ARS resources will not be sufficient to study every important weed, species should be sorted into functional groups.  A basic understanding of niche occupation and life history cycles will be necessary to design control and revegetation strategies that will be sustainable in the long term.

Goals

• Develop profiles of soil resource use by critical weedy and desirable species.
• Develop criteria by which to classify important weedy and desirable species into functional groups.
• Use information on niche occupation and functional grouping to develop and test species mixtures to compete with specific weeds.
• Identify causes of reinvasion in cases where initial weed control and revegetation appeared successful.

Approach
Niche occupation and life histories of important weedy and desirable species will be determined in a variety of environments.  A combination of new and published information will be used to design and test revegetation and restoration strategies that have a high probability to resist reinvasion by weedy species.

 Outcomes

• Restoration strategies, including combinations of weed control and revegetation, that successfully restore land productivity and biological diversity

Impact
Restoration and improved management of lands owned by public entities, nonprofit organizations, and private individuals

Linkages to Other ARS National Programs
Rangeland, Pasture, and Forages (205)
Integrated Agricultural Systems (207)
Soil Resource Management (202)
Water Quality and Management (201)