Aromatic plants and their volatile compounds affect seed germination and plant growth, and therefore hold potential for agriculture uses as plant growth regulators and bioherbicides. In the present study 17 major monoterpenes were selected, and their mechanisms of plant toxicity were elucidated using transgenic Arabidopsis thaliana at various growth stages. Microtubulin and the plant cell membrane were identified as the focal targets through which phytotoxicity and herbicidal activity acted. Variability in monoterpene mechanisms was observed. Limonene and (+)-citronellal had strong antimicrotubule efficacy, whereas citral, geraniol, (−)-menthone, (+)-carvone, and (−)-citronellal demonstrated moderate antimicrotubule efficacy. Pulegone, (−)-carvone, carvacrol, nerol, geranic acid, (+)/(−)-citronellol, and citronellic acid lacked antimicrotubule capacity. An enantioselective disruption of microtubule assembly was recorded for (+)/(−)-citronellal and (+)/(−)-carvone. The (+) enatiomers were more potent than their (−) counterparts. Citral, limonene, carvacrol, and pulegone were also tested for phytotoxicity and herbicidal activity. Pulegone had no detectable effect on microtubules or membranes. Citral disrupted microtubules but did not cause membrane damage. Carvacrol lacked a detectable effect on microtubules but incited membrane leakage, and limonene disrupted microtubules and membrane leakage. Therefore, only limonene was herbicidal at the tested concentrations. In planta quantification of residues revealed that citral was biotransformed into nerol and geraniol, and limonene was converted into carvacrol, which could explain its dual capacity with respect to microtubules and membrane functionality. The results obtained are an important added value to commercial efforts in selecting appropriate aromatic plants to be sources of bioherbicidal compounds for sustainable weed management with a limited potential for herbicide resistance evolution in weed populations.

Italian ryegrass is a major weed problem in wheat production worldwide. Field studies were conducted at Fayetteville, AR, to assess morphological characteristics of ryegrass accessions from Arkansas and differences among other Lolium spp.: Italian, rigid, poison, and perennial ryegrass. Plant height, plant growth habit, plant stem color, and node color were recorded every 2 wk until maturity. The number of tillers per plant, spikes per plant, and seeds per plant were recorded at maturity. All ryegrass accessions from Arkansas were identified as Italian ryegrass, which had erect to prostrate growth habit, green to red stem color, green to red nodes, glume (10 mm) shorter than spikelet (19 mm), and medium seed size (5 to 7 mm) with 1 to 3 mm awns. However, significant variability in morphological characteristics was found among Arkansas ryegrass accessions. When Lolium species at the seedling stage (1- to 2-wk-old plants) were compared, poison ryegrass was characterized as having a large main-stem diameter and wide droopy leaves, whereas perennial ryegrass exhibited a short and a very narrow leaf blade. These two can be distinguished from Italian and rigid ryegrass, which have leaf blades wider than perennial ryegrass but narrower than poison ryegrass. Italian and rigid ryegrass are difficult to distinguish at the seedling stage but are distinct at the reproductive stage. At maturity, Italian ryegrass and poison ryegrass seeds are awned, but perennial and rigid ryegrass seeds are awnless. Poison ryegrass awns were at least 4-fold longer than Italian ryegrass awns. Perennial ryegrass flowered 3 wk later than the other species. Poison ryegrass glumes were longer than the spikelets, whereas Italian ryegrass glumes were shorter than the spikelets. Morphological traits indicate that some Italian ryegrass populations are potentially more competitive and more fecund than others.

Greenhouse studies were conducted to investigate the absorption, translocation, and metabolism of foliar-applied [14C]halosulfuron-methyl in cucumber, summer squash, pitted morningglory, and velvetleaf. Cucumber and summer squash were treated at the 4-leaf stage, whereas velvetleaf and pitted morningglory were treated at 10 cm. All plants were collected at 4, 24, 48, and 72 h after treatment (HAT) for absorption and translocation studies and an additional 96-HAT interval was included in the metabolism study. Absorption did not exceed 45% in summer squash, whereas it plateaued around 60% in velvetleaf and cucumber and reached 80% in pitted morningglory 72 HAT. None of the four species translocated more than 23% of absorbed halosulfuron out of the treated leaf. Translocation in cucumber and summer squash was predominantly basipetal, while acropetal movement prevailed in velvetleaf. No significant direction of movement was observed for pitted morningglory. Negligible translocation occurred toward the roots, regardless of plant species. Of the total amount of [14C]halosulfuron-methyl absorbed into the plants at 96 HAT, more than 80% remained in the form of the parent compound in velvetleaf, summer squash, and pitted morningglory, whereas less than 20% was detected in cucumber. Rapid and high herbicide metabolism may explain cucumber tolerance to halosulfuron-methyl, while lack of metabolism contributes to summer squash and velvetleaf susceptibility. Pitted morningglory tolerance may be due to limited translocation associated with some level of metabolism, but further research would be needed to investigate other potential causes.

Fenoxaprop-P-ethyl, a phenoxy herbicide of the aryloxy–phenoxy–propionic acid group, had a strong control effect when applied POST to weedy rice in this study, with the effective concentrations of 294 μM and 218 μM of herbicide causing 50% inhibition (IC50) in plant height and fresh weight values, respectively. However, fenoxaprop-P-ethyl caused phytotoxicity in cultivated rice. Isoxadifen-ethyl, a widely used herbicide safener in rice, can decrease the phytotoxicity caused by fenoxaprop-P-ethyl. Owing to the extremely similar morphological features and physiological properties of weedy and cultivated rice, it is not practical to spray isoxadifen-ethyl directly on cultivated rice plants to safen them. Applying the safener directly to cultivated rice seeds may be a practical alternative method. To improve the biological activity of isoxadifen-ethyl seed treatments, novel compounds were designed by splicing other groups, including amines, amino acids, and 2- methoxy-5-nitrophenol sodium salt, to the parental structure of isoxadifen-ethyl. Through hydrolysis, acyl chlorination, acyl amination, and esterification, a series of isoxadifen-ethyl derivatives were synthesized and their structures were determined by mass spectrometry and 1H nuclear magnetic resonance spectroscopy. The biological activities of five of the isoxadifen-ethyl derivatives, which possessed recovery effects similar to isoxadifen-ethyl, were able to relieve herbicide phytotoxicity. In pot experiments, isoxadifen-ethyl showed almost no activity as a seed treatment, while three derivative compounds, when used independently as seed treatments, were able to prevent the damage caused by fenoxaprop-P-ethyl. The results will help to develop a new control method for weedy rice, thereby decreasing production costs and increasing farmers’ incomes.

Glyphosate-resistant (GR) kochia is an increasing concern for growers across the U.S. Great Plains and Canadian prairies. Integrated strategies to mitigate resistance will require an improved understanding of the seed germination dynamics of GR kochia populations. Experiments were conducted to characterize the germination of GR vs. glyphosate-susceptible (GS) kochia populations under different constant (5 to 35 C) and alternating (5/10 to 30/35 C) temperatures. Seven GR and two GS populations were collected from wheat–fallow fields in northern Montana. Selected lines of GR and GS were obtained after three generations of recurrent group selection in the greenhouse. The GR-selected lines had 4.1 to 10.8 average EPSPS copies compared with a single EPSPS gene copy for the GS selected lines. Four out of seven GR selected lines had lower final germination (d parameter) and took more time to complete 50% cumulative germination (I50 values) under all constant and alternating temperatures, compared with the GS selected lines. Those GR selected lines also had a delayed germination initiation (I10 values), particularly at lower temperatures (5 to 10 C constant or 5/10 C alternating). In contrast, the final germination (d) of the other three GR selected lines did not differ from GS lines at a majority of temperatures tested. The I50 values of those GR lines were also comparable to GS lines under a majority of the temperatures. There was no significant correlation of observed percent cumulative germination and EPSPS gene copy number of selected kochia lines. The temperature-dependent dormancy and altered germination characteristics of the four GR kochia lines reflect the common selection of resistance and avoidance (glyphosate or other preseeding treatments) mechanisms. This is most likely attributed to long-term, intensive cropping practices and less diverse weed control methods, rather than a fitness cost or pleiotropic effect of multiple copies of the EPSPS gene.

Common ragweed is one of the most important weeds in the soybean-producing areas of the United States and Canada. Recently, glyphosate-resistant (GR) biotypes have been reported in 15 states and one Canadian province. Reducing the proliferation of GR common ragweed biotypes is complicated by the high fecundity and complex seed germination behavior exhibited by this species. An experiment was conducted to evaluate the efficacy of late herbicide applications for reducing seed production, seed weight, and seed viability of a GR common ragweed biotype. Herbicide treatments included: water control, glyphosate, 2,4-D, dicamba, 2,4-D plus glyphosate, and dicamba plus glyphosate. Treatments were applied at the appearance of male flower buds (Biologische Bundesanstalt, Bundessortenamt and Chemical industry scale [BBCH] 51) or at the early female flowering stage (BBCH 61 to 63). At BBCH 51, 2,4-D or dicamba applied alone or in a tank mix with glyphosate reduced seed production by an average of 80%. Conversely, seed production following these same treatments applied at BBCH 61 to 63 was not significantly different from when glyphosate was applied alone. At this stage of development, all herbicide treatments reduced seed viability relative to the control; however, treatments containing 2,4-D or dicamba resulted in significantly lower viability than when glyphosate was applied alone. These results suggest that the application of tank mixes containing 2,4-D or dicamba have the potential to limit seed production of GR common ragweed when applied on or before BBCH 51. The development of new technologies that facilitate the in-crop application of tank mixes containing 2,4-D or dicamba may therefore be an effective option for limiting population establishment, seedbank replenishment, and future spread of glyphosate-resistant alleles.

Understanding the mechanism of herbicide resistance is fundamental for designing sustainable weed control strategies and exploiting herbicides rationally. Shortawn foxtail is a problem grass weed infesting several important crops in China. The repeated use of acetyl-CoA carboxylase (ACCase)-inhibiting herbicides has resulted in herbicide resistance in this weed. The ACCase gene of resistant individuals of a shortawn foxtail population (JSLS-1) has an Ile-2041-Thr mutation. F2 generation seeds, originated from the same heterozygous plant, were harvested, and two homozygous mutant (JSLS-1RR) and wild (JSLS-1SS) populations for the Ile-2041-Thr mutation were obtained. In whole plants, the JSLS-1RR population conferred high resistance to fenoxaprop and clodinafop, moderate resistance to haloxyfop, low resistance to pinoxaden, and no obvious resistance to clethodim and sethoxydim, compared with JSLS-1SS and a proven susceptible population (HNXY-1). A derived cleaved amplified polymorphic sequence (dCAPS) marker was developed to rapidly detect the rare Ile-2041-Thr mutation in the shortawn foxtail population. This is the first report of the cross-resistance pattern of Ile-2041-Thr mutation, and the robust dCAPS marker could quickly detect this mutation in shortawn foxtail.

Silky windgrass and annual bluegrass are among the most troublesome weeds in northern European winter crops, while problems with rattail fescue have been especially linked to direct-drilling practices. This study investigated the germination patterns of silky windgrass, annual bluegrass, and rattail fescue in multiple water potentials and temperature regimes. Temperature and water potential effects were similar between silky windgrass and rattail fescue, but differed from annual bluegrass. The three grass weeds were able to germinate under low water potential (−1.0 MPa), although water potentials ≤−0.25 MPa strongly delayed their germination. Silky windgrass and rattail fescue seeds were able to germinate at 1 C, while the minimum temperature for annual bluegrass germination was 5 C. Germination of silky windgrass and rattail fescue was very similar across temperature and water potentials, which implies similar emergence flushes under field conditions, allowing management interventions to follow the same scheme.

Glyphosate and 2,4-D have been commonly used for control of common and giant ragweed before planting of corn and soybean in the midwestern United States. Because these herbicides are primarily applied in early spring, environmental factors such as temperature may influence their efficacy. The objectives of this study were to (1) evaluate the influence of temperature on the efficacy of 2,4-D or glyphosate for common and giant ragweed control and the level of glyphosate resistance and (2) determine the underlying physiological mechanisms (absorption and translocation). Glyphosate-susceptible (GS) and glyphosate-resistant (GR) common and giant ragweed biotypes from Nebraska were used for glyphosate dose–response studies, and GR biotypes were used for 2,4-D dose–response studies conducted at two temperatures (day/night [d/n]; low temperature [LT]: 20/11 C d/n; high temperature [HT]: 29/17 C d/n). Results indicate improved efficacy of 2,4-D or glyphosate at HT compared with LT for common and giant ragweed control regardless of susceptibility or resistance to glyphosate. The level of glyphosate resistance decreased in both the species at HT compared with LT, primarily due to more translocation at HT. More translocation of 2,4-D in GR common and giant ragweed at HT compared with LT at 96 h after treatment could be the reason for improved efficacy. Similarly, higher translocation in common ragweed and increased absorption and translocation in giant ragweed resulted in greater efficacy of glyphosate at HT compared with LT. It is concluded that the efficacy of 2,4-D or glyphosate for common and giant ragweed control can be improved if applied at warm temperatures (29/17 C d/n) due to increased absorption and/or translocation compared with applications during cooler temperatures (20/11 C d/n).

The widespread occurrence of Palmer amaranth resistant to acetolactate synthase inhibitors and/or glyphosate led to the increased use of protoporphyrinogen oxidase (PPO)-inhibiting herbicides. This research aimed to: (1) evaluate the efficacy of foliar-applied fomesafen to Palmer amaranth, (2) evaluate cross-resistance to foliar PPO inhibitors and efficacy of foliar herbicides with different mechanisms of action, (3) survey the occurrence of the PPO Gly-210 deletion mutation among PPO inhibitor–resistant Palmer amaranth, (4) identify other PPO target-site mutations in resistant individuals, and (5) determine the resistance level in resistant accessions with or without the PPO Gly-210 deletion. Seedlings were sprayed with fomesafen (263 gaiha−1), dicamba (280 gaiha−1), glyphosate (870 gaiha−1), glufosinate (549 g ai ha−1), and trifloxysulfuron (7.84 gaiha−1). Selected fomesafen-resistant accessions were sprayed with other foliar-applied PPO herbicides. Mortality and injury were evaluated 21 d after treatment (DAT). The PPX2L gene of resistant and susceptible plants from a selected accession was sequenced. The majority (70%) of samples from putative PPO-resistant populations in 2015 were confirmed resistant to foliar-applied fomesafen. The efficacy of other foliar PPO herbicides on fomesafen-resistant accessions was saflufenacil>acifluorfen=flumioxazin>carfentrazone=lactofen>pyraflufen-ethyl>fomesafen>fluthiacet-methyl. With small seedlings, cross-resistance occurred with all foliar-applied PPO herbicides except saflufenacil (i.e., 25% with acifluorfen, 42% with flumioxazin). Thirty-two percent of PPO-resistant accessions were multiple resistant to glyphosate and trifloxysulfuron. Resistance to PPO herbicides in Palmer amaranth occurred in at least 13 counties in Arkansas. Of 316 fomesafen survivors tested, 55% carried the PPO Gly-210 deletion reported previously in common waterhemp. The PPO gene (PPX2L) in one accession (15CRI-B), which did not encode the Gly-210 deletion, encoded an Arg-128-Gly substitution. The 50% growth reduction values for fomesafen in accessions with Gly-210 deletion were 8- to 15-fold higher than that of a susceptible population, and 3- to 10-fold higher in accessions without the Gly-210 deletion.

Seedling emergence traits of susceptible (S) and resistant (R) blackgrass subpopulations isolated from a single non–target-site resistant (NTSR) population were studied in controlled conditions. The seedling emergence of the R subpopulation was lower and slower than that of the S subpopulation, especially at low temperature and deep burial. The burial depth inhibiting final emergence by 50% for the R subpopulation was significantly lower than that of the S subpopulation at low temperature. The present study revealed that under suboptimal conditions the NTSR loci conferring herbicide resistance were correlated with a fitness cost in relation to seedling emergence traits. The results suggest that deep soil cultivation and delayed sowing of autumn-sown crops can hamper germination of the R more than of the S subpopulation and thus potentially reduce the prevalence of the R subpopulation in the blackgrass population.

Lack of understanding the effects of single- and multiple-weed interference on soybean yield has led to inadequate weed management in Primorsky Krai, resulting in much lower average yield than neighboring regions. A 2 yr field experiment was conducted in a soybean field located in Bogatyrka (43.82°N, 131.6°E), Primorsky Krai, Russia, in 2013 and 2014 to investigate the effects of single and multiple interference caused by naturally established weeds on soybean yield and to model these effects. Aboveground dry weight was negatively affected the most by weed interference, followed by number of pods and seeds. Soybean yield under single-weed interference was best demonstrated by a rectangular hyperbolic model, showing that common ragweed and barnyardgrass were the most competitive weed species, followed by annual sowthistle, American sloughgrass, and common lambsquarters. In the case of multiple-weed interference, soybean yield loss was accurately described by a multivariate rectangular hyperbolic model, with total density equivalent as the independent variable. Parameter estimates indicated that weed-free soybean yields were similar in 2013 and 2014, i.e., estimated as 1.72 t and 1.75 t ha−1, respectively, and competitiveness of each weed species was not significantly different between the two years. Economic thresholds for single-weed interference were 0.74, 0.66, 1.15, 1.23, and 1.45 plants m−2 for common ragweed, barnyardgrass, annual sowthistle, American sloughgrass, and common lambsquarters, respectively. The economic threshold for multiple-weed interference was 0.70 density equivalent m−2. These results, including the model, thus can be applied to a decision support system for weed management in soybean cultivation under single and multiple-weed interference in Primorsky Krai and its neighboring regions of Russia.

A giant ragweed population with putative resistance to cloransulam was identified in a long-term corn–soybean rotation located in southern Wisconsin. The population represented the first potential instance of giant ragweed resistance to acetolactate synthase (ALS) inhibitors in the state. Seeds were collected from several plants and pooled for subsequent experiments. Whole-plant dose–response experiments showed a high level of resistance (>500-fold) of the resistant (R) accession to cloransulam compared with a sensitive (S) accession. In vivo ALS bioassays showed that the target enzyme was 10.6- to 13.6-fold less sensitive to cloransulam in R than in S plants. Partial sequence analysis of the ALS gene found a tryptophan-to-leucine substitution at the 574 amino acid position (W574L) in the R phenotype. To better understand the potential fitness costs associated with the target-site substitution, replacement series experiments performed under greenhouse conditions characterized the relative growth, development, and fecundity of the R accession compared with an S accession in the absence of cloransulam. Growth over time did not differ between the R and S accessions for plant height during the vegetative phase (21 to 98 d after planting [DAP]) or for plant leaf area (21 to 80 DAP). At the estimated maximum, proportional shoot dry mass of each accession did not differ from theoretical proportions representing competitive equivalence, indicating no difference in vegetative competitive ability. Fecundity of R plants (430±53 seeds plant−1) did not differ from that of S plants (451±47 seeds plant−1), nor did seed viability (74 to 75% across accessions). This is the first report of equal competitive ability, fecundity, and seed viability between giant ragweed accessions R or S to cloransulam. The results suggest that the cloransulam resistance trait may persist and spread in the giant ragweed field population over time, even in the absence of selection by cloransulam.

Waterhemp is a weed indigenous to the midwestern United States and is problematic in agronomic crop production. This weed is well suited to inhabit minimally tilled environments and is increasing in prevalence across many agricultural production areas and systems. A common garden experiment was established in Indiana in 2014 and 2015 with waterhemp populations from Indiana, Illinois, Missouri, Iowa, and Nebraska to compare the growth and development of waterhemp from these regions. Three establishment dates (May, June, and July) were used each year to simulate discontinuous germination. Mean biomass accumulations from the May (1,120 g plant−1) and June (1,069 g plant−1) establishment dates were higher than from the July (266 g plant−1) establishment date. There were no differences in biomass accumulations between the five populations in the May and June establishments, but biomass accumulations ranged from 195 to 338 g plant−1 in the July establishment. Mean seed yields were higher from the May (926,629 seeds plant−1) and June (828,905 seeds plant−1) establishment dates compared with the July (276,258 seeds plant−1) establishment. In the May and June establishments, seed yields ranged from 469,939 seeds plant−1 to 1,285,556 seeds plant−1. The Illinois population flowered the latest of all the populations yet also grew the tallest. The July establishment flowered the most rapidly after establishment, accumulated less biomass, and also had the largest seeds. This study demonstrated differences among waterhemp populations when grown in a common environment and the effect of establishment timing on waterhemp growth and development.

The effect of reduced light intensity on the growth and development of three common grass weeds, blackgrass, silky windgrass, and annual bluegrass, was studied. Two identical greenhouse experiments displaced in time were performed with six light levels aiming at 0%, 20%, 50%, 80%, 90%, and 95% shade corresponding to a mean daily light integral (DLI) of 12.4, 9.63, 7.13, 2.74, 0.95, and 0.69 mol m−2 d−1 in experiment 1 and 21.2, 18.0, 10.7, 3.71, 1.64, 1.20 mol m−2 d−1 in experiment 2. Climate screens of acrylic fabric were used to create the light levels. A DLI of 0.69 to 3.71 mol m−2 d−1 substantially reduced the plant height, the number of leaves, leaf chlorophyll content index, stomatal conductance, maximum photochemical efficiency of photosystem II, and dry matter of blackgrass. It also reduced plant height, the number of leaves, and dry matter and delayed flowering of windgrass and annual bluegrass. Annual bluegrass reacted most rapidly when light levels increased from the lowest levels by producing more leaves. DLI thresholds for blooming were estimated to be about 7.13 mol m−2 d−1 for windgrass and 1.64 mol m−2 d−1 for annual bluegrass. Annual bluegrass was able to bloom and sustain biomass even at a DLI of 1.64 mol m−2 d−1. This ability may contribute to an explanation of why annual bluegrass is among the most common weed species in highly competitive and well-fertilized crops even though it is much smaller than the two other grass species.

Silky windgrass is a serious weed in central and northern Europe. Its importance has escalated in recent years because of its growing resistance to acetolactate synthase (ALS)-inhibiting herbicides. This study investigated the resistance level for three herbicide sites of action in eight silky windgrass populations, collected in fields neighboring a field where iodosulfuron sodium salt–resistant silky windgrass had previously been found. Target site resistance (TSR) and non–target site resistance (NTSR) mechanisms were identified, and a spatial gradient distribution hypothesis of ALS resistance was tested. Populations showed large variations in ED50 values to iodosulfuron, with resistance indices (RIs) ranging from 0.1 to 372. No cross-resistance was found to other herbicide groups with the same site of action as iodosulfuron. In contrast, resistance was observed to the acetyl-CoA carboxylase inhibitor, fenoxaprop ethyl ester (RI from 0.7 to 776), while the activity of prosulfocarb, an inhibitor of long-chain fatty-acid synthesis, was unaffected. Iodosulfuron-resistant phenotypes were associated with NTSR, while fenoxaprop ethyl ester resistance was caused by both NTSR and TSR (Ile-1781-Leu mutation). A large-scale trend in the spatial distribution of resistance to ALS indicated a decreasing resistance with increased distance from an epicenter. After finer-scale analysis, less than 0.05% of the residual variation could be attributed to spatial autocorrelation. The spatial resistance pattern was not correlated with the dominant wind direction, while there was a correlation between the resistant phenotype and type of crop. This study underlines that NTSR mechanisms do not always confer broad resistance to different herbicide subclasses and site of action, hence the complex relationship to resistant phenotype. NTSR mechanisms, in particular detoxification, were present at different levels for the herbicides tested in the silky windgrass populations of this study. The factors contributing to the spatial distribution of resistance remain elusive.

American burnweed is an increasingly common annual weed in wild blueberry fields in Atlantic Canada and Maine. Knowledge of seed dormancy characteristics and potential for this species to form persistent seedbanks in wild blueberry soils, however, is lacking. A series of experiments were therefore conducted to investigate potential mechanisms regulating American burnweed seed dormancy in wild blueberry fields. Seeds were dormant at maturity and did not germinate in dark or light under warm conditions. Cold moist stratification (CMS) at 4 C for 90 d followed by exposure to warm conditions (22/15 C day/night) and light caused >90% germination, and germination was generally maximized following 80 d CMS. Exogenous potassium nitrate applied as a 5% solution did not stimulate germination, but nearly all seeds (>95%) germinated following treatment with 200, 400, 600, or 800 ppm (w/v) gibberellic acid (GA3) solution. Physical removal of the seed coat or seed exposure to short durations of dry heat did not increase germination. Seed exposure to 1 s of direct flame increased germination, but germination was low relative to germination following CMS and treatment with GA3. Based on these results, we conclude that American burnweed seeds in wild blueberry fields exhibit non–deep physiological dormancy that is most readily broken by CMS and light or seed treatment with GA3. Seeds will likely be exposed to favorable conditions for breaking dormancy (cold temperatures and light) in wild blueberry fields due to lack of tillage and seed burial, indicating high potential for this weed species to proliferate in wild blueberry fields if not properly managed.

Sown-grass margin strips, historically established to limit pesticide drift and soil erosion, are now also promoted for enhancing floral diversity and associated ecosystem services. To better understand weed community assembly in grass margin strips, we performed floral surveys in 75 sown-grass margin strips in two regions in France and characterized each species using information from trait databases. We hypothesized that traits of dominant species would differ between newly sown-grass margin strips and older strips. Weed species were separated into functional groups based on their traits using multiple correspondence analysis and hierarchical ascendant classification. Functional group trajectories were investigated in sown-grass margin strips that differed in age using a space-for-time substitution approach. We found that geophyte, competitor, and monocotyledon species were more frequent and abundant in grass margin strips than therophyte, ruderal, and dicotyledon species. Results also showed that floral diversity was greatest in grass margin strips of intermediate age. Our findings have implications for optimizing diversity and ecosystem services on land enrolled in conservation programs and suggest that mowing later in the season and periodic soil disturbance can increase floral diversity. The analytical framework that we introduced in this research can also be used to explore weed community assembly in other systems.

In the midwestern United States, biotypes of giant ragweed resistant to multiple herbicide biochemical sites of action have been identified. Weeds with resistance to multiple herbicides reduce the utility of existing herbicides and necessitate the development of alternative weed control strategies. In two experiments in southeastern Minnesota, we determined the effect of six 3 yr crop-rotation systems containing corn, soybean, wheat, and alfalfa on giant ragweed seedbank depletion and emergence patterns. The six crop-rotation systems included continuous corn, soybean–corn–corn, corn–soybean–corn, soybean–wheat–corn, soybean–alfalfa–corn, and alfalfa–alfalfa–corn. The crop-rotation system had no effect on the amount of seedbank depletion when a zero-weed threshold was maintained, with an average of 96% of the giant ragweed seedbank being depleted within 2 yr. Seedbank depletion occurred primarily through seedling emergence in all crop-rotation systems. However, seedling emergence tended to account for more of the seedbank depletion in rotations containing only corn or soybean compared with rotations with wheat or alfalfa. Giant ragweed emerged early across all treatments, with on average 90% emergence occurring by June 4. Duration of emergence was slightly longer in established alfalfa compared with other cropping systems. These results indicate that corn and soybean rotations are more conducive to giant ragweed emergence than rotations including wheat and alfalfa, and that adopting a zero-weed threshold is a viable approach to depleting the weed seedbank in all crop-rotation systems.

Knowledge of Palmer amaranth demographics and biology is essential for the development and implementation of weed management strategies. A field experiment was conducted to investigate the effects of Palmer amaranth density on seedling mortality, flowering initiation, and flowering progress throughout the growing season and biomass production and fecundity in wide-row soybean. The experimental design was a randomized complete block design with three levels of Palmer amaranth density-clusters: high, medium, and low. Palmer amaranth mortality rate was greater at high Palmer amaranth population density-cluster, reaching a peak within 30 to 40 d after Palmer amaranth emergence (DAE) (0.55 and 0.80 for 2014 and 2015, respectively), in comparison with mortality rate at medium and lower density-clusters. Likewise, as Palmer amaranth density increased, biomass and seed production per unit area of the weed also increased. Biomass production at the high density-cluster in 2014 was 664.7 g m−2 compared with 542.9 and 422.1 g m−2 at medium and low density-clusters, respectively. Similarly, biomass production at high density-cluster in 2015 was 100.6 g m−2 compared with 37.3 and 34.2 at medium and low density-clusters, respectively. In addition, seeds produced at high density-cluster were 1.5 million and 245,400 seeds m−2 for 2014 and 2015, respectively. Seed production was reduced by 29% and 54% in 2014 and by 65% and 75% in 2015 at medium and low density-clusters, respectively. Earlier flowering initiation (i.e., between 30 to 40 DAE) occurred in higher Palmer amaranth density-clusters, indicating a trade-off between reproduction and survival at high densities and more stressed environments for species survival. Palmer amaranth male-to-female sex ratio was greater at high densities, 1.3 and 1.9, compared with lower densities of 0.6 to 0.7 and 0.7 to 0.8 in 2014 and 2015, respectively. The plasticity of Palmer amaranth population and population-structure regulation, vegetative growth, and flowering shifts at various levels of intraspecific competition (i.e., high vs. low population density-clusters) and the trade-off between these biological transitions merits further investigation.