Late-season weed escapes are often ignored because they rarely cause crop yield penalty. Traditional weed management recommendations are based on the economic threshold (ET) approach, wherein management is required if the predicted current-season yield loss is greater than the cost of control interventions. While ET-based weed management can reduce current-season production costs and promote farmland biodiversity, it does not consider the long-term biological and economic consequences associated with late-season weed seed production. An important concern is that late-season weed seed production will replenish the soil seedbank, ensuring future weed problems. In the context of herbicide resistance evolution, allowing late-season weed seed production can be problematic because the probabilities of occurrence of resistant mutants rise with increases in seed production. A key component of herbicide resistance mitigation and management is preventing seed production and buildup of the soil seedbank. Late-season weed management efforts constitute additional expenses to growers, which cannot be recouped in that growing season, but any such investment must be weighed against the perceived long-term benefits. It appears that management of late-season weed escapes is valuable in a number of situations, and the degree to which management interventions should be employed can be case-specific. Adoption of economic optimum thresholds (EOTs), which can be established using bio-economic models, will be useful for making management decisions for late-season weed escapes. In systems vulnerable to herbicide resistance evolution, bio-economic resistance thresholds (BERTs) will be appropriate and bio-economic resistance models (BERMs) will be helpful for establishing such thresholds for specific production scenarios. Management considerations for late-season weed escapes are discussed, and knowledge gaps for future research are identified.

Molecular techniques are useful tools for solving taxonomic confusion among species. Polymerase chain reaction (PCR) and polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) methods were applied for the identification of barnyardgrass, early watergrass, and late watergrass. Total DNA was extracted from 266 accessions, which were collected from different rice growing areas of Turkey. The two primer sets (trn-a and trn-b1, and trn-c and trn-d) specific to a target region of the intergenic spacer between trnT (UGU) and trnL (UAA) and the entire intron region of trnL (UAA), respectively, were used in PCR amplifications. Of the 266 accessions of Echinochloa spp., only eight accessions gave a similar fragment size, which was slightly shorter than 495 bp. The PCR product obtained with the primers trn-a and trn-b1 gave two fragments when EcoRI restriction enzyme was used in barnyardgrass and early watergrass. However, not all accessions of late watergrass were digested with this enzyme. In contrast to EcoRI, the PCR product obtained using the trn-c and trn-d primer set was digested into two fragments by using AluI restriction enzyme in all accessions of late watergrass; whereas, it was not digested in barnyardgrass and early watergrass. This molecular differentiation among barnyardgrass, early watergrass, and late watergrass supports the hypothesis that late watergrass is not a synonym of early watergrass in Turkish accessions.

Flumioxazin has recently (2010) been registered for aquatic use for control of hydrilla and other noxious invasive aquatic plant species. Due to the rapid degradation of flumioxazin, especially in high pH water, some hydrilla research trials have produced less than desirable results with rapid plant regrowth. Therefore, laboratory experiments were conducted to evaluate the influence of pH on flumioxazin's effect on photosynthesis. Flumioxazin applied at concentrations ≥ 200 µg ai L−1 in high (9.0) pH water and ≥ 100 µg L−1 in low (6.0) pH water required 68 to 123 h to reduce photosynthesis by 50% (ET50). The effect of 400 µg L−1 flumioxazin on photosynthesis of apical hydrilla tips was also compared at low (20 µmol m−2 s−1), medium (170 µmol m−2 s−1), and high (400 µmol m−2 s−1) light levels at pH 9.0. Low light–treated tips were still photosynthetic at approximately 73% of the nontreated control plants 168 h after treatment. Low light–treated hydrilla required an estimated 303 h to achieve a 50% reduction in photosynthesis, while high light plants only required 99 h. Chlorophyll content of hydrilla was reduced as flumioxazin concentration was increased from 100 to 1,600 µg L−1. These data indicate that flumioxazin activity on hydrilla photosynthesis is influenced by herbicide concentration, water pH, and light intensity.

Changes in the weed flora of cropping systems reflect the impacts of factors that create safe sites for weed establishment and facilitate the influx and losses to and from the soil seedbank. This analysis of the annual surveys of the Southern Weed Science Society documents changes in the weed flora of the 14 contiguous southern states since the advent of transgenic, herbicide-resistant crops. In 1994 and 2009, the top five weeds in corn were morningglories, Texas millet, broadleaf signalgrass, johnsongrass, and sicklepod; in this same period Palmer amaranth, smartweeds, and goosegrass had the greatest increases in importance in corn. In cotton, morningglories and nutsedges were among the top five most troublesome weeds in 1995 and 2009. Palmer amaranth, pigweeds, and Florida pusley were also among the five most troublesome species in 2009; the weeds with the largest increases in importance in cotton were common ragweed and two species with tolerance to glyphosate, Benghal dayflower and Florida pusley. In soybean, morningglories, nutsedges, and sicklepod were among the top five weed species in 1995 and 2009. Two species with glyphosate resistance, Palmer amaranth and horseweed, were the second and fourth most troublesome weeds of soybean in 2009. In wheat, the top four weeds in 2008 were the same as those in 1994 and included Italian ryegrass, wild garlic, wild radish, and henbit. Crop production in the southern region is a mosaic of various crop rotations, soil types, and types of tillage. During the interval between the surveys, the predominant change in weed management practices in the region and the nation was the onset and rapid dominance of the use of glyphosate in herbicide-resistant cultivars of corn, cotton, and soybean. Because of the correspondence between the effects of glyphosate on the respective weed species and the observed changes in the weed flora of the crops, it is likely the very broad use of glyphosate was a key component shaping the changes in weed flora. Only eight of the top 15 most troublesome weeds of cotton and soybean, the crops with the greatest use of glyphosate, were the same in 1995 and 2009. In contrast, in corn and wheat where adoption of glyphosate-resistant cultivars lags or is absent, 12 of the 15 most troublesome weeds were the same in 1994 and 2008. These findings show on a regional scale that weeds adapt to recurrent selection from herbicides, currently the predominant weed management tool. Future research should seek methods to hinder the rapid spread of herbicide-tolerant and evolution of herbicide-resistant weed species. As new tools are developed, research should focus on ways to preserve the efficacy of those tools through improved stewardship.

Simazine is an important management tool for weed control in vineyards because of its relatively low price, reliable control of several problem weeds, and long residual activity. After repeated and extensive use of simazine, several growers in the Central Valley of California expressed concerns about reduced, residual weed control with this herbicide. Experiments were conducted to evaluate the rate of simazine dissipation in soils with differing simazine-use histories and to determine whether residual weed control differed among sites. Two raisin vineyards were used in all studies, one with extensive simazine-use history (adapted) and one with no recent simazine-use history (nonadapted). Results indicated that simazine dissipation from biotic processes was fourfold greater in soil with a long simazine-use history relative to soil with no recent simazine applications. In the field, simazine persisted longer at the nonadapted site, and weed-control duration was affected by dissipation rate. Central Valley vineyard soils that have had repeated simazine applications can develop enhanced, microbial degradation, and reduced, residual weed control is possible; however, weed control is also affected by environmental conditions and other crop management practices.

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.

A population of common ragweed from Delaware was not controlled in the field by herbicides that inhibit acetolactate synthase (ALS) or protoporphyrinogen oxidase (PPO). Research was conducted to ascertain if this population was resistant to these herbicidal modes of action and, if so, to determine the resistance mechanism(s). Resistance was confirmed by dose-response studies on greenhouse-grown plants with multiple ALS- and PPO-inhibiting herbicides. DNA sequence data revealed that resistance to ALS-inhibiting herbicides was due to the previously reported W574L ALS mutation. To assist in determining the mechanism of resistance to PPO-inhibiting herbicides, an F2 population was derived from a cross between the resistant biotype (Del-R) and a sensitive biotype (DV1-S). This population segregated in the ratio of three resistant : one sensitive when treated with fomesafen, indicating that resistance to PPO-inhibiting herbicides was conferred by a single, (incompletely) dominant, nuclear gene. Sequences of the target-site genes, PPX1 and PPX2, for PPO-inhibiting herbicides were obtained through the screening of a common ragweed cDNA library and subsequent cDNA extension (5′-RACE). Molecular marker analysis with the F2 population revealed that the PPX2 gene cosegregated with resistance to PPO-inhibiting herbicides. A mutation substituting an arginine codon for a leucine codon at a conserved location (R98L) of the PPX2 gene was suspected of being responsible for resistance. By using a transgenic Escherichia coli system, it was demonstrated that the R98L mutation was sufficient to confer resistance to PPO-inhibiting herbicides. The level of resistance to acifluorfen conferred by the R98L mutation in the E. coli system was about 31-fold, similar to the level of resistance seen in the whole-plant dose-response study. Last, a DNA-based assay was developed to identify the presence or absence of the common ragweed PPX2 R98L mutation. The R98L PPX2 mutation is the second mechanism identified for evolved resistance to PPO-inhibiting herbicides.

Research was conducted to determine the efficacy of aminocyclopyrachlor in comparison to glyphosate, clopyralid, fluroxypyr, and triclopyr for silk tree (commonly known as mimosa) control. In the greenhouse, aminocyclopyrachlor was applied at 8.75, 17.5, 35, and 70 g ha−1 with and without methylated seed oil (MSO) at 0.5% v/v. Efficacy of these treatments was compared to glyphosate and triclopyr at 1,350 g ha−1, fluroxypyr at 103 g ha−1, and clopyralid at 100 g ha−1. Few differences in silk tree control were detected by 28 d after treatment (DAT), as aminocyclopyrachlor with MSO controlled silk tree 87 to 100% compared to 53 to 100% for aminocyclopyrachlor without MSO. Aminocyclopyrachlor at 35 g ha−1 provided silk tree control similar to glyphosate, triclopyr, clopyralid, and fluroxypyr regardless of adjuvant. Inclusion of MSO enhanced initial activity of aminocyclopyrachlor after application. At 7 DAT, 8.75 g ha−1 of aminocyclopyrachlor plus MSO controlled silk tree similar to aminocyclopyrachlor alone at 70 g ha−1. In laboratory studies, absorption of 14C-aminocyclopyrachlor 2 h after treatment (HAT) with MSO measured 93% compared to only 62% for 14C-aminocyclopyrachlor without MSO. By 24 HAT, absorption of 14C-aminocyclopyrachlor measured 99 and 71% for applications with and without MSO, respectively. Increased foliar absorption with MSO may explain enhanced activity observed 7 DAT in greenhouse studies, as no effects in 14C-aminocyclopyrachlor translocation due to adjuvant were observed. Responses suggest MSO increased the speed of silk tree control with aminocyclopyrachlor and may also improve rainfastness of aminocyclopyrachlor applications for control of silk tree and other woody species.

Wild oat is the second-most abundant, but most economically important, weed across the Canadian Prairies of western Canada. Despite the serious economic effects of resistance to acetyl-CoA carboxylase (ACC) or acetolactate synthase (ALS) inhibitors or both in this weed throughout the Northern Great Plains of North America, little research has examined the basis for herbicide resistance. We investigated target-site and nontarget-site mechanisms conferring ACC- and ALS-inhibitor resistance in 16 wild oat populations from across western Canada (four ACC-inhibitor resistant, four ALS-inhibitor resistant, and eight ACC- and ALS-inhibitor resistant). The ACC1 mutations were found in 8 of the 12 ACC inhibitor-resistant populations. The Ile1781Leu mutation was detected in three populations, the Trp2027Cys and Asp2078Gly mutations were in two populations each, and the Trp1999Cys, Ile2041Asn, Cys2088Arg, and Gly2096Ser substitutions were in one population each. Three populations had two ACC1 mutations. Only 2 of the 12 ALS inhibitor-resistant populations had an ALS target-site mutation—Ser653Thr and Ser653Asn substitutions. This is the first global report of ALS target-site mutations in Avena spp. and four previously undocumented ACC1 mutations in wild oat. Based on these molecular analyses, seedlings of five ACC + ALS inhibitor-resistant populations (one with an ACC1 mutation; four with no ACC or ALS mutations) were treated with malathion, a known cytochrome P450 monooxygenase inhibitor, followed by application of one of four ACC- or ALS-inhibiting herbicides. Malathion treatment often resulted in control or suppression of these populations, suggesting involvement of this enzyme system in contributing to resistance to both ACC and ALS inhibitors.

Transgenic volunteer corn is a competitive weed in soybean that decreases soybean yield at densities as low as 0.5 plants m−2, yet the competitive effects of volunteer corn in corn have yet to be quantified in the peer-reviewed literature. In order to quantify competition between volunteer corn and hybrid corn, seed was harvested from transgenic hybrid corn. The seed was then hand-planted at two locations (Lafayette, IN and Wanatah, IN) into 3 by 9 m plots of hybrid corn at five densities: 0 (control), 0.5, 2, 4, and 8 plants m−2. Volunteer corn competition reduced leaf area and biomass of hybrid corn plants. Hybrid corn grain yield at Lafayette, IN, was reduced by 23 and 22% due to competition with volunteer corn growing in densities of 8 plants m−2 in 2010 and 2011, respectively, but when volunteer corn grain yield was combined with the hybrid corn grain yield, there was no reduction in total grain yield. This study demonstrates that the competitive effects on the grain yield of the hybrid corn will be offset by the grain yield of the volunteer plants. However, because the unpredictable locations and densities of volunteer corn plants present challenges to machine harvesting, future studies should examine what proportion of the volunteer crop is actually harvestable.

Weed interference with crop growth is often attributed to water, nutrient, or light competition; however, specific physiological responses to these stresses are not well described. This study's objective was to compare growth, yield, and gene expression responses of corn to nitrogen (N), low light (40% shade), and weed stresses. Corn vegetative parameters from V2 to V12 stages, yield parameters, and gene expression using transcriptome (2008) and quantitative polymerase chain reaction (qPCR) (2008/09) analyses at V8 were compared among the stresses and with nonstressed corn. N stress did not affect vegetative parameters, although grain yield was reduced by 40% compared with nonstressed plants. Shade, present until V2, reduced biomass and leaf area > 50% at V2, and recovering plants remained smaller than nonstressed plants at V12. However, grain yields of shade-stressed and nonstressed plants were similar, unless shade remained until V8. Weed stress reduced corn growth and yield in 2008 when weeds remained until V6. In 2009, weed stress until V2 reduced corn vegetative growth, but yield reductions occurred only if weed stress remained until V6 or later. Principle component analysis of differentially expressed genes indicated that shade and weed stress had more similar gene expression patterns to each other than they did to nonstressed or N-stressed tissues. However, corn grown in N-stressed conditions shared 252 differentially expressed genes with weed-stressed plants. Ontologies associated with light/photosynthesis, energy conversion, and signaling were down-regulated in response to all three stresses. Shade and weed stress clustered most tightly together, based on gene expression, but shared only three ontologies, O-METHYLTRANSFERASE activity (lignification processes), POLY(U)-BINDING activity (posttranscriptional gene regulation), and stomatal movement. Based on morphologic and genomic observations, weed stress to corn was not explained by individual effects of N or light stress. Therefore, we hypothesize that these stresses share limited signaling mechanisms.

Herbicides are the foundation of weed control in commercial crop-production systems. However, herbicide-resistant (HR) weed populations are evolving rapidly as a natural response to selection pressure imposed by modern agricultural management activities. Mitigating the evolution of herbicide resistance depends on reducing selection through diversification of weed control techniques, minimizing the spread of resistance genes and genotypes via pollen or propagule dispersal, and eliminating additions of weed seed to the soil seedbank. Effective deployment of such a multifaceted approach will require shifting from the current concept of basing weed management on single-year economic thresholds.

Giant ragweed germination is delayed by both a physiological dormancy of the embryo (embryo dormancy) and an inhibitory influence of embryo-covering structures (covering structure-enforced [CSE] dormancy). To clarify the roles of embryo and CSE dormancy in giant ragweed seedling emergence timing, we conducted two experiments to address the following objectives: (1) determine changes in germinability for giant ragweed dispersal units (hereafter “involucres”) and their components under natural burial conditions, and (2) compare embryo and CSE dormancy alleviation and emergence periodicity between successional and agricultural populations. In Experiment 1, involucres were buried in crop fields at Columbus, OH, periodically excavated, and brought to the laboratory for dissection. Involucres, achenes, and embryos were then subjected to germination assays at 20 C. In Experiment 2, temporal patterns of seedling emergence were determined at a common burial site. Reductions in embryo and CSE dormancy were compared with controlled-environment stratification followed by germination assays at 12 and 20 C, temperatures representative of soil conditions in spring and summer. Results indicated that overwinter dormancy loss involved sequential reductions in embryo and CSE dormancy. CSE dormancy, which may limit potential for fatal germination during fall, was caused by the pericarp and/or embryo-covering structures within the pericarp. In Experiment 2, successional populations emerged synchronously in early spring, whereas agricultural populations emerged throughout the growing season. Levels of embryo dormancy were greater in the agricultural populations than the successional populations, but CSE dormancy levels were similar among populations. In 12 C germination assays, embryo dormancy levels were positively correlated with time required to reach 95% cumulative emergence (run 1: r = 0.81, P = 0.03; run 2: r = 0.76, P = 0.05). These results suggest that late-season emergence in giant ragweed involves high levels of embryo dormancy that prevent germination at low temperatures in spring.

Imazamox is an imidazolinone herbicide used to control many grasses and broadleaf weeds in leguminous crops such as soybean, alfalfa, and dry bean; however, imazamox cannot be used on red lentil due to unacceptable injury. Studies were conducted to compare imazamox absorption, translocation, and metabolism in red lentil and dry bean to determine if any or all of these factors contributed to differential crop sensitivity. Radiolabeled imazamox was applied to three young red lentil leaves and the youngest, fully expanded dry bean trifoliolate leaf. Absorption, translocation, and metabolism were followed over a 96-h time course. Red lentil had more rapid absorption compared to dry beans with 64 and 54% of the applied dose absorbed 12 h after treatment (HAT), respectively. Maximum absorption was also greater in red lentil than dry bean, 79 and 61%, respectively. Translocation out of the treated leaf was significantly higher in red lentil compared with dry bean, 16 and 0.5%, respectively, at 96 HAT. Translocation was greater to red lentil roots compared to shoots, 9 and 7%, respectively, at 96 HAT. In dry bean only 14% of applied 14C-imazamox remained intact 24 HAT, while 79% of the radioactivity was imazamox in red lentil 24 HAT. Both species metabolized the herbicide to more polar metabolites. The inherent sensitivity of aceolactate synthase (ALS) from dry bean and red lentil was also evaluated. ALS from both species had similar I50 values for imazamox, 7.2 and 8.2 µM, respectively. The combined effects of increased imazamox absorption and reduced imazamox metabolism are the major contributors to differential selectivity between dry bean and red lentil. Rapid imazamox metabolism in dry bean significantly limited herbicide translocation out of the treated leaf.

Early physiological mechanisms that occur in crop plants in response to neighboring weeds are not well understood. In this experiment, it was hypothesized that, in the absence of direct competition for resources, low red to far red ratio (R:FR) reflected from neighboring weeds will modulate the phenylpropanoid pathway, increase hydrogen peroxide (H2O2), and up-regulate the expression of ethylene biosynthesis and auxin transport genes. Laboratory experiments were conducted under conditions of nonlimiting resources using perennial ryegrass as a model weed species. We discovered that the detection by phytochrome (Phy) of low R:FR signals reflected from both biological and nonbiological sources triggered an up-regulation of ethylene biosynthesis genes and stimulated an auxin transport gene. The low R:FR also modulated the phenylpropanoid pathway resulting in a reduction in anthocyanin content and an enhancement of lignin synthesis. The presence of neighboring weeds also caused an accumulation of H2O2 in the first leaf and crown root tissues of the maize seedling. Stomata were observed to be closed as H2O2 accumulated in leaf tissue. This is the first study to report the modulation of phenylpropanoid pathway and the accumulation of H2O2 attributed to low R:FR. We further suggest that these physiological changes that occur in response to early weed competition result in a physiological cost to the crop plant, which contributes to the rapid loss in yield observed in weed competition studies conducted under field conditions.

Japanese foxtail is one of the most common and competitive annual grass weeds of wheat in China. Whole-plant dose-response experiments were conducted with fenoxaprop and pinoxaden to confirm and characterize resistant and susceptible Japanese foxtail populations and to elucidate the basis of resistance to these herbicides. The resistant Japanese foxtail population was 49-fold resistant to fenoxaprop and 16-fold (cross) resistant to pinoxaden relative to the susceptible population, which was susceptible to both fenoxaprop and pinoxaden herbicides. Molecular analysis of resistance confirmed that the Ile1781 to Leu mutation in the resistant population conferred resistance to both fenoxaprop and pinoxaden. This is the first report of cross resistance of Japanese foxtail to pinoxaden in the world and of a target site mutation that corresponded to resistance to both fenoxaprop and pinoxaden in Japanese foxtail. Prior selection pressure from fenoxaprop could result in evolution of resistance to fenoxaprop and cross resistance to pinoxaden in Japanese foxtail population.

Weed composition may vary because of natural environment, management practices, and their interactions. In this study we presented a systematic approach for analyzing the relative importance of environmental and management factors on weed composition of the most conspicuous species in sugarcane. A data-mining approach represented by k-means cluster and classification and regression trees (CART) were used for analyzing the 11 most frequent weeds recorded in sugarcane cropping systems of northern Argentina. Data of weed abundance and explanatory factors contained records from 1976 sugarcane fields over 2 consecutive years. The k-means method selected five different weed clusters. One cluster contained 44% of the data and exhibited the lowest overall weed abundance. The other four clusters were dominated by three perennial species, bermudagrass, johnsongrass, and purple nutsedge, and the annual itchgrass. The CART model was able to explain 44% of the sugarcane's weed composition variability. Four of the five clusters were represented in the terminal nodes of the final CART model. Sugarcane burning before harvesting was the first factor selected in the CART, and all nodes resulting from this split were characterized by low abundance of weeds. Regarding the predictive power of the variables, rainfall and the genotype identity were the most important predictors. These results have management implications as they indicate that the genotype identity would be a more important factor than crop age when designing sugarcane weed management. Moreover, the abiotic control of crop–weed interaction would be more related to rainfall than the environmental heterogeneity related to soil type, for example soil fertility. Although all these exploratory patterns resulting from the CART data-mining procedure should be refined, it became clear that this information may be used to develop an experimental framework to study the factors driving weed assembly.