Transgressive segregation refers to the phenomenon whereby the progeny of a diverse cross exhibit phenotypes that fall outside the range of the parents for a particular trait of interest. Segregants that exceed the parental values in life-history traits contributing to survival and reproduction may represent beneficial new allelic combinations that are fitter than respective parental genotypes. In this research, we use geographically disparate paraquat-resistant biotypes of horseweed (Canada fleabane) [Erigeron canadensis L.; syn.: Conyza canadensis (L.) Cronquist] to explore transgressive segregation in biomass accumulation and the inheritance of the paraquat resistance trait in this highly self-fertilizing species. Results of this research indicate that the paraquat resistance traits in E. canadensis biotypes originating in California, USA, and Ontario, Canada, were not conferred by single major gene mechanisms. Segregating generations from crosses among resistant and susceptible biotypes all displayed transgressive segregation in biomass accumulation in the absence of the original selective agent, paraquat. However, when challenged with a discriminating dose of paraquat, progeny from the crosses of susceptible × resistant and resistant × resistant biotypes displayed contrasting responses with those arising from the cross of two resistant biotypes no longer displaying transgressive segregation. These results support the prediction that transgressive segregation is frequently expressed in self-fertilizing lineages and is positively correlated with the genetic diversity of the parental genotypes. When exposed to a new environment, transgressive segregation was observed regardless of parental identity or history. However, if hybrid progenies were returned to the parental environment with exposure to paraquat, the identity of the fittest genotype (i.e., parent or segregant) depends on the history of directional selection in the parental lineages and the dose to which the hybrid progeny was exposed. It is only in the original selective environment that the impact of allelic fixation on transgressive segregation can be observed.

Foliar-applied postemergence herbicides are a critical component of corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] weed management programs in North America. Rainfall and air temperature around the time of application may affect the efficacy of herbicides applied postemergence in corn or soybean production fields. However, previous research utilized a limited number of site-years and may not capture the range of rainfall and air temperatures that these herbicides are exposed to throughout North America. The objective of this research was to model the probability of achieving successful weed control (≥85%) with commonly applied postemergence herbicides across a broad range of environments. A large database of more than 10,000 individual herbicide evaluation field trials conducted throughout North America was used in this study. The database was filtered to include only trials with a single postemergence application of fomesafen, glyphosate, mesotrione, or fomesafen + glyphosate. Waterhemp [Amaranthus tuberculatus (Moq.) Sauer], morningglory species (Ipomoea spp.), and giant foxtail (Setaria faberi Herrm.) were the weeds of focus. Separate random forest models were created for each weed species by herbicide combination. The probability of successful weed control deteriorated when the average air temperature within the first 10 d after application was <19 or >25 C for most of the herbicide by weed species models. Additionally, drier conditions before postemergence herbicide application reduced the probability of successful control for several of the herbicide by weed species models. As air temperatures increase and rainfall becomes more variable, weed control with many of the commonly used postemergence herbicides is likely to become less reliable.

Weeds are one of the greatest challenges to snap bean (Phaseolus vulgaris L.) production. Anecdotal observation posits certain species frequently escape the weed management system by the time of crop harvest, hereafter called residual weeds. The objectives of this work were to (1) quantify the residual weed community in snap bean grown for processing across the major growing regions in the United States and (2) investigate linkages between the density of residual weeds and their contributions to weed canopy cover. In surveys of 358 fields across the Northwest (NW), Midwest (MW), and Northeast (NE), residual weeds were observed in 95% of the fields. While a total of 109 species or species-groups were identified, one to three species dominated the residual weed community of individual fields in most cases. It was not uncommon to have >10 weeds m^–2 with a weed canopy covering >5% of the field's surface area. Some of the most abundant and problematic species or species-groups escaping control included amaranth species such as smooth pigweed (Amaranthus hybridus L.), Palmer amaranth (Amaranthus palmeri S. Watson), redroot pigweed (Amaranthus retroflexus L.), and waterhemp [Amaranthus tuberculatus (Moq.) Sauer]; common lambsquarters (Chenopodium album L.); large crabgrass [Digitaria sanguinalis (L.) Scop.]; and ivyleaf morningglory (Ipomoea hederacea Jacq.). Emerging threats include hophornbeam copperleaf (Acalypha ostryifolia Riddell) in the MW and sharppoint fluvellin [Kickxia elatine (L.) Dumort.] in the NW. Beyond crop losses due to weed interference, the weed canopy at harvest poses a risk to contaminating snap bean products with foreign material. Random forest modeling predicts the residual weed canopy is dominated by C. album, D. sanguinalis, carpetweed (Mollugo verticillata L.), I. hederacea, amaranth species, and A. ostryifolia. This is the first quantitative report on the weed community escaping control in U.S. snap bean production.

Barley (Hordeum vulgare L.) and brown mustard [Brassica juncea (L.) Czern.] are winter cover crops known to produce allelochemicals that suppress plant growth. Incorporating barley or brown mustard residues into the soil before planting a spring-seeded cash crop may suppress early-season weeds in the cash crop; however, the comparative levels of weed suppression offered by barley and brown mustard cover crops incorporated into soil have not been determined. This study analyzed the relative capacities of barley and brown mustard cover crops to suppress early-season weeds of spring-seeded chile pepper (Capsicum annuum L.). Reductions in weed density or hand-hoeing time as a result of barley and/or brown mustard cover crop treatment were determined in two chile pepper fields in New Mexico over two growing seasons. For cover crop species that suppressed weeds in multiple site-years, a controlled environment study clarified possible growth stages adversely affected by determining the effects of cover crop–amended soil on the germination and seedling development of Palmer amaranth (Amaranthus palmeri S. Watson). Field study results indicated barley reduced early-season weed densities of chile pepper by up to 80% compared with the noncover control. Barley also reduced hoeing time in 3 of 4 site-years without affecting chile pepper fruit yield. Mustard cover crops reduced weed density in only 1 site-year (56% reduction relative to noncover control) and did not decrease hoeing time. The controlled environment study indicated that soil amended with barley slowed germination of A. palmeri without inhibiting seedling development. The results of this study indicate that a barley cover crop is more effective than brown mustard for early-season weed control of chile pepper in the southwestern United States.

Interrow weed control is used in a wide range of crops, traditionally applied via physical cultivation or banded herbicide application. However, these methods may result in crop damage, development of herbicide resistance, or off-target environmental impacts. Electric interrow weed control presents an alternative, although its potential impact on crop yield requires further investigation. One of the modes of action of electric weed control is the continuous electrode–plant contact method, which passes a current through the weed and into the roots. As the current passes into the roots, it can potentially disperse through the soil to neighboring root systems. Such off-target current dispersion, particularly in moist topsoil with low resistance, poses potential concern for neighboring crops when electric interrow weed control is applied. This research evaluated the continuous electrode–plant contact method, using a Zasso™ XPower machine, in comparison with mowing across three trials conducted in 2022 and 2023. Both treatments were used to remove target lupine (Lupinus albus L.) plants adjacent to a row of non-target lupine. Electric weed control was applied to plants in dry soil or following a simulated rainfall event. The trials demonstrated that electric weed control and mowing did not reduce density and biomass of neighboring non-target lupine plants compared with the untreated control. Likewise, pod and seed production, grain size, and protein, as well as grain germinability and vigor of the resulting seedlings, were not reduced by these weed control tactics. This research used technology that was not fit for purpose in broadscale grain crops but concludes that electric weed control via the continuous electrode–plant contact method or mowing did not result in crop damage. Therefore, it is unlikely that damage will occur using commercial-grade electric weed control or mowing technology designed for large-acreage interrow weed control, thus offering nonchemical weed management options.

Annual bluegrass (Poa annua L.) populations in turfgrass have evolved resistance to several herbicides in the United States, but there has been no confirmed resistance from an agricultural field. Recently, glyphosate failed to control a P. annua population found in a field in a soybean [Glycine max (L.) Merr.] and rice (Oryza sativa L.) rotation in Poinsett County, AR. The present study focused on determining the sensitivity of a putatively resistant accession (R1) to glyphosate compared with two susceptible accessions (S1 and S2). The experiments included a dose–response study, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene copy number and expression analysis, and assessment of mutations in EPSPS. Based on the dose–response analysis, R1 required 1,038 g ae ha–¹ of glyphosate to cause 50% biomass reduction, whereas S1 and S2 only required 148.2 and 145.5 g ae ha–¹, respectively. The resistance index (RI) was approximately 7-fold relative to the susceptible accessions. Real-time polymerase chain reaction data revealed at least a 15-fold increase in the EPSPS copy number in R1, along with a higher gene expression. No mutations in EPSPS were found. Gene duplication was identified as the main mechanism conferring resistance in R1. The research presented here reports the first incidence of glyphosate resistance in P. annua from an agronomic field crop situation in the United States.

Two separate field experiments were conducted during the 2021 to 2022 and 2022 to 2023 growing seasons at Kansas State University Agricultural Research Center near Hays, KS, to understand the emergence dynamics of glyphosate-resistant (GR) kochia [Bassia scoparia (L.) A. J. Scott] as influenced by fall- and spring-planted cover crops (CC) and residual herbicide. Study sites were under winter wheat (Triticum aestivum L.)–sorghum [Sorghum bicolor (L.) Moench]–fallow rotation with a natural seedbank of GR B. scoparia. In Experiment 1, fall-planted CC mixture (triticale/winter peas/radish/canola) was planted after wheat harvest and terminated at triticale [×Triticosecale Wittm. ex A. Camus [Secale × Triticum] heading stage (next spring before sorghum planting). In Experiment 2, spring-planted CC mixture (oats/barley/spring peas) was planted in sorghum stubbles and terminated at oats (Avena sativa L.) heading stage. Four treatments were established in each experiment: (1) nontreated control (no CC and no herbicide), (2) chemical fallow (no CC but glyphosate acetochlor/atrazine or flumioxazin/pyroxasulfone dicamba were used to control weeds), (3) CC terminated with glyphosate, and (4) CC terminated with glyphosate plus residual herbicide (acetochlor/atrazine for fall-planted CC and flumioxazin/pyroxasulfone for spring-planted CC). Results indicated that fall-planted CC delayed GR B. scoparia emergence by 3 to 5 wk, whereas spring-planted CC delayed emergence by 0 to 2 wk compared with nontreated control. Fall-planted CC terminated with glyphosate plus acetochlor/atrazine reduced the cumulative emergence of GR B. scoparia by 90% to 95% compared with nontreated control across both years. Similarly, spring-planted CC terminated with glyphosate plus flumioxazin/pyroxasulfone reduced the cumulative emergence of GR B. scoparia by 83% to 90% compared with nontreated control. These results suggest that fall- or spring-planted CC in combination with residual herbicide at termination can be utilized for GR B. scoparia suppression. Results from this study will help in developing prediction models for GR B. scoparia emergence under different CC strategies.

Russian thistle (Salsola tragus L.) is a significant summer annual weed in the semiarid Pacific Northwest, causing yield losses of up to 50%. Understanding the biology and ecology of S. tragus is vital for developing effective integrated weed management strategies. This study focused on (1) S. tragus emergence and seedbank persistence in two cropping systems: fallow–winter wheat (Triticum aestivum L.) and spring wheat–fallow–winter wheat rotations, and (2) S. tragus plant biomass and viable seed production in fallow and spring wheat fields. A 4-yr experiment (2020 to 2023) was conducted at the Columbia Basin Agriculture Research Center in Adams, OR, using a randomized block design with four replications. Salsola tragus seeds were sprinkled only at the beginning of the experiment, and seedling numbers were recorded throughout. Most seedlings emerged in the first year, with the highest rates in spring wheat (72%) and fallow (32%), followed by significantly lower rates (0.25% to 5%) in subsequent years. Seedling emergence began in late March and early April in the first and second years but was delayed to May in the third year. Plant biomass and viable seed production were greater in fallow than in spring wheat, with early-season plants having more biomass than later-emerging plants. Plants emerged between early and mid-May produced the most viable seeds. Viable seed production was very low until it peaked in mid-September. Findings indicated that most S. tragus seedlings emerged in the first year after dispersal coinciding with spring precipitation and lasting approximately 2 mo. Additionally, most S. tragus plants produce viable seeds in September, and seeds persist in the soil for more than 2 yr. These results demonstrate the need for growers to control S. tragus emergence to prevent reinfestations and ultimately the need to control S. tragus plants before September to prevent the species from producing viable seed.

Weed management in California water-seeded rice (Oryza sativa L.) is challenging due to herbicide-resistant weeds. Research on additional herbicide options is necessary to control herbicide-resistant weeds. Pendimethalin is a dinitroaniline herbicide commonly used in dry-seeded rice; however, it is not registered in water-seeded rice. This study was conducted to determine the pendimethalin fate in water-seeded rice after application to 1-, 3-, and 5-leaf stage rice. Ultra-high performance liquid chromatography–tandem mass spectrometry (UPLC-MS/MS) was utilized to quantify pendimethalin and degradants in the water, soil, and rice seedling tissue at 1, 5, and 14 d after treatment (DAT). More than 50% of recovered pendimethalin was observed in the rice tissue and more than 25% in the soil, with the least amounts observed in the water at all application timings and sampling dates. Three pendimethalin degradants were observed at low concentrations: p36, [1-(1-ethylpropyl)-5,6-dimethyl-7-nitro-1H-benximidazole]; p44, [4-[(1-ethylpropyl) amino]-2-methyl-3,5-ditrobenzoic acid]; and p48 [4,5-dimethyl-3-nitro-N²-(pentan-3-yl) benzene-1,2-diamine]. Degradant p36 was observed in all samples and most abundant in the soil. Degradants p36 and p44 increased in concentration in the water by 14 DAT. Degradants p44 and p48 were at low concentrations or below the lowest level of quantification in water, soil, and tissue samples. The pendimethalin parent molecule remained intact and was not readily metabolized in rice tissue. The crown region and shoots of the rice seedlings demonstrated greater pendimethalin concentrations compared with the roots at all rice stages; however, pendimethalin concentrations remained similar across the three sample timings. Rice root and shoot reduction was 16% and 13%, respectively, after the 1-leaf stage application averaged over sample timings, and 6% and 4%, respectively, after the 5-leaf stage application. The results suggest the rice stage at the application timing is an important factor for pendimethalin tolerance; therefore, encouraging early root development can be beneficial for pendimethalin tolerance in water-seeded rice.

Multiple herbicide–resistant (MHR) kochia [Bassia scoparia (L.) A.J. Scott] is a concern for farmers in the Great Plains. A total of 82 B. scoparia populations were collected from western Kansas (KS), western Oklahoma (OK), and the High Plains of Texas (TX) during fall of 2018 and 2019 (from the various locations), and their herbicide resistance status was evaluated. The main objectives were to (1) determine the distribution and frequency of resistance to atrazine, chlorsulfuron, dicamba, fluroxypyr, and glyphosate; and (2) characterize the resistance levels to glyphosate, dicamba, and/or fluroxypyr in selected B. scoparia populations. Results indicated that 33%, 100%, 48%, 30%, and 70% of the tested B. scoparia populations were potentially resistant (≥20% survival frequency) to atrazine, chlorsulfuron, dicamba, fluroxypyr, and glyphosate, respectively. A three-way premixture of dichlorprop/dicamba/2,4-D provided 100% control of all the tested populations. Dose–response studies further revealed that KS-9 and KS-14 B. scoparia populations were 5- to 10-fold resistant to dicamba, 3- to 6-fold resistant to fluroxypyr, and 4- to 5-fold resistant to glyphosate as compared with the susceptible (KS-SUS) population. Similarly, OK-10 and OK-11 populations were 10- to 13-fold resistant to dicamba and 3- to 4-fold resistant to fluroxypyr and glyphosate compared with the OK-SUS population. TX-1 and TX-13 B. scoparia populations were 2- to 4-fold resistant to dicamba, and TX-1 was 5-fold resistant to glyphosate compared with the TX-SUS population. These results confirm the first report of dicamba- and fluroxypyr-resistant B. scoparia from Oklahoma and glyphosate- and dicamba-resistant B. scoparia from Texas. These results imply that adopting effective integrated weed management strategies (chemical and nonchemical) is required to mitigate the further spread of MHR B. scoparia in the region.

Knotroot foxtail [Setaria parviflora (Poir.) Kerguélen], a perennial Setaria species, is becoming more problematic in forage and grazing systems across the southeastern United States. Setaria parviflora reproduces through the production of rhizomes and seeds, further complicating management strategies. Significant knowledge gaps exist regarding the biology and control of this species. This research aimed to understand the influence of burial depth on S. parviflora propagules and the physiological differences between it and other Setaria spp. Experiments were conducted between October 2019 and February 2021 in Clarke County, GA, to investigate the influence of burial depth (1, 2, 4, 8, and 16 cm) on the emergence and growth of S. parviflora rhizomes and seeds. Zero emergence was estimated at 8.7, 10.8, and 11.2 cm for small rhizomes, large rhizomes, and seeds, respectively. Therefore, producers could implement tillage events to a depth of 11.2 cm or greater to control S. parviflora. A separate study compared S. parviflora, yellow foxtail [Setaria pumila (Poir.) Roem. & Schult.], green foxtail [Setaria viridis (L.) P. Beauv.], and giant foxtail (Setaria faberi Herrm.) plant morphology. Despite similar aboveground appearances, S. pumila and S. parviflora had different total and belowground biomass 2 to 5 mo after emergence, which suggests differences in root formation and perennialization of S. parviflora. The present research determined that burying propagules using tillage could be included in management recommendations concerning S. parviflora; however, it should be complemented with herbicide applications during the growing season to assist in controlling S. parviflora plants produced by seeds.

A multiyear study was carried out at two citrus groves with mature trees in southwest Florida in the United States to evaluate the effects of cover cropping on the citrus interrow as a sustainable weed management strategy in the Florida citrus production system. Two cover crop (CC) mixes (legume + non-legume species and only non-legume species) were compared with a no-CC grower standard management (GSM) that utilized the herbicide paraquat for weed suppression in the citrus tree interrow spaces. We gathered data on the biomass and density of both CCs and weeds, during the spring and summer/fall CC planting seasons throughout the study years. Both mixes of CCs effectively reduced weed density in the citrus interrow by 58% to 99% (P < 0.05), depending on the growing season and study locations, compared with GSM. Additionally, there were no significant differences observed between the different CC mixes. Similarly, both CC mixes reduced the weed biomass by 95% to 99% (P < 0.05) in the citrus interrow compared with the GSM. However, weed suppression by CCs varied between growing seasons, mainly due to differences in germination and establishment of the CCs in each season.

Tetflupyrolimet (Dodhylex™ Active, FMC Corporation) is a novel herbicide inhibiting de novo pyrimidine biosynthesis that controls grassy weeds preemergence in rice (Oryza sativa L.) production. Field trials were conducted from 2021 to 2024 to evaluate turfgrass tolerance to tetflupyrolimet applications for annual bluegrass (Poa annua L.) and smooth crabgrass [Digitaria ischaemum (Schreb.) Schreb. ex Muhl.] control. Tolerance was evaluated on seven turfgrass species, including creeping bentgrass (Agrostis stolonifera L.), Kentucky bluegrass (Poa pratensis L.), tall fescue [Schedonorus arundinaceus (Schreb.) Dumort.; syn.: Festuca arundinacea Schreb.], hybrid bermudagrass [Cynodon dactylon (L.) Pers. × Cynodon transvaalensis Burtt-Davy], and manilagrass [Zoysia matrella (L.) Merr.] at various mowing heights ranging from 3.8 to 12.5 mm. Separate experiments were conducted on each turfgrass species to evaluate tolerance in both fall and spring. Tetflupyrolimet was applied at rates of 0, 25, 50, 100, 200, 400, 800, 1600, 3200, or 6400 g ai ha–¹. No injury was observed on any warm-season turfgrass species in either season, whereas cool-season grass tolerance varied among species each season; however, cool-season turfgrass tolerance for all species was greater in spring than fall. While efficacy of tetflupyrolimet (400 g ha–¹) for preemergence D. ischaemum control varied among years, mixtures of tetflupyrolimet (400 g ha–¹), pyroxasulfone (128 g ai ha–¹), and rimsulfuron (35 g ai ha–¹) applied preemergence or early postemergence effectively controlled multiple-resistant P. annua in both seasons. Overall, these findings highlight that warm-season turfgrasses are highly tolerant of tetflupyrolimet applications for P. annua or D. ischaemum control.

Trifludimoxazin is a new herbicide that inhibits protoporphyrinogen oxidase (PPO) and is targeted for commercial market introduction in North America, South America, and Asia. It will be available both as a stand-alone product and in a 1:2 mixture with saflufenacil. The herbicide is intended for use in preplant burndown and preemergence applications in cereal, corn (Zea mays L.), soybean [Glycine max (L.) Merr.], and pulse crops to control a variety of annual broadleaf and grass weed species. Additionally, it is intended to be used in tree crops, oil palm (Elaeis guineensis Jacq.), and non-crop areas. In this study, we meticulously evaluated the performance and effectiveness of both the stand-alone herbicide and the innovative mixture concept in combating prevalent weeds commonly encountered in corn and soybean fields. Our findings revealed that both products exhibited exceptional efficacy, significantly reducing the presence of these troublesome weeds. Furthermore, the mixture concept not only demonstrated commendable soil mobility but also showcased impressive residual activity, positioning it as a powerful tool for sustainable weed control. These promising effects are further substantiated by our comprehensive adsorption–distribution–metabolism–extraction (ADME) studies, which provide insight into the behavior and longevity of the herbicides in the agricultural ecosystem.

Accounting for 53% of U.S. peanuts (Arachis hypogaea L.), Georgia is the top peanut-producing state, with approximately 1.42 billion kg produced in 2023. Peanut producers often use the acetolactate synthase (ALS) imidazolinone herbicide imazapic, but reduced yellow nutsedge (Cyperus esculentus L.) control was reported in Georgia peanuts after 4 yr of continuous imazapic use. This study aimed to determine the level of resistance (LD_50, I₅₀, and GR₅₀) and potential cross-resistance for the suspected resistant population and to identify the associated genetic mutations conferring resistance. A susceptible biotype was treated with 0, 0.0088, 0.0175, 0.035, 0.07, 0.14, 0.28, and 0.56 kg ai ha^–1, and a resistant biotype was sprayed with 0, 0.07, 0.14, 0.28, 0.56, 1.13, 2.26, and 4.5 kg ai ha^–1 of imazapic. To determine whether the suspected resistant biotype was cross-resistant to halosulfuron-methyl, an ALS herbicide used to control Cyperus spp., both biotypes were treated with 0, 0.0117, 0.0233, 0.0466, 0.0933, 0.187, 0.373, and 0.746 g ai ha^–1 of halosulfuron-methyl. Plants were rated for injury at 7, 14, and 28 d after treatment (DAT), and aboveground biomass was harvested at 28 DAT. For imazapic, LD₅₀ was 0.041 and 1.503 kg ai ha^–1 and the GR₅₀ was estimated to be 0.0128 and 1.853 kg ha^–1 for Sus and Res biotypes, respectively, indicating 36- and 145-fold increase in resistance of the Res biotype for I₅₀ and GR₅₀, respectively. Both biotypes responded similarly to applications of halosulfuron-methyl, with biomass reduction at rates greater than 0.023 kg ai ha^–1. Transcriptome profiles revealed a mutation in the target-site gene of the resistant biotype causing an amino acid substitution from alanine to valine at position 205 (Ala-205-Val). Growers should continue to rotate chemistries and implement integrated weed management approaches for control of C. esculentus, as the use of imazapic over consecutive years has led to resistance in C. esculentus.

Italian ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot] has become a major annual weed in wheat (Triticum aestivum L.) production systems in the inland Pacific Northwest. With large genetic variability and abundant seed production, L. perenne ssp. multiflorum has developed globally 74 documented cases of herbicide resistance covering 8 different mechanisms of action. Harvest weed seed control (HWSC) systems were introduced in Australia in response to the widespread evolution of multiple herbicide resistance in rigid ryegrass (Lolium rigidum Gaudin) and wild radish (Raphanus raphanistrum L.). The efficacy of these systems for any given weed species is directly related to the proportion of total seed retained by that species at harvest time. From 2017 to 2020, ten L. perenne ssp. multiflorum plants were collected from three different slope aspects at each location in Washington, USA. Collections were initiated in each field when it was visually apparent that seed fill was nearly complete, and seed shatter had not yet occurred. Collection continued at near-weekly intervals until the fields were harvested. The number of filled florets on a spikelet was used to assess the degree of seed shatter over time. Seed shatter at harvest was 67% of the total number of florets on each spikelet. Seed shatter was closely aligned with wheat kernel development in both spring and winter wheat. The high percentage of L. perenne ssp. multiflorum seeds that are shattered by harvest may make HWSC less effective than for L. rigidum in Australia; however, seeds with the greatest biomass tend to not shatter before harvest, which may increase the efficacy of HWSC for managing the soil seedbank. Strategies like planting earlier-maturing wheat cultivars could help HWSC be more effective by having wheat harvest begin earlier, when more L. perenne ssp. multiflorum seeds are still on the mother plant.

Furrow-irrigated rice (Oryza sativa L.) hectares are increasing in the Midsouth. The lack of sustained flooding creates a favorable environment for weed emergence and persistence, which makes Palmer amaranth (Amaranthus palmeri S. Watson) difficult to control throughout the growing season. The negative yield impacts associated with season-long A. palmeri interference in corn (Zea mays L.), cotton (Gossypium hirsutum L.), and soybean [Glycine max (L.) Merr.] have been evaluated. However, there is limited knowledge of the weed's ability to influence rice grain yield. Research was initiated in 2022 and 2023 to determine the effect of A. palmeri time of emergence relative to rice on weed seed production and grain yield. Cotyledon-stage A. palmeri plants were marked every 7 d, beginning 1 wk before rice emergence through 4 wk after rice emergence. Amaranthus palmeri seed production decreased exponentially as emergence timing was delayed relative to rice, and seed production increased by 447 seed plant^–1 for every 1-g increase in weed biomass. Without rice competition and from the earliest emergence timing, A. palmeri produced 540,000 seeds plant^–1. Amaranthus palmeri that emerged 1 wk before the crop had the greatest spatial influence on rice, with grain yield loss of 5% and 50% at a distance of 1.4 m and 0.40 m from the weed, respectively. As A. palmeri emergence was delayed, the area of influence decreased. However, A. palmeri plants emerging 3.5 wk after rice emergence still negatively affected grain yield and produced sufficient seed to replenish the soil seedbank, potentially impacting long-term crop management decisions. These results show that the time of A. palmeri emergence is a crucial factor influencing rice grain yield and weed seed production, which can be used to determine the consequences of escapes in rice.