The utility of biological control for weed management in agroecosystems will increase with a greater understanding of the relationships between common weed and granivore species. Giant foxtail is an introduced, summer annual grass weed that is common throughout the United States and problematic in numerous crops. Harpalus pensylvanicus (DeGeer) (Coleoptera: Carabidae) is a common, native, omnivorous carabid beetle with a range that overlaps giant foxtail. In 2004 and 2005, H. pensylvanicus was captured from farm fields in Centre County, PA, and subjected to laboratory feeding trials to test the preference of giant foxtail and other species on predation by H. pensylvanicus. Weed species seed preference experiments that included “Choice” and “No Choice” treatments were conducted using giant foxtail, common lambsquarters, and velvetleaf. When given a choice amongst the three weed species, H. pensylvanicus preferred giant foxtail and common lambsquarters seeds equally compared to velvetleaf seeds. When given the choice, H. pensylvanicus preferred newly dispersed giant foxtail seeds over field-aged seeds. Phenology of giant foxtail seed shed relative to H. pensylvanicus activity density was also quantified in field experiments in 2005 and 2006. Giant foxtail seed rain was determined by collecting shed seeds from August through October using pan traps. Activity density of H. pensylvanicus was monitored for 72-h periods using pitfall traps from June to October. Peak activity density of H. pensylvanicus occurred at the onset of giant foxtail seed shed in both years; however, giant foxtail seed shed peaked approximately 30 to 50 d after H. pensylvanicus activity density. Future research should focus on management practices that enhance and support H. pensylvanicus populations later in the growing season to maximize suppression of giant foxtail and other weeds that shed palatable seeds later in the season.

Experiments were conducted in laboratory and screenhouse to determine the effects of light, temperature, salt and water stress, seed burial depth, and amount of rice residue on seed germination and seedling emergence of wild bushbean and to evaluate the response of that weed to commonly available POST herbicides in the Philippines. Germination (81 to 85%) was not influenced by light; however, it was higher at 30/20 C (88%) than at 25/15 C (82%) and 35/25 C (80%) day/night temperatures. The sodium chloride and osmotic potential required for 50% inhibition of maximum germination were 149 mM and −0.28 MPa, respectively. Seedling emergence (81%) was similar for seeds placed on the soil surface and for seeds buried up to 4 cm deep. No seedlings emerged from a burial depth of 8 cm. Rice residue applied on the soil surface up to 8 t ha−1 did not affect seedling emergence (72 to 80%) of wild bushbean. The herbicide bentazon provided 100% control of wild bushbean when applied at the three- to five-leaf stages. However, to achieve 100% control by 2,4-D and metsulfuron-methyl + chlorimuron-ethyl, herbicides had to be applied at the three-leaf stage. The information gained from this study can help in developing components of integrated weed management programs for wild bushbean. A deep-tillage operation to bury weed seeds below their maximum depth of emergence and early application of an effective POST herbicide can help in managing this weed in rice fields.

Junglerice is one of the world's most problematic C4 grass weeds present in dry-seeded rice in many countries. A screenhouse study was conducted to determine the effect of four rice planting densities (0, 100, 200, and 400 plants m−2) and four nitrogen (N) rates (0, 50, 100, and 150 kg ha−1) on the growth and morphological plasticity of junglerice. Junglerice plant height was reduced by 15 to 35%, tiller number by 54 to 77%, leaf number by 61 to 85%, leaf area by 69 to 90%, leaf biomass by 63 to 88%, stem biomass by 70 to 92%, and inflorescence biomass by 66 to 94% at rice planting densities ranging from 100 to 400 plants m−2 relative to the junglerice plants grown alone. However, all these growth parameters increased with increasing N rates. Junglerice biomass increased by 125 to 472%, whereas rice biomass increased by 122 to 285% with the application of 50 to 150 kg N ha−1. Additional N favored junglerice biomass production relative to rice. Rice crop interference (200 to 400 plants m−2) reduced junglerice growth and biomass and overshaded the junglerice plants when no N was applied. Increasing N application resulted in taller plants and higher biomass of junglerice, while it reduced root-shoot weight ratio. These results suggest that increasing N rate increased the competitive ability of the junglerice over rice regardless of crop planting density. Information generated in this study could be useful in devising appropriate combinations of planting density and fertilizer management strategies for cultural junglerice management, particularly in situations where junglerice species are more responsive than rice to N.

The role of weed biomass in the nutrient balance of agro-ecosystems remains poorly understood. To measure the rate of decomposition and nutrient release of common weeds, litterbag methodology was employed using waterhemp and giant foxtail desiccated by glyphosate at heights of 10, 20, 30, and 45 cm in two southern Illinois soybean fields. Losses were then expressed as a decay constant (k) regressed over time according to the single exponential decay model. Concentrations of the recalcitrant cell wall components (cellulose, hemicellulose, and lignin) were generally greatest as weed height (maturity) increased in giant foxtail compared with waterhemp. Sixteen weeks after desiccation by glyphosate, 10-cm waterhemp and giant foxtail detritus had lost 10 and 12% more mass, respectively, compared to the 45-cm height of each species. Decomposition rates revealed mass loss was highest for 10-cm waterhemp (kD = 0.022) and lowest for 45-cm giant foxtail (kD = 0.011) and this process was negatively correlated to the overall amount of cell wall constituents (r = −0.73). Nutrient release rates followed a similar trend in that both shorter (younger) weeds and waterhemp liberated nutrients more readily. Across all tested plant material, K was the nutrient most rapidly released, whereas, Ca was the most strongly retained nutrient.

The implementation of a successful glyphosate resistance management strategy requires a simple and cost-effective method for detecting resistance in key weeds. To date, however, glyphosate resistance is still routinely confirmed via laborious and time consuming whole-plant pot assays using seeds collected at the end of the growing season. Here, we describe a simple, early-season bioassay for detecting evolved glyphosate resistance in grass and broadleaf weeds. It involves transplanting suspected glyphosate resistant seedlings alongside known sensitive and resistant standards into agar containing informative rates of herbicide and recording percentage survival 14 d after plating. The method was validated using sensitive and resistant populations of Lolium, Eleusine, Conyza, and Amaranthus species encompassing the main glyphosate resistance mechanisms, namely, impaired translocation, EPSPS gene duplication, and mutations. The whole plant pot and agar-based seedling tests generated comparable resistance indices in dose-response assays and percentage survival at discriminating glyphosate rates. The method was applied successfully to detect resistance in a rigid ryegrass population collected from a French vineyard well before glyphosate was applied in the field for the current season. Additionally, the test was shown to be highly transferable to several other grass and broadleaf weeds that have evolved resistance to glyphosate. One major attribute of the method is that it is capable of detecting resistance regardless of the mechanism involved. In addition to being very simple, quick and, cost-effective, it allows determination of glyphosate resistance in weeds prior to field application. It thus offers the opportunity for an informed choice of herbicides for effective weed control.

The effect of weed interference on corn yield and the critical period for weed control (CPWC) were determined in Germany and Benin. Treatments with weed control starting at different crop growth stages and continuously kept weed-free until harvest represented the “weed-infested interval.” Treatments that were kept weed-free from sowing until different crop growth stages represented the “weed-free interval.” Michaelis–Menten, Gompertz, logistic and log–logistic models were employed to model the weed interference on yield. Cross-validation revealed that the log–logistic model fitted the weed-infested interval data equally well as the logistic and slightly better than the Gompertz model fitted the weed-free interval. For Benin, economic calculations considered yield revenue and cost increase due to mechanical weeding operations. Weeding once at the ten-leaf stage of corn resulted already profitable in three out of four cases. One additional weeding operation may optimize and assure profit. Economic calculations for Germany determined a CPWC starting earlier than the four-leaf stage, challenging the decade-long propagated CPWC for corn. Differences between Germany and Benin are probably due to the higher yields and high costs in Germany. This study provides a straightforward method to implement economic data in the determination of the CPWC for chemical and nonchemical weed control strategies.

Indirect effects of insect control strategies on weed populations are important to consider when developing robust integrated pest management strategies. Weed seed predation rates were investigated in corn managed under three contrasting treatments based on control practices for corn rootworm: (1) the transgenic crop Cry3Bb Bt corn, (2) the broad-spectrum insecticide tefluthrin, and (3) no insecticide control. This 2-yr field study conducted near Ithaca, NY, involved quantifying seed loss from velvetleaf, common lambsquarters, and giant foxtail in arenas with and without vertebrate exclosures. Velvetleaf and giant foxtail were unaffected by the insecticide treatment; however, average seed predation of common lambsquarters was lower in both the Bt corn (11.9%) and insecticide-treated plots (11.8%) compared with control plots (17.5%) that did not receive any insecticide. Seed predation of common lambsquarters was not affected by the vertebrate exclosure. Lower seed predation in the transgenic Bt corn and insecticide treatments was likely due to nontarget effects on carabids (Coleoptera: Carabidae). Although the reduction in seed predation was modest and limited to only one of the three weed species tested, our results highlight the need for greater risk assessment that includes the ecosystem service of weed seed predation when considering insect pest management options.

Increased soybean seed cost has generated recent interest in reducing seeding rates to improve economic returns. However, low seeding rates result in reduced established plant stands with slower canopy development, and canopy development is an important element of integrated weed management (IWM). Field studies were conducted in 2012 and 2013 in Wisconsin to determine the trade-off between reduced seeding rates and PRE residual herbicide use for POST herbicide exposure. Soybean was planted in mid May in 38-cm-wide rows at five seeding rates ranging from 148,200 to 469,300 seeds ha−1. A PRE application of metolachlor plus fomesafen was made to half of the plots. One of two POST herbicide programs were sprayed at the V4 soybean growth stage to determine whether blending herbicide-resistant (HR) and non-HR soybean cultivars could be a practical alternative to reduce soybean seed expenses while maintaining the potential benefit of weed suppression before the POST herbicide application. An increase in seeding rate did not reduce the density or size of weeds exposed to the POST herbicide, and furthermore, end-of-season weed density and biomass were not influenced. In contrast, the use of a PRE herbicide reduced total weed density and biomass before POST application by 93 and 95%, respectively, in both years. In 2012, the season was dry early and harvest stands of 161,100 and 264,100 plants ha−1 produced 95% of the maximum yield for the PRE and no-PRE treatments, respectively. The difference was not repeated in 2013 with adequate early season rainfall. In conclusion, PRE herbicide use produced maximum yield with fewer plants per hectare by limiting early season weed competition and reduced weeds exposed to POST herbicide application thus contributing to HR management (HRM). In contrast, higher plant densities generated within the seeding rate range of this study did little to improve IWM or HRM.

Weeds persist and cause economic losses in agricultural systems because they exploit underused portions of that system. Reducing the effect of weeds on agroecosystems begins with minimizing the number of propagules (e.g., seeds and tubers) that are produced and returned to the soil. Purple nutsedge is a problematic weed around the globe, persisting between growing seasons as tubers in the soil. Halosulfuron is an effective herbicide for controlling purple nutsedge foliage and is used in corn and several vegetable crops. Studies were conducted to evaluate the effect of various rates of halosulfuron on purple nutsedge tuber production. Single, presprouted purple nutsedge tubers were transplanted into outdoor microplots and treated after 6 wk of growth with six rates of halosulfuron (7 to 208 g ai ha−1) POST with a nontreated control (NTC). All shoots that had emerged at the time of application were marked with plastic rings; this allowed for classification of tubers at exhumation of (1) tubers attached to shoots that had emerged by the time of application, (2) tubers attached to shoots that emerged after application, and (3) tubers without an aerial shoot during the study. Seven weeks after application, the tubers in the microplots were exhumed and tubers were classified, quantified, and their ability to sprout was evaluated. In the NTC, there were 530 total tubers, with a log-logistic regression model describing the tuber population with increasing halosulfuron rate. The rate of halosulfuron that reduced total tuber population 50% (I50) was 8 g ha−1. In the NTC, 200 tubers were attached to shoots that emerged following halosulfuron application, and this class of tubers had an I50 of 19 g ha−1. Viability of tubers with shoots that emerged following halosulfuron application was 28% at the 52 g ha−1 halosulfuron, suggesting the action of the herbicide may have rendered the tuber nonviable after new shoots were produced. The final classification of tubers was those that did not have an aerial shoot during the study. These were tubers in which apical dominance suppressed shoot development or were likely the most-recent tubers to develop. Of the three classes, the tubers without shoots were the most numerous in the NTC, with 294 tubers and an I50 of 1 g ha−1. Halosulfuron is an effective herbicide that not only controls purple nutsedge foliage but also reduces the number of new tubers produced and overall tuber viability. This could be an important component to reduce the long-term population density of the weed.

Difficulties in chemically controlling large crabgrass in corn in South Africa have recently been attributed to the occurrence of naked crabgrass. In contrast to large crabgrass, naked crabgrass is not easily controlled with acetanilide herbicides. Critical periods of weed control (CPWC) for naked crabgrass in corn was determined in field studies during the 2009/2010 and 2010/2011 growing seasons at two separate localities for an early and late planting date of corn. Weed-free and weed–crop interference treatments of increasing duration were maintained at various crop growth stages in the presence of naked crabgrass. Biomass of naked crabgrass was determined as dry weight per square meter quadrant, which yielded 428 g m−2 at Potchefstroom and 594 g m−2 at Wesselsbron. An exponential regression model was used to determine the CPWC expressed as growing degree days after crop emergence, on the basis of an estimated 10% relative yield loss in corn. The onset and ending, as well as the duration of the CPWC, differed between seasons and localities. At 10% relative yield loss, the onset of the CPWC ranged between the two-leaf (V2) and six-leaf (V6) stages, and the ending between the 12-leaf (V12) stage and 2 wk after tasseling (T + 2). The duration of the CPWC ranged between 22 and 80 d for the respective planting dates, years, and localities. Yield losses ranged from 28 to 82% in the season-long weedy plots. The shifting of planting dates alone did not reduce yield losses since the effect of late infestations of naked crabgrass is significant. Naked crabgrass control from crop emergence is essential, followed by POST herbicide application during the critical period of weed control to lower the risk of corn yield losses.

Saflufenacil, a pyrimidinedione herbicide, is used for contact and residual broadleaf weed control in various crops. Bioactivity of saflufenacil in soil was tested in greenhouse and laboratory studies on 29 soils representing a wide range of soil properties and geographic areas across the United States. A greenhouse bioassay method was developed using various concentrations of saflufenacil applied PPI to each soil. Whole canola plants were harvested 14 d after treatment, and fresh and dry weights were recorded. Nonlinear regression analysis was used to determine the effective saflufenacil doses for 50% (ED50,), 80% (ED80), and 90% (ED90) inhibition of total plant fresh weight. Bioactivity of saflufenacil in soil was strongly correlated to soil organic (R = 0.85) and humic matter (R = 0.81), and less correlated to cation exchange capacity (R = 0.49) and sand content (R = −0.32). Stepwise regression analysis indicated that organic matter was the major soil constituent controlling bioactivity in soil and could be used to predict the bioactivity of saflufenacil. Saflufenacil phytotoxicity was found to be dependent on soil property; therefore, efficacy and crop tolerance from PRE and PPI applications may vary based on soil organic matter content and texture classification.

Fomesafen, saflufenacil, and sulfentrazone had similar parameters for their mass spectrometry analysis, all being analyzed in negative mode with similar ionization energies. Flumioxazin was analyzed in positive mode using different ionization temperatures and voltage energies, and a larger injection volume (10 µl compared with 2–5 µl) due to lower liquid chromatography–mass spectrometry (LC-MS) detector response. Quantitative limits of detection in soil were < 5 parts per billion by weight for all herbicides. The field study was conducted three times (2010, 2011, 2012) with four blocks of each treatment each year. Herbicide concentrations over time were based on field samples that were later extracted and quantified using the described LC-MS procedures. Data were examined using a simple first-order (SFO) equation with each year-by-herbicide treatment combination regressed using SigmaPlot version 12.5 to determine regression parameters. The SFO rate constant was used to determine a half life, or DT50 (in days) for each curve. All data were analyzed using a GLMMix ANOVA procedure using SAS version 9.3 and contrast statements were used to directly compare each herbicide comparison. Slopes for each herbicide use the SFO curve and were estimated using SAS. The order from shortest to longest DT50 was flumioxazin (21.1 d) = saflufenacil (21.4 d) < fomesafen (45.6 d) < sulfentrazone (70.8 d). These results concur with the labeled recrop recommendations after application for flumioxazin and saflufenacil, which have shorter cotton plant-back restrictions compared with sulfentrazone and fomesafen. In these studies, none of the herbicides was highly persistent (all half-lives < 100 d), so none would be expected to be persistent pollutants in the environment, although further research is needed in this area.

Two broad aims drive weed science research: improved management and improved understanding of weed biology and ecology. In recent years, agricultural weed research addressing these two aims has effectively split into separate subdisciplines despite repeated calls for greater integration. Although some excellent work is being done, agricultural weed research has developed a very high level of repetitiveness, a preponderance of purely descriptive studies, and has failed to clearly articulate novel hypotheses linked to established bodies of ecological and evolutionary theory. In contrast, invasive plant research attracts a diverse cadre of nonweed scientists using invasions to explore broader and more integrated biological questions grounded in theory. We propose that although studies focused on weed management remain vitally important, agricultural weed research would benefit from deeper theoretical justification, a broader vision, and increased collaboration across diverse disciplines. To initiate change in this direction, we call for more emphasis on interdisciplinary training for weed scientists, and for focused workshops and working groups to develop specific areas of research and promote interactions among weed scientists and with the wider scientific community.