Efficacy of Selected Herbicide Programs in 2 , 4-D Tolerant Cotton ( Gossypium hirsutum L . )

The use of transgenic crops has grown significantly over the past couple of decades. Many agronomic crops produced today are tolerant to glyphosate. Glyphosate-tolerant crops were commercially introduced in 1996, and, about nine years later, glyphosate-resistant Palmer amaranth was confirmed in Georgia. Glyphosate-resistant weeds arose from reliance on postemergence only glyphosate programs to control weeds in crops. New transgenic traits for glufosinate and 2,4-D choline have been developed, and evaluations of stacked traits and concurrent use of multiple herbicides have provided additional tools in the management of glyphosate-resistant weeds. Field experiments were conducted in 2012 and 2013 at the Edisto Research and Education Center near Blackville, SC, USA to determine the efficacy of 2,4-D-based herbicide programs in transgenic cotton tolerant to 2,4-D choline, glyphosate, and glufosinate. The treatments provided good to excellent Palmer amaranth and pitted morningglory control in 2012 and 2013. Seed cotton yields across treatments ranged from 0 to 2057 kg ha. This new trait technology package in cotton permits in-season postemergence use of 2,4-D choline, a herbicide mode of action not previously used postemergence in cotton, which can control resistant weeds, including Palmer amaranth if applied at the proper growth stage.


Introduction
In 1996, cotton and other crops tolerant to the herbicide glyphosate were commercialized and released to the market [1].The adoption and use of genetically modified crops have increased dramatically over the past 20 years in corn (Zea mays L.), soybean (Glycine max L.), and cotton.For example, in 2016, 93% of the USA cotton hectares were planted with seed containing tolerance to the herbicide glyphosate [2] [3].Initially, glyphosate was very effective in managing a broad spectrum of weeds; however, reliance on a single site of action over a broad geographic area dramatically increased the selection pressure which led to the selection and spread of several glyphosate-resistant weed biotypes in cotton [4] [5].
Palmer amaranth (Amaranthus palmeri S. Wats) is a warm season annual dioecious broadleaf plant [6].Currently, the most common and troublesome weed in cotton in the Southern US is Palmer amaranth due to its rapid growth rate, prolific seed production, and high competitiveness [5] [6] [7].Cotton lint yield can be drastically reduced by the presence of Palmer amaranth [8] [9] [10].
Previous research has shown that lint yields can be reduced from as few as 1 to 2 Palmer amaranth plants per row meter [8].Palmer amaranth competition not only causes yield losses, but it can also increase the cost of production and its large stature can impede the harvesting efficiency of cotton [9].
Palmer amaranth has previously developed resistance to several herbicide groups, including to the dinitroanilines, ALS-inhibitors, and glyphosate groups [5].Dinitroaniline resistant Palmer amaranth biotypes were confirmed in 1992 in South Carolina [11].In a peanut field in Georgia, Palmer amaranth biotypes were confirmed to be resistant to ALS-inhibitor herbicides in 2000 [12].The over reliance on glyphosate postemergence only programs has led to rapid selection and spread of resistant Palmer amaranth biotypes throughout the Southern US and is causing huge economic losses in these crops.Therefore, glyphosate-resistant biotypes of Palmer amaranth have spread rapidly since their confirmation in 2005 [4] [13].
Synthetic auxin herbicides, such as 2,4-D, selectively control broadleaf weeds, including Palmer amaranth [14].Auxinic herbicides cause uncontrolled increases in cell wall growth and cell division which leads to abnormal development in the meristematic tissue in susceptible plants [15].Dow AgroSciences has developed a new salt formulation of 2,4-D called 2,4-D choline that is significantly less volatile than previous salt formulations.Enlist Duo™ is a prepackaged mixture of 0.19 kg ae L −1 of 2,4-D choline and 0.2 kg ae L −1 of glyphosate [16].Dow AgroSciences has recently developed and released transgenic cotton varieties with genetic tolerance to the postemergence applications of

Materials and Methods
Field experiments were conducted on a Varina sandy loam (pH of 6.The experimental design was a randomized complete block of four row plots with four replications.All field maintenance processes, such as fertilizing, defoliation, and insect control, were followed according to recommended production practices for cotton in South Carolina [20].The middle two rows represented the treated area and outside rows were used as an untreated control.In the two studies conducted in 2012 and 2013, the herbicide treatments are presented in Table 1.The application rates selected for the 2,4-D choline and glyphosate (Enlist Duo) treatments were based on the use rates recommended on the herbicide label [16].The remaining treatments in the studies were selected based on the standard Extension herbicide programs used in cotton production in South Carolina.Herbicides were applied in water using a CO Wheaton, IL, USA) at a ground speed of 5 km h −1 .
All plots, except the untreated control, was treated with a preemergence (PRE) application of fomesafen at 0.28 kg ai ha −1 at planting and followed up with two postemergence herbicide combinations (POST1 and POST2).Ratings of weed control, estimates of weed density, and cotton yield data were subjected to ANOVA using the PROC GLM procedure in SAS (SAS 9.4, SAS ® Institute Inc. Cary, NC, USA), with herbicide treatment and years as the main effect and replication as the random effect.Percent weed control and population densities were combined over trial years if no significant treatment by year was observed.Means of significant main effects were separated using Fisher's Protected LSD at P ≤ 0.05.

Results and Discussion
This research showed differences among treatments and treatment by year across all rating periods.Weed control and population density data were presented separately if there was a treatment by year interaction, and averaged over both years when there was no significant interaction.
The monthly precipitation accumulation and average temperature data at the experimental sites during 2012 and 2013 are presented in Table 2.In 2012, a total of 740 mm rainfall was received during the growing season and 693 mm rainfall was received in 2013 at the study sites.
Soil active herbicides, such as fomesafen, require a minimum amount of precipitation after application for proper activation in the soil.In June 2012, rainfall received when the studies were initiated in the field was 81 mm.However, rainfall accumulation in June 2013 was much higher (178 mm).Despite, the lower amount of precipitation received during planting in 2012, fomesafen was properly activated based on the level of weed control observed after planting in both years.During July, August, and September, precipitation accumulation was 427 and 403 mm in 2012 and 2013, respectively (Table 2).The highest average air temperature was observed during the month of July (29˚C in 2012 and 27˚C in 2013).However, in 2013, May and August were also comparable to July average air temperatures indicating that effects of heat stress on the plants were experienced over a much longer period than in 2012.Average air temperatures decreased rapidly after September in 2012 and 2013.

Palmer Amaranth Control
Estimates for control and density of Palmer amaranth resulted in some differences between treatments (Table 3).Palmer amaranth visual control and population density did not have a significant treatment by year interaction; as a result, data was combined for 2012 and 2013.The PRE application of fomesafen provided early season control of Palmer amaranth across all treatments ranging 91% -100% at 3 weeks after planting, excluding the untreated check.

Pitted Morningglory Control
There was greater variability in the pitted morningglory population counts and visual percent control between treatments compared to Palmer amaranth (Table 4).A treatment by year interaction was observed with the 3 WAP1 and  pitted morningglory rating dates; therefore, data are presented by year.Treatments were sprayed with fomesafen PRE, and significant differences in control of pitted morningglory (83% to 100%) were observed between treatments at 3 weeks after planting, excluding the untreated check.At 3 WAP1, treatments  In 2012, the seed cotton yield was consistent across all treatments, especially in the treatments containing 2,4-D choline which indicates that cotton is very tolerant to the over-the-top herbicides in this study.The use of 2,4-D choline in the 2,4-D tolerant cotton will be very effective tool in managing broadleaf weeds and, most importantly, reduce the possibility of selecting new herbicide resistant biotype weeds.However, growers must utilize a comprehensive management program that includes a PRE herbicide, such as fomesafen, at planting followed by tank mixing residual herbicides, such as acetochlor and s-metolachlor, with each 2,4-D choline POST application (i.e., overlapping residuals).
1 and organic matter of 1.3%), (fine, kaolinitic, thermic Plinthic Paleudults) at the Clemson University Edisto Research and Education Center (33.36˚N, −81.32˚W) located near Blackville, SC, USA in 2012 and 2013 to determine the efficacy of 2,4-D based herbicide programs for weed control in 2,4-D tolerant cotton.The cotton variety "pDAB4468" (Dow AgroSciences; Indianapolis, IN, USA) was seeded using a four-row Almaco cone plot planter (Almaco, Nevada, IA, USA) 1.25 cm deep in rows spaced 96 cm apart on 15 June 2012 and 21 June 2013 in a conventionally-tilled soil at 10 seed m −1 .Treated plot dimensions were two rows wide and 9.4 m long.In both study years, soybean was the previous year's crop grown at each location.

2 ( 2
The POST1 treatments were applied 26 days after planting (DAP) when Palmer amaranth and pitted morningglory (Ipomoea lacunosa L.) sizes ranged from 5 to 10 cm in height.The POST2 combinations were sprayed 47 DAP when Palmer amaranth and pitted morningglory sizes ranged from 10 to 15 cm in height.Data collected included visual ratings of percent control of weeds, estimates of weed populations in plots, crop response or injury, and seed cotton yield.Visual ratings for crop injury and percent weed control were taken on a scale of 0% -100%, with 0% indicating no effect on cotton or weed populations, respectively, and 100% indicating crop death or complete weed control, respectively.Visual ratings of control were collected 3 weeks after POST 1 (3 WAP1) and 2 weeks after POST WAP2).Weed species population densities were estimated at the 2 WAP2 application timing by randomly tossing a 0.5 m 2 quadrat down the middle of the 2 treated rows and each weed species present was identified and counted.The middle two treated rows were harvested using a two-row spindle type picker and weighed in kg per plot on 11 November 2012 and 11 December 2013.Harvest weights for each plot were then converted into kg per hectare.
at the time of POST application will be a critical consideration for economical control.The application of fomesafen PRE reduced the early season emergence of Palmer amaranth and pitted morningglory.The overall success of 2,4-D choline containing POST herbicide programs is reliant on the use of a soil residual herbicide at planting plus the use of residual herbicides at each application timing (overlapping residuals).Without an at-plant PRE herbicide, weed size and density will quickly reach beyond the recommendations by the time that the over-the-top application of 2,4-D choline is completed reducing the efficacy of the herbicide application.

Table 1 .
Herbicide treatments, application timing, and rates for 2,4-D based herbicide program evaluations.

Table 2 .
Monthly rainfall totals and average temperatures observed at the study sites during the growing season at Edisto Research and Education Center located near Blackville, SC, USA, for the months of May through November in 2012 and 2013.

Table 3 .
Palmer amaranth (AMAPA) percent visual control and population density counts as affected by herbicide treatments in 2012 & 2013.Refer to Table1for treatment (TRT) names and rates; b Means followed by the same letter do not differ significantly according to Fishers Protected LSD at 5%; c Palmer amaranth percent control and population density rating periods: 3 weeks after POST1 (3 WAP1) and 2 weeks after POST2 application (2 WAP2).

Table 4 .
Pitted morningglory (IPOLA) percent visual control and population density counts as affected by herbicide treatments for pitted morningglory in 2012 & 2013.

Table 5 .
Mean seed cotton yield as affected by selected herbicide programs in 2012 & 2013.Refer to Table1for treatment (TRT) names and rates; b Means followed by the same letter do not differ significantly according to Fishers Protected LSD at 5%.