Introduction
Palmer amaranth is one of the most troublesome broadleaf weeds in North American agriculture. Native to the arid southwestern United States and northwestern Mexico, this summer annual weed species has rapidly expanded its range across the continent, with significant infestations now reported throughout the southern, midwestern, and even northeastern United States (Aulakh et al. Reference Aulakh, Chahal, Kumar, Price and Guillard2021, Reference Aulakh, Kumar, Brunharo, Veron and Price2024; Chahal et al. Reference Chahal, Aulakh, Jugulam, Jhala, Price, Kelton and Sarunaite2015; Kumar et al. Reference Kumar, Liu and Stahlman2020; Ward et al. Reference Ward, Webster and Steckel2013). The rapid spread of Palmer amaranth to the north has been particularly problematic in the last 2 decades, rising from relative obscurity to a significant threat to multiple cropping systems (Aulakh et al. Reference Aulakh, Chahal, Kumar, Price and Guillard2021, Reference Aulakh, Kumar, Brunharo, Veron and Price2024; Kumar et al. Reference Kumar, Aulakh, Liu and Jhala2023). The unique biological characteristics provide Palmer amaranth with exceptional competitive abilities compared to other pigweed species. For instance, Palmer amaranth exhibits an extended emergence period with multiple cohorts emerging throughout the growing season (Jha et al. Reference Jha, Norsworthy and Riley2009; Liu et al. Reference Liu, Kumar, Jha and Stahlman2022). As a C4 plant, Palmer amaranth can grow rapidly with an average height increment of 5 to 8 cm per day under optimal conditions (Ward et al. Reference Ward, Webster and Steckel2013). Full-grown Palmer amaranth plants can exceed 2 m in height, and a single female Palmer amaranth plant can produce more than 600,000 seeds, highlighting its remarkable growth and reproductive potential (Horak and Loughin Reference Horak and Loughin2000; Keeley et al. Reference Keeley, Carter and Thullen1987). The dioecious nature of flowering (male and female flowers on separate plants) and high outcrossing of Palmer amaranth ensures high genetic diversity within and among field populations, facilitating its rapid evolutionary adaptation (Adhikary and Pratt Reference Adhikary and Pratt2015; Sosnoskie et al. Reference Sosnoskie, Webster, Kichler, MacRae, Grey and Culpepper2012).
Season-long interference of Palmer amaranth at densities ranging from 0.11 to 10.55 plants m−2 can cause substantial crop yield losses, with documented reductions up to 91% in corn, 79% in soybean, and 59% in cotton (Bensch et al. Reference Bensch, Horak and Peterson2003; Klingaman and Oliver Reference Klingaman and Oliver1994; Massinga et al. Reference Massinga, Currie, Horak and Boyer2001; Morgan et al. Reference Morgan, Baumann and Chandler2001). In addition to direct competition, the aggressive growth of Palmer amaranth can interfere with mechanical harvesting operations, further compromising crop productivity and quality, while increasing production costs (Smith et al. Reference Smith, Baker and Steele2000).
Palmer amaranth has a high tendency to evolve herbicide resistance (Heap Reference Heap2025). Currently, Palmer amaranth populations with resistance to 10 distinct herbicide sites of action (SOAs) have been documented in the United States (Heap Reference Heap2025). For instance, Palmer amaranth populations that are resistant to herbicides that inhibit acetolactate synthase (ALS; categorized as Group 2 herbicide by the Weed Science Society of America [WSSA]), microtubule assembly (WSSA Group 3), 4-hydroxyphenylpyruvate dioxygenase (HPPD; WSSA Group 27), 5-enolpyruvyl shikimate-3-phosphate synthase (EPSPS; WSSA Group 9), photosystem II (PS II; WSSA Groups 5 and 6), protoporphyrinogen oxidase (PPO; WSSA Group 14), synthetic auxins (WSSA Group 4), glutamine synthetase (WSSA Group 10), and very-long-chain fatty acid elongase (WSSA Group 15) have been reported (Carvalho-Moore et al. Reference Carvalho-Moore, Norsworthy, González-Torralva, Hwang, Patel, Barber, Butts and McElroy2022; Chahal et al. Reference Chahal, Varanasi, Jugulam and Jhala2017; Foster and Steckel Reference Foster and Steckel2022; Heap Reference Heap2025; Kouame et al. Reference Kouame, Bertucci, Savin, Bararpour, Steckel, Butts, Willett, Machado and Roma-Burgos2022; Kumar et al. Reference Kumar, Liu, Boyer and Stahlman2019, Reference Kumar, Liu and Stahlman2020; Priess et al. Reference Priess, Norsworthy, Godara, Mauromoustakos, Butts, Roberts and Barber2022). Furthermore, multiple resistance to two to six different herbicide SOAs have been reported in Palmer amaranth populations from Connecticut, Kansas, and Arkansas (Aulakh et al. Reference Aulakh, Chahal, Kumar, Price and Guillard2021; Heap Reference Heap2025; Kouame et al. Reference Kouame, Bertucci, Savin, Bararpour, Steckel, Butts, Willett, Machado and Roma-Burgos2022; Kumar et al. Reference Kumar, Liu, Boyer and Stahlman2019, Reference Kumar, Liu and Stahlman2020). Glyphosate resistance has been widely reported among Palmer amaranth populations across various regions of the United States (Aulakh et al. Reference Aulakh, Chahal, Kumar, Price and Guillard2021, Reference Aulakh, Kumar, Brunharo, Veron and Price2024; Chahal et al. Reference Chahal, Aulakh, Jugulam, Jhala, Price, Kelton and Sarunaite2015; Heap Reference Heap2025; Kumar et al. Reference Kumar, Liu and Stahlman2020; Ward et al. Reference Ward, Webster and Steckel2013). EPSPS gene amplification is the most common underlying mechanism conferring glyphosate resistance in the majority of these glyphosate-resistant (GR) Palmer amaranth populations (Aulakh et al. Reference Aulakh, Kumar, Brunharo, Veron and Price2024; Chahal et al. Reference Chahal, Varanasi, Jugulam and Jhala2017; Gaines et al. Reference Gaines, Zhang, Wang, Bukun, Chisholm, Shaner, Nissen, Patzoldt, Tranel, Culpepper, Grey, Webster, Vencill, Sammons, Jiang, Preston, Leach and Westra2010; Patterson et al. Reference Patterson, Pettinga, Ravet, Neve and Gaines2018). The EPSPS gene amplification enables resistant plants to produce excessive EPSPS enzyme to counteract glyphosate while maintaining normal metabolic function (Gaines et al. Reference Gaines, Zhang, Wang, Bukun, Chisholm, Shaner, Nissen, Patzoldt, Tranel, Culpepper, Grey, Webster, Vencill, Sammons, Jiang, Preston, Leach and Westra2010). New reports on GR Palmer amaranth in northern states pose a significant threat to crop production and warrant effective alternative postemergence herbicide options for its management (Aulakh et al. Reference Aulakh, Chahal, Kumar, Price and Guillard2021; Butler-Jones et al. Reference Butler-Jones, Maloney, McClements, Kramer, Morran, Gaines, Besançon and Sosnoskie2024).
Several previous studies have reported the effectiveness of various postemergence herbicides for Palmer amaranth control with mixed results. For instance, Scruggs et al. (Reference Scruggs, VanGessel, Holshouser and Flessner2021) reported 78% to 88% control of Palmer amaranth 28 d after treatment (DAT) with postemergence-applied glufosinate, 2,4-D + glyphosate, dicamba + glyphosate, and glufosinate + glyphosate to Enlist soybeans when herbicides were applied at the first visible female inflorescence of Palmer amaranth. However, only 62% and 72% control was achieved with postemergence-applied 2,4-D or dicamba to Enlist or Xtend soybean, respectively (Scruggs et al. Reference Scruggs, VanGessel, Holshouser and Flessner2021). In contrast, Lawrence et al. (Reference Lawrence, Bond, Eubank, Golden, Cook and Mangialardi2018) reported 87% control and ≥81% dry weight reduction in 5- to 10-cm-tall GR Palmer amaranth at 28 DAT with 2,4-D in a greenhouse study. In a field study, a mixture of glyphosate + glufosinate and 2,4-D + glyphosate provided 75% and 81% control of GR Palmer amaranth, respectively, 28 DAT (Lawrence et al. Reference Lawrence, Bond, Eubank, Golden, Cook and Mangialardi2018). In field studies conducted in Mississippi and Nebraska, Franca et al. (Reference Franca, Dodds, Butts, Kruger, Reynolds, Anthony Mills, Bond, Catchot and Peterson2020) reported 56% to 62% control of Palmer amaranth with lactofen when applied with nozzles varying in droplet size from 150 to 900 μm. In contrast, Aulakh et al. (Reference Aulakh, Chahal, Kumar, Price and Guillard2021) reported 100% control of GR Palmer amaranth population from Connecticut with postemergence-applied lactofen at 21 DAT in a greenhouse study.
During the 2024 growing season, a new Palmer amaranth population (NY_PA) was identified in a soybean field in Ontario County, New York. Based on the grower’s observations, this population appeared to be resistant to both glyphosate and atrazine, because Palmer amaranth plants had survived postemergence applications of these herbicides in the previous corn crop. Information on managing multiple herbicide–resistant (MHR) Palmer amaranth populations with alternative postemergence herbicide mixtures remains somewhat limited in New York cropping systems. In addition, producers in New York have a limited number of registered herbicides available to them to effectively control MHR Palmer amaranth. Information generated through this applied research is critically necessary to support education and outreach efforts aimed at helping New York producers adopt more diversified and effective herbicide strategies for MHR Palmer amaranth control. To bridge these knowledge gaps, the main objectives of this research were to 1) determine whether the NY_PA population is resistant to glyphosate and atrazine, and 2) evaluate the effectiveness of various alternative postemergence herbicides applied alone or in mixtures for the control of suspected MHR Palmer amaranth populations in New York and Connecticut.
Materials and Methods
Plant Materials
A new Palmer amaranth population (NY_PA) was identified in a soybean field (42.83°N, 76.74°W) in Ontario County, New York, during the 2024 growing season. About 15 to 20 female seedheads of this population were collected at maturity. The seed heads were manually threshed, and the seeds were cleaned using sieves with various mesh sizes. After cleaning, seeds from each female seedhead were composited into one sample and stored in a plastic Ziploc bag at 4 C. In addition to the NY_PA population, seeds of two previously confirmed GR Palmer amaranth populations from Connecticut (CT_PA) and Kansas (KS_PA), and a glyphosate-susceptible population from Alabama (AL_SUS) were also used (Aulakh et al. Reference Aulakh, Kumar, Brunharo, Veron and Price2024). A previously known glyphosate-resistant KS_PA population was included to compare and validate the response of GR Palmer amaranth populations identified from northeastern region (NY_PA and CT_PA) with alternative postemergence herbicides.
Determination of EPSPS Gene Copy Numbers
In 2024, seedlings from the NY_PA population were grown at Cornell University’s Guterman Bioclimatic Laboratory in Ithaca, NY. Seeds of the NY_PA population were planted on the surface of 54- by 34- by 6-cm germination flat filled with a Cornell potting mixture (a mixture of Canadian peat moss, vermiculite, perlite, dolomite lime, Jack’s 10-5-10 media mix plus II, and calcium sulfate). Once the seedlings emerged, they were manually transplanted into 10-cm-diam plastic pots containing the same potting mixture as mentioned above. Greenhouse conditions were maintained at day/night temperatures of 27/24 ± 3 C, with 16/8-h day/night photoperiods. Young seedlings of NY_PA and previously frozen leaf tissue of the AL_SUS population (three plants per population) were shipped overnight to the University of Florida Tropical Research and Education Center in Homestead. Genomic DNA was extracted from leaf tissue of the NY_PA and AL_SUS populations using the standard cationic detergent cetyltrimethylammonium bromide extraction method to determine the number of EPSPS gene copies. The quality of the extracted DNA was assessed using 0.8% agarose gel electrophoresis and was quantified using a NanoDrop 1000 spectrophotometer (Thermo Fisher Scientific, Waltham, MA). The relative quantitative polymerase chain reaction (qPCR) was conducted to determine the EPSPS gene copy number using ALS as a reference (single copy) gene and the following primers: ALS (forward, 5′-GCTGCTGAAGGCTACGCT-3′; reverse, 5′-GCGGGACTGAGTCAAGAAGTG-3′; 118-base pair product) and EPSPS (forward, 5′-ATGTTGGACGCTCTCAGAACTCTTGGT-3′; reverse, 5′-TGAATTTCCTCCAGCAACGGCAA-3′; 195-base pair product) designed by Dillon et al. (Reference Dillon, Varanasi, Danilova, Koo, Nakka, Peterson, Tranel, Friebe, Gill and Jugulam2017). The qPCR assay we used was QuantStudio 3 (Thermo Fisher Scientific) real-time PCR. The 20-μl qPCR reaction mixture consisted of 10 μl of GoTaq qPCR Master Mix (2×, Promega, Madison, WI), 1 µl each of forward and reverse primers (10 μM; Integrated DNA Technologies, Coralville, IA), 2 μl of genomic DNA (20 ng/μl), 0.2 µl CXR reference Dye (30 µM; Promega), and 5.8 μl of nuclease-free water to make up the volume. Three biological and three technical replicates were performed for each population. The following reaction conditions were used: initial denaturation at 95 C for 15 min, followed by 40 cycles of denaturing at 95 C for 30 s, and then annealing and extension at 60 C for 1 min. The relative EPSPS copies were calculated as ∆Ct = CTALS – CTEPSPS, and the EPSPS copy number was shown as 2∆Ct. The 2−ΔΔCt method was used to quantify the EPSPS gene copy number relative to the ALS gene (Gaines et al. Reference Gaines, Zhang, Wang, Bukun, Chisholm, Shaner, Nissen, Patzoldt, Tranel, Culpepper, Grey, Webster, Vencill, Sammons, Jiang, Preston, Leach and Westra2010).
Resistance to Atrazine
Greenhouse experiments were initiated in the spring and repeated in the summer of 2025 at Cornell University Guterman Bioclimatic Laboratory to confirm and characterize the level of atrazine resistance in NY_PA and CT_PA populations. Greenhouse conditions were set at temperatures and photoperiods as described above. Seeds of selected Palmer amaranth populations from the NY_PA, CT_PA, and AL_SUS populations were separately sown on the surface of 54- by 34- by 6-cm germination flats filled with Cornell greenhouse potting mixture. Seedlings from each population were separately transplanted into 10-cm-square plastic pots (Greenhouse Megastore, Danville, IL) containing the same Cornell greenhouse potting mixture. Experiments were conducted in a randomized complete block (blocked by populations) design with nine replications (one plant in a pot = one replication). Actively growing seedlings (at the 3- to 4-leaf stage and 7- to 10 cm tall) from the NY_PA population were treated with various doses of atrazine (0, 281, 562, 1,125, 2,250, 4,500, 9,000, and 18,000 g ha−1) containing crop oil concentrate (COC) at 10 ml L−1. In contrast, the AL_SUS population was treated with atrazine doses (0, 35, 70, 140, 281, 562, 1,125, 2,250, 4,500, 9,000, and 18,000 g ha−1) along with COC at 10 ml L−1. Treatments were applied using a stationary cabinet spray chamber (Research Track Sprayer, De Vries Manufacturing, Hollandale, MN) equipped with a flat-fan 8002XR nozzle tip (TeeJet Technologies, Glendale Heights, IL) calibrated to deliver 141 L ha−1 of spray solution at 276 kPa. After spraying, all treated plants were kept in the isolation room for 24 h following restricted entry interval (REI) for atrazine. Plants were returned to the greenhouse and watered daily to avoid soil moisture stress. At 21 d after treatment (DAT), the shoot biomass of all treated Palmer amaranth populations was collected and dried at 72 C for 5 d to measure aboveground shoot dry weights. The aboveground shoot dry weight reduction (%) was calculated using Equation 1:
$$\;Shoot\,dry\,biomass\,reuction\,\left( \% \right) = \left[ {{{NT - T} \over {NT}}} \right]*100$$
where NT is the average aboveground shoot dry weights from nontreated plants and T is the aboveground shoot dry weights from treated plants.
Effectiveness of Alternative Postemergence Herbicides
Greenhouse experiments were initiated at Cornell University Guterman Bioclimatic Laboratory during fall 2024 and repeated in spring 2025 to evaluate the effectiveness of alternative postemergence herbicides alone or in mixtures to control the three Palmer amaranth populations (NY_PA, CT_PA, and KS_PA). Seeds of all three Palmer amaranth populations were separately sown in germination trays, and seedlings were transplanted in the same manner and conditions described above. A randomized complete block (blocked by population) design was used with 10 to 12 replications (1 replication = 1 pot = 1 plant). A total of 12 postemergence herbicides alone or in mixtures (Table 1) were evaluated in this study. A nontreated control (a group of 10 to 12 plants) for each population was included for treatment comparison. Selected postemergence herbicides alone or in mixtures were tested at their field-use rates in corn, soybean, and/or fallow situations. All selected postemergence herbicides were applied to 8- to 10-cm-tall Palmer amaranth plants (the 3- to 4-leaf stage) using a stationary cabinet spray chamber as described above. All treated plants were watered daily to avoid moisture stress. Percent control of each Palmer amaranth plant on the scale of 0% to 100% (0% means no control and 100% means complete plant death) was determined at 28 DAT. At 28 DAT, Palmer amaranth plants from all three populations were harvested at the soil level and dried at 72 C for 5 d to determine the aboveground shoot dry biomass. For each Palmer amaranth population, shoot dry biomass were expressed as a percentage reduction relative to the nontreated control.
List of alternative postemergence herbicides tested for controlling multiple herbicide-resistant Palmer amaranth populations.a
a Abbreviations: EPSPS, 5-enolpyruvylshikimate-3-phosphate synthase; GS, glutathione synthetase; PPO, protoporphyrinogen oxidase.
b Ammonium sulfate at 20 g L−1 was included.
c Crop oil concentrate at 10 ml L−1 was included.
Statistical Analyses
Data from each study (atrazine dose-response and alternative postemergence herbicide experiments) were checked for ANOVA assumptions using the Shapiro-Wilk and Levene tests with the UNIVARIATE and GLM procedures, respectively, with SAS software (v.9.3; SAS Institute Inc., Cary, NC), and all data met ANOVA assumptions. Data from both experimental runs of atrazine dose-response studies were combined due to a nonsignificant interaction between experimental run by herbicide interactions (P = 0.321). Aboveground shoot dry weights (% of nontreated) of each Palmer amaranth population were subjected to three-parameter log-logistic model (Ritz et al. Reference Ritz, Baty, Streibig and Gerhard2015) using Equation 2:
$$Y = d/\{ 1 + exp\;[b\left( {log\;\left( x \right) - log\;\left( e \right)} \right]\} $$
where y is the aboveground shoot dry weight reduction (% of nontreated), d is the maximum aboveground shoot dry weight reduction, e is the atrazine dose required for 50% reduction in aboveground shoot dry weights (indicated as GR50 values), x is the atrazine dose, and b represents the slope of each curve.
The Akaike information criterion was used to select the nonlinear three-parameter model. A lack-of-fit test (P > 0.05) was used to confirm that the selected model described the aboveground shoot dry biomass reduction of each Palmer amaranth population (Ritz et al. Reference Ritz, Baty, Streibig and Gerhard2015). All nonlinear regression parameter estimates, standard errors, and GR90 values (the atrazine dose required for a 90% reduction in aboveground shoot dry weights) were determined using the drc package in R software (v.4.3.0; R Core Team 2025). The resistance factor (indicated as RF), the ratio of the GR50 or GR90 value of the NY_PA and CT_PA populations to that of the GR50 or GR90 value of the AL_SUS population, was determined.
Data from alternative postemergence herbicide experiments were combined across experimental runs due to nonsignificant experimental runs by herbicide interaction (P = 0.419). Data were subjected to ANOVA using the MIXED procedure in SAS software to test the significance of fixed effects (population, postemergence herbicide, and their interactions). Random effects in the model were experiment run, and replications nested within experimental runs (due to differences in number of replications for each run). Means were separated using Fisher’s protected LSD test at P ≤ 0.05.
Results and Discussion
EPSPS Gene Copy Numbers
The average EPSPS gene copy number in the NY_PA population (180.01 ± 10.43) was approximately 134-fold higher than the AL_SUS population (1.36 ± 0.19). These results confirm that the EPSPS gene amplification is the underlying mechanism conferring glyphosate resistance in the newly identified Palmer amaranth population from Ontario County, New York. These results are consistent with previously reports of GR Palmer amaranth populations in Connecticut, New York, and elsewhere (Aulakh et al. Reference Aulakh, Kumar, Brunharo, Veron and Price2024; Butler-Jones et al. Reference Butler-Jones, Maloney, McClements, Kramer, Morran, Gaines, Besançon and Sosnoskie2024; Gaines et al. Reference Gaines, Zhang, Wang, Bukun, Chisholm, Shaner, Nissen, Patzoldt, Tranel, Culpepper, Grey, Webster, Vencill, Sammons, Jiang, Preston, Leach and Westra2010).
Resistance to Atrazine
Results from dose-response studies confirmed the first case of multiple herbicide resistance in Palmer amaranth populations in New York and Connecticut. Both populations were resistant to glyphosate and atrazine (Figure 1; Tables 2 and 3). The atrazine dose that resulted in GR50 values for NY_PA and CT_PA was 1,725 g ha−1 and 1,165 g ha−1, respectively, which is significantly higher than that of the AL_SUS population (159 g ha−1). Based on GR50 values, the NY_PA population was 11-fold resistant to atrazine, and the CT_PA population was 7-fold resistant to atrazine compared with the AL_SUS population (Table 2). Furthermore, the estimated atrazine dose to obtain a GR90 value for the NY_PA and CT_PA populations was 20,770 g ha−1 and 6,817 g ha−1, respectively, doses that are significantly higher than that to achieve a GR90 for AL_SUS (897 g ha−1). Based on the GR90 values, the NY_PA and CT_PA Palmer amaranth populations were 23-fold and 8-fold resistant to atrazine, respectively, compared with the AL_SUS population (Figure 1 and Table 2). These results are consistent with those reported by Jhala et al. (Reference Jhala, Sandell, Rana, Kruger and Knezevic2014), that 9-fold to 14-fold resistance to postemergence applied atrazine was found in an MHR Palmer amaranth from Nebraska that was also resistant to HPPD inhibitors. Similarly, 14.4-fold resistance to postemergence-applied atrazine has been reported in an MHR Palmer amaranth population from Kansas that was resistant to five SOAs: ALS, PS II, EPSPS, HPPD, and synthetic auxins (Kumar et al. Reference Kumar, Liu, Boyer and Stahlman2019).
Shoot dry weight reduction (% of nontreated) response of selected Palmer amaranth populations from Alabama (AL_SUS), Connecticut (CT_PA), and New York (NY_PA) treated with various doses of atrazine in greenhouse experiments at 21 d after treatment. Vertical bars indicate model-based standard errors (±) of the predicted mean.
Regression parameter estimates from shoot dry weight reduction (% of nontreated) response of suspected multiple herbicide–resistant Palmer amaranth populations from Connecticut and New York along with a known susceptible population from Alabama treated with various doses of atrazine 21 d after treatment.a
a Abbreviations: AL_SUS, Palmer amaranth population from Alabama; CI, confidence interval; CT_PA, Palmer amaranth population from Connecticut; GR50, effective dose of herbicide needed for a 50% shoot dry biomass reduction; GR90, herbicide dose required for a 90% reduction in aboveground shoot dry weights NY_PA, Palmer amaranth population from New York; RF, resistance factor; SE, standard error.
b Parameter d is the maximum shoot dry weight reduction (upper asymptote, fixed to 100%), b is the slope of each curve with standard error in parentheses, and GR50 is the effective dose of atrazine needed for 50% shoot dry biomass reduction (% of nontreated) for each tested Palmer amaranth population.
c RF is the ratio of the GR50 or GR90 value of the NY_PA and CT_PA populations to that of the GR50 or GR90 value of the AL_SUS population.
Average percent of visual control and shoot dry weight reductions in glyphosate-resistant Palmer amaranth populations from New York, Kansas, and Connecticut with various herbicides at labeled field-use rates 28 d after treatment (DAT) in greenhouse experiments.a,b
a Abbreviations: NY_PA, Palmer amaranth population from New York; KS_PA, Palmer amaranth population from Kansas; CT_PA, Palmer amaranth population from Connecticut.
b Means followed by the same letters within a column are not significantly different using the Fisher’s protected least square difference at α = 0.05.
c Ammonium sulfate (AMS) at 20 g L−1 was included.
d Crop oil concentrate (COC) at 10 ml L−1 was included.
Effectiveness of Alternative Postemergence Herbicides
A significant interaction (P = 0.0345) was observed between postemergence herbicides and populations for both visual control and shoot dry weight reduction; therefore, data were analyzed and presented by population. As expected, glyphosate provided poor control (22% to 35%) of all three tested Palmer amaranth populations at 28 DAT, further confirming glyphosate resistance among the tested populations (Table 3). However, all postemergence-applied herbicides (alone or in mixtures) provided excellent control (90% to 100%) of Palmer amaranth populations from Connecticut and Kansas at 28 DAT (Table 3). For the NY_PA population, most postemergence herbicides also provided effective control, except for glufosinate alone and the glyphosate + dicamba combination, which showed slightly lower control (79% to 86%) than other treatments. Among all postemergence herbicides, 2,4-D, dicamba, saflufenacil, and lactofen alone or in mixtures with 2,4-D, dicamba, and glufosinate were effective against all three Palmer amaranth populations. These findings are consistent with previous reports on the efficacy of these herbicides on Palmer amaranth populations (Aulakh et al. Reference Aulakh, Chahal, Kumar, Price and Guillard2021; Chahal et al. Reference Chahal, Varanasi, Jugulam and Jhala2017; Joseph et al. Reference Joseph, Marshall and Sanders2018; Scruggs et al. Reference Scruggs, VanGessel, Holshouser and Flessner2021). Postemergence herbicide mixtures, especially lactofen + dicamba, lactofen + glufosinate, and glyphosate + glufosinate provided excellent control (100%) of all three Palmer amaranth populations. Use of postemergence herbicide mixtures comprising multiple SOAs can help reduce the selection pressure from glyphosate or other single SOA-based herbicide programs, thereby preventing evolution and spread of MHR among Palmer amaranth populations.
Consistent with visual control, the majority of tested postemergence herbicides reduced shoot dry weights (≥90%) of KS_PA and CT_PA populations at 28 DAT (Table 3). Glyphosate resulted in the lowest shoot dry weight reduction (4% to 69%) across all three populations, further confirming glyphosate resistance in these populations. For the NY_PA population, glufosinate, lactofen, glyphosate + 2,4-D, and glyphosate + dicamba resulted in 82% to 88% shoot dry weight reduction compared to other treatments, which were consistent with visual control assessments.
Practical Implications
Results from this research confirm that the newly identified Palmer amaranth population from Ontario County, NY, is resistant to both glyphosate and atrazine. Recent confirmation of MHR Palmer amaranth populations in New York and Connecticut raises serious concerns for crop producers in the northeastern United States (Aulakh et al. Reference Aulakh, Chahal, Kumar, Price and Guillard2021, Reference Aulakh, Kumar, Brunharo, Veron and Price2024; Butler-Jones et al. Reference Butler-Jones, Maloney, McClements, Kramer, Morran, Gaines, Besançon and Sosnoskie2024). Results from this study also demonstrate that several effective alternative postemergence herbicide options are available to manage these MHR Palmer amaranth populations. Based on these findings, producers can use the postemergence herbicides tested in this research in their weed management programs. Specifically, 2,4-D, dicamba, glufosinate, saflufenacil, and lactofen, applied alone or in various combinations, can effectively manage MHR Palmer amaranth populations. Furthermore, integrating these postemergence herbicides into comprehensive management strategies that include soil-residual herbicides (preemergence), diversified crop rotations, cover crops, and other cultural practices will be crucial for sustainable, cost-effective management of MHR Palmer amaranth. Future research should focus on elucidating the underlying mechanism of atrazine resistance and evaluating the combination of preemergence and postemergence herbicides (two-pass programs) and other nonchemical tactics (such as cover crops, harvest weed seed control, electrocution, interrow mowing) for managing MHR Palmer amaranth in New York and northeastern cropping systems.
Acknowledgments
We thank Midhat Z. Tugoo, Henrique Scatena, and Preetaman Bajwa for their technical assistance in greenhouse experiments.
Funding
Funding for this research was provided in part by New York Corn and Soybean Growers Association Soybean Checkoff (Grant 168758) and by the U.S. Department of Agriculture–National Institute of Food and Agriculture, Hatch project 2024-25-142.
Competing Interests
The authors declare they have no competing interests.
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