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Biological effects on Palmer amaranth surviving glufosinate

Biological effects on Palmer amaranth surviving glufosinate AbbreviationsDATdays after treatmentfbfollowed byGR5050% germination rateHGherbicide groupINTRODUCTIONPalmer amaranth (Amaranthus palmeri S. Watson) is one of the most pervasive weeds of row crops in the United States due to prolonged germination periods, rapid seedling growth, dioecy, and high fecundity (Bravo et al., 2017; Horak & Loughin, 2000; Keeley et al., 1987). Female plants can produce 50,000 to 1,000,000 seeds plant−1 and thus replenish the soil seed bank facilitating the persistence of the population within the agroecosystem (Mahoney et al., 2021; Walsh et al., 2013). Difficulty to control Palmer amaranth is exacerbated by evolved resistance to almost all labeled row crop herbicides (Heap, 2022).Glufosinate (herbicide group [HG] 10) is a nonselective, fast‐acting contact herbicide that inhibits glutamine synthetase (Enzyme Commission 6.3.1.2) ceasing the production of glutamate and disrupting chlorophyll production (Takano et al., 2019, 2020). Because Palmer amaranth has evolved resistance to nine unique herbicide groups and multiple herbicide‐resistant populations are common in crop fields, glufosinate is among the few effective postemergence options for some cotton (Gossypium L.) and soybean [Glycine max (L.) Merr.] farmers in North Carolina (Everman et al., 2007; Heap, 2022; Mahoney et al., 2020). However, glufosinate resistance has evolved in isolated populations of Palmer amaranth (Norsworthy et al., 2021; Priess et al., 2022).While glufosinate is efficacious on susceptible Palmer amaranth populations, control is plant height/size dependent (Everman et al., 2007; Steckel et al., 1997). Although the glufosinate label recommends treating weeds before reaching 7.5 cm in height, it is common to find Palmer amaranth individuals exceeding this height at the time of application (Anonymous, 2017; Reinhardt Piskackova et al., 2021). Furthermore, once Palmer amaranth height exceeds 10 cm, control is greatly reduced and treated plants can produce viable seed (Scruggs et al., 2020; Steckel et al., 1997). Preventing seed production is vital for preventing future weed problems, specifically if the weed population has evolved herbicide resistance (Bagavathiannan & Norsworthy, 2012; Leon et al., 2016). Because glufosinate control diminishes as Palmer amaranth height increases, quantifying the fecundity of surviving Palmer amaranth is important. Previous research demonstrated that applying glufosinate to Palmer amaranth plants in reproductive growth stages can significantly reduce seed production compared with nontreated plants (Jha & Norsworthy, 2012; Scruggs et al., 2020).Currently, there is no peer‐reviewed literature providing data on quantifying vegetative‐stage Palmer amaranth fecundity after surviving glufosinate and the implication of different application timings. If Palmer amaranth remains fecund after glufosinate treatment, it would be important to determine whether the offspring exhibit differential germination or susceptibility to glufosinate (Oreja et al., 2021). Previous research suggests Palmer amaranth can evolve resistance to herbicide(s) when recurrently treated with sublethal doses (Neve & Powles, 2005; Tehranchian et al., 2017; Vieira et al., 2020). Evolution of glufosinate resistance was observed with recurrent selection with sublethal doses in Italian ryegrass (Lolium perenne ssp. multiflorum) after three generations (Matzrafi et al., 2020). Although the evolution of herbicide resistance is a multigenerational event, measurable reductions in control after one generation could provide insight on the duration before evolution of glufosinate resistance occurs (Neve et al., 2011). Fecundity will also be a key variable for predicting the rapidity of Palmer amaranth evolving glufosinate resistance.Seed viability may be affected by glufosinate when applied to Palmer amaranth when inflorescences are present. Scruggs et al. (2020) previously reported no differences in seed viability from reproductive stage Palmer amaranth plants treated with glufosinate (656 g a.i. ha−1) compared with nontreated Palmer amaranth plants. Conversely, Jha and Norsworthy (2012) observed decreased seed viability when glufosinate (820 g a.i. ha−1) was applied to flowering Palmer amaranth. In other weed species, seed viability and germination have been affected by sublethal doses of efficacious herbicides (Qi et al., 2017; Shuma et al., 1995; Young & Whitesides, 1987). The differential seed viability observed in previous research could be due to the tested systemic herbicides translocating to reproductive tissue/seeds compared with desiccation of glufosinate‐treated reproductive tissue (Thomas et al., 2004; Walker & Oliver, 2008). However, the seed physiology could be affected by differential resource partitioning by plants surviving glufosinate applications in the vegetative state (Rosenthal & Kotanen, 1998; Schooler et al., 2007; Wise & Abrahamson, 2008). Thus, the objectives of this research were to (a) quantify the fecundity of Palmer amaranth plants surviving glufosinate applied at various timings, (b) determine the viability of seeds obtained from Palmer amaranth plants surviving glufosinate, and (c) determine if the offspring from Palmer amaranth plants surviving different glufosinate application timings exhibit differential susceptibility to glufosinate.Core IdeasPalmer amaranth (∼10 cm) surviving glufosinate will remain fecund (2,400–22,000 seeds female−1).Seeds from Palmer amaranth surviving glufosinate do not exhibit differential viability or germination.Plants from surviving Palmer amaranth do not exhibit reduced glufosinate susceptibility after one generation.MATERIALS AND METHODSGlufosinate control and fecundity experimentField experiments were conducted at three locations in 2019: Lenoir County (35.29 N, 77.65 W [Triangle] and 35.26 N, 77.65 W [Walnut]) and Johnston County (35.66 N, 78.51 W [Clayton]), NC. The Walnut location has a Kalmia loamy sand (fine loamy over sandy or sandy‐skeletal, siliceous, semiactive, thermic Typic Hapludult), whereas the Triangle location has a mosaic of Lumbee sandy loam (sandy‐skeletal, siliceous, subactive, thermic Typic Endoaquult) and Portsmouth loam (sandy‐skeletal, mixed, semiactive, thermic Typic Umbraquult) soils. The Clayton location has a mosaic of Norfolk loamy sand (fine‐loamy, kaolinitic, thermic Typic Kandiudult), Rains sandy loam (fine‐loamy, siliceous, semiactive, thermic Typic Paleaquult), Varina loamy sand (fine, kaolinitic, thermic Plinthic Paleudult), and a Wagram loamy sand (loamy, kaolinitic, thermic Arenic Kandiudult) soils. The Palmer amaranth populations at each location are resistant to acetolactate synthase (Enzyme Commission 2.2.1.6) inhibiting herbicides (HG 2) and glyphosate (HG 9). Fields were cultivated prior to experiment layout to control established weeds, but preemergence herbicides were not applied to ensure maximum weed seedling emergence. The Lenoir County locations were planted on 16 June 16 2019, and the Johnston County location was planted on 12 July 2019 with soybeans (CZ 6515LL; Credenz, BASF) at a population of 272,000 seeds ha−1 and row spacing of 76 and 91 cm, respectively.Treatments were arranged in a randomized complete block design with four replications. Individual plots were 3.6 m wide × 9.0 m long. Glufosinate was applied at three timings when Palmer amaranth was 5 (early postemergence), 7–10 (mid postemergence), and 10–20 cm tall (late postemergence), and all orthogonal combinations of those timings (Table 1). The glufosinate application timings were selected to represent how North Carolina farmers are currently using the herbicide (Jones et al., 2022). A nontreated check was included in the experiment as well. Glufosinate treatments were applied with a CO2‐pressurized backpack sprayer calibrated to deliver 140 L ha−1 at 165 kPa with TeeJet XR11002‐VS nozzles (TeeJet Technologies) 46 cm above the target weed height. Glufosinate was applied at a rate of 590 g a.i. ha−1 with the inclusion of 10 g L−1 of ammonium sulfate. Glufosinate treatments were applied at 2± h of solar noon and temperatures above 30 °C with relative humidity greater than 30% to avoid environment‐induced control reductions (Coetzer et al., 2001; Sellers et al., 2003a). Palmer amaranth control ratings were made using visual estimates based on scale ranging from 0 (no control) to 100 (complete control) and were conducted 35 d after treatment (DAT). Palmer amaranth plants emerging after glufosinate applications were not rated as glufosinate has no soil residual activity.1TABLEPalmer amaranth control 35 d after initial glufosinate (590 g a.i. ha−1) application in soybeans at Johnston (Clayton) and Lenoir (Triangle and Walnut) County, NCTreatmentClaytonTriangleWalnut%EPOSTa100a100a96abMPOST70de83bcd70deLPOST78cd33e56eEPOST fb MPOST100a99ab100aEPOST fb LPOST100a100a94abcMPOST fb LPOST88abc100a85abcdEPOST fb MPOST fb LPOST100a100a100aNote. EPOST, early postemergence; fb, followed by; LPOST, late postemergence; MPOST, mid postemergence. Within columns, means followed by a letter are not significantly different based on Tukey's honestly significant difference (P < .05).aEPOST, 5 cm; POST, 7–10 cm; LPOST, > 10 cm Palmer amaranth.Approximately 10 Palmer amaranth plants per plot surviving glufosinate were marked with a flag. Three surviving female Palmer amaranth from the marked plants were arbitrarily selected from each plot if present, allowed to produce seed, and then harvested. These plants were stored at 10 to 20 °C for approximately 1 mo and threshed by hand to remove seeds from the florets. Seeds were separated from plant residues using sieves and a forced air column separator (South Dakota Seed Blower; Seedburo Equipment Company). Crush tests, as described by Sawma and Mohler (2002), were conducted during the cleaning process to determine whether seeds were viable before discarding with vegetative residue. Seeds were then rubbed between pieces of rubber sheeting which supplied enough friction and pressure to remove the remaining florets. Samples were cleaned again with the forced air to further remove plant residue. The total number of seeds produced by each female plant was extrapolated by determining the mass of five subsamples of 100 seeds for each Palmer amaranth cohort (Bertucci et al., 2020; Jones et al., 2019; Sellers et al., 2003b). The total number of seeds produced for each plant was calculated using Equation 1:1T=WS×100$$\begin{equation}T\; = \left( {\frac{W}{S}} \right)\; \times 100\end{equation}$$where W equals the total seed mass, S equals the average seed mass determined based on five 100‐seed subsamples, and T equals the calculated number of seeds produced.Germination experimentA germination experiment was conducted to assess the viability of offspring seeds from nontreated and glufosinate‐treated Palmer amaranth plants. Seeds from each plot were placed in a petri dish with a small amount of tap water and stored at 5 °C for 2 wk to break dormancy (Leon et al., 2006). The petri dishes without lids were then placed into a dryer at 45 °C for 48 h to reduce their moisture content before storage. One gram of seed from each treatment replicate was pooled together within a location. Seeds from a Palmer amaranth population from a field in Johnston County, NC, collected in 2013 that had not been treated with glufosinate or near glufosinate‐treated Palmer amaranth were included as well. This population was selected as a control to determine if the seeds from nontreated plants from the field experiment germinated similarly. Seeds were stored at 5 °C until needed. The experimental design was completely randomized with four replications. The experiment was conducted four times; two runs in two different germination chambers (Percival Scientific). Fifty seeds from each treatment were then placed into a petri dish containing blue blotter paper and 7.5 ml of distilled water. The petri dishes were sealed with parafilm to maintain moisture throughout the experiment. The petri dishes containing seeds were then placed into a germination chamber for 14 d adjusted to 14‐h photoperiods with 30/20 °C diurnal temperature. Light was supplemented by fluorescent light providing 600–1,000 μmol m−2 s−1 photosynthetic photon flux density. Palmer amaranth germination was determined by counting the number of germinated seeds daily for 14 d. Palmer amaranth seeds were considered germinated when radicles were visible. Germinated seeds were removed from each petri dish and resealed with parafilm after each evaluation.Glufosinate susceptibility experimentCollected Palmer amaranth seeds were handled and pooled as described above for the germination experiment. The Palmer amaranth population isolated from glufosinate application and glufosinate‐treated plants collected in Johnston County, NC (2013), was included in this experiment as well. Seeds were sown into 22 cm × 29 cm flats containing a 4:1 ratio of Sunshine Mix #2 potting soil and sand with approximately 5 g of Osmocote Flower Food Granules (14‐14‐14). Plants were maintained in the greenhouse at 24 °C and topically watered to maintain field capacity water content. Sunlight was supplemented by metal halide lights with 600—1,000 μmol m−2 s−1 photosynthetic photon flux density set to a 14‐h photoperiod. Plants were transplanted when approximately 2—5 cm in height to 5‐cm pots containing the same potting media with 1 g of pellet fertilizer. Palmer amaranth plants were treated with glufosinate when they reached 5—7 cm in height. Glufosinate was applied at 0, 1/8, 1/4, 3/8, 1/2, 5/8, 1, and 2× of the rate used in the field experiment (590 a.i. ha−1). All rates of glufosinate included 10 g L−1 of ammonium sulfate. Glufosinate was applied with a CO2‐pressurized backpack sprayer calibrated to deliver 140 L ha−1 at 165 kPa with TeeJet XR11002‐VS nozzles 46 cm above the target weed height. Glufosinate‐treated Palmer amaranth plants were not randomized until 24 h after treatment. The experimental design was completely randomized with four replications. The experiment was conducted twice. Palmer amaranth control ratings were made using estimates described above for the field study 21 DAT.Statistical analysisGlufosinate control and fecundity data from the field experiment were subjected to analysis of variance (ANOVA) using the Glimmix procedure in SAS 9.4 (SAS Institute). Location, glufosinate application timing, and their interactions were considered fixed effects, whereas block was considered a random effect. Treatment means were separated using Tukey's honestly significant difference test (α = 0.05). Control and fecundity data were also subjected to the Corr procedure using Pearson correlation coefficients in SAS 9.4 to determine the relationship between the two variables.Palmer amaranth seed germination data were modeled using an exponential growth model to determine the days until reaching 50% germination rate (GR50) (see Equation 2):2y=y0×1−e−kt$$\begin{equation}y\; = \;{y_0} \times \left( {1 - {e^{ - kt}}} \right)\end{equation}$$where y equals germination (percent), y0 equals initial germination, t equals time in days, and k equals a constant that determines the steepness of the curve. Seed viability was determined by taking the cumulative seeds germinated during the 14‐d evaluation. The GR50 and viability data were then subjected to an ANOVA using the GLIMMIX procedure in SAS 9.4. Location and glufosinate application timing survival were considered fixed effects, whereas the germination chamber, run, and replication were considered a random effects. Treatment means were separated using Tukey's honestly significant difference test (α = 0.05). Germination rate and viability data were also subjected to the Corr procedure using Pearson correlation coefficients in SAS 9.4 to determine the relationship between the two variables.The glufosinate susceptibility of Palmer amaranth offspring surviving field glufosinate applications was subjected to an ANOVA using the GLIMMIX procedure in SAS 9.4. Run, location, and glufosinate rate were considered fixed effects, whereas the replication was considered a random effect.RESULTS AND DISCUSSIONPalmer amaranth control with glufosinateControl differed across application timing (P < .0001) but not locations (P = .33). A significant interaction between location × application timing was detected (P = .006). Thus, control means were separated across locations and application timings. Early postemergence and the sequential applications of glufosinate provided the highest level of Palmer amaranth control (85—100%) (Table 1). The mid postemergence application of glufosinate provided 70—83% control depending on location (Table 1). The late postemergence application of glufosinate controlled Palmer amaranth differently across locations. Late postemergence applications of glufosinate provided higher control at Clayton (78%) compared with Triangle (33%) and Walnut (56%). Low level of Palmer amaranth control with the late postemergence glufosinate application at Triangle was likely due to plant size at herbicide applications (approximately 20 cm). Control with the different glufosinate application timings observed in this experiment were similar to previously conducted research (Coetzer et al., 2002; Randell et al., 2020).Palmer amaranth fecundityPalmer amaranth seed size differed across locations (P < .0001) but was not affected by the application timing (P = .42). Although the interaction between location and application timing was significant for Palmer amaranth seed size (P = .0003), this interaction was mainly driven by the location effect (location F‐value: 40.92; application timing F‐value: 0.96; location × application timing F‐value: 6.00), thus the mass of 100 Palmer amaranth seeds were averaged over locations. Palmer amaranth seed mass was largest at Walnut followed by Clayton and Triangle (Table 2). The experiment provides evidence that glufosinate does not influence the size of seed produced by Palmer amaranth surviving tested application timings. The differences in seed size across locations rather than across application timing was not unexpected as this is a quantitative trait likely to be influenced more by environment than sublethal herbicide doses (Colquhoun et al., 2001; Kegode et al., 1999).2TABLEThe average mass of five 100‐seed subsamples from nontreated Palmer amaranth and Palmer amaranth plants surviving glufosinate applications pooled across application timingsLocation100 seed massgClayton0.029bTriangle0.030bWalnut0.034aNote. Values that share the same letters are not statistically different based on Tukey's honestly significant difference (P < .05).Palmer amaranth fecundity was not affected by location (P = .46), whereas the application timing had a significant effect (P < .0001). No significant interaction between location × application timing was detected (P = .98). Thus, Palmer amaranth fecundity was averaged over locations. Palmer amaranth was controlled when treated at early postemergence, early postemergence followed by (fb) mid postemergence, early postemergence fb late postemergence, and early postemergence fb mid postemergence fb late postemergence; resulting in no surviving plants to produce seed (Table 3). Nontreated Palmer amaranth plants and plants treated at the mid postemergence and late postemergence application did not differ in fecundity (Table 3). Palmer amaranth plants surviving the mid postemergence fb late postemergence treatment still produced > 2,000 seeds plant−1 despite the high level of efficacy achieved with this treatment (Tables 1 and 3). These results suggest that Palmer amaranth fecundity can differ with glufosinate application timings. The correlation between control 35 DAT and fecundity was not significant (R = −0.22; P = .48) suggesting that control may not accurately predict Palmer amaranth fecundity. The experiment provided evidence that Palmer amaranth plants surviving glufosinate were fecund (2,400 to 22,000 seeds plant−1) and sequential applications of glufosinate significantly reduced seed production. The fecundity of Palmer amaranth surviving glufosinate applied at the vegetative stage in this research was similar to the fecundity of Palmer amaranth surviving glufosinate and other unrelated herbicides applied in the reproductive stage (de Sanctis et al., 2021; Jha & Norsworthy, 2012; Scruggs et al., 2020).3TABLEFecundity of nontreated Palmer amaranth and Palmer amaranth plants surviving glufosinate (590 g a.i. ha−1) applications pooled across experiment locationsTreatmentFecunditySeeds plant−1NTC35,000aEPOSTaNSMPOST20,000aLPOST22,000aEPOST fb MPOSTNSEPOST fb LPOSTNSMPOST fb LPOST2,400bEPOST fb MPOST fb LPOSTNSNote. EPOST, early postemergence; fb, followed by; LPOST, late postemergence; MPOST, postemergence; NS, no survivors; NTC, nontreated. Values that share the same letters are not statistically different based on Tukey's honestly significant difference (P < .05).aEPOST, 5 cm; MPOST, 7–10 cm; LPOST, > 10 cm Palmer amaranth.Palmer amaranth germinationAll main effects and interactions were significant for the GR50 and viability; thus, data were analyzed considering the full ANOVA model (Table 4). The GR50 and seed viability were negatively correlated (R = −0.73; P < .0001) suggesting that seed lots with lower viability also exhibit low vigor. Palmer amaranth GR50 ranged from 0.7 to 4.5 d, whereas viability ranged from 63 to 90% for all locations and application timings, respectively (Table 4). Across both germination chambers and runs, Palmer amaranth treated mid postemergence fb late postemergence at the Walnut location had the highest GR50 and lowest viability and Palmer amaranth treated mid postemergence fb late postemergence at Clayton did not (Table 4). Aside from the Palmer amaranth treated mid postemergence fb late postemergence at Walnut, there was no distinct pattern of germination or viability across location and glufosinate application timing (Table 4). Scruggs et al. (2020) found a similar result when testing the viability of seeds from Palmer amaranth at reproductive stages surviving glufosinate application with a similar rate. Jha and Norsworthy (2012) found decreased viability of seeds from Palmer amaranth treated with higher rate of glufosinate at reproductive stages. Differences between the differential germination exhibited by the Palmer amaranth collected from different locations and from plants surviving different glufosinate application timings was not unexpected as this dioecious species has high germination variability (Camacho et al., 2021; Ehleringer, 1983; Steckel et al., 2004).4TABLEDays to reach 50% germination rate (GR50) and the viability of seeds from nontreated Palmer amaranth and Palmer amaranth plants surviving glufosinate (590 g a.i. ha−1) applications across experiment locations. Germination was evaluated daily for 14 dLocationTiming survivedGR50Viabilityd%CNTC1.4bc82a–dPOST2.2bc74deLPOST0.9c89abPOST fb LPOST1.2c86a–dCJNTC1.0c87abcTNTC4.5ab74deMPOSTa0.7c90aLPOST1.8bc78a–dWNTC2.1bc78bcdMPOST2.2bc77cdLPOST1.6bc78bcdPOST fb LPOST6.0a63eNote. C, Clayton; CJ, Clayton field edge; fb, followed by; LPOST, late postemergence; MPOST, mid postemergence; NTC, nontreated; T, Triangle; W, Walnut. Values that share the same letters are not statistically different based on Tukey's honestly significant difference (P < .05). Seed from Palmer amaranth population was collected in 2013 from a field edge in Johnston County, NC (CJ) isolated from glufosinate use was included as well.aMPOST, 7–10 cm; LPOST, > 10 cm Palmer amaranth.Palmer amaranth offspring susceptibility to glufosinateAll rates of glufosinate resulted in control of Palmer amaranth plants 21 DAT (data not shown). These results suggest that the offspring from large Palmer amaranth plants that can survive glufosinate treatments will still exhibit susceptibility to glufosinate in the subsequent growing season if applied at the correct timing. Glufosinate‐susceptible Palmer amaranth populations were controlled with similar low glufosinate rates (95 g a.i. ha−1) in this experiment compared with glufosinate‐resistant Palmer amaranth surviving much higher rates (> 220 g a.i. ha−1) (Noguera et al., 2022; Priess et al., 2022). These results provide further evidence that the offspring from Palmer amaranth surviving glufosinate do not exhibit reduced susceptibility to glufosinate. Although previous research has demonstrated reduced susceptibility to herbicides after treatment with sublethal doses, the mechanism(s) facilitating Palmer amaranth to survive tested glufosinate applications may not be an epigenetic effect, maternally heritable, or provide enough selection pressure to be heritable after one season (Ghanizadeh et al., 2019; Markus et al., 2017; Neve & Powles, 2005).Results from this research conclude that vegetative‐stage Palmer amaranth surviving glufosinate will remain fecund (2,400–22,000 seeds female−1) but differential fecundity can be observed if the plant survived one or two glufosinate applications; however, the offspring would not exhibit differential germination or susceptibility to glufosinate after one growing season. Furthermore, applications of glufosinate should be applied early postemergence (the appropriate weed size according to the label) to eliminate seed production of Palmer amaranth to further deplete the soil seedbank and mitigate recurrent Palmer amaranth population control. Although sequential applications of glufosinate ceased or decreased Palmer amaranth seed production, these applications should be recommended to farmers with caution to reduce the selection pressure on Palmer amaranth evolving resistance to glufosinate. Sequential applications of glufosinate are common in North Carolina row crops and many North Carolina farmers utilize additional tactics for herbicide resistance management (Jones et al., 2022).The collected fecundity data compared across glufosinate‐treated and nontreated Palmer amaranth plants is not a true representation of fecundity reduction as nontreated Palmer amaranth plants were subjected to extreme levels of inter‐ and intraspecific competition. Future research should determine the fecundity of Palmer amaranth plants grown with minimal interspecific competition (i.e., grown in a weed‐free crop) and no competition (i.e., fallow) to determine the reduction of fecundity when surviving glufosinate at a specific timing. In tandem, future research should include other glufosinate‐resistant crops to determine if the different crops influence glufosinate control and fecundity of Palmer amaranth surviving glufosinate applications. In addition, cross pollination was not controlled in the study; thus, fertilization of female Palmer amaranth surviving glufosinate treatment may be a product of pollen from nontreated male Palmer amaranth in proximity. Similar research conducted did not control for volunteer pollen and the results were similar across tested variables (de Sanctis et al., 2021; Jha & Norsworthy, 2012; Scruggs et al., 2020). However, a true representation of decreased glufosinate susceptibility would be to evaluate biparental crosses of glufosinate‐ and nontreated plants. Future research investigating concomitant effects on the susceptibility to glufosinate exhibited on the offspring should focus on controlled crosses between Palmer amaranth plants surviving glufosinate treatment.ACKNOWLEDGMENTSProject funding was provided by the North Carolina Soybean Producers Association. The authors thank John Sanders, Diego Contreras, and Marco Fajardo for technical support. The authors thank Dr. Micheal Owen for reviewing the manuscript. No conflicts of interest have been declared.AUTHOR CONTRIBUTIONSEric Jones: Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Writing‐original draft; Writing‐review & editing. Ramon G. Leon: Conceptualization; Funding acquisition; Methodology; Project administration; Supervision; Writing‐review & editing. 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Weed Science, 35, 554–559. https://doi.org/10.1017/S0043174500060549 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png "Agrosystems, Geosciences & Environment" Wiley

Biological effects on Palmer amaranth surviving glufosinate

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Abstract

AbbreviationsDATdays after treatmentfbfollowed byGR5050% germination rateHGherbicide groupINTRODUCTIONPalmer amaranth (Amaranthus palmeri S. Watson) is one of the most pervasive weeds of row crops in the United States due to prolonged germination periods, rapid seedling growth, dioecy, and high fecundity (Bravo et al., 2017; Horak & Loughin, 2000; Keeley et al., 1987). Female plants can produce 50,000 to 1,000,000 seeds plant−1 and thus replenish the soil seed bank facilitating the persistence of the population within the agroecosystem (Mahoney et al., 2021; Walsh et al., 2013). Difficulty to control Palmer amaranth is exacerbated by evolved resistance to almost all labeled row crop herbicides (Heap, 2022).Glufosinate (herbicide group [HG] 10) is a nonselective, fast‐acting contact herbicide that inhibits glutamine synthetase (Enzyme Commission 6.3.1.2) ceasing the production of glutamate and disrupting chlorophyll production (Takano et al., 2019, 2020). Because Palmer amaranth has evolved resistance to nine unique herbicide groups and multiple herbicide‐resistant populations are common in crop fields, glufosinate is among the few effective postemergence options for some cotton (Gossypium L.) and soybean [Glycine max (L.) Merr.] farmers in North Carolina (Everman et al., 2007; Heap, 2022; Mahoney et al., 2020). However, glufosinate resistance has evolved in isolated populations of Palmer amaranth (Norsworthy et al., 2021; Priess et al., 2022).While glufosinate is efficacious on susceptible Palmer amaranth populations, control is plant height/size dependent (Everman et al., 2007; Steckel et al., 1997). Although the glufosinate label recommends treating weeds before reaching 7.5 cm in height, it is common to find Palmer amaranth individuals exceeding this height at the time of application (Anonymous, 2017; Reinhardt Piskackova et al., 2021). Furthermore, once Palmer amaranth height exceeds 10 cm, control is greatly reduced and treated plants can produce viable seed (Scruggs et al., 2020; Steckel et al., 1997). Preventing seed production is vital for preventing future weed problems, specifically if the weed population has evolved herbicide resistance (Bagavathiannan & Norsworthy, 2012; Leon et al., 2016). Because glufosinate control diminishes as Palmer amaranth height increases, quantifying the fecundity of surviving Palmer amaranth is important. Previous research demonstrated that applying glufosinate to Palmer amaranth plants in reproductive growth stages can significantly reduce seed production compared with nontreated plants (Jha & Norsworthy, 2012; Scruggs et al., 2020).Currently, there is no peer‐reviewed literature providing data on quantifying vegetative‐stage Palmer amaranth fecundity after surviving glufosinate and the implication of different application timings. If Palmer amaranth remains fecund after glufosinate treatment, it would be important to determine whether the offspring exhibit differential germination or susceptibility to glufosinate (Oreja et al., 2021). Previous research suggests Palmer amaranth can evolve resistance to herbicide(s) when recurrently treated with sublethal doses (Neve & Powles, 2005; Tehranchian et al., 2017; Vieira et al., 2020). Evolution of glufosinate resistance was observed with recurrent selection with sublethal doses in Italian ryegrass (Lolium perenne ssp. multiflorum) after three generations (Matzrafi et al., 2020). Although the evolution of herbicide resistance is a multigenerational event, measurable reductions in control after one generation could provide insight on the duration before evolution of glufosinate resistance occurs (Neve et al., 2011). Fecundity will also be a key variable for predicting the rapidity of Palmer amaranth evolving glufosinate resistance.Seed viability may be affected by glufosinate when applied to Palmer amaranth when inflorescences are present. Scruggs et al. (2020) previously reported no differences in seed viability from reproductive stage Palmer amaranth plants treated with glufosinate (656 g a.i. ha−1) compared with nontreated Palmer amaranth plants. Conversely, Jha and Norsworthy (2012) observed decreased seed viability when glufosinate (820 g a.i. ha−1) was applied to flowering Palmer amaranth. In other weed species, seed viability and germination have been affected by sublethal doses of efficacious herbicides (Qi et al., 2017; Shuma et al., 1995; Young & Whitesides, 1987). The differential seed viability observed in previous research could be due to the tested systemic herbicides translocating to reproductive tissue/seeds compared with desiccation of glufosinate‐treated reproductive tissue (Thomas et al., 2004; Walker & Oliver, 2008). However, the seed physiology could be affected by differential resource partitioning by plants surviving glufosinate applications in the vegetative state (Rosenthal & Kotanen, 1998; Schooler et al., 2007; Wise & Abrahamson, 2008). Thus, the objectives of this research were to (a) quantify the fecundity of Palmer amaranth plants surviving glufosinate applied at various timings, (b) determine the viability of seeds obtained from Palmer amaranth plants surviving glufosinate, and (c) determine if the offspring from Palmer amaranth plants surviving different glufosinate application timings exhibit differential susceptibility to glufosinate.Core IdeasPalmer amaranth (∼10 cm) surviving glufosinate will remain fecund (2,400–22,000 seeds female−1).Seeds from Palmer amaranth surviving glufosinate do not exhibit differential viability or germination.Plants from surviving Palmer amaranth do not exhibit reduced glufosinate susceptibility after one generation.MATERIALS AND METHODSGlufosinate control and fecundity experimentField experiments were conducted at three locations in 2019: Lenoir County (35.29 N, 77.65 W [Triangle] and 35.26 N, 77.65 W [Walnut]) and Johnston County (35.66 N, 78.51 W [Clayton]), NC. The Walnut location has a Kalmia loamy sand (fine loamy over sandy or sandy‐skeletal, siliceous, semiactive, thermic Typic Hapludult), whereas the Triangle location has a mosaic of Lumbee sandy loam (sandy‐skeletal, siliceous, subactive, thermic Typic Endoaquult) and Portsmouth loam (sandy‐skeletal, mixed, semiactive, thermic Typic Umbraquult) soils. The Clayton location has a mosaic of Norfolk loamy sand (fine‐loamy, kaolinitic, thermic Typic Kandiudult), Rains sandy loam (fine‐loamy, siliceous, semiactive, thermic Typic Paleaquult), Varina loamy sand (fine, kaolinitic, thermic Plinthic Paleudult), and a Wagram loamy sand (loamy, kaolinitic, thermic Arenic Kandiudult) soils. The Palmer amaranth populations at each location are resistant to acetolactate synthase (Enzyme Commission 2.2.1.6) inhibiting herbicides (HG 2) and glyphosate (HG 9). Fields were cultivated prior to experiment layout to control established weeds, but preemergence herbicides were not applied to ensure maximum weed seedling emergence. The Lenoir County locations were planted on 16 June 16 2019, and the Johnston County location was planted on 12 July 2019 with soybeans (CZ 6515LL; Credenz, BASF) at a population of 272,000 seeds ha−1 and row spacing of 76 and 91 cm, respectively.Treatments were arranged in a randomized complete block design with four replications. Individual plots were 3.6 m wide × 9.0 m long. Glufosinate was applied at three timings when Palmer amaranth was 5 (early postemergence), 7–10 (mid postemergence), and 10–20 cm tall (late postemergence), and all orthogonal combinations of those timings (Table 1). The glufosinate application timings were selected to represent how North Carolina farmers are currently using the herbicide (Jones et al., 2022). A nontreated check was included in the experiment as well. Glufosinate treatments were applied with a CO2‐pressurized backpack sprayer calibrated to deliver 140 L ha−1 at 165 kPa with TeeJet XR11002‐VS nozzles (TeeJet Technologies) 46 cm above the target weed height. Glufosinate was applied at a rate of 590 g a.i. ha−1 with the inclusion of 10 g L−1 of ammonium sulfate. Glufosinate treatments were applied at 2± h of solar noon and temperatures above 30 °C with relative humidity greater than 30% to avoid environment‐induced control reductions (Coetzer et al., 2001; Sellers et al., 2003a). Palmer amaranth control ratings were made using visual estimates based on scale ranging from 0 (no control) to 100 (complete control) and were conducted 35 d after treatment (DAT). Palmer amaranth plants emerging after glufosinate applications were not rated as glufosinate has no soil residual activity.1TABLEPalmer amaranth control 35 d after initial glufosinate (590 g a.i. ha−1) application in soybeans at Johnston (Clayton) and Lenoir (Triangle and Walnut) County, NCTreatmentClaytonTriangleWalnut%EPOSTa100a100a96abMPOST70de83bcd70deLPOST78cd33e56eEPOST fb MPOST100a99ab100aEPOST fb LPOST100a100a94abcMPOST fb LPOST88abc100a85abcdEPOST fb MPOST fb LPOST100a100a100aNote. EPOST, early postemergence; fb, followed by; LPOST, late postemergence; MPOST, mid postemergence. Within columns, means followed by a letter are not significantly different based on Tukey's honestly significant difference (P < .05).aEPOST, 5 cm; POST, 7–10 cm; LPOST, > 10 cm Palmer amaranth.Approximately 10 Palmer amaranth plants per plot surviving glufosinate were marked with a flag. Three surviving female Palmer amaranth from the marked plants were arbitrarily selected from each plot if present, allowed to produce seed, and then harvested. These plants were stored at 10 to 20 °C for approximately 1 mo and threshed by hand to remove seeds from the florets. Seeds were separated from plant residues using sieves and a forced air column separator (South Dakota Seed Blower; Seedburo Equipment Company). Crush tests, as described by Sawma and Mohler (2002), were conducted during the cleaning process to determine whether seeds were viable before discarding with vegetative residue. Seeds were then rubbed between pieces of rubber sheeting which supplied enough friction and pressure to remove the remaining florets. Samples were cleaned again with the forced air to further remove plant residue. The total number of seeds produced by each female plant was extrapolated by determining the mass of five subsamples of 100 seeds for each Palmer amaranth cohort (Bertucci et al., 2020; Jones et al., 2019; Sellers et al., 2003b). The total number of seeds produced for each plant was calculated using Equation 1:1T=WS×100$$\begin{equation}T\; = \left( {\frac{W}{S}} \right)\; \times 100\end{equation}$$where W equals the total seed mass, S equals the average seed mass determined based on five 100‐seed subsamples, and T equals the calculated number of seeds produced.Germination experimentA germination experiment was conducted to assess the viability of offspring seeds from nontreated and glufosinate‐treated Palmer amaranth plants. Seeds from each plot were placed in a petri dish with a small amount of tap water and stored at 5 °C for 2 wk to break dormancy (Leon et al., 2006). The petri dishes without lids were then placed into a dryer at 45 °C for 48 h to reduce their moisture content before storage. One gram of seed from each treatment replicate was pooled together within a location. Seeds from a Palmer amaranth population from a field in Johnston County, NC, collected in 2013 that had not been treated with glufosinate or near glufosinate‐treated Palmer amaranth were included as well. This population was selected as a control to determine if the seeds from nontreated plants from the field experiment germinated similarly. Seeds were stored at 5 °C until needed. The experimental design was completely randomized with four replications. The experiment was conducted four times; two runs in two different germination chambers (Percival Scientific). Fifty seeds from each treatment were then placed into a petri dish containing blue blotter paper and 7.5 ml of distilled water. The petri dishes were sealed with parafilm to maintain moisture throughout the experiment. The petri dishes containing seeds were then placed into a germination chamber for 14 d adjusted to 14‐h photoperiods with 30/20 °C diurnal temperature. Light was supplemented by fluorescent light providing 600–1,000 μmol m−2 s−1 photosynthetic photon flux density. Palmer amaranth germination was determined by counting the number of germinated seeds daily for 14 d. Palmer amaranth seeds were considered germinated when radicles were visible. Germinated seeds were removed from each petri dish and resealed with parafilm after each evaluation.Glufosinate susceptibility experimentCollected Palmer amaranth seeds were handled and pooled as described above for the germination experiment. The Palmer amaranth population isolated from glufosinate application and glufosinate‐treated plants collected in Johnston County, NC (2013), was included in this experiment as well. Seeds were sown into 22 cm × 29 cm flats containing a 4:1 ratio of Sunshine Mix #2 potting soil and sand with approximately 5 g of Osmocote Flower Food Granules (14‐14‐14). Plants were maintained in the greenhouse at 24 °C and topically watered to maintain field capacity water content. Sunlight was supplemented by metal halide lights with 600—1,000 μmol m−2 s−1 photosynthetic photon flux density set to a 14‐h photoperiod. Plants were transplanted when approximately 2—5 cm in height to 5‐cm pots containing the same potting media with 1 g of pellet fertilizer. Palmer amaranth plants were treated with glufosinate when they reached 5—7 cm in height. Glufosinate was applied at 0, 1/8, 1/4, 3/8, 1/2, 5/8, 1, and 2× of the rate used in the field experiment (590 a.i. ha−1). All rates of glufosinate included 10 g L−1 of ammonium sulfate. Glufosinate was applied with a CO2‐pressurized backpack sprayer calibrated to deliver 140 L ha−1 at 165 kPa with TeeJet XR11002‐VS nozzles 46 cm above the target weed height. Glufosinate‐treated Palmer amaranth plants were not randomized until 24 h after treatment. The experimental design was completely randomized with four replications. The experiment was conducted twice. Palmer amaranth control ratings were made using estimates described above for the field study 21 DAT.Statistical analysisGlufosinate control and fecundity data from the field experiment were subjected to analysis of variance (ANOVA) using the Glimmix procedure in SAS 9.4 (SAS Institute). Location, glufosinate application timing, and their interactions were considered fixed effects, whereas block was considered a random effect. Treatment means were separated using Tukey's honestly significant difference test (α = 0.05). Control and fecundity data were also subjected to the Corr procedure using Pearson correlation coefficients in SAS 9.4 to determine the relationship between the two variables.Palmer amaranth seed germination data were modeled using an exponential growth model to determine the days until reaching 50% germination rate (GR50) (see Equation 2):2y=y0×1−e−kt$$\begin{equation}y\; = \;{y_0} \times \left( {1 - {e^{ - kt}}} \right)\end{equation}$$where y equals germination (percent), y0 equals initial germination, t equals time in days, and k equals a constant that determines the steepness of the curve. Seed viability was determined by taking the cumulative seeds germinated during the 14‐d evaluation. The GR50 and viability data were then subjected to an ANOVA using the GLIMMIX procedure in SAS 9.4. Location and glufosinate application timing survival were considered fixed effects, whereas the germination chamber, run, and replication were considered a random effects. Treatment means were separated using Tukey's honestly significant difference test (α = 0.05). Germination rate and viability data were also subjected to the Corr procedure using Pearson correlation coefficients in SAS 9.4 to determine the relationship between the two variables.The glufosinate susceptibility of Palmer amaranth offspring surviving field glufosinate applications was subjected to an ANOVA using the GLIMMIX procedure in SAS 9.4. Run, location, and glufosinate rate were considered fixed effects, whereas the replication was considered a random effect.RESULTS AND DISCUSSIONPalmer amaranth control with glufosinateControl differed across application timing (P < .0001) but not locations (P = .33). A significant interaction between location × application timing was detected (P = .006). Thus, control means were separated across locations and application timings. Early postemergence and the sequential applications of glufosinate provided the highest level of Palmer amaranth control (85—100%) (Table 1). The mid postemergence application of glufosinate provided 70—83% control depending on location (Table 1). The late postemergence application of glufosinate controlled Palmer amaranth differently across locations. Late postemergence applications of glufosinate provided higher control at Clayton (78%) compared with Triangle (33%) and Walnut (56%). Low level of Palmer amaranth control with the late postemergence glufosinate application at Triangle was likely due to plant size at herbicide applications (approximately 20 cm). Control with the different glufosinate application timings observed in this experiment were similar to previously conducted research (Coetzer et al., 2002; Randell et al., 2020).Palmer amaranth fecundityPalmer amaranth seed size differed across locations (P < .0001) but was not affected by the application timing (P = .42). Although the interaction between location and application timing was significant for Palmer amaranth seed size (P = .0003), this interaction was mainly driven by the location effect (location F‐value: 40.92; application timing F‐value: 0.96; location × application timing F‐value: 6.00), thus the mass of 100 Palmer amaranth seeds were averaged over locations. Palmer amaranth seed mass was largest at Walnut followed by Clayton and Triangle (Table 2). The experiment provides evidence that glufosinate does not influence the size of seed produced by Palmer amaranth surviving tested application timings. The differences in seed size across locations rather than across application timing was not unexpected as this is a quantitative trait likely to be influenced more by environment than sublethal herbicide doses (Colquhoun et al., 2001; Kegode et al., 1999).2TABLEThe average mass of five 100‐seed subsamples from nontreated Palmer amaranth and Palmer amaranth plants surviving glufosinate applications pooled across application timingsLocation100 seed massgClayton0.029bTriangle0.030bWalnut0.034aNote. Values that share the same letters are not statistically different based on Tukey's honestly significant difference (P < .05).Palmer amaranth fecundity was not affected by location (P = .46), whereas the application timing had a significant effect (P < .0001). No significant interaction between location × application timing was detected (P = .98). Thus, Palmer amaranth fecundity was averaged over locations. Palmer amaranth was controlled when treated at early postemergence, early postemergence followed by (fb) mid postemergence, early postemergence fb late postemergence, and early postemergence fb mid postemergence fb late postemergence; resulting in no surviving plants to produce seed (Table 3). Nontreated Palmer amaranth plants and plants treated at the mid postemergence and late postemergence application did not differ in fecundity (Table 3). Palmer amaranth plants surviving the mid postemergence fb late postemergence treatment still produced > 2,000 seeds plant−1 despite the high level of efficacy achieved with this treatment (Tables 1 and 3). These results suggest that Palmer amaranth fecundity can differ with glufosinate application timings. The correlation between control 35 DAT and fecundity was not significant (R = −0.22; P = .48) suggesting that control may not accurately predict Palmer amaranth fecundity. The experiment provided evidence that Palmer amaranth plants surviving glufosinate were fecund (2,400 to 22,000 seeds plant−1) and sequential applications of glufosinate significantly reduced seed production. The fecundity of Palmer amaranth surviving glufosinate applied at the vegetative stage in this research was similar to the fecundity of Palmer amaranth surviving glufosinate and other unrelated herbicides applied in the reproductive stage (de Sanctis et al., 2021; Jha & Norsworthy, 2012; Scruggs et al., 2020).3TABLEFecundity of nontreated Palmer amaranth and Palmer amaranth plants surviving glufosinate (590 g a.i. ha−1) applications pooled across experiment locationsTreatmentFecunditySeeds plant−1NTC35,000aEPOSTaNSMPOST20,000aLPOST22,000aEPOST fb MPOSTNSEPOST fb LPOSTNSMPOST fb LPOST2,400bEPOST fb MPOST fb LPOSTNSNote. EPOST, early postemergence; fb, followed by; LPOST, late postemergence; MPOST, postemergence; NS, no survivors; NTC, nontreated. Values that share the same letters are not statistically different based on Tukey's honestly significant difference (P < .05).aEPOST, 5 cm; MPOST, 7–10 cm; LPOST, > 10 cm Palmer amaranth.Palmer amaranth germinationAll main effects and interactions were significant for the GR50 and viability; thus, data were analyzed considering the full ANOVA model (Table 4). The GR50 and seed viability were negatively correlated (R = −0.73; P < .0001) suggesting that seed lots with lower viability also exhibit low vigor. Palmer amaranth GR50 ranged from 0.7 to 4.5 d, whereas viability ranged from 63 to 90% for all locations and application timings, respectively (Table 4). Across both germination chambers and runs, Palmer amaranth treated mid postemergence fb late postemergence at the Walnut location had the highest GR50 and lowest viability and Palmer amaranth treated mid postemergence fb late postemergence at Clayton did not (Table 4). Aside from the Palmer amaranth treated mid postemergence fb late postemergence at Walnut, there was no distinct pattern of germination or viability across location and glufosinate application timing (Table 4). Scruggs et al. (2020) found a similar result when testing the viability of seeds from Palmer amaranth at reproductive stages surviving glufosinate application with a similar rate. Jha and Norsworthy (2012) found decreased viability of seeds from Palmer amaranth treated with higher rate of glufosinate at reproductive stages. Differences between the differential germination exhibited by the Palmer amaranth collected from different locations and from plants surviving different glufosinate application timings was not unexpected as this dioecious species has high germination variability (Camacho et al., 2021; Ehleringer, 1983; Steckel et al., 2004).4TABLEDays to reach 50% germination rate (GR50) and the viability of seeds from nontreated Palmer amaranth and Palmer amaranth plants surviving glufosinate (590 g a.i. ha−1) applications across experiment locations. Germination was evaluated daily for 14 dLocationTiming survivedGR50Viabilityd%CNTC1.4bc82a–dPOST2.2bc74deLPOST0.9c89abPOST fb LPOST1.2c86a–dCJNTC1.0c87abcTNTC4.5ab74deMPOSTa0.7c90aLPOST1.8bc78a–dWNTC2.1bc78bcdMPOST2.2bc77cdLPOST1.6bc78bcdPOST fb LPOST6.0a63eNote. C, Clayton; CJ, Clayton field edge; fb, followed by; LPOST, late postemergence; MPOST, mid postemergence; NTC, nontreated; T, Triangle; W, Walnut. Values that share the same letters are not statistically different based on Tukey's honestly significant difference (P < .05). Seed from Palmer amaranth population was collected in 2013 from a field edge in Johnston County, NC (CJ) isolated from glufosinate use was included as well.aMPOST, 7–10 cm; LPOST, > 10 cm Palmer amaranth.Palmer amaranth offspring susceptibility to glufosinateAll rates of glufosinate resulted in control of Palmer amaranth plants 21 DAT (data not shown). These results suggest that the offspring from large Palmer amaranth plants that can survive glufosinate treatments will still exhibit susceptibility to glufosinate in the subsequent growing season if applied at the correct timing. Glufosinate‐susceptible Palmer amaranth populations were controlled with similar low glufosinate rates (95 g a.i. ha−1) in this experiment compared with glufosinate‐resistant Palmer amaranth surviving much higher rates (> 220 g a.i. ha−1) (Noguera et al., 2022; Priess et al., 2022). These results provide further evidence that the offspring from Palmer amaranth surviving glufosinate do not exhibit reduced susceptibility to glufosinate. Although previous research has demonstrated reduced susceptibility to herbicides after treatment with sublethal doses, the mechanism(s) facilitating Palmer amaranth to survive tested glufosinate applications may not be an epigenetic effect, maternally heritable, or provide enough selection pressure to be heritable after one season (Ghanizadeh et al., 2019; Markus et al., 2017; Neve & Powles, 2005).Results from this research conclude that vegetative‐stage Palmer amaranth surviving glufosinate will remain fecund (2,400–22,000 seeds female−1) but differential fecundity can be observed if the plant survived one or two glufosinate applications; however, the offspring would not exhibit differential germination or susceptibility to glufosinate after one growing season. Furthermore, applications of glufosinate should be applied early postemergence (the appropriate weed size according to the label) to eliminate seed production of Palmer amaranth to further deplete the soil seedbank and mitigate recurrent Palmer amaranth population control. Although sequential applications of glufosinate ceased or decreased Palmer amaranth seed production, these applications should be recommended to farmers with caution to reduce the selection pressure on Palmer amaranth evolving resistance to glufosinate. Sequential applications of glufosinate are common in North Carolina row crops and many North Carolina farmers utilize additional tactics for herbicide resistance management (Jones et al., 2022).The collected fecundity data compared across glufosinate‐treated and nontreated Palmer amaranth plants is not a true representation of fecundity reduction as nontreated Palmer amaranth plants were subjected to extreme levels of inter‐ and intraspecific competition. Future research should determine the fecundity of Palmer amaranth plants grown with minimal interspecific competition (i.e., grown in a weed‐free crop) and no competition (i.e., fallow) to determine the reduction of fecundity when surviving glufosinate at a specific timing. In tandem, future research should include other glufosinate‐resistant crops to determine if the different crops influence glufosinate control and fecundity of Palmer amaranth surviving glufosinate applications. In addition, cross pollination was not controlled in the study; thus, fertilization of female Palmer amaranth surviving glufosinate treatment may be a product of pollen from nontreated male Palmer amaranth in proximity. Similar research conducted did not control for volunteer pollen and the results were similar across tested variables (de Sanctis et al., 2021; Jha & Norsworthy, 2012; Scruggs et al., 2020). However, a true representation of decreased glufosinate susceptibility would be to evaluate biparental crosses of glufosinate‐ and nontreated plants. Future research investigating concomitant effects on the susceptibility to glufosinate exhibited on the offspring should focus on controlled crosses between Palmer amaranth plants surviving glufosinate treatment.ACKNOWLEDGMENTSProject funding was provided by the North Carolina Soybean Producers Association. The authors thank John Sanders, Diego Contreras, and Marco Fajardo for technical support. The authors thank Dr. Micheal Owen for reviewing the manuscript. No conflicts of interest have been declared.AUTHOR CONTRIBUTIONSEric Jones: Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Writing‐original draft; Writing‐review & editing. Ramon G. Leon: Conceptualization; Funding acquisition; Methodology; Project administration; Supervision; Writing‐review & editing. 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"Agrosystems, Geosciences & Environment" – Wiley

Published: Jan 1, 2022

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