Response of a Multiple-Resistant Palmer Amaranth (Amaranthus palmeri) Population to Four HPPD-Inhibiting Herbicides Applied Alone and with Atrazine

Weed Science ◽  
2017 ◽  
Vol 65 (4) ◽  
pp. 534-545 ◽  
Author(s):  
Jonathon R. Kohrt ◽  
Christy L. Sprague
Keyword(s):  
Weed Management ◽  
Palmer Amaranth ◽  
Amaranthus Palmeri ◽  
Joint Activity ◽  
Application Timing ◽  
Greenhouse Experiments ◽  
Synergistic Response

Control of multiple-resistant Palmer amaranth populations in corn will rely heavily on the use of POST 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides. Therefore, field and greenhouse experiments were conducted to: (1) evaluate Palmer amaranth control with four HPPD inhibitors alone and in combination with atrazine at two application timings and (2) investigate the joint activity of HPPD-inhibiting herbicides and atrazine in atrazine-resistant (AR) and atrazine-susceptible (AS) Palmer amaranth populations. Control of the AR Palmer amaranth population varied among the HPPD-inhibiting herbicides with tolpyralate>tembotrione=topramezone>mesotrione based on GR50values in the greenhouse. In the field, Palmer amaranth control was lower when the HPPD-inhibiting herbicides, with the exception of tolpyralate, were applied to 15- vs. 8-cm-tall Palmer amaranth. Tolpyralate controlled Palmer amaranth ≥95% at both application timings. The addition of atrazine (560 g ai ha−1) improved Palmer amaranth control with mesotrione and topramezone at the 8-cm application timing and with mesotrione and tembotrione at the 15-cm application timing. In the greenhouse, joint activity of mesotrione and atrazine and tembotrione and atrazine was synergistic with both the AR and AS Palmer amaranth populations. In the AR population, an additional 980 g ai ha−1of atrazine (8X) was needed to cause a synergistic response compared with the AS population. Synergistic responses with mesotrione were detected with all atrazine rates for the AS population and for atrazine rates ranging from 280 to 2,240 g ai ha−1for the AR population. Only additive responses were observed when atrazine was applied with tolpyralate and topramezone, indicating that joint activity in the form of synergism occurs more readily with the triketones compared with the benzopyrazoles. When faced with an AR Palmer amaranth population, the addition of atrazine to HPPD inhibitors may increase the overall success of weed management due to joint activity.

scholarly journals Effects of Palmer Amaranth (Amaranthus palmeri) Establishment Time and Distance from the Crop Row on Biological and Phenological Characteristics of the Weed: Implications on Soybean Yield

Weed Science ◽  
2019 ◽  
Vol 67 (1) ◽  
pp. 126-135 ◽  
Author(s):  
Nicholas E. Korres ◽  
Jason K. Norsworthy ◽  
Andy Mauromoustakos
Keyword(s):  
Seed Production ◽  
Weed Management ◽  
Cropping Systems ◽  
Management Strategies ◽  
Palmer Amaranth ◽  
Yield Reduction ◽  
Amaranthus Palmeri ◽  
Soybean Yield ◽  
Management Measures ◽  
Weed Population Dynamics

AbstractInformation about weed biology and weed population dynamics is critical for the development of efficient weed management programs. A field experiment was conducted in Fayetteville, AR, during 2014 and 2015 to examine the effects of Palmer amaranth (Amaranthus palmeriS. Watson) establishment time in relation to soybean [Glycine max(L.) Merr.] emergence and the effects ofA. palmeridistance from the soybean row on the weed’s height, biomass, seed production, and flowering time and on soybean yield. The establishment time factor, in weeks after crop emergence (WAE), was composed of six treatment levels (0, 1, 2, 4, 6, and 8 WAE), whereas the distance from the crop consisted of three treatment levels (0, 24, and 48 cm). Differences inA. palmeribiomass and seed production averaged across distance from the crop were found at 0 and 1 WAE in both years. Establishment time had a significant effect onA. palmeriseed production through greater biomass production and height increases at earlier dates.Amaranthus palmerithat was established with the crop (0 WAE) overtopped soybean at about 7 and 10 WAE in 2014 and 2015, respectively. Distance from the crop affectedA. palmeriheight, biomass, and seed production. The greater the distance from the crop, the higherA. palmeriheight, biomass, and seed production at 0 and 1 WAE compared with other dates (i.e., 2, 4, 6, and 8 WAE).Amaranthus palmeriestablishment time had a significant impact on soybean yield, but distance from the crop did not. The earlierA. palmeriinterfered with soybean (0 and 1 WAE), the greater the crop yield reduction; after that period no significant yield reductions were recorded compared with the rest of the weed establishment times. Knowledge ofA. palmeriresponse, especially at early stages of its life cycle, is important for designing efficient weed management strategies and cropping systems that can enhance crop competitiveness. Control ofA. palmeriwithin the first week after crop emergence or reduced distance between crop and weed are important factors for an effective implementation of weed management measures againstA. palmeriand reduced soybean yield losses due to weed interference.

Susceptibility of Palmer amaranth (Amaranthus palmeri) to herbicides in accessions collected from the North Carolina Coastal Plain

Weed Science ◽  
2020 ◽  
Vol 68 (6) ◽  
pp. 582-593
Author(s):  
Denis J. Mahoney ◽  
David L. Jordan ◽  
Nilda Roma-Burgos ◽  
Katherine M. Jennings ◽  
Ramon G. Leon ◽  
...  
Keyword(s):  
North Carolina ◽  
Dose Response ◽  
Coastal Plain ◽  
Weed Management ◽  
Acetolactate Synthase ◽  
Fold Increase ◽  
Palmer Amaranth ◽  
Amaranthus Palmeri ◽  
Sites Of Action ◽  
Two Populations

AbstractPalmer amaranth (Amaranthus palmeri S. Watson) populations resistant to acetolactate synthase (ALS)-inhibiting herbicides and glyphosate are fairly common throughout the state of North Carolina (NC). This has led farm managers to rely more heavily on herbicides with other sites of action (SOA) for A. palmeri control, especially protoporphyrinogen oxidase and glutamine synthetase inhibitors. In the fall of 2016, seeds from A. palmeri populations were collected from the NC Coastal Plain, the state’s most prominent agricultural region. In separate experiments, plants with 2 to 4 leaves from the 110 populations were treated with field use rates of glyphosate, glufosinate-ammonium, fomesafen, mesotrione, or thifensulfuron-methyl. Percent visible control and survival were evaluated 3 wk after treatment. Survival frequencies were highest following glyphosate (99%) or thifensulfuron-methyl (96%) treatment. Known mutations conferring resistance to ALS inhibitors were found in populations surviving thifensulfuron-methyl application (Ala-122-Ser, Pro-197-Ser, Trp-574-Leu, and/or Ser-653-Asn), in addition to a new mutation (Ala-282-Asp) that requires further investigation. Forty-two populations had survivors after mesotrione application, with one population having 17% survival. Four populations survived fomesafen treatment, while none survived glufosinate. Dose–response studies showed an increase in fomesafen needed to kill 50% of two populations (LD50); however, these rates were far below the field use rate (less than 5 g ha−1). In two populations following mesotrione dose–response studies, a 2.4- to 3.3-fold increase was noted, with LD90 values approaching the field use rate (72.8 and 89.8 g ha−1). Screening of the progeny of individuals surviving mesotrione confirmed the presence of resistance alleles, as there were a higher number of survivors at the 1X rate compared with the parent population, confirming resistance to mesotrione. These data suggest A. palmeri resistant to chemistries other than glyphosate and thifensulfuron-methyl are present in NC, which highlights the need for weed management approaches to mitigate the evolution and spread of herbicide-resistant populations.

scholarly journals Evaluation of POST-Harvest Herbicide Applications for Seed Prevention of Glyphosate-Resistant Palmer amaranth (Amaranthus palmeri)

2015 ◽  
Vol 29 (3) ◽  
pp. 405-411 ◽  
Author(s):  
Whitney D. Crow ◽  
Lawrence E. Steckel ◽  
Robert M. Hayes ◽  
Thomas C. Mueller
Keyword(s):  
Weed Management ◽  
Control Strategies ◽  
Palmer Amaranth ◽  
Amaranthus Palmeri ◽  
Wheat Grain ◽  
Herbicide Application ◽  
Post Harvest ◽  
Herbicide Treatments ◽  
Soil Seedbank ◽  
Residual Herbicide

Recent increases in the prevalence of glyphosate-resistant (GR) Palmer amaranth mandate that new control strategies be developed to optimize weed control and crop performance. A field study was conducted in 2012 and 2013 in Jackson, TN, and in 2013 in Knoxville, TN, to evaluate POST weed management programs applied after harvest (POST-harvest) for prevention of seed production from GR Palmer amaranth and to evaluate herbicide carryover to winter wheat. Treatments were applied POST-harvest to corn stubble, with three applications followed by a PRE herbicide applied at wheat planting. Paraquat alone or mixed withS-metolachlor controlled 91% of existing Palmer amaranth 14 d after treatment but did not control regrowth. Paraquat tank-mixed with a residual herbicide of metribuzin, pyroxasulfone, saflufenacil, flumioxazin, pyroxasulfone plus flumioxazin, or pyroxasulfone plus fluthiacet improved control of regrowth or new emergence compared with paraquat alone. All residual herbicide treatments provided similar GR Palmer amaranth control. Through implementation of POST-harvest herbicide applications, the addition of 1,200 seed m−2or approximately 12 million seed ha−1to the soil seedbank was prevented. Overall, the addition of a residual herbicide provided only 4 to 7% more GR Palmer amaranth control than paraquat alone. Wheat injury was evident (< 10%) in 2012 from the PRE applications, but not in 2013. Wheat grain yield was not adversely affected by any herbicide application.

Evaluation of Herbicide Programs for Use in a 2,4-D–Resistant Soybean Technology for Control of Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri)

2016 ◽  
Vol 30 (2) ◽  
pp. 366-376 ◽  
Author(s):  
M. Ryan Miller ◽  
Jason K. Norsworthy
Keyword(s):  
Weed Management ◽  
Field Experiments ◽  
Palmer Amaranth ◽  
Integrated Weed Management ◽  
Amaranthus Palmeri ◽  
Early Season ◽  
Fully Integrated ◽  
Soil Seedbank ◽  
High Level ◽  
Herbicide Programs

Two separate field experiments were conducted over a 2-yr period in Fayetteville, AR, during 2012 and 2013 to (1) evaluate POST herbicide programs utilizing a premixture of dimethylamine (DMA) salt of glyphosate + choline salt of 2,4-D in a soybean line resistant to 2,4-D, glyphosate, and glufosinate and (2) determine efficacy of herbicide programs that begin with PRE residual herbicides followed by POST applications of 2,4-D choline + glyphosate DMA on glyphosate-resistant Palmer amaranth. In the first experiment, POST applications alone that incorporated the use of residual herbicides with the glyphosate + 2,4-D premixture provided 93 to 99% control of Palmer amaranth at the end of the season. In the second experiment, the use of flumioxazin, flumioxazin + chlorimuron methyl, S-metolachlor + fomesafen, or sulfentrazone + chloransulam applied PRE provided 94 to 98% early-season Palmer amaranth control. Early-season control helped maintain a high level of Palmer amaranth control throughout the growing season, in turn resulting in fewer reproductive Palmer amaranth plants present at soybean harvest compared to most other treatments. Although no differences in soybean yield were observed among treated plots, it was evident that herbicide programs should begin with PRE residual herbicides followed by POST applications of glyphosate + 2,4-D mixed with residual herbicides to minimize late-season escapes and reduce the likelihood of contributions to the soil seedbank. Dependent upon management decisions, the best stewardship of this technology will likely rely on the use multiple effective mechanisms of action incorporated into a fully integrated weed management system.

Increased Absorption and Translocation Contribute to Improved Efficacy of Dicamba to Control Early Growth Stage Palmer amaranth (Amaranthus palmeri)

Weed Science ◽  
2019 ◽  
pp. 1-25 ◽  
Author(s):  
Ivan Cuvaca ◽  
Randall Currie ◽  
Kraig Roozeboom ◽  
Jack Fry ◽  
Mithila Jugulam
Keyword(s):  
Effective Dose ◽  
Logistic Model ◽  
Growth Stage ◽  
Fold Increase ◽  
Early Growth ◽  
Palmer Amaranth ◽  
Shoot Biomass ◽  
Amaranthus Palmeri ◽  
Application Timing ◽  
Early Growth Stage

Abstract Rapid growth of Palmer amaranth (Amaranthus palmeri S. Watson) poses a challenge for timely management of this weed. Dose response studies were conducted in 2017 and 2018 under field and greenhouse conditions near Garden City and Manhattan, KS, respectively, to evaluate the efficacy of dicamba to control ≤10 cm, 15 cm, and 30 cm tall-Palmer amaranth that mimics three herbicide application timing: on time application (Day 0), and 1 (Day 1) and 4 days (Day 4) delay. Visual injury rating and reduction in shoot biomass (% of non-treated), and mortality were assessed at four weeks after treatment using a three- and four-parameter log-logistic model, in R software program. Increasing dicamba doses increased A. palmeri control regardless of plant height both in the field and greenhouse studies. The results suggest that delaying application one (15 cm) and four days (30 cm), resulted in a two- and 27-fold increase in the effective dose of dicamba on A. palmeri, respectively, under field conditions. However, in the greenhouse, for the same level of A. palmeri control, more than one- and two-fold increase in dicamba dose, respectively was required. Similarly, the effective dose of dicamba required for 50% reduction in A. palmeri shoot biomass (GR50) increased more than four- and eight-fold or more than one- and two-fold when dicamba application was delayed by one (15 cm) and four days (30 cm), in the field or in the greenhouse, respectively. To understand the basis of increased efficacy of dicamba in controlling early growth stage of A. palmeri, dicamba absorption and translocation studies were conducted. Results indicate a significant reduction in dicamba absorption (7%) and translocation (15%) with increase in A. palmeri height. Therefore, increased absorption and translocation of dicamba results in increased efficacy in improving A. palmeri control at early growth stage.

Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri) Increases Herbicide Use, Tillage, and Hand-Weeding in Georgia Cotton

Weed Science ◽  
2014 ◽  
Vol 62 (2) ◽  
pp. 393-402 ◽  
Author(s):  
Lynn M. Sosnoskie ◽  
A. Stanley Culpepper
Keyword(s):  
Weed Management ◽  
Glyphosate Resistance ◽  
Palmer Amaranth ◽  
Extension Agents ◽  
Amaranthus Palmeri ◽  
Field Site ◽  
Timely Manner ◽  
Biological Attributes ◽  
Hand Weeding ◽  
Herbicide Use

In 2005, the existence of glyphosate-resistance in Palmer amaranth was confirmed at a single 250 ha field site in Macon County, Georgia. Currently, all cotton producing counties in Georgia are infested, to some degree, with glyphosate-resistant Palmer amaranth. In 2010 and 2011, surveys were administered to Georgia growers and extension agents to determine how the development of glyphosate-resistance has affected weed management in cotton. According to respondents, the numbers of cotton acres that were treated with paraquat, glufosinate and residual herbicides effective against Palmer amaranth more than doubled between 2000 to 2005 and 2006 to 2010. Glyphosate use declined between 2000 to 2005 and 2006 to 2010 although, on average, the active ingredient was still applied to a majority of cotton acres. Although grower herbicide input costs have more than doubled following the evolution and spread of glyphosate resistance, chemically-based control of Palmer amaranth is still not adequate. As a consequence, Georgia cotton growers hand weeded 52% of the crop at an average cost of $57 per hand-weeded ha; this represents a cost increase of at least 475% as compared to the years prior to resistance. In addition to increased herbicide use and hand weeding, growers in Georgia are also using mechanical, in-crop cultivation (44% of acres), tillage for the incorporation of preplant herbicides (20% of the acres), and post-harvest deep-turning (19% of the acres every three years) for weed control. Current weed management systems are more diverse, complex and expensive than those employed only a decade ago, but are effective at controlling glyphosate-resistant Palmer amaranth in glyphosate-resistant cotton. The success of these programs may be related to producers improved knowledge about herbicide resistance, and the biological attributes that make Palmer amaranth so challenging, as well as their ability to implement their management programs in a timely manner.

Seed Retention of Palmer amaranth (Amaranthus palmeri) and Barnyardgrass (Echinochloa crus-galli) in Soybean

2017 ◽  
Vol 31 (4) ◽  
pp. 617-622 ◽  
Author(s):  
Lauren M. Schwartz-Lazaro ◽  
Jeremy K. Green ◽  
Jason K. Norsworthy
Keyword(s):  
Weed Management ◽  
Palmer Amaranth ◽  
Amaranthus Palmeri ◽  
Weed Seed ◽  
Weed Species ◽  
Seed Retention ◽  
The Us ◽  
Amaranth Seed ◽  
Soil Seedbank ◽  
Crop Harvest

Harvest weed seed control is an alternative non-chemical approach to weed management that targets escaped weed seeds at the time of crop harvest. Relatively little is known on how these methods will work on species in the US. Two of the most prominent weeds in soybean production in the midsouthern US are Palmer amaranth and barnyardgrass. Typically, when crop harvesting occurs the weed seed has already either shattered or is taken into the combine and may be redistributed in the soil seedbank. This causes further weed seed spread and may contribute to the addition of resistant seeds in the seedbank. There is little research on how much seed is retained on different weed species at or beyond harvest time. Thus, the objective of this study was to determine the percentage of total Palmer amaranth and barnyardgrass seed production that was retained on the plant during delayed soybean harvest. Retained seed over time was similar between 2015 and 2016, but was significantly different between years for only Palmer amaranth. Seed retention did not differ between years for either weed species. Palmer amaranth and barnyardgrass retained 98 and 41% of their seed at soybean maturity and 95 and 32% of their seed one month after soybean maturity, respectively. Thus, this research indicates that if there are escaped Palmer amaranth plants and soybean is harvested in a timely manner, most seed will enter the combine and offer potential for capture or destruction of these seeds using harvest weed seed control tactics. While there would be some benefit to using HWSC for barnyardgrass, the utility of this practice on mitigating herbicide resistance would be less pronounced than that of Palmer amaranth because of the reduced seed retention or early seed shatter.

Soybean Canopy and Tillage Effects on Emergence of Palmer Amaranth (Amaranthus palmeri) from a Natural Seed Bank

Weed Science ◽  
2009 ◽  
Vol 57 (6) ◽  
pp. 644-651 ◽  
Author(s):  
Prashant Jha ◽  
Jason K. Norsworthy
Keyword(s):  
Weed Management ◽  
Field Experiments ◽  
Seedling Recruitment ◽  
Palmer Amaranth ◽  
Amaranthus Palmeri ◽  
No Tillage ◽  
Recruitment Pattern ◽  
Emergence Period ◽  
Soybean Canopy ◽  
Uniform Population

Field experiments were conducted in 2004, 2005, and 2006, at Pendleton, SC, to determine the effects of soybean canopy and tillage on Palmer amaranth emergence from sites with a uniform population of Palmer amaranth. In 2006, the effect of soybean canopy was evaluated only in no-tillage plots. Palmer amaranth emerged from May 10 through October 23, May 13 through September 2, and April 28 through August 25 in 2004, 2005, and 2006, respectively. Two to three consistent emergence periods occurred from early May through mid-July. Shallow (10-cm depth) spring tillage had minimal influence on the cumulative emergence of Palmer amaranth. Increase in light interception following soybean canopy formation was evident by early July, resulting in reduced Palmer amaranth emergence, especially in no-tillage conditions. In no-tillage plots, from 32 or 33 d after soybean emergence through senescence, Palmer amaranth emergence was reduced by 73 to 76% in plots with soybean compared with plots without soybean. Emergence of Palmer amaranth was favored by high-thermal soil amplitudes (10 to 16 C) in the absence of soybean. Of the total emergence during a season, > 90% occurred before soybean canopy closure. The seedling recruitment pattern of Palmer amaranth from this research suggests that, although Palmer amaranth exhibits an extended emergence period, cohorts during the peak emergence periods from early May to mid-July need greater attention in weed management.

Palmer Amaranth (Amaranthus palmeri) Management in GlyTol®LibertyLink®Cotton

2014 ◽  
Vol 28 (4) ◽  
pp. 592-600 ◽  
Author(s):  
Jacob D. Reed ◽  
J. Wayne Keeling ◽  
Peter A. Dotray
Keyword(s):  
Weed Management ◽  
Mixed Model ◽  
Synergistic Effects ◽  
Field Studies ◽  
Field Trials ◽  
Palmer Amaranth ◽  
Amaranthus Palmeri ◽  
Glufosinate Ammonium ◽  
Model Method ◽  
Expected Values

Field trials were conducted in Lubbock, TX in 2010 and 2011 to evaluate tank-mix combinations of glyphosate and glufosinate in GlyTol®LibertyLink®cotton for control of Palmer amaranth. Herbicide treatments included glyphosate and glufosinate applied at various tank-mix rate combinations (1X:1X, 1X:0.75X, 1X:0.5X, 1X:0.25X and 1X:0X of glyphosate plus glufosinate), proportional tank-mix rate combinations (1X:0X, 0.75X:0.25X, 0.5X:0.5X, 0.25X:0.75X, and 0X:1X of glyphosate plus glufosinate, where X is 0.84 kg ae ha−1of glyphosate or 0.58 kg ai ha−1of glufosinate ammonium), and in sequential (1X followed by 1X) applications of both herbicides in an overall weed management system. Greenhouse studies were conducted to quantify antagonistic or synergistic effects. Treatments included a nontreated control; glyphosate at 0.84, 0.63, 0.42, and 0.21 kg ha−1; glufosinate at 0.58, 0.44, 0.29, and 0.15 kg ha−1; and all tank-mix combinations of each herbicide rate. Dry weights were converted to percent growth values for each rate of the two herbicides alone, and these values were used to calculate expected responses of tank-mix combinations with the use of Colby's method. Expected values were compared to observed percent growth values using an augmented mixed-model method. Results of field studies indicated that tank mixes of glyphosate and glufosinate were less effective at controlling Palmer amaranth than glyphosate applied alone. The addition of any rate of glufosinate to a 1X rate of glyphosate reduced Palmer amaranth control compared to glyphosate alone. Greenhouse studies confirmed antagonism seen in the field. These results indicate that sequential applications of these two herbicides are a better option for Palmer amaranth weed management.

Influence of Glyphosate Timing and Row Width on Palmer Amaranth (Amaranthus palmeri) and Pusley (Richardiaspp.) Demographics in Glyphosate-Resistant Soybean

Weed Science ◽  
2008 ◽  
Vol 56 (3) ◽  
pp. 408-415 ◽  
Author(s):  
Prashant Jha ◽  
Jason K. Norsworthy ◽  
William Bridges ◽  
Melissa B. Riley
Keyword(s):  
Seed Production ◽  
Soybean Seed ◽  
Mixed Population ◽  
Palmer Amaranth ◽  
Amaranthus Palmeri ◽  
Single Application ◽  
Application Timing ◽  
The Third ◽  
Row Width ◽  
Vegetative Biomass

The influence of soybean row width and glyphosate application timing was determined on survival, biomass, and seed production of cohorts from a mixed population of Palmer amaranth and pusley species (Florida and Brazil pusley) along with soybean seed yield. The first Palmer amaranth and pusley cohort comprised plants that emerged from soybean planting through the V3 (3 wk after soybean emergence [WAE]) soybean stage (cohort 1). The second cohort comprised plants that emerged between the V3 to V6 (5 WAE) soybean stages (cohort 2), and the third cohort emerged after the V6 through the R2 soybean stage (cohort 3). Glyphosate at 840 g ae ha−1was applied at V3; V6; V3 and V6; and V3, V6, and R2 in rows either 19 or 97 cm wide. A nontreated control was included for comparison in each row width. Sequential glyphosate applications at V3 and V6 or at V3, V6, and R2 soybean stages resulted in 1 to 3% survival of cohort 1 compared with 23 to 28% survival after a single glyphosate application. Vegetative biomass production by cohort 1 accounted for 71% of the total pusley biomass produced in the nontreated plots. Cohort 1, 2, and 3 contributed 68, 31, and 1%, respectively, of the total 37,900 seeds m−2produced by pusley plants in nontreated plots. Delaying a glyphosate application to the V6 stage resulted in higher biomass and more than twice the seed produced from cohort 1 when compared with cohort 2. Glyphosate applied at V3 and V6 stages prevented pusley seed production from cohort 1, and an additional glyphosate application at the R2 stage prevented seed production from cohorts 2 and 3. No Palmer amaranth emergence occurred after the V6 soybean stage in either row width. A single glyphosate application at the V3 or V6 stage eliminated cohort 1 of Palmer amaranth in narrow rows. Palmer amaranth plants from cohort 1 in wide rows that survived the V3 glyphosate application produced 3.3 g m−2biomass and 600 seeds m−2. Averaged over years and row widths, soybean yields after sequential glyphosate applications were 2,490 to 2,640 kg ha−1compared with 1,850 to 2,020 kg ha−1after a single glyphosate application at the V3 or V6 stage. This research confirms that sequential glyphosate applications are superior to a single application for minimizing pusley and Palmer amaranth survival, biomass, and seed production along with an improvement in soybean yields.

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