Seed germination responses to light influence the timing of weed emergence and management outcomes. We evaluated (1) variation in light-dependent germination among germplasm sources, (2) the relationship between primary dormancy and far-red light–enforced germination inhibition, and (3) the role of seed size in light-dependent dormancy. Between 2022 and 2025, germination of 70 kochia [Bassia scoparia (L.) A. J. Scott] populations and 49 common lambsquarters (Chenopodium album L.) accessions was evaluated under three light environments—dark, red, and far red—for both freshly harvested and stored seed. Bassia scoparia germination did not differ among light environments (ANOVA, P = 0.67), indicating weak photoblastic regulation at the species level. In contrast, C. album exhibited pronounced differences in light-dependent germination among accessions (P < 0.0001). Accessions with greater primary dormancy, defined as the difference between germination of freshly harvested and stored seed, exhibited stronger far-red-enforced germination inhibition, indicating coordinated regulation of dormancy depth and light sensitivity. Seed weight was not strongly associated with primary dormancy (Spearman’s ρ = 0.057, P = 0.11) or far-red suppression (ρ = 0.50, P = 0.06). These results indicate that variation in light-dependent dormancy and germination in C. album is governed primarily by genetic and physiological regulation of light perception rather than by seed size.

Understanding how crop species or communities influence weed seed mortality could effectively build ecological weed management systems. Therefore, we examined whether perennial forage monocultures or mixtures can accelerate weed seed mortality and affect the microbial composition of seeds. We buried mesh bags containing weed seeds of either Powell amaranth (Amaranthus powellii S. Watson) or velvetleaf (Abutilon theophrasti Medik.) in perennial forage treatments consisting of monocultures and mixtures of alfalfa (Medicago sativa L.), forage chicory (Cichorium intybus L.), and orchardgrass (Dactylis glomerata L.). Throughout a 2.5-yr duration, we evaluated seed mortality of both weed species and used 16S rRNA and internal transcribed spacer (ITS) amplicon sequencing to characterize A. powellii seed bacterial and fungal composition, respectively. We found limited effects of perennial forage treatment on A. theophrasti seed mortality, as the alfalfa–chicory biculture resulted in greater seed mortality compared with the orchardgrass monoculture. However, we found no other effects of perennial forage treatment on A. theophrasti or A. powellii seed mortality, nor did forage treatment affect the composition of bacteria or fungi associated with A. powellii seeds. We also found no effect of perennial forage richness on seed mortality of either weed species. Interestingly, soil cations (Ca, Mg, and K) tended to be negatively associated with weed seed mortality. Our research provided limited evidence that perennial forage communities can vary in their ability to accelerate weed seed mortality in the soil. However, we did uncover insights into microbial communities associated with weed seeds that could be promising for further research.

High-quality bermudagrass [Cynodon dactylon (L.) Pers.] forage production is a vital component of southeastern U.S. agriculture. Previous research indicates that environmental stresses like elevated CO2 may significantly affect crop productivity, weed competitiveness, and ecological shifts. Although the effects of elevated CO2 on C4 plants like bermudagrass could be marginal, the growth, vigor, and herbicide tolerance of C3 weeds can be affected profoundly. In the final 2 yr of a 7-yr study investigating the effects of management practices and elevated CO2 levels on bermudagrass forage production, we aimed to evaluate how these treatments impacted weed competition and diversity. From 2018 to 2025, bermudagrass was grown in a coarse-textured soil bin at the USDA-ARS National Soil Dynamics Laboratory in Auburn, AL. Open-top chambers delivered either ambient or elevated CO2 (+ 200 mg kg−1), and plots were either managed annually with fertilizer and herbicide or left unmanaged. In the final 2 yr, elevated CO2 had no significant effect on total biomass production (bermudagrass + weeds), but in unmanaged plots, elevated CO2 resulted in a significantly greater proportion of weeds. While some C4 grasses and sedges were observed in this experiment, most weed species were C3. Consequently, C3 species dominance was generally high, especially in managed plots exposed to elevated CO2. Weeds were observed and identified in all plots, but those managed with fertilizer and herbicide had a greater proportion of bermudagrass plants to weeds, as well as lower weed densities. Species diversity indices yielded significantly greater species richness under elevated CO2 conditions. Moreover, we observed greater weed diversity with elevated CO2, which was exacerbated without proper nutrient and weed management. This provides compelling evidence that substantial shifts in weed diversity could occur due to environmental change factors like elevated CO2 and the lack of a proper crop management program.

Wild radish (Raphanus raphanistrum L.) is a highly competitive annual broadleaf weed that significantly constrains wheat (Triticum aestivum L.) production, particularly under increasing herbicide resistance and limited chemical control options. Optimizing sowing geometry offers a practical, nonchemical approach to enhance crop competitiveness and suppress weed growth. A field study was conducted during 2022 and 2023 to evaluate the impact of different sowing geometries on R. raphanistrum suppression and wheat productivity. The experiment was arranged in a randomized complete block design with seven treatments, including broadcast sowing; line sowing at 11, 22, and 33 cm; ridge sowing (30 cm); bed sowing (60 cm); and cross sowing (22 cm). Sowing geometry significantly influenced R. raphanistrum density and biomass at all growth stages (15 to 45 d after sowing). Narrow spacing (11 cm) consistently resulted in the lowest weed density and biomass, while the wider 33-cm spacing resulted in the highest weed pressure. Crop growth and yield responses were consistent across years, with 11-cm line sowing producing the highest number of productive tillers (393.7 m−2), grains per spike (42.9), biological yield (15.4 Mg ha−1) and grain yield (6.0 Mg ha−1) averaged across 2 yr. This treatment was closely followed by cross sowing (22 cm). In contrast, wider spacing (33 cm) reduced grain yield by approximately 25% to 30% due to increased weed competition and reduced crop competitiveness. Correlation and principal component analyses revealed a strong negative association between late-season weed biomass and wheat productivity, emphasizing the importance of sustained weed suppression during critical growth stages. Overall, narrow row spacing, particularly 11-cm line sowing, enhanced crop competitiveness, effectively suppressed R. raphanistrum, and maximized wheat yield, demonstrating its potential as an eco-friendly strategy for integrated weed management.

Palmer amaranth (Amaranthus palmeri S. Watson) is a highly competitive and drought-tolerant weed that poses significant challenges to U.S. agriculture due to its adaptability, prolific seed production, and herbicide resistance. Understanding how water stress at specific growth stages affects its development and seed production is critical for optimizing management strategies, particularly in water-limited environments. This study evaluated the effects of growth stage–specific water stress on growth and fecundity of A. palmeri under controlled greenhouse conditions. Four irrigation levels (100%, 80%, 60%, and 40% field capacity [FC]) were applied during vegetative and reproductive stages. Increasing water stress significantly reduced plant height, stem diameter, and biomass, with reductions of 17% to 35% under severe stress (40% FC). Total biomass declined by approximately 17% under severe water limitation. Root biomass at 60% FC decreased by 13% to 14% relative to 100% FC, whereas root length density increased by 11% to 16%, indicating adaptive root elongation under severe water deficit conditions. Despite reductions in vegetative growth, A. palmeri maintained high reproductive capacity. Seed production at 100% FC was 48% to 72% higher than at 80% FC and more than 2- to 3-fold greater than at 60% FC. Furthermore, plants subjected to reproductive-stage stress produced approximately 29% more seeds than those stressed during the vegetative stage, highlighting strong stage-dependent reproductive plasticity. Seed germination was highest at 100% FC but declined by approximately 55% at 60% FC. In contrast, germination at 80% FC remained within 10% of optimal levels and was comparable under 40% FC, suggesting a nonlinear response and potential maternal effects enhancing seed resilience under stress. The study revealed the importance of early-season weed control to exploit periods of vulnerability and limit seedbank replenishment. Overall, this study provides valuable insights for integrating weed management strategies in dryland cropping systems, where water scarcity and herbicide resistance present ongoing challenges.

Winning the battle against weeds is crucial for sustainable rice (Oryza sativa L.) production in sub-Saharan Africa (SSA), where weeds remain a leading cause of yield losses and continue to threaten the livelihoods of millions of smallholder farmers, with farms below 1 ha. This review evaluates the dynamic landscape of weed control strategies by examining weed ecology; the limitations of traditional hand weeding; and the growing risks associated with overreliance on herbicides, including escalating health concerns, environmental impacts, and the rapid rise of herbicide resistance. The central finding advanced in this review is that, despite the proven potential of integrated weed management (IWM) to provide sustainable and resilient weed control, its widespread adoption remains considerably low. Key barriers include weak extension services, low farmer awareness, and insufficient policy support, which collectively prevent timely and effective uptake of diversified weed control strategies. While approaches such as biological control, cover cropping, crop rotation, and precision tools hold promise, they remain underutilized without strong institutional backing. Drawing from case studies across the region, the review argues that IWM could deliver the most resilient and context-appropriate results if embedded within robust advisory systems and supportive incentives. The paper concludes with recommendations to strengthen extension capacity, promote farmer-centered innovation, and align policies to accelerate sustainable, scalable adoption of IWM across SSA.

Palmer amaranth (Amaranthus palmeri S. Watson) is one of the most problematic weeds in U.S. agriculture, capable of rapidly adapting to environmental and management pressures. This study assessed temporal changes in glyphosate response in A. palmeri by comparing ED50 values, shikimic acid accumulation, and [14C]glyphosate absorption and translocation in four biotypes collected from two Georgia fields, Jones (J) and Little Jones (LJ), in 2008 and 2023. Glyphosate ED50 increased 9-fold (J08 vs. J23) and 25-fold (LJ08 vs. LJ23), indicating a marked reduction in glyphosate sensitivity between collection periods. Shikimic acid accumulation increased with glyphosate dose in all biotypes but remained substantially lower in biotypes collected in 2023, indicating reduced 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) inhibition. Radiolabeled assays revealed differences in early uptake, with populations collected in 2023 reaching near maximum absorption more rapidly, as reflected by shorter times to 95% absorption (A95), although total absorption continued to increase across all biotypes through 48 h after treatment. Translocation patterns varied only slightly among biotypes, suggesting that changes in glyphosate response are associated more closely with altered uptake kinetics and EPSPS-related mechanisms than with major reductions in systemic movement. These results demonstrate a temporal shift in glyphosate response in Georgia A. palmeri populations and highlight the importance of integrating kinetic analyses with traditional resistance metrics.

Herbicide resistance in Palmer amaranth (Amaranthus palmeri S. Watson) continues to threaten the sustainability of cotton (Gossypium hirsutum L.) production in the United States, partly because management programs often emphasize in‑season suppression without sufficiently limiting the number of individuals repeatedly exposed to postemergence herbicide selection. A 5-yr large-plot field experiment (2019 to 2023) was conducted to evaluate four integrated weed management (IWM) components: zero tolerance for seedbank replenishment (ZT–SBR), occasional deep inversion tillage (Occ_DIT), cereal rye cover crop (CRCC), and dicamba‑inclusive herbicide programs (Dic_inCrop), applied singly, in combination, or absent altogether in a conventional four-pass base program lacking all four components. Preemergence escapes, defined here as emerged A. palmeri surviving the residual herbicide preceding each pass, were quantified at each timing and analyzed as annual trajectories and 5-yr cumulative exposure (a proxy for accumulated postemergence selection). In 2024, legacy emergence was measured under unmanaged conditions. Occ_DIT strongly structured temporal trajectories, inducing an immediate low‑density state (12% of the base program in Year 1) versus gradual decline without Occ_DIT (∼75% in Year 1; steep early slope), yielding markedly lower cumulative preemergence-escape pressure. Across 5-yr, cumulative exposure was most reduced by Occ_DIT, followed by Dic_inCrop, CRCC, and ZT–SBR, with significant interactions indicating non‑additive benefits. In the legacy year, main‑effect incidence rate ratios (IRRs) showed substantial suppression by Dic_inCrop (IRR = 0.02), ZT–SBR (IRR = 0.02), Occ_DIT (IRR = 0.06), and CRCC (IRR = 0.24), with four‑way combinations reducing emergence by >98% relative to the base program. Positive-interaction IRRs reflected diminishing marginal returns near the ecological floor, not antagonism. Collectively, these results demonstrate that IWM efficacy at low weed densities is governed less by additive suppression than by how mortality is repartitioned across independent demographic bottlenecks, reinforcing the value of diversified IWM as an evolutionary risk-management strategy.

Trifludimoxazin + saflufenacil is a premix protoporphyrinogen oxidase (PPO)-inhibiting herbicide with preemergence and postemergence activity pending USEPA registration for burndown application ahead of planting corn (Zea mays L.), soybean [Glycine max (L.) Merr.], grain sorghum [Sorghum bicolor (L.) Moench], and wheat (Triticum aestivum L.). With early-season soybean growth often occurring simultaneously with preplant burndown of fields in later-planted soybean and grain sorghum, odds of negative impacts associated with off-target herbicide movement to the crop increase. Field studies were conducted in 2024 to evaluate effect of reduced rates of trifludimoxazin + saflufenacil (12.5% to 0.4% of the lowest proposed labeled application rate of 38.3 g ai ha−1) applied to 1- to 2-leaf soybean. Reduced rates of 1/8×, 1/16×, 1/32×, and 1/64× resulted in 53%, 43%, 32%, and 21% visual phytotoxicity at 1 wk after treatment (WAT), while at 4 WAT response was 31%, 23%, 17%, and 12%. Trifludimoxazin + saflufenacil at these rates reduced soybean height 30% to 7% and 18% to 4% at 1 and 4 WAT, respectively. Soybean yield was negatively impacted 8%, 5%, 2%, and 0% at these respective rates. Off-target movement of trifludimoxazin + saflufenacil to soybean in early vegetative growth will result in severe visual injury and height reduction, especially with exposure at the three highest rates evaluated. Soybean should not be expected to fully recover and significant negative impact on yield (5% or greater) will be observed at the two highest rates evaluated in this research.

Adoption of organic no-till soybean [Glycine max (L.) Merr.] production practices within grain cropping systems in the U.S. Northeast remains limited due to variability in performance and crop rotation challenges. During soybean production, weed control is provided by rolled-crimped cereal rye (Secale cereale L.) mulch, which can be insufficient due to late sowing within grain cropping systems. In two experiments, we evaluated rye cultural management strategies for rye biomass, weed suppression, and soybean yield. We tested (1) four rye seeding rates (31 to 188 kg ha−1) and two sowing arrangements (grid vs. row sowing) and (2) fall-applied poultry litter (0, 3.36, 6.63 Mg ha−1) with two soybean planting dates (planting green or standard planting). Increasing cereal rye seeding rate did not lead to increased rye biomass but did increase weed suppression. Soybean yield was unaffected by rye seeding rates. Sowing arrangement did not affect any response. Fall poultry litter significantly increased rye biomass, but weed suppression was unaffected. In 1 yr, planting green reduced soybean establishment and yield. Of the cultural practices evaluated, increasing seeding rate was most effective at reducing weed biomass, while poultry litter effectively increased rye biomass but not weed suppression. These results highlight the limitations of organic no-till soybean within grain crop rotations in the U.S. Northeast when using cereal rye as a stand-alone weed-suppression method. Increasing cereal rye seeding rates or applying fall fertility could be effective cultural practices when integrated with other weed control tactics to supplement weed suppression by rye surface mulch.

This study investigated the effects of essential oils (EOs) from rosemary (Rosmarinus officinalis L.) and Shirazi thyme (Zataria multiflora Boiss.) on the early growth and physiological characteristics of three weed species: redroot pigweed (Amaranthus retroflexus L.), common lambsquarters (Chenopodium album L.), and Canada thistle [Cirsium arvense (L.) Scop.] conducted at Razi University. the experiment utilized a factorial design with varying concentrations of EOs in laboratory (0.5, 1, 2, 5, 10, and 20 µl ml−1) and greenhouse (5, 10, and 20 µl ml−1) settings. Control treatments included distilled water, distilled water + Tween 20 (1%), and trifluralin and glyphosate. Gas chromatography–mass spectrometry analysis revealed 15 major compounds in rosemary (Rosmarinus officinalis L.) EO and 23 in Shirazi thyme EO. A significant inhibition of plumule and radicle lengths was reported across all species. In greenhouse trials, Shirazi thyme EO at 10 and 20 µl ml−1 reduced stem length by up to 75.2% and 74.5% in A. retroflexus, surpassing reductions caused by rosemary EO (57.2% at 20 µl ml−1) and glyphosate (65.2%). Root length reductions were most pronounced with Shirazi thyme EO, reaching up to 83.9% inhibition in A. retroflexus at 10 µl ml−1, exceeding effects of rosemary EO and glyphosate. Shirazi thyme EO causing up to 54.4% reduction in stem dry weight of C. arvense and 82.7% reduction in root dry weight of A. retroflexus. Leaf greenness, photosynthetic efficiency, and maximum quantum yield of photosystem II also decreased markedly with increasing EO concentrations, particularly under Shirazi thyme EO, which in some cases completely abolished photosynthetic efficiency, outperforming glyphosate.

Weedy plants display enormous variation in the phenological traits that make up their life cycles both within and between populations. Facultative winter annual species are particularly interesting, because they can adopt either a fall-emerging/spring-flowering or spring-emerging/summer-flowering life cycle at the population level via evolution or at the individual level via within-generation and transgenerational plasticity. Responses of phenological traits to the environment have often been found to be mediated by changes in hormone levels, especially the growth hormone gibberellic acid (GA). We conducted growth chamber and greenhouse experiments using the facultative winter annual Canadian horseweed [Erigeron canadensis L.; syn.: Conyza canadensis (L.) Cronquist] to investigate the interactive effects of genetic variation; parent plant life cycle; and plastic responses to temperature, light, and GA treatments. We found that contrary to a prior report, exposing imbibed seeds to 3 to 4 wk of cold (i.e., seed vernalization) does not always result in summer annual type growth, with considerable variation found among field-collected seeds from 10 populations. Further, seed vernalization and exogenous application of GA both tended to increase summer annual characteristics, interacting in ways that were largely consistent with the hypothesis that GA is a mechanism for cold-induced life-cycle differentiation. Light treatment did not significantly affect life-cycle traits, while parent life-cycle type had marginal effects on offspring life-cycle type. Finally, genetic variation among and within sites explained far less of the variation in life-cycle traits than the plastic responses to seed vernalization and GA treatments. Our study proposes that the seasonality of this harmful agricultural weed is influenced by a GA-mediated response to vernalization of seeds during winter, yet highlights the need for further study, given the variability in this response. Insight into the phenology of E. canadensis is important for management, given that intervention success depends on the timing of deployment relative to the weed’s life cycle.

It is well established that cytochrome P450 monooxygenases (P450s) play a crucial role in herbicide metabolism and resistance evolution in weeds. In most documented cases, P450-mediated resistance is primarily conferred through the overexpression of P450 enzymes. However, the regulatory mechanisms underlying this overexpression remain poorly understood. In insects, amino acid substitutions that enhance P450-mediated metabolic detoxification have been clearly demonstrated as a key mechanism of insecticide resistance. In contrast, their potential role in herbicide resistance in weeds remains unclear. In this study, two CYP96A146 variants from flixweed [Descurainia sophia (L.) Webb ex Prantl], designated CYP96A146-S and CYP96A146-R, were heterologously expressed in Saccharomyces cerevisiae. These variants, which differ by four amino acid residues, were examined for their ability to metabolize model substrates and herbicides. The results indicated that both variants exhibited catalytic activity toward model substrates of p-nitroanisole, methoxyresorufin, ethoxyresorufin, 7-ethoxycoumarin, and benzo[a]pyrene, as well as toward the herbicides tribenuron-methyl, bensulfuron-methyl, and carfentrazone-ethyl. Notably, CYP96A146-R showed significantly higher catalytic activity than CYP96A146-S to both the model substrates (p-nitroanisole, methoxyresorufin, ethoxyresorufin, and 7-ethoxycoumarin) and the herbicides (tribenuron-methyl and carfentrazone-ethyl). These findings suggest that the amino acid substitutions are likely responsible for the enhanced metabolic capability of CYP96A146-R. Such mutations may induce conformational changes of CYP96A146 enzyme, facilitating more frequent molecular collisions between CYP96A146 and the substrates or herbicides, thereby improving catalytic efficiency.

A trial involving two types of biochar was conducted in two greenhouse experimental runs in 2025 to determine weed control efficiencies of two preemergence herbicides (metribuzin and S-metolachlor) at respective label-recommended rates to control Palmer amaranth (Amaranthus palmeri S. Watson) and crowfootgrass [Dactyloctenium aegyptium (L.) Willd]. Plants were grown in a loamy soil amended with commercially available sugarcane (Saccharum officinale L.) and pinewood (Pinus spp.) biochars ( 2 Mg ha−1). Complete control of both weeds was observed with the label-recommended doses of metribuzin and S-metolachlor in pinewood biochar-amended soils, whereas in sugarcane biochar-amended soils, complete control of both weeds was achieved only with metribuzin at the label-recommended dose. The label-recommended dose of S-metolachlor provided nearly 47% and 74% control of A. palmeri and D. aegyptium, respectively, in sugarcane biochar-amended soils. Application of sugarcane biochar decreased the total residue of S-metolachlor compared with soil with no or pinewood biochar and showed poor weed control efficiency (WCE). On day 7, nearly 3% and 24% germination for D. aegyptium and A. palmeri, respectively, was observed with S-metolachlor in sugarcane biochar-amended soils, whereas no germination was recorded in other treatments, which indicated biochar–herbicide–species specificity. Pearson correlation analysis showed negative associations between weed germination and soil herbicide residues across sampling times, although these relationships were not statistically significant (P > 0.05). Results from this greenhouse study indicated that commercial biochars from different raw materials could influence the WCE of preemergence herbicides even at a low biochar dose (2 Mg ha−1). Field evaluations are necessary to understand the weed control dynamics.

Spray drift and volatilization of the herbicide dicamba from agricultural uses has been an area of significant study in recent years. While only the low-volatility formulations of dicamba have been approved for use in the United States in postemergence dicamba-tolerant (DT) cotton (Gossypium hirsutum L.) and soybean [Glycine max (L.) Merr.], a variety of other dicamba products that are not specifically identified as low-volatility have been approved for decades and can be used as preemergence herbicides in certain crops, including soybean and cotton, and in early postemergence corn (Zea mays L.) applications. This study compared the volatility and spray drift deposition resulting from an approved dicamba use on DT soybean, an approved dicamba use on corn, and an example of an unapproved use of dicamba on soybean all side by side in two locations. Results showed that the downwind spray drift deposition of dicamba was highest in the approved corn applications at both field locations due to medium droplet size and higher release height. The volatility of dicamba during the first sampling interval after application as measured by air concentrations was between 13 and 74 times higher for approved corn or unapproved soybean treatments at 1.5 m above the crop compared with approved soybean at both study locations. Results from this study suggest that the low-volatility formulations and label measures in DT soybean and cotton uses are beneficial in keeping dicamba off-target movement potential relatively low.

Seed position on the mother plant, time of seed germination, and soil moisture can affect life-history characteristics of annual plants. However, most studies considered only one level of seed position, and how seed position combined with emergence season and watering regime affect life history traits of annual invasive plants is not clear. In this study, seeds in different positions within and between diaspores at different positions in compound spikes of the invasive diaspore heteromorphic annual grass Tausch’s goatgrass (Aegilops tauschii Coss.) sown under different watering conditions and life-history traits were monitored. Seeds of A. tauschii germinated in autumn and in spring under both natural and increased soil moisture. Autumn-germinated plants were larger but allocated lower proportions of resources to reproductive organs than spring-germinated plants. Regardless of emergence season and soil watering regime, A. tauschii exhibited two levels of seed position effects on life-history traits. For distal diaspores/seeds, vegetative growth stage was longer and seedling emergence percentages, plant height, number of diaspores per tiller, and vegetative allocation were higher, while reproductive growth stage was shorter, and seedling survival percentages, number of tillers per plant, number of seeds per diaspore, and reproductive allocation were lower than for basal diaspores/seeds. Thus, in A. tauschii, distal diaspores/seeds represent a “high-risk” strategy and basal ones a “low-risk” strategy. This apparent “bet-hedging” strategy likely increases survival and geographic/habitat expansion of A. tauschii and also may contribute to its invasiveness.

Palmer amaranth (Amaranthus palmeri S. Watson) is among the most economically damaging weeds in the United States and has evolved resistance to numerous herbicides, including glyphosate. This fast-growing species has expanded its range into different regions, including New York State, where glyphosate-resistant (GR) populations are now found. We evaluated how temperature modifies two components of weed risk, sensitivity to glyphosate and competitiveness against corn (Zea mays L.), under growth chamber conditions. First, we performed a dose–response study testing glyphosate efficacy for a GR A. palmeri biotype from New York State and a glyphosate-susceptible (GS) biotype from Nebraska. Tests were conducted at low (25/15 C), medium (30/20 C), and high (35/25 C) day/night temperatures. The nontreated GS biotype produced greater biomass at low versus high temperatures. This biotype was highly sensitive to glyphosate across all temperatures. In contrast, biomass of the nontreated GR biotype did not vary with temperature. This biotype was less sensitive to glyphosate across all temperatures. Our second study tested whether temperature modifies the impacts of A. palmeri on corn biomass at four planting densities (zero, one, two, or four A. palmeri with one corn plant). We used the same three temperature regimes and the same two A. palmeri biotypes as the dose–response study. There was no evidence that the biotypes, at any planting density, were more competitive with corn under elevated temperatures. In fact, the biomass of A. palmeri decreased with increasing temperatures while corn biomass did not vary. In the competition study, the GS biotype from Nebraska had higher biomass than the GR biotype from New York. Overall, these results do not support the hypothesis that increasing temperatures would exacerbate the risk posed by the tested A. palmeri biotypes in corn. Instead, these two A. palmeri biotypes may be highly problematic under temperatures already occurring in New York State.

Johnsongrass [Sorghum halepense (L.) Pers.] is a problematic, perennial weed species that is increasingly difficult to control across a range of agroecosystems; especially under intensifying pressures from climate change and herbicide resistance. Glyphosate has historically been effective at controlling S. halepense; however, repeated and widespread use has imposed strong selection pressure resulting in resistant biotypes. A suspected glyphosate-resistant biotype was identified in Texas, and this study was performed to characterize potential target- and non–target site resistance mechanism(s) in this biotype. Dose–response assays revealed an observed ED50 of 1,036 g ae ha−1, a 2.3-fold resistance ratio compared with a glyphosate-sensitive biotype. Target-site resistance mechanisms were not detected; no amino acid substitutions or increased copy number of the EPSPS gene were observed. Liquid chromatography−mass spectrometry was used to quantify glyphosate. At 7 and 10 d after application, glyphosate concentrations in aboveground tissue were 48% and 50% lower, respectively, in the resistant compared with the susceptible biotype; however, there were no decreases in belowground glyphosate concentrations. Collectively, these results provide putative evidence for altered glyphosate fate as a resistance mechanism in S. halepense. Additionally, this study describes the development of novel molecular markers for EPSPS amplification and copy number variation, addressing a key limitation in resistance diagnostics for this polyploid weed species.

A cereal rye (Secale cereale L.) cover crop provides a multitude of benefits, including soil conservation and weed suppression. Cereal rye biomass is positively correlated with weed suppression; however, high biomass is not always feasible. The weed-suppression potential of cereal rye grown under conditions that do not support high biomass is unknown. In such cases, other mechanisms, such as reduced soil temperature and nutrient depletion, may contribute to weed suppression. The objective of this study was to determine the impact of cereal rye biomass levels on soil water, temperature, nutrients, and in turn, weed emergence patterns. Cereal rye was planted at four seeding rates (0, 20, 40, and 80 kg ha−1) and terminated at three timings (6, 4, and 2 wk before planting cotton [Gossypium hirsutum L.]). Soil water and temperature were continuously monitored using automatic sensors. Soil nutrient content was analyzed from samples taken before cereal rye planting and at each termination timing. Weed seedling emergence was assessed throughout the summer. Cover crop termination timing had a greater influence on biomass production than seeding rate; delaying termination by 4 wk resulted in approximately 80 to 210% more biomass. High cereal rye biomass levels reduced maximum soil surface (0 to 10 cm) temperature by up to 7 C and thermal amplitude by 10 C before crop planting. Cereal rye significantly reduced soil nitrogen content but had minimal effect on phosphorus and potassium. A minimum biomass production of 2 Mg ha−1 is necessary for 30% to 50% weed suppression, whereas moderate (2 to 4 Mg ha−1) and high biomass levels (∼6 Mg ha−1) provided 60% and 70% and >90% weed suppression, respectively. The time for 50% weed seedling emergence was delayed by 18 d under high cereal rye biomass compared with fallow. Overall, our findings indicate that cereal rye suppresses weeds through multiple mechanisms, explaining suppression even at low biomass levels.

Weed competition remains a major constraint to productivity in rice (Oryza sativa L.)-based cropping systems, yet the ecological factors structuring weed communities across environments and management regimes remain insufficiently resolved in South Asia. This study examined weed community composition, diversity, and dominance in rice systems of Bangladesh and Nepal to assess how environmental conditions, crop rotation, and herbicide use shape weed assemblages. Weed community surveys were conducted in major rice-growing areas during 2016 and 2019 in Bangladesh and during 2019 in Nepal, covering 120 farmers’ fields per season across contrasting crop rotation systems (rice–fallow–rice and wheat (Triticum aestivum L.)–jute (Corchorus olitorius L.)–rice in Bangladesh; rice–wheat–fallow and rice–lentil (Lens culinaris Medik.)–fallow in Nepal) and management regimes with and without preemergence herbicide use. We studied species richness, diversity, relative abundance, and frequency and their association with cropping system, weed management, land type, soil texture, and water management. Weed diversity and community composition varied significantly by site, year, cropping system, and herbicide use. Fields managed without herbicides consistently exhibited higher species richness and evenness, while herbicide use was associated with reduced diversity and dominance by fewer species. In Bangladesh, the wheat–jute–rice system supported higher weed diversity (0.96 to 1.01) than the rice–fallow–rice system (0.84 to 0.91), whereas in Nepal, weed community structure differed more strongly among locations than among crop rotations. Dominant species in Bangladesh included yellow nutsedge (Cyperus rotundus L.), bermudagrass [Cynodon dactylon (L.) Pers.], barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.], pickerelweed [Monochoria vaginalis (Burm. f.) C. Presl. ex Kunth], cosmopolitan bulrush [Bolboschoenus maritimus (L.) Palla]; syn.: Scirpus maritimus L.], and Paspalum spp., with relative abundance varying by land type and water regime. In Nepal, weed communities were characterized by recurring dominance of fimbry (Fimbristylis littoralis Gaudich.), ricefield flatsedge (Cyperus iria L.), C. dactylon, junglerice [Echinochloa colona (L.) Link], pimpernels (Lindernia spp.), and joyweeds (Alternanthera spp.), with shifts in abundance linked primarily to site-specific hydrological and soil conditions. Farmer identification of troublesome weeds closely matched measured dominance patterns. These findings provide an ecological foundation for refining site-specific weed management strategies.