Integrated Palmer Amaranth Management in Glufosinate-resistant Cotton: I. Soil-inversion, High-residue Cover Crops and Herbicide Regimes

A three year field experiment was conducted to evaluate the role of soil-inversion, cover crops and herbicide regimes for Palmer amaranth between-row (BR) and within-row (WR) management in glufosinate-resistant cotton. The main plots were two soil-inversion treatments: fall inversion tillage (IT) and non-inversion tillage (NIT). The subplots were three cover crop treatments: crimson clover, cereal rye and winter fallow; and sub subplots were four herbicide regimes: preemergence (PRE) alone, postemergence (POST) alone, PRE + POST and a no herbicide check (None). The PRE herbicide regime consisted of a single application of pendimethalin at 0.84 kg ae ha −1 plus fomesafen at 0.28 kg ai ha −1. The POST herbicide regime consisted of a single application of glufosinate at 0.60 kg ai ha −1 plus S-metolachlor at 0.54 kg ai ha −1 and the PRE + POST regime combined the prior two components. At 2 weeks after planting (WAP) cotton, Palmer amaranth densities, both BR and WR, were reduced ≥90% following all cover crop treatments in the IT. In the NIT, crimson clover reduced Palmer amaranth densities >65% and 50% compared to winter fallow and cereal rye covers, respectively. At 6 WAP, the PRE and PRE + POST herbicide regimes in both IT and NIT reduced BR and WR Palmer amaranth densities >96% over the three years. Additionally, the BR density was reduced 296 ≥59% in no-herbicide (None) following either cereal rye or crimson clover when compared to no-herbicide in the winter fallow. In IT, PRE, POST and PRE + POST herbicide regimes controlled Palmer amaranth >95% 6 WAP. In NIT, Palmer amaranth was controlled ≥79% in PRE and ≥95% in PRE + POST herbicide regimes over three years. POST herbicide regime following NIT was not very consistent. Averaged across three years, Palmer amaranth controlled ≥94% in PRE and PRE + POST herbicide regimes regardless of cover crop. Herbicide regime effect on cotton yield was highly significant; the maximum cotton yield was produced by the PRE + POST herbicide regime. Averaged over three years, the PRE, POST and PRE + POST cotton yields were about three times higher than no herbicide regime. In a conservation tillage production system, a PRE + glufosinate POST herbicide based regime coupled with a cereal rye cover crop may effectively control Palmer amaranth and maximize cotton yields.


Introduction
Palmer amaranth (Amaranthus palmeri S.Wats) is a highly aggressive dioecious row crop weed in the Southeastern US [1].It has several unique weedy characteristics including rapid growth of >5 inches a day during peak growth and can attain a final height of ≥2 m [2].It is a prolific seed producer; a single female plant can produce more than 600,000 seeds, which have an average diameter of 1.0 mm [3].In addition, Palmer amaranth has exceptional drought-tolerance and can endure moisture stress very well [4][5][6][7].
The occurrence of glyphosate resistance in Palmer amaranth has challenged cotton-weed managers in the Southeaster US.Until 2005, glyphosate had been very efficacious on Palmer amaranth [22][23][24].However, the extensive adoption of glyphosate-resistant technology virtually replaced the conventional weed control technology consisting of preplant incorporated (PPI), preemergence (PRE), postemergence (POST) and post-directed (PDS) applied herbicides [25].The sole reliance on glyphosate has resulted in selection for resistant biotypes of Palmer amaranth.Glyphosate resistant in Palmer amaranth was first documented in Macon County Georgia, in 2006 [26,27].As of 2010, glyphosate-resistant Palmer amaranth populations have been confirmed in 10 states [28].Additionally, the Palmer amaranth populations resistant to dinitroaniline herbicides have been also reported [28,29].
Historically, cotton was grown in conventional tillage utilizing primary and secondary tillage including moldboard plowing, disking and cultivation.However, increasing input costs, low commodity prices, and concerns for declining soil organic quality, and in some regions subsoil compaction, necessitated the adoption of alternative tillage options such as strip-tillage production systems that utilize a within row subsoiler to disrupt soil compaction under the crop row and minimizes surface residue disturbance [30,31].Consequently, inadequate weed control has been reported in some conservation-tillage cotton production [32].The infestation of small-seeded annual weeds such as Palmer amaranth has often been attributed to conservation tillage systems that preclude burial of weed seed.Moldboard plowing with soil inversion to the depth of 30 cm (12 in) has been shown to reduce glyphosate-resistant Palmer amaranth emergence 46% to 60% because many of the weed seeds are placed at depths which prevent emergence [33,34].However, the return to conventional tillage to control glyphosate-resistant Palmer amaranth threatens to reduce conservation tillage practices.Some researchers [35,36] advocate the integration of traditional and alternative weed control strategies, such as the utilization of crop and herbicide rotation and integration of high residue cereal cover crops in order to sustain conservation tillage practices.
Over the last decade, the inclusion of cover crops in conservation-tillage systems has been researched due to the ability of some cover crops to suppress early-season weed density and growth either through direct competition from cover crop biomass [37][38][39][40][41] or through allelopathy [42][43][44][45][46][47].Cereal rye (Secale cereale L.) has been well documented for both high biomass potential and allelopathic properties by several researchers [42,43,[48][49][50][51][52].Several studies on cover crops have reported excellent early-season weed control that can preclude the use of preemergence herbicides in crops [39,41,[52][53][54][55][56][57][58][59].However, the success of a cover crop in early-season weed suppression is determined by the biomass production potential which varies with year, location and management practices [37,48,53,56,60].It has been observed that cereal rye residue alone was effective in reducing the glyphosate-resistant Palmer amaranth emergence by 94% in the row middle and 50% within the row [34].The use of high residue cover crops in conjunction with chemical and cultural weed control tactics could provide effective Palmer amaranth control in established glyphosate-resistant populations as well as help prevent the development of resistance in the remaining glyphosate-susceptible populations [61].Thus, the inclusion of cover crops in conservation tillage system may provide weed control benefits similar to those realized from inversion tillage in conventional tillage system.
With the widespread appearance of glyphosate-resistant Palmer amaranth, cotton producers are considering other weed management options such as inversion tillage, surface tillage, and increased integration of soil active herbicides, cover crops and adoption of alternative GMO herbicide-crop systems such as glufosinate resistant cotton technology.Although inversion tillage can improve control of glyphosate-resistant Palmer amaranth, increased input costs and potential soil erosion are significant challenges.However the integration of cover crops and glufosinate-resistant cotton technology are possible viable alternatives.Therefore, a field study was conducted to evaluate the role of soil inversion, cover crops and herbicide regimes for Palmer amaranth management in glufosinate-resistant cotton.

Materials and Methods
A three year field experiment was conducted from fall 2008 through 2011 at the E.V. Smith Research Center, Field Crops Unit near Shorter, AL on a Compass sandy loam soil (coarse-loamy, siliceous, subactive, thermic Plinthic Paleudults) with 1.9% to 2.1% organic matter and pH 6.2 to 6.4.The experiment occupied a site that had been in continuous strip-tillage for the previous six years prior to experiment establishment, infested with a mixed population of resistant and susceptible Palmer amaranth, and subsequent treatments remained in the same location for three years without re-randomization of treatments.The experimental design consisted of a split-split plot treatment restriction in a randomized complete block design with three replicates.The main plots (43.9 by 9.1 m) were two soil-inversion treatments: fall inversion tillage (IT) and non-inversion tillage (NIT).After establishment, all IT plots reverted to NIT in future years.The subplots (14.6 by 9.1 m) were three different cover crops: crimson clover, cereal rye and winter fallow.The sub subplots (3.6 by 9.1 m) were four different herbicide regimes: preemergence (PRE) alone, postemergence (POST) alone, PRE + POST, and a no-herbicide check (None).A schedule of operations performed each year is given in Table 1.

Soil-Inversion, Cover Crops, and Cover Crop Management
In the fall 2008, approximately 28 million native glyphosate susceptible Palmer amaranth seeds were broadcast per hectare to ensure a sizeable seedbank.Half of each replicate was subjected to fall inversion tillage (IT) by moldboard plowing (30 cm) immediately fb one pass each of a disk and field cultivator, and half was under non-inversion tillage (NIT) using a within-row subsoiler equipped with pneumatic tires only to close the subsoiling slot.Subsequently each year in the fall, cereal rye (var."Elbon" in 2009 and 2010 and "Wrens Abruzzi" in 2011) and crimson clover (Trifolium incarnatum L.) var.'Dixie' cover crops were planted using 101 and 28 kg ha −1 seed, respectively in both the IT and NIT.Different cereal rye varieties were planted due to seed availability; Wrens Abruzzi has been shown to be more allelopathic [62].In 2009 and 2010, frequent rain delayed both the harvesting of cotton and subsequent planting of cover crops [36].Cereal rye cover was fertilized using 34 kg•ha −1 of a 33-0-0 fertilizer.A winter fallow control was also included as check.
Cover crops were rolled with a three section straight bar roller/crimper (Bigham Brothers, Inc., Lubbock, TX, USA) in late April or early May using a JD 7730 equipped with an AutoSteer GPS.Cover crop rolling was immediately followed by an application of glyphosate (Roundup Weathermax ® , Monsanto Company, St. Louis, MO, USA) at 0.84 kg ae ha −1 plus glufosinate (Ignite ® , Bayer Crop Science, Research Triangle Park, NC, USA) at 0.49 kg ae ha −1 .The mixture was needed to enhance crimson clover termination.Cover crop biomass samples were taken prior to desiccation and oven dry biomass was recorded.The entire experimental area was sub-soiled in May using the previously described equipment to remove hardpan induced interactions; thus, no hardpans existed throughout the experimental area which could likely bias the yield results.Subsoiling was followed by planting of glufosinate-resistant cotton (FM 1845 LLB2 in 2009, and FM 1735 LL, in 2010 and 2011, Bayer Crops Science, Research Triangle Park, NC).Each year, cotton was fertilized using 211 kg•ha −1 of 16-16-16 fertilizer at the time of planting.

Herbicide Regimes
Four herbicide regimes constituted the sub-sub plot treatments.The PRE herbicide regime consisted of a single application of pendimethalin (Prowl ® , BASF Ag.Products, Research Triangle Park, NC, USA) at 0.84 kg ae ha −1 plus fomesafen (Reflex ® , Syngenta Crop Protection, Inc., Greensboro, NC, USA) at 0.28 kg ai ha −1 .The POST herbicide regime consisted of a single application of glufosinate at 0.60 kg ai ha −1 plus S-metolachlor (Dual II Magnum ® , Syngenta Crop Protection, Inc., Greensboro, NC, USA) at 0.54 kg ai ha −1 and the PRE + POST regime consisted of both the aforementioned PRE and POST regimes.PRE herbicides were applied with a CO 2 -pressurized backpack sprayer calibrated to deliver 145 L ha −1 with 8002 flat-fan nozzles.POST herbicides were applied to 3 to 4 lf Palmer amaranth between 15 and 20 days after planting cotton with an ATV-mounted sprayer delivering 145 L ha −1 with 8002 flat-fan spray nozzles.A last application (LAYBY) directed spray consisting of a prometryn (Caporal ® , Syngenta Crop Protection, Inc., Greensboro, NC, USA) at 0.84 kg ai ha −1 + MSMA (Drexel Chemical Company, Memphis, TN) at 1.4 kg ai ha −1 was applied.Sethoxydim (Poast Plus ® , Bayer AG.Products, Research Triangle Park, NC, USA) was applied at 0.28 kg ai ha −1 as needed to maintain grass control.

Palmer Amaranth Sampling and Control Ratings
Palmer amaranth density was recorded once before the application of POST and again before the LAYBY application.Between-row (BR) Palmer amaranth densities were recorded as number of plants in a quadrat (0.25 m −2 ) randomly placed at 4 different positions between the 2nd and 3rd row of a four-row cotton plot.Similarly, the within-row (WR) Palmer densities were recorded from a quadrant (0.25 m −2 ) randomly placed at 4 different positions within the 2nd and 3rd rows.Palmer amaranth control was assessed visually at weekly intervals, starting a week after application of PRE until LAYBY application.A 0-100 scale was used where 0 and 100 indicate no control and complete control, respectively.Each year, the Palmer amaranth was hand removed from all the plots before application of LAYBY to facilitate harvest.Therefore, Palmer amaranth was 100% controlled in each plot after LAYBY until cotton harvest.Cotton yields were recorded by mechanically harvesting two center 9 m rows within each four-row plot with a spindle picker.

Statistical Analysis
Three years data were subjected to combined ANOVA using Proc GLIMMIX in SAS (version 9.2, SAS Institute, Inc., Cary, NC, USA).Year, soil-inversion, cover crop, herbicide regime and their interactions were treated as fixed effects, while replications, replication × soil-inversion, replication × soil-inversion × cover crop were treated as random effects.When year and its interaction with other factors were significant, data were analyzed and presented by year.Palmer amaranth visual control data were arcsine-transformed and Palmer amaranth density data were square root transformed.However, the original and transformed data analyses gave similar results, thus non-transformed data are presented.Multiple mean comparisons were made using the "adj = simulate" option in the statistical analysis system at the 5% significance level.

Cover Crop Biomass
Analysis of the three year data revealed significant effect of type of cover crop only.Averaged over three years, the maximum cover crop biomass was produced by cereal rye (4047 kg•ha −1 ) fb crimson clover (3570 kg•ha −1 ) that was significantly more than and winter fallow (1253 kg•ha −1 ).

Palmer Amaranth Density
Palmer amaranth densities at 2 WAP revealed significant year by treatment interactions.Therefore data are presented by year.A soil-inversion by cover crop interaction was observed for both BR and WR densities at 2 WAP in 2008-2009 and 2010-2011 while only the IT main effect was significant in 2009-2010.The highest BR and WR densities of 49 and 35 plants m −2 , respectively, occurred in winter fallow following NIT in 2009-2010 (Table 2).In 2008-2009 and 2010-2011, crimson clover reduced Palmer amaranth density by as much as 96% BR and 82% WR in NIT.Similar reductions in Palmer amaranth and other weed densities by cover crop residues have been reported [63,[64][65][66].Both BR and WR densities were reduced >90% following all cover crops in the IT.Each year with IT both BR and WR densities of Palmer amaranth, were ≥90% lower than with NIT (Table 2).Furthermore each year at 2 WAP, Palmer amaranth was 100% controlled by the PRE and PRE + POST herbicide regimes that received a PRE application within two days of planting (data not shown).
At 6 WAP, the effect of year and its interactions with other factors were not significant.However, both the BR and WR densities demonstrated strong interaction of soil-inversion by herbicide regime.Additionally, a cover crop by herbicide regime interaction (P < 0.0001) was detected, for BR densities only.The BR and WR densities were markedly reduced (≤1 plant m −2 ) under PRE, POST and PRE + POST herbicide regimes following IT and PRE and PRE + POST herbicide regimes following NIT (Table 3).The Palmer amaranth density, both BR and WR, was reduced ≥77% in no-herbicide regime (None) following IT when compared to no-herbicide regime following NIT.The PRE and PRE + POST herbicide regimes in both IT and NIT reduced BR and WR densities ≥96%.With the cover crop by herbicide regime interaction, the BR density was reduced ≥55% in no-herbicide (None) following either cereal rye or crimson clover when compared to no-herbicide in the winter fallow (Table 4).PRE, POST and PRE + POST herbicide regimes' Palmer amaranth densities were similar but lower than the no herbicide (None) regime following any cover crop.However, the PRE and PRE + POST herbicide regimes were very consistent in reducing Palmer amaranth density (>95%) following all the cover crops.Earlier research also indicated the need of either a PRE or PRE + POST herbicide regime to supplement partial weed control obtained following different cover crops in a conservation tillage system [63].Previous researchers also reported similar cover crop by herbicide interaction effect [67].Excellent control of Palmer amaranth with a combination of pendimethalin and fomesafen has been reported [68].

Palmer Amaranth Visual Control
Palmer amaranth visual percent control at 6 WAP reflected significant year by treatment interactions.Analysis by year also indicated significant two way interactions between soil-inversion and herbicide regime and cover crop by herbicide regime each year.Additionally, a soil-inversion by cover crop interaction was highly significant in 2010-2011 (P = 0.0007).All main effects were also highly significant each year (P < 0.0001).In 2008-2009, Palmer amaranth was controlled 38%, 79% and 95% in POST, PRE and PRE + POST herbicide regimes, respectively, following NIT while all herbicide regimes following IT provided ≥91% control (Table 5).Cover crop by herbicide regime interaction over years revealed ≥94% control of Palmer amaranth in PRE and PRE + POST herbicide regimes regardless of type of cover crop (Table 6).
The POST herbicide regime following both winter fallow and cereal rye provided 83% control of Palmer amaranth and was similar to the POST following crimson clover.However, Palmer amaranth control varied from 36% to 63% in no-herbicide (None) regime following different cover crops.Analysis revealed a soil-inversion by cover crop interaction in 2010-2011; Palmer amaranth control following different cover crops varied from 75% to 82% in NIT and 89% to 100% in IT, respectively (Figure 1).
Both cereal rye and crimson clover in IT gave significantly higher Palmer amaranth control (≥97%) than winter fallow in both IT and NIT and crimson clover and cereal rye in NIT.Previous research indicates the need to utilize residual herbicides throughout the season to aid in management of glyphosate resistant Palmer amaranth [69][70][71][72].

Cotton Yield
Analysis of the yield data revealed significant effect of herbicide regimes only.Although the cotton yield differences were not significant between PRE, POST and PRE + POST herbicide regimes, the maximum cotton yield was produced by the PRE + POST herbicide regime (1878 kg•ha −1 ) followed by POST (1658 kg•ha −1 ) and PRE (1532 kg•ha −1 ) alone regimes.PRE, POST and PRE + POST herbicide regimes' cotton yields were ≥2.7 times higher than no-herbicide (None) regime (Table 7).
Table 7. Influence of herbicide regimes on cotton yield with cover crop, soil-inversion, and three production years' data combined.

Discussion and Conclusions
Recent evolution of herbicide resistance in Palmer amaranth has revealed that an urgent restructuring of weed management tactics is needed.Consequently, integration of various weed management approaches such as IT, cover crops, crop rotations, competitive cultivars, herbicide rotation, soil residual chemistries and tank mixture of herbicides with different modes of action could diversify the weed control practices and thereby preclude the selection pressure for herbicide resistance.Furthermore, the longevity of herbicide resistant technology itself necessitates the inclusion of multiple tactics in weed management systems.
Fall-inversion tillage offers improved Palmer amaranth control by allowing the deep burial of seed.Considering the rapid loss of Palmer amaranth seed viability with time, IT would help reduce the amount of viable seed near the surface [73].However, IT is well known to deplete soil quality parameters such as soil organic matter while simultaneously increasing soil erosion.An occasional rotation with IT immediately followed by a cover crop conservation-tillage system could diversify weed management systems and prevent soil erosion.There is a great need of practical weed management solutions on farms severely impacted by glyphosate resistant Palmer amaranth [74].
Our research evaluated soil-inversion, cover crops and alternative herbicide regimes as an integrated approach to managing Palmer amaranth.Results indicate that IT alone resulted in ≥77% control of Palmer amaranth 6 WAP due to Palmer amaranth seed burial; addition of winter cover crops further increased Palmer amaranth control.Cover crops alone in NIT provided ≤50% control of Palmer amaranth; thus indicating the need in both IT and NIT to integrate other effective weed management practices to protect cotton yields.In an IT-cereal rye cover crop situation, a PRE or POST alone herbicide regime was as effective as a PRE + POST regime due to lower Palmer amaranth densities.However, with NIT, an effective and timely PRE + POST herbicide regime was necessary to control the higher Palmer amaranth densities present in this situation.Overall, the PRE + POST herbicide regime resulted in the maximum Palmer amaranth control and higher cotton yields in both soil-inversion treatments (Table 8).The highest Palmer amaranth densities, regardless of soil-inversion treatment and herbicide regime, were consistently recorded in the winter fallow situation.Therefore, in a conservation tillage production system, a PRE + glufosinate POST herbicide based regime coupled with a cover crop may effectively control Palmer amaranth and maximize cotton yields (Table 8).

*
Palmer amaranth density at 6 WAP * and cotton yield with cover crop and three production years' data combined.Abbreviations: WAP, weeks after cotton planting; BR, between row; WR, within row; PRE, only preemergence; POST, only postemergence; PRE + POST, both preemergence and postemergence; ** Means within a column followed by same letter are not significant (P = 0.05).

In 2009 -
2010 and 2010-2011, both PRE and PRE + POST herbicide regimes controlled Palmer amaranth ≥93% regardless of the soil-inversion treatment.In 2009-2010, the POST herbicide regime controlled Palmer amaranth 84 and 97% in NIT and IT, respectively.However in 2010-2011, POST herbicide regime controlled Palmer amaranth 100% in both soil-inversion treatments.The reason for poor performance of the POST herbicide regime in NIT in 2008-2009 is likely attributed to the oversized (>10 cm) Palmer amaranth plants at the time of application.

Figure 1 .
Figure 1.Soil-inversion by cover crop interaction on Palmer amaranth control in 2011.Different letters indicate significant differences at P = 0.05.

Table 1 .
Schedule of operations performed during the experiment.

Table 2 .
Influence of soil-inversion and cover crop on Palmer amaranth density at 2 WAP over three production years.

variable Palmer amaranth density (plants m −2 )
* Abbreviations: WAP, weeks after cotton planting; BR, between row; WR, within row; ** Means within a column followed by the same letter are not significant (P = 0.05).

Table 4 .
Influence of cover crop and herbicide regime on BR * Palmer amaranth density at 6 WAP and cotton yield with soil-inversion and three production years' data combined.
* Abbreviations: BR, between row; WAP, weeks after cotton planting; PRE, only preemergence; POST, only postemergence; PRE + POST, both preemergence and postemergence; ** Means within a column followed by same letter are not significant (P = 0.05).

Table 5 .
Influence of soil-inversion by herbicide regime on Palmer amaranth control at 6 WAP * with cover crop data combined in three production years.
* Abbreviations: WAP, weeks after cotton planting; PRE, only preemergence; POST, only postemergence; PRE + POST, both preemergence and postemergence; ** Means within a column followed by same letter are not significant (P = 0.05).

Table 6 .
Cover crop by herbicide regime interaction effect on Palmer amaranth control at 6 WAP * with soil-inversion and three production years' data combined.

Table 8 .
Palmer amaranth density and visual percent control at 6 WAP * and cotton yield from selected treatments with data combined over three production years.

The following treatments were the best in terms of both Palmer amaranth control and cotton yield.
* Abbreviations: WAP, weeks after cotton planting; PRE, only preemergence; POST, only postemergence; PRE + POST, both preemergence and postemergence.