Herbicides targeting grass plastidic acetyl coenzyme A carboxylase (ACC) are effective selective graminicides. Their intensive use worldwide has selected for resistance genes in a number of grass weed species. Biochemistry and molecular biology have been the means of determining the herbicidal activity and selectivity toward crop plants of ACC-inhibiting herbicides. In recent years, elucidation of the tridimensional structure of ACC and identification of five amino acid residues within the ACC carboxyl transferase domain that are critical determinants for herbicide sensitivity shed light on the basis of ACC-based resistance to herbicides. However, metabolism-based resistance to ACC-inhibiting herbicides is much less well known, although this type of resistance seems to be widespread. A number of genes thus endow resistance to ACC-inhibiting herbicides, with the possibility for various resistance genes that confer dominant resistance at the herbicide field rate to accumulate within a single weed population or plant. This, together with a poor knowledge of the genetic parameters driving resistance, renders the evolution of resistance to ACC-inhibiting herbicides unpredictable. Future research should consider developing tactics to slow the spread of resistance. For this purpose, it is crucial that our understanding of metabolism-based resistance improves rapidly because this mechanism is complex and can confer resistance to herbicides with different target sites.

Total weed density increased after 1 yr of no-tillage and after 2 yr of conventional tillage in a 4-yr experiment with repeated assignment of the same treatment to the same plots. Large crabgrass, goosegrass, and carpetweed densities were higher in the no-tillage compared with the conventional-tillage treatment in at least 1 yr whereas common lambsquarters density was greater in the conventional-tillage treatment the last year of the experiment. Within the no-tillage treatment, rye or hairy vetch residue reduced total weed density an average of 78% compared to the treatment without cover crop when cover crop biomass exceeded 300 g m–2 and when residue covered more than 90% of the soil. Goosegrass, stinkgrass, and carpetweed densities were reduced by cover crop residue in at least 1 yr whereas large crabgrass was unaffected. Common lambsquarters density increased where rye was grown as a cover crop prior to conventional tillage. Despite differences in weed density among treatments, weed biomass was equivalent in all treatments during the last 2 yr.

Laboratory and greenhouse experiments were conducted to determine the herbicidal effect of plant-derived oils and to identify the active ingredient in an oil with herbicide activity. Twenty-five different oils were applied to detached leaves of dandelion in the laboratory. Essential oils (1%, v/v) from red thyme, summer savory, cinnamon, and clove were the most phytotoxic and caused electrolyte leakage resulting in cell death. Each of these essential oils in aqueous concentrations from 5 to 10% (v/v) plus two adjuvants (nonionic surfactant and paraffinic oil blend at 0.2% [v/v]) were applied to shoots of common lambsquarters, common ragweed, and johnsongrass in the greenhouse; shoot death occurred within 1 h to 1 d after application. Essential oil of cinnamon had high herbicidal activity, and eugenol (2-methoxy-4-[2-propenyl]phenol) was determined to be this oil's major component (84%, v/v). Dandelion leaf disk and whole-plant assays verified that eugenol was the active ingredient in the essential oil of cinnamon. Essential oils are extracted from plants and thus may be useful as “natural product herbicides” for organic farming systems.

Interactions between weeds and arthropods occur frequently. This review covers the topic of weed/arthropod interactions, and provides the reader with access to literature in the subject area that is scattered in weed science, entomological, crop production, and ecological journals. We first analyze the current status of weed and arthropod management in the context of multidisciplinary integrated pest management (IPM). The remainder of the review is organized according to the mechanisms driving interactions. The first section deals with interactions driven by trophic relationships, and is subdivided into direct and indirect trophic interactions. Direct trophic interactions occur when pest or beneficial arthropods feed directly on weeds. Indirect trophic interactions occur when arthropod feeding damage to crops impacts weeds through alteration of ecosystem resource availability, or through weeds serving as hosts for alternate prey for beneficial arthropods, or via tritrophic interactions. The second mechanism driving interactions is considered in relation to alteration of the physical habitat by the presence of weeds, such as alteration of temperature within the plant canopy. The third major mechanism driving interactions is based on control tactics for the two types of pests. These are considered from the aspect of direct physical effects, such as tillage, and from the aspect of interactions resulting from the use of pesticides. The latter is divided into direct effects of herbicides and insecticides on non-target pests and beneficials, and on interactions that result from alteration of host plant physiology by pesticides. A conclusion section attempts to place the impact of interactions into an IPM framework, and to indicate where multidisciplinary research involving weed and arthropod management should be focused in the future.

The species composition and density of weed seed in the soil vary greatly and are closely linked to the cropping history of the land. Altering tillage practices changes weed seed depth in the soil, which plays a role in weed species shifts and affects efficacy of control practices. Crop rotation and weed control practices also affect the weed seedbank. Information on the influence of cropping practices on the weed seedbank should be a useful tool for integrated weed management. Decision aid models use information on the weed seedbank to estimate weed populations, crop yield loss, and recommend weed control tactics. Understanding the light requirements of weed seed may provide new approaches to weed management. Improving and applying our understanding of weed seedbank dynamics is essential to developing improved weed management systems. The principles of plant ecology must be integrated with the science of weed management to develop strategies that take advantage of basic plant responses in weed management systems for agronomic crops.

Development of herbicide-resistant crops has resulted in significant changes to agronomic practices, one of which is the adoption of effective, simple, low-risk, crop-production systems with less dependency on tillage and lower energy requirements. Overall, the changes have had a positive environmental effect by reducing soil erosion, the fuel use for tillage, and the number of herbicides with groundwater advisories as well as a slight reduction in the overall environmental impact quotient of herbicide use. However, herbicides exert a high selection pressure on weed populations, and density and diversity of weed communities change over time in response to herbicides and other control practices imposed on them. Repeated and intensive use of herbicides with the same mechanisms of action (MOA; the mechanism in the plant that the herbicide detrimentally affects so that the plant succumbs to the herbicide; e.g., inhibition of an enzyme that is vital to plant growth or the inability of a plant to metabolize the herbicide before it has done damage) can rapidly select for shifts to tolerant, difficult-to-control weeds and the evolution of herbicide-resistant weeds, especially in the absence of the concurrent use of herbicides with different mechanisms of action or the use of mechanical or cultural practices or both.

Field studies were conducted in 1997 and 1998 at Manhattan and Topeka, KS, to examine the competitive effects of redroot pigweed, Palmer amaranth, and common waterhemp on soybean yield. The experiments were established as a randomized complete block design in a factorial arrangement of three pigweed species, two pigweed planting dates (soybean planting and cotyledon stage), and seven weed densities (0.25, 0.5, 1, 2, 4, and 8 plants m−1 of row, plus a weed-free control). The effect of weed density on soybean yield loss, pigweed biomass, and pigweed seed production were described using a rectangular hyperbola model. Soybean yield loss varied between locations depending on the weed species, density, and time of emergence. Yield loss increased with weed density for each species and location with the first pigweed emergence time. The maximum soybean yield loss occurred at the first planting and 8 plants m−1 of row density, and was 78.7, 56.2, and 38.0% as determined by the model for Palmer amaranth, common waterhemp, and redroot pigweed, respectively. The second planting of pigweed did not significantly reduce soybean yield. The relative ranking of the pigweed species biomass was Palmer amaranth > common waterhemp > redroot pigweed. Maximum seed production for Palmer amaranth, common waterhemp, and redroot pigweed was 32,300, 51,800, and 9,500 seeds m−2. Palmer amaranth produced a larger quantity of seed than did common waterhemp or redroot pigweed at low weed densities (0.25 to 4 plants m−1 of row). But common waterhemp seed production equaled or surpassed Palmer amaranth at high weed densities.

Fluridone {1-methyl-3-phenyl-5-[3-(trifluromethyl)phenyl]-4(1H)-pyridinone} and norflurazon [4-chloro-5-(methylamino)-2-(α,α,α-trifluoro-m-tolyl)-3(2H)-pyridazinone] inhibited carotenoid synthesis in wheat (Triticum aestivum L. ‘Maricopa’) seedlings and caused the accumulation of colorless carotenoid precursors, phytoene, and phytofluene. These data suggested that dehydrogenation reactions following phytoene formation were inhibited by the herbicides. Chlorophyll accumulated in the treated seedlings grown under 10 lux of light but was destroyed in seedlings illuminated with 16 klux of light. Seedlings illuminated with 16 klux of light lacked chloroplast ribosomes and normal ultrastructure whereas dark grown seedlings contained pastid ribosomes and had normal etioplast ultrastructure. The disruption of the chloroplasts and loss of chlorophyll was due to the absence of carotenoids in the treated plants, which would normally act to protect the chloroplasts from photodestruction.

Changes in the weed seedbank due to crop production practices are an important determinant of subsequent weed problems. Research was conducted to evaluate effects of primary tillage (moldboard plowing and chisel plowing), secondary tillage (row cultivation), and herbicides on weed species changes in the soil seedbank in three irrigated row crop rotational sequences over a 3-yr period. The cropping sequences consisted of continuous corn for 3 yr, continuous pinto beans for 3 yr, or sugarbeets for 2 yr followed by corn in the third year. Cropping sequence was the most dominant factor influencing species composition in the seedbank. This was partly due to herbicide use in each cropping sequence producing a shift in the weed seedbank in favor of species less susceptible to applied herbicides. A comparison between moldboard and chisel plowing indicated that weed seed of predominant species were more prevalent near the soil surface after chisel plowing. The number of predominant annual weed seed over the 3-yr period increased more rapidly in the seedbank after chisel plowing compared to moldboard plowing unless effective weed control could be maintained to produce a decline in seedbank number. In this case, seedbank decline was generally more rapid after moldboard plowing. Row cultivation generally reduced seedbanks of most species compared to uncultivated plots in the pinto bean and sugarbeet sequences. A simple model was developed to validate the observation that rate of change in the weed seedbank is influenced by type of tillage and weed control effectiveness.

A field in which Brassica napus volunteers were not controlled by several applications of glyphosate was investigated in 1998. This field had been planted with glufosinate-resistant and imidazolinone-resistant B. napus in 1997 and was adjacent to a field that had grown glyphosate-resistant B. napus. Mature volunteer B. napus were collected on a 50- by 100-m grid in the field. Progeny from 34 volunteers were sprayed with glyphosate at 440 g ae ha−1, and the survivors were sprayed with either glufosinate or imazethapyr at 400 or 50 g ai ha−1, respectively. Where seed numbers permitted (14 volunteers), seedlings were also sprayed sequentially with glyphosate, glufosinate, and imazethapyr, at 440 g ae ha−1, 400 g ai ha−1, and 50 g ai ha−1, respectively. In total, 15 volunteers had progeny that were between 66 and 82% resistant to glyphosate, consistent with the predicted 3:1 resistant : susceptible ratio. Volunteer B. napus plants with glyphosate-resistant seedlings were most common close to the putative pollen source; however, a plant with glyphosate-resistant progeny was collected 500 m from the adjacent field edge. Seedlings from all nine volunteers collected from the glufosinate-resistant area showed multiple resistance to glyphosate and glufosinate, whereas seedlings from 10 of 20 volunteers collected from the imidazolinone-resistant area showed resistance to imazethapyr and glyphosate. DNA extraction and restriction fragment length polymorphism (RFLP) analysis of seedlings confirmed that mature B. napus volunteers were hybrids resulting from pollen transfer rather than inadvertent seed movement between fields. Two seedlings from the 924 screened were resistant to all three herbicides. Progeny from these self-pollinated individuals were resistant to glyphosate and glufosinate at the predicted 3:1 resistant : susceptible ratio and resistant to imazethapyr at the predicted 15:1 resistant : susceptible ratio. Sequential crossing of three herbicide-resistant varieties is the most likely explanation for the observed multiple herbicide resistance. Integrated management techniques, including suitable crop and herbicide rotations, herbicide mixtures, and nonchemical controls should be used to reduce the incidence and negative effect of B. napus volunteers with multiple herbicide resistance.

Weeds are often more competitive with crops at higher soil nutrient levels. This can result in increased dependency on herbicides and tillage to maintain adequate weed control. A number of studies have shown weeds accumulate higher concentrations of nitrogen, phosphorus, potassium, calcium, and magnesium than crops, thus depleting soil nutrient levels more quickly and reducing crop yield. Understanding basic mechanisms and timing of nutrient uptake in weeds and crops can lead to fertilization strategies which will enhance the competitive ability of crops while reducing interference from weeds. Such strategies can include deep band application of fertilizers to the crop row, as opposed to broadcast applications. Banding techniques also reduce the potential for nitrate contamination of surface or groundwater. The addition of nitrification inhibitors or increasing the proportion of nitrogen as ammonium or urea in fertilizer mixtures can restrict growth of ammonium- or urea-sensitive weeds. The timing of fertilizer applications can take advantage of maximal rates of nutrient uptake into crop roots at specific developmental stages. In addition, nutrient use efficiency can be enhanced by choosing appropriate crop cultivars, maintaining effective weed control practices, or altering row spacing or seeding rate to increase accumulation of nitrogen, phosphorus, and potassium in crops.

The mechanism of glyphosate resistance in horseweed was investigated. Eleven biotypes of putative sensitive (S) and resistant (R) horseweed were obtained from regions across the United States and examined for foliar retention, absorption, translocation, and metabolism of glyphosate. Initial studies used spray application of 14C-glyphosate to simulate field application. When S and R biotypes were compared in the absence of toxicity at a sublethal dose, we observed comparable retention and absorption but reduced root translocation in the R biotypes. S and R biotypes from Delaware were further examined at field use rates and results confirmed similar retention and absorption but reduced root translocation in the R biotypes. Application of 14C-glyphosate to a single leaf demonstrated reduced export out of the treated leaf and lower glyphosate import into other leaves, the roots, and the crown in R relative to S biotypes. Examination of the treated leaf by autoradiography showed that glyphosate loading into the apoplast and phloem was delayed and reduced in the R biotype. Our results consistently showed a strong correlation between impaired glyphosate translocation and resistance. Tissues from both S and R biotypes showed elevated levels of shikimate suggesting that 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) remained sensitive to glyphosate. Analysis of tissue shikimate levels demonstrated reduced efficiency in EPSPS inhibition in the R biotypes. Our results suggest that resistance is likely due to altered cellular distribution that impaired phloem loading and plastidic import of glyphosate resulting in reduced overall translocation as well as inhibition of EPSPS.

Community assembly is a branch of ecology that looks at how communities are assembled as they follow trajectories through time. A trajectory is controlled by biotic and abiotic constraints (filters) that act at multiple scales. From a total species pool, environmental and dispersal constraints control which species enter an ecological species pool. Within this pool, internal dynamics determine which of these species becomes part of the extant community. Environmental filters act by removing species that lack specific traits. Thus, traits are filtered, and with them, species. In this paper, we present the basic ecological theory of community assembly and address how it can be used in conjunction with a trait-based approach to understand and possibly predict how weed community structure changes in response to imposed filters such as tillage or crop rotation. Weed ecologists have struggled with the need to place our practical knowledge of agriculture and weeds into a broader theory, and there have been many calls to integrate ecology with agronomy and weed science. Community assembly might be one way to do so.

Palmer amaranth (Amaranthus palmeri S. Wats. # AMAPA) planted in a field at monthly intervals from March through October at Shafter, CA, began to emerge in March when soil temperatures at a depth of 5 cm reached 18 C. With the exception of March and April plantings, at least 50% of the seed of later plantings produced seedlings within 2 weeks after planting. Although growth of plants was initially slower for early plantings, plantings from March to July reached 2 m or greater in height by fall. Due to longer growing times, plantings from March to June eventually produced more dry matter and a greater number of inflorescences than later plantings. Plants began flowering 5 to 9 weeks after planting in March through June and 3 to 4 weeks after planting in July through October. Some viable seed was produced as early as 2 to 3 weeks after flowering began. Total seed production in the fall ranged from 200 000 to 600 000 seed/plant for the March through June plantings, and 115 to 80 000 seed/plant for the July through September plantings. Killing frosts in November prevented Palmer amaranth planted in October from producing seed.

Increasing concerns about pesticide use and a steadily increasing conversion to organic farming have been major factors driving research in physical and cultural weed control methods in Europe. This article reviews some of the major results achieved with nonchemical methods and strategies, especially adapted for row crops (e.g., corn, sugar beet, onion, leek, and carrot) and small-grain cereals (e.g., barley and wheat). In row crops, intrarow weeds constitute a major challenge, and research has mainly aimed at replacing laborious hand-weeding with mechanization. A number of investigations have focused on optimizing the use of thermal and mechanical weeding methods against intrarow weeds, such as flaming, harrowing, brush weeding, hoeing, torsion weeding, and finger weeding. And new methods are now under investigation such as robotic weeding for row crops with abundant spacing between individual plants and band-steaming for row crops developing dense crop stands. The strategic use of mechanical weed control methods in small-grain cereals has been another area of considerable interest. Weed harrowing and interrow hoeing provide promising results when they are part of a strategy that also involves cultural methods such as fertilizer placement, seed vigor, seed rate, and competitive varieties. Although research in preventive, cultural, and physical methods have improved weed control in row crops and small-grain cereals, effective long-term weed management in low external input and organic systems can only be achieved by tackling the problem in a wider context, i.e., at the cropping system level. Basic principles of this approach, examples of cover crop and intercropping use for weed suppression, and an application in a 2-yr rotation are presented and discussed.

While witchweed is nearing eradication in the United States, it continues to thrive in other parts of the world, especially in Africa, together with other witchweed species. The continuing problems from witchweeds and other parasitic weeds, the broomrapes, dodders and mistletoes, are outlined, including their extent, the degrees of damage caused, and the difficulties in their control. While a small minority are being successfully controlled by the use of immune varieties, most are currently controlled by existing techniques only partially, or on a local basis, and they may even be spreading or intensifying. The challenges they present are emphasised.

Information on responses of weeds to various soil fertility levels is required to develop fertilizer management strategies as components of integrated weed management programs. A controlled environment study was conducted to determine shoot and root growth response of 23 agricultural weeds to N fertilizer applied at 0, 40, 80, 120, 180, or 240 mg kg−1 soil. Wheat and canola were included as control species. Shoot and root growth of all weeds increased with added N, but the magnitude of the response varied greatly among weed species. Many weeds exhibited similar or greater responses in shoot and root biomass to increasing amounts of soil N, compared with wheat or canola. With increasing amounts of N, 15 weed species showed a greater increase in shoot biomass, and 8 species showed a greater increase in root biomass, compared with wheat. Ten weed species exhibited increases in shoot biomass similar to that exhibited by canola, and five weed species showed greater increases in root biomass than did canola, as N dose was increased. All crop and weed species extracted > 80% of available N at low soil N levels. At the highest N dose, 17 of 23 weed species took up similar or greater amounts of soil N than did wheat, and 6 weed species took up N in amounts similar to that taken up by canola. These findings have significant implications as to how soil fertility affects crop–weed competition. The high responsiveness of many weed species to N may be a weakness to be exploited through development of fertilizer management methods that enhance crop competitiveness with weeds.

Several commercial and experimental herbicides such as p-nitrodiphenyl ethers, oxadiazoles, and cyclic imides inhibit protoporphyrinogen IX oxidase (Protox), the enzyme that converts protoporphyrinogen IX to protoporphyrin IX (Proto). This leads to uncontrolled autooxidation of the substrate and results in accumulation of Proto. Blockage of the porphyrin pathway at this site inhibits synthesis of both chlorophylls and heme. Heme is a feedback regulator of the porphyrin pathway. Thus, inhibition of Protox also deregulates the pathway, causing increased carbon flow to the accumulating pool of Proto. Proto is a potent photosensitizer that generates high levels of singlet oxygen in the presence of molecular oxygen and light. In plants treated with these herbicides, damage is light dependent and closely correlated with the level of Proto that accumulates. Proto accumulation is apparently largely extraplastidic, resulting in rapid photodynamic damage to the plasmalemma and tonoplast. After high levels of Proto accumulate in response to these herbicides, protochlorophyllide (PChlide) levels can increase also; however, Proto appears to be the primary photodynamic pigment responsible for the herbicidal activity.

Seed of 41 economically important weed species of the Great Plains region of the United States were buried 20 cm deep in soil in eastern and western Nebraska in 1976. The 41 species consisted of 11 annual grass, 14 annual broadleaf, 4 biennial broadleaf, and 12 perennial broadleaf species. Weed seeds were exhumed annually for germination tests the first 9 yr, then after 12 and 17 yr. Germination percentages at the two burial locations averaged over 0, 1 to 4, 5 to 8, and 9 to 17 yr of burial were 57, 28, 9, and 4% for annual grass; 47, 26, 16, and 11 % for annual broadleaf; 52, 49, 44, and 30 % for biennial broadleaf; 36, 18, 13, and 8% for perennial broadleaf; and 47, 26, 16, and 10% for all 41 weed species, respectively. Biennial broadleaf weeds showed the greatest seed germination over years. Annual grass weeds showed less seed germinability over 17 yr of burial than annual broadleaf weeds and perennial broadleaf weed species were intermediate. Weed seed germinability in soil was greater in the reduced rainfall and more moderate soil temperatures of western Nebraska than in the greater rainfall and more fluctuating soil temperatures of eastern Nebraska. The greatest seed survival among the 41 weed species was shown by common mullein, which had 95% germination after 17 yr of burial in western Nebraska. Decay rates of individual weed species in soil will be of most value to weed scientists, agriculturalists, and modelers evaluating past or designing future weed management systems.

Herbicides applied to soils potentially affect soil microbial activity. Quantity and frequency of glyphosate application have escalated with the advent of glyphosate-tolerant crops. The objective of this study was to determine the effect of increasing glyphosate application rate on soil microbial biomass and activity. The soil used was Weswood silt loam. The isopropylamine salt of glyphosate was added at rates of 47, 94, 140, and 234 µg ai g−1 soil based on an assumed 2-mm glyphosate–soil interaction depth. Glyphosate significantly stimulated soil microbial activity as measured by C and N mineralization but did not affect soil microbial biomass. Cumulative C mineralization, as well as mineralization rate, increased with increasing glyphosate rate. Strong linear relationships between mineralized C and N and the amount of C and N added as glyphosate (r 2 = 0.995, 0.996) and slopes approximating one indicated that glyphosate was the direct cause of the enhanced microbial activity. An increase in C mineralization rate occurred the first day following glyphosate addition and continued for 14 d. Glyphosate appeared to be directly and rapidly degraded by microbes, even at high application rates, without adversely affecting microbial activity.