Dose-response studies are an important tool in weed science. The use of such studies has become especially prevalent following the widespread development of herbicide resistant weeds. In the past, analyses of dose-response studies have utilized various types of transformations and equations which can be validated with several statistical techniques. Most dose-response analysis methods 1) do not accurately describe data at the extremes of doses and 2) do not provide a proper statistical test for the difference(s) between two or more dose-response curves. Consequently, results of dose-response studies are analyzed and reported in a great variety of ways, and comparison of results among various researchers is not possible. The objective of this paper is to review the principles involved in dose-response research and explain the log-logistic analysis of herbicide dose-response relationships. In this paper the log-logistic model is illustrated using a nonlinear computer analysis of experimental data. The log-logistic model is an appropriate method for analyzing most dose-response studies. This model has been used widely and successfully in weed science for many years in Europe. The log-logistic model possesses several clear advantages over other analysis methods and the authors suggest that it should be widely adopted as a standard herbicide dose-response analysis method.

Advances in statistical software allow statistical methods for nonlinear regression analysis of dose-response curves to be carried out conveniently by non-statisticians. One such statistical software is the program R with the drc extension package. The drc package can: (1) simultaneously fit multiple dose-response curves; (2) compare curve parameters for significant differences; (3) calculate any point along the curve at the response level of interest, commonly known as an effective dose (e.g., ED30, ED50, ED90), and determine its significance; and (4) generate graphs for publications or presentations. We believe that the drc package has advantages that include: the ability to relatively simply and quickly compare multiple curves and select ED-levels easily along the curve with relevant statistics; the package is free of charge and does not require licensing fees, and the size of the package is only 70 MB. Therefore, our objectives are to: (1) provide a review of a few common issues in dose-response-curve fitting, and (2) facilitate the use of up-to-date statistical techniques for analysis of dose-response curves with this software. The methods described can be utilized to evaluate chemical and non-chemical weed control options. Benefits to the practitioners and academics are also presented.

A growing awareness of environmental issues in Canada has had a major influence on government policies. An initiative was launched by the government of Ontario to promote research toward the development of an integrated weed management (IWM) system. Research in IWM must take all aspects of the cropping system into consideration and evolve in a progressive manner. This approach must encompass the role of conservation tillage, knowledge of the critical period of weed interference, alternative methods of weed control, enhancement of crop competitiveness, modeling of crop-weed interference, influence of crop rotation and seed bank dynamics, and education and extension of the findings. The complexity involved in addressing these issues requires a multi-disciplinary approach.

Eight species of Tamarix were first brought to North America in the 1800s from southern Europe or the eastern Mediterranean region. Many of the species escaped cultivation and by the 1920s invaded about 4,000 ha of riparian habitat in the southwestern United States. By 1987, it was estimated to have increased to at least 600,000 ha. The success of saltcedar in the southwest can be attributed to several factors related to its growth habit, reproduction, water usage, ability to tolerate highly saline conditions, and redistribution of salt from deep in the soil profile to the soil surface. The flowers produce small, numerous, and tufted seeds that can be carried long distances by wind or water. The seeds, however, have a short period of viability, and must come in contact with suitable moisture within a few weeks of dispersal. Unlike obligate phreatophytes, such as willows and cottonwoods, saltcedar is a facultative phreatophyte and is often able to survive under conditions where groundwater is inaccessible. The high evapotranspiration rates of saltcedar can lower the water table and alter the floristic composition in heavily infested areas. Mature plants are tolerant to a variety of stress conditions, including heat, cold, drought, flooding, and high salinity. Saltcedar is not an obligate halophyte but survives in areas where groundwater concentrations of dissolved solids can average 8,000 ppm or higher. In addition, the leaves of saltcedar excrete salts that are deposited on the soil surface under the plant, inhibiting germination and growth of competing species.

In input-intensive cropping systems around the world, farmers rarely proactively manage weeds to prevent or delay the selection for herbicide resistance. Farmers usually increase the adoption of integrated weed management practices only after herbicide resistance has evolved, although herbicides continue to be the dominant method of weed control. Intergroup herbicide resistance in various weed species has been the main impetus for changes in management practices and adoption of cropping systems that reduce selection for resistance. The effectiveness and adoption of herbicide and nonherbicide tactics and practices for the proactive and reactive management of herbicide-resistant (HR) weeds are reviewed. Herbicide tactics include sequences and rotations, mixtures, application rates, site-specific application, and use of HR crops. Nonherbicide weed-management practices or nonselective herbicides applied preplant or in crop, integrated with less-frequent selective herbicide use in diversified cropping systems, have mitigated the evolution, spread, and economic impact of HR weeds.

In little over 20 yr, Palmer amaranth has risen from relative obscurity to its current status as one of the most widespread, troublesome, and economically damaging agronomic weeds in the southeastern U.S. Numerous factors have enabled Palmer amaranth to become such a dominant and difficult-to-control weed, including its rapid growth rate, high fecundity, genetic diversity, ability to tolerate adverse conditions, and its facility for evolving herbicide resistance. It is both a serious threat to several U.S. cropping systems and a fascinating model weed. In this paper, we review the growing body of literature on Palmer amaranth to summarize the current state of knowledge on the biology, agricultural impacts, and management of this weed, and we suggest future directions for research.

Integrated weed management (IWM) can be defined as a holistic approach to weed management that integrates different methods of weed control to provide the crop with an advantage over weeds. It is practiced globally at varying levels of adoption from farm to farm. IWM has the potential to restrict weed populations to manageable levels, reduce the environmental impact of individual weed management practices, increase cropping system sustainability, and reduce selection pressure for weed resistance to herbicides. There is some debate as to whether simple herbicidal weed control programs have now shifted to more diverse IWM cropping systems. Given the rapid evolution and spread of herbicide-resistant weeds and their negative consequences, one might predict that IWM research would currently be a prominent activity among weed scientists. Here we examine the level of research activity dedicated to weed control techniques and the assemblage of IWM techniques in cropping systems as evidenced by scientific paper publications from 1995 to June 1, 2012. Authors from the United States have published more weed and IWM-related articles than authors from any other country. When IWM articles were weighted as a proportion of country population, arable land, or crop production, authors from Switzerland, the Netherlands, New Zealand, Australia, and Canada were most prominent. Considerable evidence exists that research on nonherbicidal weed management strategies as well as strategies that integrate other weed management systems with herbicide use has increased. However, articles published on chemical control still eclipse any other weed management method. The latter emphasis continues to retard the development of weed science as a balanced discipline.

Rice is a principal source of food for more than half of the world population, and more than 90% of rice worldwide is grown and consumed in Asia. A change in establishment method from manual transplanting of rice seedlings to dry-seeded rice (DSR) has occurred in some countries as growers respond to increased costs or decreased availability of labor or water. However, weeds are a major constraint to DSR production because of the absence of the size differential between the crop and the weeds and the suppressive effect of standing water on weed growth at crop establishment. Herbicides are used to control weeds in DSR, but because of concerns about the evolution of herbicide resistance and a scarcity of new and effective herbicides, there is a need to integrate other weed management strategies with herbicide use. In addition, because of the variability in the growth habit of weeds, any single method of weed control cannot provide effective and season-long control in DSR. Various weed management approaches need to be integrated to achieve effective, sustainable, and long-term weed control in DSR. These approaches may include tillage systems; the use of crop residue; the use of weed-competitive cultivars with high-yield potential; appropriate water depth and duration; appropriate agronomic practices, such as row spacing and seeding rates; manual or mechanical weeding; and appropriate herbicide timing, rotation, and combination. This article aims to provide a logical perspective of what can be done to improve weed management strategies in DSR.

Since its discovery and initial commercialization in the 1940s, 2,4-D has been an important tool for weed control in a wide variety of crop and noncrop uses. Work studying its chemistry, physiology, mode of action, toxicology, environmental behavior, and efficacy has not only helped elucidate the characteristics of 2,4-D but also provided basic methods that have been used to investigate the properties of hundreds of herbicides that followed it. Much of the information published by researchers over 60 yr ago is still pertinent to understanding the performance of 2,4-D today. Further, new studies continue to be published, especially regarding the mechanisms of 2,4-D action at the molecular level. New uses for 2,4-D, sometimes enabled by biotechnology, continue to be developed. This review strives to provide an overall understanding of 2,4-D activity in plants, plant sensitivity to 2,4-D, toxicological impacts, and current and future uses.

Herbicide resistance jeopardizes the usefulness of valuable chemical tools and, therefore, weed management in many crop systems. Models must be developed to evaluate management tactics that prevent, delay, or reduce resistance. The complexity of biological processes involved in herbicide resistance also requires models to focus research and to integrate experiments. A population model was developed that improves upon previous attempts to predict herbicide resistance dynamics. The model incorporates plant population demographics with the Hardy-Weinberg concept for gene segregation. The model simulates the evolution, spread, and subsequent dynamics of resistance in the presence and absence of a herbicide. Analysis of model simulations identified two sets of biological processes as key factors in the evolution and dynamics of herbicide-resistant weed populations. These are processes that influence ecological fitness and gene flow. Several options are suggested as examples for the management of resistant weed populations.

The lack of evolution of weed resistance to the herbicide glyphosate has been considered from several perspectives. Few plant species are inherently resistant to glyphosate. Furthermore, the long history of extensive use of the herbicide has resulted in no verified instances of weeds evolving resistance under field situations. Unique properties of glyphosate such as its mode of action, metabolism, chemical structure and lack of residual activity in soil may explain this observation. Selection for glyphosate resistance of crops using intense whole plant and cell/tissue culture techniques, including mutagenesis, has had only limited success and is unlikely to be duplicated under normal field conditions. Information obtained in the development of glyphosate-resistant crops suggests that target-site alterations that decrease the herbicidal activity of glyphosate also may lead to reduced survival of a weed. In addition, the complex manipulations that were required for the development of glyphosate-resistant crops are unlikely to be duplicated in nature to evolve glyphosate-resistant weeds.

Recent shifts in herbicide use patterns can be attributed to rapid, large-scale adoption of glyphosate-resistant soybean and cotton. A dramatic increase in glyphosate use is the most obvious change associated with the adoption of glyphosate-resistant crops. Consequently, the diversity of herbicides used for weed management in these crops has declined, particularly in soybean. To date, the availability of glyphosate-resistant corn has limited the use of glyphosate in corn. While exploiting the benefits of glyphosate-resistant crops, many growers have abandoned the principles of sound weed and herbicide-resistance management. Instead of incorporating glyphosate into a resistance management strategy utilizing multiple herbicide sites of action, many growers rely exclusively upon glyphosate for weed control. Although it is difficult to establish a clear relationship between the adoption of glyphosate-resistant crops and changes in other crop production practices, the increase in no-till and strip-till production of cotton and soybean between 1995 and 2002 may have been facilitated by glyphosate-resistant crops.

The widespread evolution of multiple herbicide resistance in the most serious annual weeds infesting Australian cropping fields has forced the development of alternative, non-chemical weed control strategies, especially new techniques at grain harvest. Harvest weed seed control (HWSC) systems target weed seed during commercial grain harvest operations and act to minimize fresh seed inputs to the seedbank. These systems exploit two key biological weaknesses of targeted annual weed species: seed retention at maturity and a short-lived seedbank. HWSC systems, including chaff carts, narrow windrow burning, bale direct, and the Harrington Seed Destructor, target the weed seed bearing chaff material during commercial grain harvest. The destruction of these weed seeds at or after grain harvest facilitates weed seedbank decline, and when combined with conventional herbicide use, can drive weed populations to very low levels. Very low weed populations are key to sustainability of weed control practices. Here we introduce HWSC as a new paradigm for global agriculture and discuss how these techniques have aided Australian grain cropping and their potential utility in global agriculture.

The design and analysis of competition experiments should be based on an unambiguous objective. Recent criticisms of particular designs have been made without reference to objectives and may be misleading. Three common designs are discussed: additive, replacement series, and response surface. Additive designs are well suited to agronomic objectives; replacement series are useful for comparing pairs of species; response surface designs can be used for most objectives but may be unnecessarily complex. The published criticisms of additive and replacement series designs are argued to be acceptable limitations within the bounds of the objectives for which they are used. Concerns about these designs confounding density and proportion are irrelevant to the objectives for which they are most suited. The continued use of multiple comparison tests is argued to be illogical. Regression approaches to analysis are more relevant, many non-linear equations are now easy to fit to data and these can be used without the need for linearization. However, there are various pitfalls not adequately reported to date. In particular, error structures need to be checked carefully and over-elaborate equations should be avoided.

A naturally occurring prickly lettuce biotype resistant to a 5:1 formulated mixture of chlorsulfuron:metsulfuron (DPX-G8311) was identified in a no-till winter wheat field near Lewiston, ID, in April, 1987. Field and greenhouse studies were established to evaluate its resistance to other sulfonylureas, imidazolinones, and herbicides with alternate sites of action. The resistant biotype resisted eight sulfonylurea herbicides; resisted the imidazolinone herbicides, imazapyr and imazethypyr, but not imazaquin; and resisted no other herbicides included in the studies. The resistant biotype was identified in seven of nine fields on the farm where it was discovered.

Resistance to the herbicide glyphosate is currently known in at least eight weed species from many countries. Some populations of goosegrass from Malaysia, rigid ryegrass from Australia, and Italian ryegrass from Chile exhibit target site–based resistance to glyphosate through changes at amino acid 106 of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene. Mutations change amino acid 106 from proline to either serine or threonine, conferring an EPSPS weakly resistant to glyphosate. The moderate level of resistance is sufficient for commercial failure of the herbicide to control these plants in the field. Conversely, a nontarget site resistance mechanism has been documented in glyphosate-resistant populations of horseweed and rigid ryegrass from the United States and Australia, respectively. In these resistant plants, there is reduced translocation of glyphosate to meristematic tissues. Both of these mechanisms are inherited as a single, nuclear gene trait. Although at present only two glyphosate-resistance mechanisms are known, it is likely that other mechanisms will become evident. The already very large and still increasing reliance on glyphosate in many parts of the world will inevitably result in more glyphosate-resistant weeds, placing the sustainability of this precious herbicide resource at risk.

Cover crop–based organic rotational no-till soybean production has attracted attention from farmers, researchers, and other agricultural professionals because of the ability of this new system to enhance soil conservation, reduce labor requirements, and decrease diesel fuel use compared to traditional organic production. This system is based on the use of cereal rye cover crops that are mechanically terminated with a roller-crimper to create in situ mulch that suppresses weeds and promotes soybean growth. In this paper, we report experiments that were conducted over the past decade in the eastern region of the United States on cover crop–based organic rotational no-till soybean production, and we outline current management strategies and future research needs. Our research has focused on maximizing cereal rye spring ground cover and biomass because of the crucial role this cover crop plays in weed suppression. Soil fertility and cereal rye sowing and termination timing affect biomass production, and these factors can be manipulated to achieve levels greater than 8,000 kg ha−1, a threshold identified for consistent suppression of annual weeds. Manipulating cereal rye seeding rate and seeding method also influences ground cover and weed suppression. In general, weed suppression is species-specific, with early emerging summer annual weeds (e.g., common ragweed), high weed seed bank densities (e.g. > 10,000 seeds m−2), and perennial weeds (e.g., yellow nutsedge) posing the greatest challenges. Due to the challenges with maximizing cereal rye weed suppression potential, we have also found high-residue cultivation to significantly improve weed control. In addition to cover crop and weed management, we have made progress with planting equipment and planting density for establishing soybean into a thick cover crop residue. Our current and future research will focus on integrated multitactic weed management, cultivar selection, insect pest suppression, and nitrogen management as part of a systems approach to advancing this new production system.

Herbicide-resistant populations have evolved only in monoculture and/or monoherbicide conditions at predictable rates for each compound and weed. No populations of triazine-resistant weeds have appeared in corn where rotations of crops and herbicides or herbicide mixtures were used. This is due to the greatly reduced competitive fitness of the resistant individuals, which could be expressed only during rotational cycles, and also to the greater sensitivity of resistant individuals to other herbicides, pests, and control practices (“negative cross-resistance”). The model presented here describes how an understanding of all of these factors can provide strategies to decrease the frequency of the resistant individuals during rotation. Rotations or mixtures may not delay the rate of appearance of resistance to inhibitors of acetolactate synthase (ALS), where the fitness of resistant biotypes is claimed to be near normal. The best way to delay resistance to ALS inhibitors is to use those compounds with less persistence so that the selection pressure will be lowered. Too little is known about the frequency of resistance to other herbicides with target-site resistance–to dinitroanilines, to acetyl CoA carboxylase inhibitors, or to those situations where a single enzyme system confers resistance to a broad spectrum of seemingly unrelated herbicides.

Herbicide rotations and mixtures are widely recommended to manage herbicide resistance. However, little research has quantified how these practices actually affect the selection of herbicide resistance in weeds. A 4-yr experiment was conducted in western Canada from 2004 to 2007 to examine the impact of herbicide rotation and mixture in selecting for acetolactate synthase (ALS) inhibitor resistance in the annual broadleaf weed, field pennycress, co-occurring in wheat. Treatments consisted of the ALS-inhibitor herbicide, ethametsulfuron, applied in a mixture with bromoxynil/MCPA formulated herbicide (photosystem-II inhibitor/synthetic auxin), or in rotation with the non-ALS inhibitor at an ALS-inhibitor application frequency of 0, 25, 50, 75, and 100% (i.e., zero to four applications, respectively) over the 4-yr period. The field pennycress seed bank at the start of the experiment contained 5% ethametsulfuron-resistant seed. Although weed control was only marginally reduced, resistance frequency of progeny of survivors increased markedly after one ALS-inhibitor application. At the end of the experiment, the level of resistance in the seed bank was buffered by susceptible seed, increasing from 29% of recruited seedlings after one application to 85% after four applications of the ALS inhibitor. The level of resistance in the seed bank for the mixture treatment after 4 yr remained similar to that of the nontreated (weedy) control or 0% ALS-inhibitor rotation frequency treatment. The results of this study demonstrate how rapidly ALS-inhibitor resistance can evolve as a consequence of repeated application of herbicides with this site of action, and supports epidemiological information from farmer questionnaire surveys and modeling simulations that mixtures are more effective than rotations in mitigating resistance evolution through herbicide selection.

Failure of glyphosate to control Palmer amaranth was first reported in Arkansas in Mississippi County in June, 2005. The objectives of this research were to (a) confirm glyphosate-resistant Palmer amaranth in Arkansas, and (b) determine the effectiveness of 15 postemergence- (POST) applied herbicides comprising eight modes of action in controlling the glyphosate-resistant biotype compared to glyphosate-susceptible accessions. The LD50 values were similar among three susceptible Palmer amaranth accessions, ranging from 24.4 to 35.5 g ae/ha glyphosate. The resistant biotype had an LD50 of 2,820 g/ha glyphosate, which was 79- to 115-fold greater than that of the susceptible biotypes and 3.4 times a normal glyphosate-use rate of 840 g/ha. The glyphosate-resistant biotype was effectively controlled with most of the evaluated herbicides, but the use of acetolactate synthase-inhibiting herbicides such as pyrithiobac, trifloxysulfuron, and imazethapyr is not a viable option for control of this Palmer amaranth population.