Confirmation and Transcriptomic Characterization of Glufosinate-ammonium Resistance in Waterhemp ( <i>Amaranthus tuberculatus</i> ) Populations from Illinois

bioRxiv (Cold Spring Harbor Laboratory) · 2026-02-09

Abstract Glufosinate-ammonium (GA) has been widely used in Midwestern fields, and in recent years a growing number of failures to control waterhemp [ Amaranthus tuberculatus (Moq.) Sauer] have raised concerns about the potential evolution of resistance. The goal of this study was to investigate four independent cases of suspected resistance to GA in A. tuberculatus from Illinois using greenhouse, field, and transcriptomics studies. Greenhouse dose-response experiments revealed resistance ratios ranging from 2.2- to 3.4-fold based on survival and from 1.3- to 2.8-fold based on dry biomass relative to a susceptible population. A subsequent field study where one of the populations originated confirmed that twenty percent of treated plants survived the labeled GA field-recommended rate. Screening for other herbicide sites of action revealed that most populations showed reduced sensitivity to atrazine, glyphosate, and imazethapyr, surviving up to three times the field-recommended rates, and to a lesser extent, lactofen and fomesafen. Transcriptomic analysis of plants surviving GA revealed no resistance-associated mutations or differential transcript abundance in the plastidic and cytosolic isoforms of glutamine synthetase. Among the four suspected resistant populations, there were 182 genes differentially expressed relative to two susceptible populations. Different sets of genes were differentially expressed among the populations studied, with only one gene (upregulated relative to two susceptible populations) shared among all four. Many of the differentially expressed genes, including cytochrome P450s, glutathione S -transferases, glycosyltransferases, transporters, and transcriptional regulators, are commonly associated with metabolic resistance. Gene ontology enrichment analyses indicated significant overrepresentation of stress response, defense regulation, and secondary metabolism categories across the populations. Together, these findings provide evidence for the evolution of GA resistance in populations of A. tuberculatus in Illinois. While more in-depth studies are needed to fully characterize the underlying mechanisms, the consistent differential expression of metabolism-related genes and no indication of target-site mechanisms points to a potential metabolic basis for resistance.

Confirmation and transcriptomic characterization of glufosinate-ammonium resistance in waterhemp (Amaranthus tuberculatus) populations from Illinois

Weed Science · 2026-01-01

Abstract Glufosinate-ammonium (GA) has been widely used in U.S. Midwest fields, and in recent years a growing number of failures to control waterhemp [ Amaranthus tuberculatus (Moq.) Sauer] have raised concerns about the evolution of resistance. The goal of this study was to investigate four cases of suspected resistance to GA in A. tuberculatus from Illinois using greenhouse, field, and transcriptomics studies. Greenhouse dose–response experiments revealed resistance ratios ranging from 2.2-to 3.4-fold based on survival and 1.3-to 2.8-fold based on biomass relative to a susceptible population. A subsequent field study where one of the populations originated confirmed that 20% of treated plants survived the labeled GA field recommended rate. Screening with other herbicide site-of-action groups revealed that most populations showed reduced sensitivity to atrazine, glyphosate, and imazethapyr, surviving up to 3× the field recommended rates, and to a lesser extent, lactofen and fomesafen. Transcriptomic analysis of plants surviving GA revealed no resistance-associated mutations or differential transcript abundance in the plastidic and cytosolic isoforms of glutamine synthetase. Among the four suspected resistant populations, there were 182 genes differentially expressed relative to two susceptible populations. Different sets of genes were differentially expressed among the populations studied, with only one gene (upregulated relative to two susceptible populations) shared among all four. Many of the differentially expressed genes, including cytochrome P450s, glutathione S -transferases, glycosyltransferases, transporters, and transcriptional regulators, are commonly associated with metabolic resistance. Gene ontology enrichment analyses indicated significant overrepresentation of stress response, defense regulation, and secondary metabolism categories across the populations. Together, these findings provide evidence for the evolution of GA resistance in populations of A. tuberculatus in Illinois. While more in-depth studies are needed to fully characterize the underlying mechanisms, the consistent differential expression of metabolism-related genes and no indication of target-site mechanisms point to a potential metabolic basis for resistance.

Weather and glufosinate efficacy: a retrospective analysis looking forward to the changing climate

Weed Science · 2025-01-01 · 1 citations

Abstract Foliar-applied postemergence applications of glufosinate are often applied to glufosinate-resistant crops to provide nonselective weed control without significant crop injury. Rainfall, air temperature, solar radiation, and relative humidity near the time of application have been reported to affect glufosinate efficacy. However, previous research may have not captured the full range of weather variability to which glufosinate may be exposed before or following application. Additionally, climate models suggest more extreme weather will become the norm, further expanding the weather range to which glufosinate can be exposed. The objective of this research was to quantify the probability of successful weed control (efficacy ≥85%) with glufosinate applied to some key weed species across a broad range of weather conditions. A database of &gt;10,000 North American herbicide evaluation trials was used in this study. The database was filtered to include treatments with a single postemergence application of glufosinate applied to waterhemp [ Amaranthus tuberculatus (Moq.) Sauer], morningglory species ( Ipomoea spp.), and/or giant foxtail ( Setaria faberi Herrm.) &lt;15 cm in height. These species were chosen because they are well represented in the database and listed as common and troublesome weed species in both corn ( Zea mays L.) and soybean [ Glycine max (L.) Merr.] (Van Wychen 2020, 2022). Individual random forest models were created. Low rainfall (≤20 mm) over the 5 d before glufosinate application was detrimental to the probability of successful control of A. tuberculatus and S. faberi . Lower relative humidity (≤70%) and solar radiation (≤23 MJ m −1 d −1 ) on the day of application reduced the probability of successful weed control in most cases. Additionally, the probability of successful control decreased for all species when average air temperature over the first 5 d after application was ≤25 C. As climate continues to change and become more variable, the risk of unacceptable control of several common species with glufosinate is likely to increase.

Optimizing metribuzin rates for herbicide-resistant <i>Amaranthus</i> weed control in soybean

Weed Technology · 2025-01-01

Abstract Palmer amaranth and waterhemp are troublesome weeds in U.S. corn, soybean, and cotton production systems. Rapid evolution of resistance to herbicide from multiple sites of action in these species warrant alternate weed control options. Metribuzin applied preemergence can provide effective control of herbicide-resistant Amaranthus species. However, despite its decades of efficacy, many growers remain unaware of its weed control potential or are hesitant to use it due to concerns over crop injury. Field experiments were conducted in 2022 and 2023 in 15 states across the United States to investigate residual control of Palmer amaranth and waterhemp with metribuzin applied preemergence to soybean. Sites had either herbicide-resistant Palmer amaranth or waterhemp as the dominant weed species. Seventeen preemergence treatments were evaluated, including 13 doses of metribuzin (210 to 841 g ai ha -1 ), a dose of sulfentrazone (420 g ai ha -1 ), and a dose of S -metolachlor (1,790 g ai ha -1 ), along with nontreated and a weed-free control plots. Weed control and soybean injury were visually assessed and recorded at 14, 28, and 42 d after application (DAA) of preemergence herbicides. Additionally, weed density, weed biomass, and soybean height were recorded 28 DAA followed by a measure of soybean yield at maturity. Weed control was analyzed as a function of metribuzin dose and environmental factors using a generalized additive model. Crop injury of not more than 5% was predicted even with 841 g ai ha -1 of metribuzin. Metribuzin at 630 g ai ha -1 was more effective than sulfentrazone in delaying weed emergence and reducing weed density, while 315 g ai ha -1 of metribuzin outperformed S -metolachlor in both metrics. Metribuzin doses of 578 to 841 g ai ha -1 provided greater than 95%, 90%, and 80% weed control, respectively, at 14, 28, and 42 DAA. Higher metribuzin doses of 578 to 841 g ai ha -1 could be safely to effectively control herbicide-resistant Amaranthus weeds.

Early-planted soybean weed management as affected by herbicide application rate and timing

Weed Technology · 2025-01-01 · 2 citations

Abstract The opportunity to increase soybean yield has prompted Illinois farmers to plant soybean earlier than historical norms. Extending the growing season with an earlier planting date might alter the relationship between soybean growth and weed emergence timings, potentially altering the optimal herbicide application timings to minimize crop yield loss due to weed interference and ensure minimal weed seed production. The objective of this research was to examine various herbicide treatments applied at different timings and rates to assess the effect on weed control and yield in early-planted soybean. Field experiments were conducted in 2021 at three locations across central Illinois to determine effective chemical strategies for weed management in early-planted soybean. PRE treatments consisted of a S -metolachlor + metribuzin premix applied at planting or just prior to soybean emergence at 0.5X (883 + 210 g ai ha −1 ) or 1X (1,766 + 420 g ai ha −1 ) label-recommended rates. POST treatments were applied when weeds reached 10 cm tall and consisted of 1X rates of glufosinate (655 g ai ha −1 ) + glyphosate (1,260 g ae ha −1 ) + ammonium sulfate, without or with pyroxasulfone at a 0.5X (63 g ai ha −1 ) or 1X (126 g ai ha −1 ) rate. Treatments comprising both a full rate of PRE followed by a POST resulted in the greatest and most consistent weed control at the final evaluation timing. The addition of pyroxasulfone to POST treatments did not consistently reduce late-season weed emergence. The lack of a consistent effect by pyroxasulfone could be attributed to suppression of weeds by soybean canopy closure due to earlier soybean development. The full rate of PRE extended the timing of POST application 2 to 3 wk for all treatments at all locations except Urbana. Full-rate PRE treatments also reduced the time between the POST application and soybean canopy closure. Overall, a full-rate PRE reduced early-season weed interference and minimized soybean yield loss due to weed interference.

Commercial sprayer efficiency for application success on targeted weeds

Weed Technology · 2025-01-01 · 6 citations

Abstract Commercial targeted sprayer systems allow producers to reduce herbicide inputs but risks the possibility of not treating emerging weeds. Currently, targeted applications with the John Deere system have five spray sensitivity settings, and no published literature discusses the effects of these settings on detecting and spraying weeds of varying species, sizes, and positions in crops. Research was conducted in Arkansas, Illinois, Indiana, Mississippi, and North Carolina on plantings of corn, cotton, and soybean to determine how various factors might influence the ability of targeted applications to treat weeds. These data included 21 weed species aggregated to six classes with height, width, and densities ranging from 25 to 0.25 cm, 25 to 0.25 cm, and 14.3 to 0.04 plants m −2 , respectively. Crop and weed density did not influence the likelihood of treating the weeds. As expected, the sensitivity setting alters the ability to treat weeds. Targeted applications (across sensitivity settings, median weed height and width, and density of 2.4 plants m −2 ) resulted in a treatment success of 99.6% to 84.4% for Convolvulaceae , 99.1% to 68.8% for decumbent broadleaf weeds, 98.9% to 62.9% for Malvaceae , 99.1% to 70.3% for Poaceae , 98.0% to 48.3% for Amaranthaceae , and 98.5% to 55.8% for yellow nutsedge. Reducing the sensitivity setting reduced the ability to treat weeds. The size of weeds aided targeted application success, with larger weeds being more readily treated through easier detection. Based on these findings, various conditions can affect the outcome of targeted multinozzle applications. Additionally, the analyses highlight some of the parameters to consider when using these technologies.

Soybean (<i>Glycine max</i> L.) canopy response to simulated dicamba vapor drift using unmanned aerial sensing

Pest Management Science · 2025-06-23

BACKGROUND: Concerns about off-target dicamba exposure to sensitive vegetation have escalated following the commercialization of dicamba-tolerant (DT) soybean [Glycine max (L.) Merr.] and cotton (Gossypium hirsutum L.) The spectral response of plant injury at the field scale is a crucial knowledge gap that may help researchers understand dicamba's fate in the environment. Non-DT soybean is the ideal sentinel crop owing to its extreme sensitivity. Field experiments were conducted to characterize the reflectance spectra associated with dicamba vapor drift injured soybean canopies. The objective was, under field conditions and using land-based remote sensing, to determine what regions of the EM correlate to dicamba-injured soybean canopies. RESULTS: Soybean injury was observed 8 days after treatment at only 1/10000th of a labeled use rate of dicamba. Correlations between simulated vapor drift and reflectance spectra at the single channel red-edge and vegetative indices excess red (ExR) and green leaf index (GLI) were observed. The study demonstrates the potential for detecting off-target dicamba vapor drift injury in soybean, with reflectance spectra following dicamba treatments observed at all spectral channels. CONCLUSIONS: Differentiating dicamba-injured from dicamba-tolerant soybean canopies can be achieved using single spectral channels and vegetative indices. These findings usher in the possibility that remote sensing satellites, which are well-documented for identifying crop stress on the landscape, could be harnessed to understand the extent to which dicamba injury appears following over-the-top (OTT) application in DT soybean. Remote sensing technology could be instrumental in monitoring off-target herbicide injury and mitigating the effects of dicamba drift. © 2025 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Searching for consistent postemergence weed control in progressively inconsistent weather

Weed Science · 2024-11-18 · 1 citations

Abstract 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 &lt;19 or &gt;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.

Efficacy of dicamba and dicamba/tembotrione with and without ammonium sulfate for broadleaf weed control

Weed Technology · 2024-01-01 · 2 citations

Abstract Mixing ammonium sulfate (AMS) can increase dicamba volatility. Therefore, AMS cannot be used with dicamba products in dicamba-resistant soybean. However, most dicamba products applied in corn are labeled to mix with AMS. The objectives of this study were to evaluate broadleaf weed control with dicamba (DiFlexx®) and dicamba/tembotrione (DiFlexx® DUO) applied alone or with AMS or AMS substitute and their effect on broadleaf weed density and biomass. Field experiments were conducted in Illinois, Missouri, and Nebraska in 2018 and 2019. In Illinois and Nebraska, mixing AMS + crop oil concentrate (COC) with dicamba applied at 1,120 g ae ha −1 increased the control of Palmer amaranth and waterhemp ( Amaranthus species) from 78% to 92% and velvetleaf from 73% to 96% compared with dicamba applied alone 14 d after application (DAA); however, Missouri data showed no difference. Mixing AMS + COC with dicamba/tembotrione at 597 and 746 g ai ha −1 did not improve broadleaf weed control 14 DAA at any site compared with dicamba/tembotrione applied alone. Control of Amaranthus species was increased from 82% with dicamba applied at 840 g ae ha −1 to 96% when mixed with AMS + COC 28 DAA in Illinois; however, control was similar to dicamba applied at 1,120 g ae ha −1 . Broadleaf weed control did not differ among dicamba or dicamba/tembotrione 28 and 56 DAA in Missouri and Nebraska. Broadleaf weed density decreased from 64 plants m −2 to 24 plants m −2 with dicamba at 1,120 g ae ha −1 with AMS + COC 14 DAA in Nebraska; however, no differences were observed in broadleaf weed density or biomass 56 DAA in any state. The results suggest that dicamba or dicamba/tembotrione can be applied without AMS or AMS substitute, especially at higher rates, without losing broadleaf weed control efficacy.

The silver bullet that wasn’t: Rapid agronomic weed adaptations to glyphosate in North America

PNAS Nexus · 2023-12-01 · 12 citations

The rapid adoption of glyphosate-resistant crops at the end of the 20th century caused a simplification of weed management that relied heavily on glyphosate for weed control. However, the effectiveness of glyphosate has diminished. A greater understanding of trends related to glyphosate use will shed new light on weed adaptation to a product that transformed global agriculture. Objectives were to (1) quantify the change in weed control efficacy from postemergence (POST) glyphosate use on troublesome weeds in corn and soybean and (2) determine the extent to which glyphosate preceded by a preemergence (PRE) improved the efficacy and consistency of weed control compared to glyphosate alone. Herbicide evaluation trials from 24 institutions across the United States of America and Canada from 1996 to 2021 were compiled into a single database. Two subsets were created; one with glyphosate applied POST, and the other with a PRE herbicide followed by glyphosate applied POST. Within each subset, mean and variance of control ratings for seven problem weed species were regressed over time for nine US states and one Canadian province. Mean control with POST glyphosate alone decreased over time while variability in control increased. Glyphosate preceded by a labeled PRE herbicide showed little change in mean control or variability in control over time. These results illustrate the rapid adaptation of agronomically important weed species to the paradigm-shifting product glyphosate. Including more diversity in weed management systems is essential to slowing weed adaptation and prolonging the usefulness of existing and future technologies.