The root exudates produced by sorghums contain a biologically active constituent known as sorgoleone. Seven sorghum accessions were evaluated for their exudate components. Except for johnsongrass, which yielded 14.8 mg root exudate/g fresh root wt, sorghum accessions consistently yielded approximately 2 mg root exudate/g fresh root wt. Exudates contained four to six major components, with sorgoleone being the major component (> 85%). Three-dimensional structure analysis was performed to further characterize sorgoleone's mode of action. These studies indicated that sorgoleone required about half the amount of free energy (493.8 kcal/mol) compared to plastoquinone (895.3 kcal/mol) to dock into the QB-binding site of the photosystem II complex of higher plants. Light, cryo-scanning, and transmission electron microscopy were utilized in an attempt to identify the cellular location of root exudate production. From the ultrastructure analysis, it is clear that exudate is being produced in the root hairs and being deposited between the plasmalemma and cell wall. The exact manufacturing and transport mechanism of the root exudate is still unclear. Studies were also conducted on sorgoleone's soil persistence and soil activity. Soil impregnated with sorgoleone had activity against a number of plant species. Recovery rates of sorgoleone-impregnated soil ranged from 85% after 1 h to 45% after 24 h. Growth reduction of 9 14-d-old weed species was observed with foliar applications of sorgoleone.

Red rice has long been a troublesome, conspecific weed of cultivated rice. Rice varieties carrying certain herbicide-resistant traits acquired through genetic modification (herbicide-resistant varieties) now offer new options for red rice control. In concert with this innovation is the risk of gene flow, which can result in the transfer of that specific herbicide resistance to red rice and thus render this weed control measure ineffective. Gene flow in concept is simple, however, the parameters that determine the establishment of a new trait in a weed population are complex. Cross-pollination to make hybrid seed and the subsequent fate of those hybrid families in the general weed population are some of the biological factors that influence gene flow between red rice and cultivated rice. Natural outcrossing among rice plants is generally low. Most of the pollen dispersal studies published to date indicated that rice × rice outcrossing rates were less than 1.0%. Numerous reports summarized in this study suggest that outcrossing rates between rice and red rice can be highly variable but usually are similar to or lower than this level. However, once hybrids form, they may introgress into a red rice population within only a few generations. If hybrid seed families are to persist and establish herbicide-resistant red rice populations, they must successfully compete in the crop–weed complex. The ability to survive a herbicide applied to a herbicide-resistant rice variety would be a strong selective advantage for these hybrid families. Thus, the well-established principles of weed resistance management appear to be relevant for herbicide-resistant crop systems and should be used in combination with practices to minimize coincident flowering to mitigate the potential impact of gene flow from herbicide-resistant rice into red rice. For the rice–red rice crop–weed complex, there are both biological factors and agricultural practices that can work together to preserve these new weed control options.

Prescribed burning has primarily been used as a tool for the control of invasive late-season annual broadleaf and grass species, particularly yellow starthistle, medusahead, barb goatgrass, and several bromes. However, timely burning of a few invasive biennial broadleaves (e.g., sweetclover and garlic mustard), perennial grasses (e.g., bluegrasses and smooth brome), and woody species (e.g., brooms and Chinese tallow tree) also has been successful. In many cases, the effectiveness of prescribed burning can be enhanced when incorporated into an integrated vegetation management program. Although there are some excellent examples of successful use of prescribed burning for the control of invasive species, a limited number of species have been evaluated. In addition, few studies have measured the impact of prescribed burning on the long-term changes in plant communities, impacts to endangered plant species, effects on wildlife and insect populations, and alterations in soil biology, including nutrition, mycorrhizae, and hydrology. In this review, we evaluate the current state of knowledge on prescribed burning as a tool for invasive weed management.

Studies of weed and crop competition can be used to predict yield losses from weed presence and to determine optimum levels or periods of weed control. However, competition is a complex phenomenon that is governed by various biological, environmental, and proximity factors. The factors of proximity include plant density, species proportion, and spatial arrangement among individuals. Several experimental methods have been developed that attach different levels of importance to proximity factors. These methods are described, and the advantages and disadvantages of each are discussed. Density, proportion, and arrangement of plants influence the outcome of competition experiments and should be incorporated into studies of crop-weed interference, since differing estimates for the effects of weeds on crop productivity can be obtained, depending upon the experimental method used.

Knowledge of interference thresholds, biology, and growth habits of weeds of rice is essential to implement timely, effective, economical weed control technology for profitable rice production. Regression analyses were used to determine threshold levels for weed densities and durations of interference for major weeds of rice. Weed threshold information for individual species or combinations of species can be used to develop models for initiating control inputs and determining their costs and returns. Among the grass weeds in density experiments, red rice reduced rice grain yields the most followed by barnyardgrass, beared sprangletop, and broadleaf signalgrass. Among broadleaf/aquatic weeds in density experiments, hemp sesbania reduced rice grain yields the most followed by northern jointvetch, ducksalad, spreading dayflower, and eclipta. Barnyardgrass, broadleaf signalgrass, and ducksalad interfered with rice the most during early season, whereas eclipta, hemp sesbania, northern jointvetch, red rice, and spreading dayflower caused greater interference during mid- to late-season.

The vast majority of crop hectares in the United States are treated with chemical herbicides annually. The adoption of herbicides for weed control was rapid in the 1950s and 1960s. Herbicides replaced the use of millions of workers to pull and hoe weeds by hand and greatly reduced the use of tillage for weed control. Costs of production were reduced and crop yields increased because herbicides were cheaper and more effective than hand weeding and cultivation. Organic crop growers cite weed control as their greatest difficulty in crop production because they are not permitted the use of chemical herbicides. They substitute hand weeding and cultivation for herbicides at a greatly increased cost and with reduced effectiveness. Aggregate studies that estimate the value of herbicides assume that growers would substitute a certain amount of hand weeding and tillage if chemicals were not used, which would not be sufficient to prevent yield losses totaling about 20% of U.S. crop production.

Herbicide resistance in rigid ryegrass is an escalating problem in grain-cropping fields of southeastern Australia due to increased reliance on herbicides as the main method for weed control. Weed surveys were conducted between 1998 and 2009 to identify the extent of herbicide-resistant rigid ryegrass across this region to dinitroaniline, and acetolactate synthase- and acetyl coenzyme A (CoA) carboxylase-inhibiting herbicides. Rigid ryegrass was collected from cropped fields chosen at random. Outdoor pot studies were conducted during the normal winter growing season for rigid ryegrass with PRE-applied trifluralin and POST-applied diclofop-methyl, chlorsulfuron, tralkoxydim, pinoxaden, and clethodim. Herbicide resistance to trifluralin in rigid ryegrass was identified in one-third of the fields surveyed from South Australia, whereas less than 5% of fields in Victoria exhibited resistance. In contrast, resistance to chlorsulfuron was detected in at least half of the cropped fields across southeastern Australia. Resistance to the cereal-selective aryloxyphenoxypropionate-inhibiting herbicides diclofop-methyl, tralkoxydim, and pinoxaden ranged between 30 and 60% in most regions, whereas in marginal cropping areas less than 12% of fields exhibited resistance. Resistance to clethodim varied between 0 and 61%. Higher levels of resistance to clethodim were identified in the more intensively cropped, higher-rainfall districts where pulse and canola crops are common. These weed surveys demonstrated that a high incidence of resistance to most tested herbicides was present in rigid ryegrass from cropped fields in southeastern Australia, which presents a major challenge for crop producers.

Diagnosing herbicide-resistant weeds as a first step in resistance management and monitoring their nature, distribution, and abundance demands efficient and effective screening tests. This review summarizes and recommends appropriate seed sampling techniques, protocols for screening weeds for resistance to herbicides of different sites of action, interpretation of results, and information given to the grower. Elements common to all screening procedures are reviewed. Choosing appropriate discriminating doses to distinguish between resistant and susceptible weed biotypes is the most important factor in achieving accurate and consistent results. Interpretation of results is also critical because resistant weeds may comprise a small portion of the population in suspected accessions or biotypes.

Herbicide-resistant weed species have become widespread in recent years. Fifty-five weed species, including 40 dicots and 15 grasses, are known to have biotypes resistant to the triazine herbicides. One or more resistant species have arisen in 31 states of the United States, four provinces of Canada, 18 countries in Europe, and Israel, Japan, Australia, and New Zealand. Resistance to other classes of herbicides is more restricted in distribution and recent in detection but is becoming more widespread. Trifluralin resistance has spread in the southeastern United States and has been detected in Canada, while 11 species with biotypes resistant to paraquat have been reported around the world. Diclofop-methyl-resistant weed species are problems in cereal production in Australia and have been found in Oregon, South Africa, and the United Kingdom. Resistance to the substituted ureas also is present in the United Kingdom, West Germany, and Hungary. Within the last 2 yr, biotypes of at least four weed species resistant to the sulfonylurea herbicides have arisen following several annual applications of these herbicides in wheat. Some resistant biotypes have multiple resistance to different classes of herbicides, which greatly exacerbates the threat of resistance. Herbicide resistance has reached the level where more concerted efforts are needed in research, education, and development of effective management strategies to preserve herbicides as essential tools of agricultural technology.

There are nearly 75 species in the genus Amaranthus, part of the Amaranthaceae family, worldwide. In this large genus, there is a distinct group of 10 species that are dioecious (separate male and female plants). In contrast to the monoecious Amaranthus spp. which are represented by species endemic to every continent, the dioecious Amaranthus spp. are all native to North America. The Amaranthus spp. have a long documented history of being fellow travelers with humans. In recent years, three dioecious Amaranthus spp.: Palmer amaranth (Amaranthus palmeri S. Wats.), common waterhemp (Amaranthus rudis Sauer), and tall waterhemp [Amaranthus tuberculatus (Moq) Sauer] have become major weeds to row crops in North America. Palmer amaranth has become a very troublesome weed to cotton (Gossypium hirsutum L.), corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production in large parts of the southern United States, and the waterhemp complex is now a major weed pest in corn and soybean production in the midwestern United States (Horak and Loughin 2000; King 1966; Mabberly 1997; Robertson 1981; Sauer 1950, 1955, 1972; Wax 1995).

An adjuvant is a material added to a tank mix to aid or modify the action of an agrichemical, or the physical properties of the mixture. For the most recent decade, responsible researchers, suppliers, and trade organizations have made significant progress to identify, understand, and standardize adjuvants. Adjuvant components, their purpose in application, terminology, and now, even tests that establish minimum performance expectations are under development. Certification of voluntary adjuvant composition and performance standards will soon be available. The adjuvant industry has grown into respectability. This paper will attempt to serve as a useful reference for current terminology and definitions accepted by weed science, and also provide a brief discussion of the chemistry favored in current adjuvant compositions.

Seeds of suspected herbicide-resistant Palmer amaranth and common waterhemp were collected in Clay County and Douglas County, KS, respectively. An experiment was established in a greenhouse to determine if these species had developed resistance to imazethapyr and thifensulfuron. Imazethapyr was applied pre- (PRE) and postemergence (POST) at 1×, 2×, 4×, and 8× the suggested use rate (70 g/ha), and thifensulfuron was applied POST at 1×, 2×, 4×, and 8× the suggested use rate (4.5 g/ha). Both species had developed resistance to all rates of these herbicides. The occurrence of resistance at the Clay County site (Palmer amaranth) fit the typical pattern for the development of herbicide resistance, i.e., multiple applications of the same class of herbicide for several years. However, the Douglas County (common waterhemp) site had a limited history of use of ALS-inhibiting herbicides and did not follow typical models of resistance development.

Row spacing affects the time of canopy closure, thus influencing the growth and development of both crop and weeds. Field studies were conducted in 1999, 2000, and 2001 at Mead, NE, and 2000 and 2001 at Concord in eastern Nebraska to determine the effects of three row spacings (19, 38, and 76 cm) on the critical time for weed removal (CTWR) in dryland soybean. A three-parameter logistic equation was fit to data relating relative crop yield to increasing duration of weed presence. In general, earliest CTWR occurred in the 76-cm rows, and coincided with the first trifoliate stage of soybean. Latest CTWR occurred in the 19-cm rows and coincided with the third trifoliate. The CTWR in 38-cm rows occurred at the second trifoliate. Practical implications are that planting soybean in wide rows reduces early-season crop tolerance to weeds requiring earlier weed management programs than in narrower rows.

Drift of glyphosate may cause injury to nontarget plant species. Herbicidal damage caused by glyphosate may be attributed to the herbicides that inhibit acetohydroxyacid synthase (AHAS) or acetyl CoA carboxylase (ACCase), because the time taken to show herbicidal damage and the visual injury symptoms caused by these three types of herbicides are similar. Glyphosate inhibits the biosynthesis of aromatic amino acid biosynthesis which causes accumulation of shikimate in plants. A modified, simpler method of extraction and assay for detection of shikimate in plants has been adapted from previously published procedures. With this assay, rapid accumulation of shikimate was observed in glyphosate-sensitive plants that have been treated with glyphosate. Furthermore, the accumulation of shikimate in herbicide-sensitive plants was only due to glyphosate and not due to an AHAS or an ACCase inhibitor. Therefore, high levels of shikimate in a damaged plant indicate glyphosate injury. With this assay, no accumulation of shikimate was seen in a glyphosate-resistant variety of soybean. Therefore, lack of accumulation of shikimate in a glyphosate-treated weed or crop species indicates that the plant is resistant to glyphosate.

Diffuse knapweed, a biennial or short-lived perennial, and spotted knapweed, a perennial, are taprooted Eurasian weeds invading rangeland in the western United States and Canada. Knapweed (Centaurea spp.) invasion is associated with reductions in biodiversity, wildlife, and livestock forage, and increased erosion. Spotted knapweed grows to about 1 m and usually has purple flowers, whereas diffuse knapweed is slightly shorter, usually with white flowers. Persistent flower bracts on diffuse knapweed bear a rigid terminal spine about 8 mm long with four or five pairs of shorter lateral spines. Bracts on spotted knapweed have dark spotted tips. Knapweed management involves a combination of containing infestations and control efforts. Hand pulling in areas with small infestations can be effective for controlling spotted and diffuse knapweeds. Picloram applied at 0.28 kg ha−1 provides control for about 3 yr. Effective long-term control of knapweeds requires periodic applications of picloram, which are only cost-effective on highly productive range sites with a residual grass understory. About 12 insect species have been released for knapweed biocontrol. Seed production has been reduced by 46% by insects feeding in the flower heads. Although insects have not reduced spotted knapweed densities, they may stress the weed and shift the competitive balance to associated species. Sheep grazing reduces the density of very young seedlings and may limit seedling recruitment into the population. In areas without a residual understory of desired plant species, revegetation of knapweed-infested rangeland is required. Components of any integrated weed management program are sustained effort, constant evaluation, and the adoption of improved strategies.

Field experiments were conducted from 1989 to 1992 to determine whether no-tillage corn grown in 38-cm rows and a 2× population could improve weed control relative to 76-cm rows and 1× population under reduced-herbicide options. A standard treatment including 1.12 kg ai/ha of atrazine plus 2.24 kg ai/ha of metolachlor was compared with a treatment including the same herbicides applied at 25% of the standard rates. Both treatments included 0.56 kg ai/ha of paraquat which controlled annual weeds established at the time of application. Weed control was less in the 25%-herbicide treatment than in the standard treatment in two of four years when corn was grown in 76-cm rows with a l× population. The 25%-herbicide treatment provided weed control and grain yields similar to the standard treatment in each year when corn was grown in 38-cm rows with a 2× population. Weed control was poor and yield was reduced when no herbicides were applied regardless of row spacing or population. The leaf canopy of corn in the 38-cm row/2×-population treatment reduced light transmittance 1 wk earlier than corn in the 76-cm row/1×-population treatment.

Allelochemicals representing numerous chemical groups have been isolated from over 30 families of terrestrial and aquatic plants. Some of the compounds also have been isolated from soil in quantities sufficient to reduce plant growth. Although selected allelochemicals are believed to influence plant densities and distributions, none isolated from higher plants have been considered active enough for development as commercial herbicidal products. Almost all herbicidal allelochemicals exist in plants in nontoxic, conjugated forms. The toxic moiety may be released upon exposure to stress or upon death of the tissue. The most successful use of allelochemicals in weed control has been management of selectively toxic plant residues. For example, rye residues have controlled weeds effectively in a variety of cropping systems. Several weed species may interfere with crop growth through chemicals released from their residues. A number of noxious perennial species appear to exploit allelochemicals in their interference processes. This review focuses on the more recent chemical discoveries and how they might be exploited for weed control.

The replication of experiments over multiple environments such as locations and years is a common practice in field research. A major reason for the practice is to estimate the effects of treatments over a variety of environments. Environments are frequently classed as random effects in the model for statistical analysis, while treatments are almost always classed as fixed effects. Where environments are random and treatments are fixed, it is not always necessary to include all possible interactions between treatments and environments as random effects in the model. The rationale for decisions about the inclusion or exclusion of fixed by random effects in a mixed model is presented. Where the effects of treatments over broad populations of environments are to be estimated, it is often most appropriate to include only those fixed by random effects that reference experimental units.

Composition and abundance of weed populations often change in response to new or extensively used weed management practices. Glyphosate-resistant (GR) technology is one such weed management practice now used extensively. A recent survey of weed scientists was conducted to address weed shifts in GR corn, cotton, and soybean. Twelve scientists in 11 states responded to the survey. Averaged over estimates from scientists, GR corn, cotton, and soybean were planted on 15, 90, and 88% of the hectarage in 2003, respectively. Acreage of GR corn is expected to rise, whereas only minor changes in acreage of GR cotton or soybean are expected. Weed shifts have not been observed in GR corn but have occurred in GR cotton and soybean. In GR cotton, Amaranthus, Commelina, Ipomoea, and Cyperus species as well as annual grasses were noted as becoming more problematic. Similar to cotton, Ipomoea and Commelina species are becoming more troublesome in GR soybean. In addition, in GR soybean, various winter annuals, lambsquarters species, and waterhemp species were noted as becoming more problematic. All scientists felt that weeds shifts were occurring, and two-thirds of these scientists noted that weed shifts are currently of economic concern. The scientists recommend the following to help manage weed shifts: additional herbicides in mixture with glyphosate, rotation to herbicides other than glyphosate, rotation to non–GR crops, and greater use of soil-applied herbicides.

A four-year experiment was conducted near Urbana, IL to evaluate the effect of a rye cover crop on weed control, soybean yield and soil moisture. Soybeans were planted into either a rye mulch or corn stubble (with and without spring tillage). Giant foxtail, velvetleaf, smooth pigweed and common lambsquarters control in the rye mulch plots was generally greater than 90% and better than the corn residue treatments five weeks after planting. Weed control was generally better, except for lambsquarters, in the corn residue without spring tillage plots compared to the spring-tilled plots. Herbicides improved weed control in the corn residue plots but not in the no-till rye treatment, due to the excellent control by the rye mulch. Soil water content was lowest during June under the late-killed (killed at planting) rye during dry periods due to water depletion caused by the growing rye. During wet periods the rye mulch resulted in a wetter soil profile compared to the corn residue treatments. Soybean yields were reduced in late-killed rye compared to early-killed rye (killed 2 wk prior to planting) due to soybean stand reductions in the late killed rye. Yields in early-killed rye and spring-tilled treatments were similar to or better than soybeans planted in corn residue without spring tillage.