Herbicide-resistant crops (HRCs) have been used extensively during the past five decades, because crop tolerance is necessary for a selective herbicide. In the past, chemical company personnel screened a large number of organic compounds against a limited number of major crop and weed species to discover HRCs. Now a single herbicide can be evaluated for crop selectivity against a great number of biotypes of a single crop in tissue culture, or sometimes gene transfer can be used to produce tolerant genotypes. Thus, one can now choose an environmentally “benign” herbicide and search for or develop resistant crop cultivars. The advantages of these new HRCs are: (a) solutions may be more easily found for difficult weed management problems, (b) growers are more apt to use integrated weed management when they have effective backup weed control procedures if cultural or mechanical methods fail, (c) increased weed management options for growers of minor crops, (d) economic advantages to growers, and (e) increased use of more environmentally “benign” herbicides. Concerns about HRCs include: (a) these weed management procedures do not eliminate herbicides, (b) some believe biotechnology should not further herbicide use, (c) HRCs may become weeds, (d) this approach may facilitate less ecological diversity, (e) HRCs may increase herbicide use, (f) it could result in more herbicide carryover by facilitating the use of environmentally “harsh” herbicides, (g) HRCs may be less competitive to weeds, and (h) dependence on HRCs may decrease the weed control practices that a farmer uses. One needs to consider both the benefits and risks of HRCs to properly evaluate this technology.

Weed management is an integral part of agriculture; weeds lower both productivity and quality of agricultural products. A combination of mechanical, chemical, biological, and cultural methods is expected to deliver a sustainable weed management program for the next two decades. While chemical methods offer the most cost effective means of weed management, crop selectivity has hampered the use of the best chemicals for weed management. Recent progress in gene technology has facilitated the introduction and expression of genes to confer a wide range of traits to crop plants. Application of this technology has resulted in the development of crop plant genotypes that are resistant to a specific herbicide. This article describes the progress that has been made by our group toward the introduction of glyphosate tolerance to crop plants. Glyphosate [N-(phosphonomethyl)glycine] kills plants due to inhibition of the biosynthesis of aromatic compounds via the shikimate pathway. Our approach for introduction of glyphosate tolerance is based on insertion and expression in plants of a gene encoding a glyphosate-tolerant 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase, a key enzyme of the shikimate pathway. The wild type enzyme present in plants is susceptible to inhibition glyphosate; variants of EPSP synthase have been produced that are less susceptible to inhibition by glyphosate. Expression of genes encoding these variants has been shown to confer glyphosate tolerance to plants. The degree of glyphosate tolerance is related to the tolerance characteristics of the EPSP synthase variant, its substrate activity, targeting to the plastid, and the level of expression of the variant gene. The tissue specificity of expression of the variant EPSP synthase has also been shown to be critical since glyphosate is a systemic herbicide and is translocated to many growing points within the plant. Our studies on glyphosate tolerance have substantially enhanced our understanding of the mode-of-action of glyphosate, the shikimate pathway, and protein sorting within plant cells, as well as developmental and tissue specific expression of genes in plants. Commercial use of glyphosate tolerance technology is expected to affect positively, the weed management arsenal available to the farmers, the sustainability of farm land and groundwater, and promote the use of a “soft” herbicide.

There are three primary activities that characterize the discipline of Weed Science. These activities are weed technology, weed biology, and the ethics of weed control. Each of these activities needs to be considered as herbicide-tolerant crops (HTCs) are introduced. HTCs are the most recent refinement in the existing technology to control weeds. The potential benefits from the improved weed control must be weighed against possible increased costs of production and potential for genes that control herbicide tolerance to escape into non-tolerant plant populations. These questions about herbicide resistance are primarily technological and biological. They demonstrate the paucity of information in Weed Science on weed genetics, gene flow, fitness, and other aspects of weed-crop population dynamics. Other questions about HTCs are ethical. They require that we ask who benefits from the technology and what are the economic, ecological, and social consequences of it.

The primary aim of seed companies is to provide products that the customer wants to buy, and to sell them at a profit. Future demands for improved cultivars of major field crops will continue to be primarily for those with reliably high yield and desirable quality. Herbicide tolerance will not be a primary determinant in cultivar choice for most customers. Therefore, seed companies probably will move with caution in development of herbicide-tolerant cultivars.

Development of herbicide-tolerant plants is the focus of considerable research. Some projects aim to increase herbicide use or promote use of particularly environmentally damaging chemicals, and thus may lead to environmental degradation. Other projects aim to develop herbicide-tolerant plants that allow substitution of newer less environmentally damaging chemicals for older more damaging ones. To the extent they divert research dollars from development of other weed control strategies, these projects may also jeopardize environmentally sound weed control. The paper concludes with policy recommendations concerning a) public sector research priorities, b) planting of herbicide-tolerant trees in forests, and c) regulation of herbicide-tolerant crops.

Technology is now available to create herbicide-tolerant crops. This technology, when combined with environmentally and toxicologically sound herbicides and when used wisely in an integrated weed management program, can contribute positively to crop production. However, it is unlikely that herbicide-tolerant crops will revolutionize weed management. Instead, they will complement existing weed management practices and provide additional options to growers. Adoption and success of herbicide-tolerant crops will be dependent on several technical, environmental, business, economic, and societal issues. Industries that are developing herbicide-tolerant crops need to openly communicate and discuss both the value that their technology brings to agriculture and society and the strategies for ensuring its responsible use.

The regulation of transgenic plants is at the very early stages of dialog between the U.S. Environmental Protection Agency (EPA), the Food and Drug Administration (FDA), and the U.S. Department of Agriculture (USDA). Herbicide resistance is discussed in the broader terms of resistance management and of the cooperative efforts underway involving governments, industry, academia, and the environmental community on an international level. A new body, the International Organization for Resistant Pest Management (IOPRM) was formed to implement pesticide resistance management programs worldwide. The research base for integrated pest management programs which form the context for pesticide resistance management programs is in need of expansion. Public and private efforts to expand this base converge in an ambitious collaborative program between the U.S. EPA and USDA entitled the National Integrated Pest Management Forum. Finally, the potential role of EPA in the regulation of resistance management is discussed.

Weed resistance has not generally been considered a serious problem where herbicide rotations and combinations are used. Herbicide-tolerant crops present new opportunities for decreasing risk of crop injury, decreasing carryover problems, broadening control spectrum, and for using herbicides that present less risk to the environment. However, herbicide-tolerant crops that allow intensified use of some herbicides may allow tolerant weed species to proliferate and herbicide carryover problems to increase for certain crops. Those responsible for the technology transfer process will need to keep well informed and objectively provide clientele with the basis for appropriate decisions as new weed control systems are designed as a result of these developments.

The development of crop cultivars with resistance to selected herbicides has the potential to impact environmental quality, food safety, consumers, and crop producers in either a positive or negative manner. The technology that makes it possible to develop herbicide-resistant crops is neither good nor bad, it is rather how the products of this technology are used that will determine whether or not the introduction of herbicide-resistant crops is ultimately a good or bad decision. The introduction of herbicide-resistant crops will have diverse impacts leading to redundancy, diversity, and confusion in crop production systems. Often the introduction of herbicide-resistant cultivars will have the same impact on cropping systems as the introduction of a new herbicide that has the same mode-of-action and use pattern of herbicides already in use. This may add diversity of herbicide options for a given crop but will cause redundancy of product use over several years. This redundancy could lead to weed resistance and water quality concerns. Confusion at the user level will exist because not all cultivars of a crop will be resistant to the herbicide; this could be the major deterrent to widespread adoption of herbicide-resistant crops. Steps must be taken to provide information to crop producers that will insure that herbicide-resistant crops are used effectively and safely. Weed scientists will determine whether this technology will be used to improve food safety, water quality, crop production systems, and farmer profitability or have a negative impact on agriculture and the whole of society.

At the 1990 meeting of the Weed Science Society of America (WSSA) in Montreal a Symposium titled “Pesticide Transport to Groundwater: Perspectives and Field Methodology” was Chaired by Thanh H. Dao. That symposium made it clear that simulation modeling is being used more and more to integrate the field test data generated by pesticide companies as part of the environmental impact assessment required for pesticide registration with the Environmental Protection Agency (EPA). At the Soil Aspects Section Business Meeting it was decided that a Symposium on the state of the art of this technology would be timely and the authors agreed to organize it.

Computer based simulation models are increasingly being used to predict the environmental fate of crop protection chemicals. Some considerations that need to be given in selecting appropriate models for regulatory purposes include model applicability, validation, capability, ease of use, and documentation. Problems commonly encountered in modeling include limited accuracy, lack of defined objectives and standard modeling practices, and misuse of models and results. Models will continue to play an important role in the regulation of crop protection chemicals. It is important that regulators and industry agree on appropriate models and practices, and that regulatory decisions are not based solely on model results but take into account all available data.

The use of models for regulatory purposes requires accurate descriptions of important chemical fate processes, techniques for applying models for specific regulatory issues, techniques for parameterizing the model, and sufficient model distribution and support. Researchers (i.e. model developers), on the other hand, want to conceive more and more complex, sophisticated models in order to better represent the important environmental processes; unfortunately this invariably tends to lead to models with greater data requirements, parameters that are difficult to evaluate, greater expertise needed for application, and greater computational requirements, all of which makes for more difficult model application and use. The development of usable models for both current needs and future regulatory requirements demands that we achieve a balance between these ‘research needs' and ‘user needs'. Software engineering and code development issues can help to establish the needed bridge between research and user needs so that future models can more quickly escape the research arena and satisfy user needs as effective regulatory tools.

The movement and degradation of agricultural chemicals in soil and groundwater is a complex process, influenced by climatic conditions, site characteristics, chemical properties, and agricultural practices. Because of this complexity, mechanistic models are useful tools in evaluating environmental fate. Although currently available unsaturated zone models often do not provide accurate concentration profiles when compared with field measurements, they can be used to estimate the depth and extent of movement. Another application of modeling is to compare predicted movement under different simulation conditions to determine the significance of the various factors affecting the behavior of agricultural chemicals. When properly designed and interpreted, modeling studies can contribute to the understanding of the environmental fate of agricultural chemicals. When necessary, modeling can also be useful in developing management practices included on the label or in state regulations. Modeling has been a frequently used tool in developing restrictions for minimizing the movement of aldicarb residues into groundwater and preventing residues in drinking water. Examples of model use include optimizing application timing, estimating well setback distances, and identifying vulnerable soils.

Predicting the off-target movement of agrochemicals applied to turf is an essential component for assessing risk to aquatic environments. For surface transport, a fully validated model does not exist. Such a model is required to make cost effective risk assessments and regulatory evaluations. To address this deficiency, a turf model validation program is outlined in this paper. The effect of turf characteristics on the fate and movement of pesticides is presented followed by the discussion of the selection for a proper turf runoff model. The design and implementation of small plot runoff studies and their usefulness in model validation are discussed. The procedures of model calibration and model verification are explained for model validation purpose.

Computer models designed to simulate the fate and transport of pesticide residues in surface runoff and water that recharges shallow aquifers are used at several stages in the regulation of pesticides. Models can be a powerful tool to identify environments in which pesticides are more mobile and are more likely to impact ground-water quality. Models can also be used to highlight pesticides that can contaminate surface or ground water at an early stage of the registration process. At present, models are utilized primarily as comparative tools to identify which compounds should be examined on a priority basis. Results may be used to calibrate numerical models and estimate the long term impact on the environment from the use of a particular product or its alternatives. To date, field monitoring studies have been required for over 40 chemicals by the Office of Pesticide Programs (OPP) to evaluate the leaching potential of pesticides under realistic usage conditions. As the capabilities of models improve and linkages are developed between databases and the models, computer simulation models will play an increasingly important role in the Environmental Protection Agency's (EPA's) efforts to manage pesticide use to prevent adverse effects on the environment.

A screening procedure has been developed to evaluate the relative loss of pesticides from soils, to assist the Soil Conservation Service in implementing water quality for conservation planning. This screening procedure is a first-tier evaluation of the impact of using a particular pesticide on a specified soil. The screening procedure was developed from evaluating pesticide loss from over 40 thousand runs of the GLEAMS (Groundwater Loading Efects of Agricultural Management Systems) model. The model input data were a combination of soils and pesticides with a wide range of properties. The estimated pesticide losses were categorized into losses by leaching, losses adsorbed on sediment in runoff, and losses in the solution phase of runoff. Algorithms using soil properties were developed to rate soils into four loss potential classes for leaching and three loss potential classes for the two categories of runoff. Also, algorithms using pesticide properties were developed to rate pesticides into four loss potential classes for leaching and three loss potential classes for runoff. The soil and pesticide groupings are combined in a matrix to give an overall loss potential rating. Statistics of the overall loss potential indicate the low loss potential class is pure, that is, it does not contain occurrences that have medium or high losses. The medium loss potential classes does not contain occurrences of high loss, but does contain many occurrences of low loss. The high loss potential class contains incidences of medium and low loss.

Do quasi-physically based models with more detail perform better than regression or other empirical models? This is a question that was raised many years ago and still remains. In an effort to respond to this question, the author reviews the needs and concerns of users and then divides the large number of models into three classes: (1) screening, (2) research, and (3) planning, monitoring, and assessment. Empirical and causal (physically based) models are contrasted and the advantages and disadvantages of each described. Sources of model uncertainty (properties of data bases, model structure, parameter estimation methods, algorithmic implementation, verification and validation, and future users) that lead to skepticism about models' performance are investigated. Simulation scale and spatial variability are also important considerations. The leading question is then discussed from the perspective of screening, research, and planning, monitoring, and regulatory models.

Two methods were used to obtain the sensitivity of chemical leaching depth to variations in the input parameters of the Pesticide Root Zone Model (PRZM). First a Plackett-Burman (PB) screening design was used to vary 35 PRZM inputs over seven ranges around a nominal value. Six of the seven ranges were approximately 0.1, 0.25, 0.5, 1.0, 5.0, and 15%, the seventh range was chosen to cover a range appropriate for a soybean herbicide applied preemergence in the Midwestern region defined by the USDA–SCS land resource region M. Next, Fourier amplitude sensitivity testing (FAST) was then used to vary from 19 to 25 parameters over four of the ranges previously tested. For the smaller parameter ranges the two methods typically gave equivalent results but the PB method required far fewer simulations. For the simulation of the Midwestern region where some parameter varied by larger amounts the relative magnitudes of the sensitivity coefficients obtained by the two methods were similar but the magnitude of the coefficients obtained using FAST were smaller than those obtained using PB.

The GLEAMS model (Groundwater Loading Effects of Agricultural Management Systems) is used to illustrate model application in evaluating potential pesticide runoff of two similar pesticides from one soil. This limited application was chosen for simplicity in illustrating relationships between annual means and single events. When using annual totals of simulated pesticide runoff for comparing two pesticides or assessing risks, long-term 50-yr simulations are preferable to short 10-yr simulations. When short-term simulations are performed, care must be exercised in selecting representative climatic periods. For short half-life pesticides, as demonstrated in this study, initial rainfall events on or near the day of application will often contribute most to annual pesticide lost. In these cases, single event analysis may be required. Procedures are demonstrated for expressing annual total pesticide losses and single rainfall event losses probabilistically in terms of expected recurrence intervals.