Glyphosate-resistant Palmer Amaranth Research Articles

Influence of cover crop residue and residual herbicide on emergence dynamics of glyphosate-resistant Palmer amaranth (Amaranthus palmeri) in grain sorghum

Abstract A field study was conducted from 2020 to 2023 at Kansas State University Agricultural Research Center near Hays, KS, to understand the emergence dynamics and periodicity of glyphosate-resistant (GR) Palmer amaranth (Amaranthus palmeri S. Watson) as influenced by cover crop (CC) residue and residual herbicide in grain sorghum [Sorghum bicolor (L.) Moench]. The study site was under a wheat (Triticum aestivum L.)–sorghum–fallow rotation with a natural seedbank of GR A. palmeri. Treatments included (1) fall-planted CC mixture [winter triticale (×Triticosecale Wittm. ex A. Camus [Secale × Triticum])/winter peas (Pisum sativum L.)/rapeseed (Brassica napus L.)/radish (Raphanus sativus L.)] after wheat harvest and terminated at triticale heading stage (next spring before sorghum planting) with glyphosate alone or (2) glyphosate plus acetochlor/atrazine, (3) chemical fallow (no CC but treated with acetochlor/atrazine and dicamba before sorghum planting), and (4) nontreated control (no CC and no herbicide). Results indicated that CC terminated with glyphosate plus acetochlor/atrazine had a delayed and reduced cumulative emergence of GR A. palmeri as compared with chemical fallow and CC terminated with glyphosate alone across all 3 yr. Compared with chemical fallow, the CC terminated with glyphosate alone and glyphosate plus acetochlor/atrazine required 66 to 643 and 105 to 1,257 more cumulative growing degree days, respectively, to achieve 90% cumulative emergence of GR A. palmeri across all 3 yr. The combined effect of CC residue with glyphosate plus acetochlor/atrazine reduced the total emergence counts of GR A. palmeri by 42% to 56% and 82% to 94% as compared with chemical fallow and nontreated control, respectively. These results suggest that fall-planted CC combined with a residual herbicide at termination can be utilized for GR A. palmeri suppression in grain sorghum.

Effects of 2,4-D choline on fruiting in sensitive cotton

AbstractWith the increase in hectares planted to auxin-resistant cotton, the number of preplant, at-plant, and postplant applications of dicamba and 2,4-D choline to aid in the control of troublesome broadleaf weeds, including glyphosate-resistant Palmer amaranth, has increased. More dicamba and 2,4-D choline applications mean an increased risk of off-target movement. Field studies were conducted in 2019 to 2021 at the Texas Tech University New Deal Research Farm to evaluate dicamba-resistant cotton response to various rates of 2,4-D choline when applied at four growth stages (first square [FS] + 2 wk, first bloom [FB], FB + 2 wk, and FB + 4 wk). Applications of 2,4-D choline were applied at 1,060 (1X), 106 (1/10X), 21 (1/50X), 10.6 (1/100X), 2.1 (1/500X), and 1.06 (1/1000X) g ae ha−1 to Deltapine 1822 XF cotton. Relative to the nontreated control, yield losses were observed in all years at FS + 2 wk and FB from rates of 2,4-D choline ≥ 1/100X. At the FB + 4 wk application, only the 1X rate of 2,4-D choline resulted in a yield reduction in all three years. Micronaire, fiber length, and uniformity were negatively influenced by the 1/10X and 1X rates of 2,4-D choline at various timings in 2019, 2020, and 2021. In addition, short fiber content, neps, and seed coat neps increased where micronaire, fiber length, and uniformity were negatively impacted.

HPPD-resistant cotton response and weed management systems using isoxaflutole

AbstractThe southern United States produces 90% of the nation’s cotton, and the Texas High Plains is the largest contiguous cotton producing region. Since 2011, glyphosate-resistant Palmer amaranth has complicated cotton production, and alternatives to glyphosate are needed. Integrating soil residual herbicides into a weed management program is a crucial step to control glyphosate resistant weeds before emergence. The recent development of p-hydroxyphenylpyruvate dioxygenase (HPPD)-resistant cotton by BASF Corporation may allow growers to use isoxaflutole in future weed management programs. In 2019 and 2020, field experiments were conducted in New Deal, Lubbock, and Halfway, Texas, to evaluate HPPD-resistant cotton response to isoxaflutole applied preemergence (PRE) or early postemergence (EPOST) and to determine the efficacy of isoxaflutole when used as part of a season-long weed management program. At the New Deal location, cotton response was observed following the EPOST application, but it never exceeded 10%. Cotton response was greatest following the PRE application in Lubbock in 2019 but did not exceed 14%. In 2020 in Lubbock, cotton was replanted due to severe weather. There was <1% cotton response following the PRE application, and maximum cotton response observed was 9% following EPOST and mid-postemergence (MPOST) applications. Cotton lint yields were not different from those of the nontreated, weed-free control at either location. In non-crop weed control studies in Halfway, all treatments controlled Palmer amaranth ≥94% 21 d after the EPOST application. Twenty-one days after the MPOST treatment, systems with isoxaflutole applied EPOST controlled Palmer amaranth by 88% to 93%, while systems with isoxaflutole PRE controlled Palmer amaranth by 94% to 98%. End-of-season Palmer amaranth control was lowest in the system without isoxaflutole (88%) and when isoxaflutole was used EPOST (88% to 91%). These studies suggest that the use of isoxaflutole in cotton weed management systems may improve season-long control of several troublesome weeds with no adverse effects on cotton yield and quality.

Effect of cotton herbicide programs on weed population trajectories and frequency of glyphosate-resistant Palmer amaranth (Amaranthus palmeri)

AbstractThe adoption of dicamba-resistant cotton (Gossypium hirsutum L.) cultivars allows using dicamba to reduce weed populations across growing seasons. However, the overuse of this tool risks selecting new herbicide-resistant biotypes. The objectives of this research were to determine the population trajectories of several weed species and track the frequency of glyphosate-resistant (GR) Palmer amaranth (Amaranthus palmeri S. Watson) over 8 yr in dicamba-resistant cotton. An experiment was established in North Carolina in 2011, and during the first 4 yr, different herbicide programs were applied. These programs included postemergence applications of glyphosate, alone or with dicamba, with or without residual herbicides. During the last 4 yr, all programs received glyphosate plus dicamba. Biennial rotations of postemergence applications of glyphosate only and glyphosate plus dicamba postemergence with and without preemergence herbicides were also included. Sequential applications of glyphosate plus dicamba were applied to the entire test area for the final 4 yr of the study. No herbicide program was entirely successful in controlling the weed community. Weed population trajectories were different according to species and herbicide program, creating all possible outcomes; some increased, others decreased, and others remained stable. Density of resistant A. palmeri increased during the first 4 yr with glyphosate-only programs (up to 11,739 plants m−2) and decreased a 96% during the final 4 yr, when glyphosate plus dicamba was implemented. This species had a strong influence on population levels of other weed species in the community. Goosegrass [Eleusine indica (L.) Gaertn.] was not affected by A. palmeri population levels and even increased its density in some herbicide programs, indicating that not only herbicide resistance but also reproductive rates and competitive dynamics are critical for determining weed population trajectories under intensive herbicide-based control programs. Frequency of glyphosate resistance reached a maximum of 62% after 4 yr, and those levels were maintained until the end of the experiment.

Economics of reducing Palmer amaranth seed production in dicamba/glufosinate/glyphosate‐resistant soybean

AbstractIncreased prevalence of glyphosate‐resistant (GR) weeds within agronomic cropping systems has led to the readoption of pre‐emergence (PRE) herbicides and use of multiple herbicide‐resistant soybean [Glycine max (L.) Merr.] cultivars. Herbicide programs were evaluated in the recently commercialized dicamba/glyphosate/glufosinate‐resistant (DGGR) soybean for weed control, reduction of Palmer amaranth (Amaranthus palmeri S. Watson) seed production, crop safety, and economic performance. At 35 days after pre‐emergence herbicides, acetochlor plus dicamba plus metribuzin, acetochlor/fomesafen plus dicamba, dicamba plus flumioxazin, and imazethapyr/pyroxasulfone/saflufenacil provided 80–99% control of velvetleaf (Abutilon theophrasti Medik.), Palmer amaranth, common lambsquarters (Chenopodium album L.), and Poaceae species. Evaluation at 14 days after early postemergence herbicides indicated PRE followed by (fb) POST applications of mixtures of acetochlor, dicamba, glufosinate, and glyphosate provided 80–99% weed control compared with 67–93% control in POST‐only programs. Most herbicide programs provided 83–99% control of grass and broadleaf weeds, with 85–91% weed biomass reductions at 28 days after late‐POST. The PRE fb POST programs reduced Palmer amaranth seed production by 94–99%, whereas POST‐only programs provided 75–83% reduction. In 2020, most programs provided gross profit margins ≥US$1,000 ha−1, with glufosinate fb glufosinate and imazethapyr/pyroxasulfone/saflufenacil fb acetochlor plus glufosinate providing $1,481 and $1,466 ha−1, respectively. Benefit/cost ratios ranged between 0.3 and 3.9 in 2019 due to hail but increased to 2.9–10.9 in 2020. Results of this study support use of PRE herbicides with multiple sites of action in DGGR soybean and indicate that glufosinate can provide POST control of GR Palmer amaranth.

Hyperspectral plant sensing for differentiating glyphosate‐resistant and glyphosate‐susceptible johnsongrass through machine learning algorithms

Johnsongrass (Sorghum halepense) is one of the weeds that evolves resistance to glyphosate [N-(phosphonomethyl)-glycine], the most widely used herbicide, and the weed may cause agronomic troublesome in the southern USA. This paper reports a study on developing a hyperspectral plant sensing approach to explore the spectral features of glyphosate-resistant (GR) and glyphosate-sensitive (GS) plants to evaluate this approach using machine learning algorithms to differentiate between GR and GS plants. On average, GR plants have higher spectral reflectance compared with GS plants. The sensitive spectral bands were optimally selected using the successive projections algorithm respectively wrapped with the machine learning algorithms of k-nearest neighbors (KNN), random forest (RF), and support vector machine (SVM) with Fisher linear discriminant analysis (FLDA) to classify between GS and GS plants. At 3 weeks after transplanting (WAT) KNN and SVM could not acceptably classify the GR and GS plants but they improved significantly with the stages to have their overall accuracies reaching 73% and 77%, respectively, at 5 WAT. RF and FLDA had a better ability to classify the plants at 3 WAT but RF was low in accuracy at 2 WAT while FLDA dropped accuracy to 50% at 4 WAT from 57% at 3 WAT and raised it to 73% at 5 WAT. Previous studies were conducted developing the hyperspectral imaging approach to differentiate GR Palmer amaranth from GS Palmer amaranth and GR Italian ryegrass from GS Italian ryegrass with classification accuracies of 90% and 80%, respectively. This study demonstrated that the hyperspectral plant sensing approach could be developed to differentiate GR johnsongrass from glyphosate-sensitive GS johnsongrass with the highest classification accuracy of 77%. The comparison with our previous studies indicated that the similar hyperspectral approach could be used and transferred from classification across different GR and GS weed biotypes, such as Palmer amaranth, Italian ryegrass and johnsongrass, so it is highly possible for classification of more other GR and GS weed biotypes as well. On the basis of classic pattern recognition approaches the process of plant classification can be enhanced by modeling using machine learning algorithms. © 2022 Society of Chemical Industry. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

Transpiration Responses of Herbicide-Resistant and -Susceptible Palmer Amaranth (Amaranthus palmeri (S.) Wats.) to Progressively Drying Soil

Drought events are predicted to increase in the future. Evaluating the response of herbicide-resistant and -susceptible weed ecotypes to progressive drought can provide insights into whether resistance traits affect the fitness of resistant weed populations. Two experiments were conducted in the greenhouse between January and May 2021 to evaluate drought tolerance differences between Palmer amaranth accessions resistant to S-metolachlor or glyphosate and their susceptible counterparts. The accessions used were S-metolachlor-resistant (17TUN-A), a susceptible standard (09CRW-A), and glyphosate-resistant (22–165 EPSPS copies) and glyphosate-susceptible (3–10 EPSPS copies) plants from accession 16CRW-D. Daily transpiration of each plant was measured. The daily transpiration rate was converted to normalized transpiration ratio (NTR) using a double-normalization procedure. The daily soil water content was expressed as a fraction of transpirable soil water (FTSW). The threshold FTSW (FTSWcr), after which NTR decreases linearly, was estimated using a two-segment linear regression analysis. The data showed differences between S-metolachlor-resistant and -susceptible accessions (p ≤ 0.05). The FTSW remaining in the soil at the breakpoint for the S-metolachlor-susceptible accession (09CRW-A) was 0.17 ± 0.007. The FTSW remaining in the soil at the breakpoint for the S-metolachlor-resistant accession (17TUN-A) was 0.23 ± 0.004. The FTSW remaining in the soil at the breakpoint for the glyphosate-resistant and glyphosate-susceptible plants (16CRW-D) was 0.25 ± 0.007 and 0.25 ± 0.008, respectively. Although the mechanism endowing resistance to S-metolachlor might have contributed to increased drought tolerance, follow-up experiments are needed in order to verify this finding. Increased EPSPS copy numbers did not improve the drought tolerance of Palmer amaranth. As droughts are predicted to increase in frequency and severity, these results suggest that S-metolachlor-resistant and glyphosate-resistant Palmer amaranth populations will not be at a competitive disadvantage compared to susceptible genotypes. Alternative and diverse management strategies will be required for effective Palmer amaranth control, regardless of herbicide resistance status.

Weed Management and Economics in Dicamba-Tolerant Cotton

Increasing populations of glyphosate-resistant Palmer amaranth [Amaranthus palmeri (S.) Wats.] have increased weed management costs for Texas High Plains cotton [Gossypium hirsutum (L.)] producers. The introduction of dicamba-tolerant cotton varieties and registration of dicamba formulations for postemergence use, combined with residual herbicides, can effectively control Palmer amaranth. Field studies were conducted in 2018 and 2019 near Lubbock, TX, USA to evaluate Palmer amaranth control and economics of weed management in dicamba-, glufosinate-, glyphosate-, and conventional cotton systems. The most consistent season-long Palmer amaranth control was achieved with the dicamba-tolerant system in both years. In 2018, greatest lint yields were achieved with dicamba-tolerant system when compared to the conventional and glufosinate-tolerant systems. In 2018, greatest gross margin above weed management costs were achieved with the dicamba-tolerant and glyphosate-tolerant systems.  Greatest lint yield was achieved with the dicamba-tolerant and conventional systems in 2019 and greatest gross margins were achieved with the dicamba-tolerant system. Total variable costs were similar across all systems, with greater seed/technology and herbicide costs in dicamba-tolerant and glufosinate-tolerant systems, compared to higher tillage and hand hoeing costs in glyphosate-tolerant and conventional systems.

Control of acetolactate synthase inhibitor/glyphosate-resistant Palmer amaranth (Amaranthus palmeri) in isoxaflutole/glufosinate/glyphosate-resistant soybean

AbstractPalmer amaranth is the most problematic and troublesome weed in agronomic cropping systems in the United States. Acetolactate synthase (ALS) inhibitor and glyphosate-resistant (GR) Palmer amaranth has been confirmed in Nebraska and it is widespread in several counties. Soybean resistant to isoxaflutole/glufosinate/glyphosate has been developed that provides additional herbicide site of action for control of herbicide-resistant weeds. The objectives of this study were to evaluate herbicide programs for control of ALS inhibitor/GR Palmer amaranth and their effect on Palmer amaranth density and biomass, as well as soybean injury and yield in isoxaflutole/glufosinate/glyphosate–resistant soybean. Field experiments were conducted in a grower’s field infested with ALS inhibitor and GR Palmer amaranth near Carleton, Nebraska, in 2018 and 2019. Isoxaflutole applied alone or mixed with sulfentrazone/pyroxasulfone, flumioxazin/pyroxasulfone, or imazethapyr/saflufenacil/pyroxasulfone provided similar control (86%–99%) of Palmer amaranth 21 d after PRE (DAPRE). At 14 d after early-POST (DAEPOST), isoxaflutole applied PRE and PRE followed by (fb) POST controlled Palmer amaranth by 10% to 63% compared to 75% to 96% control with glufosinate applied EPOST in both years. A PRE herbicide fb glufosinate controlled Palmer amaranth 80% to 99% 21 d after late-POST (DALPOST) in 2018, and reduced density 89% to 100% in 2018 and 58% to 100% in 2019 at 14 DAEPOST. No soybean injury was observed from any of the herbicide programs tested in this study. Soybean yield in 2019 was relatively higher due to higher precipitation compared with 2018 with generally no differences between herbicide programs. This research indicates that herbicide programs are available for effective control of ALS inhibitor/GR Palmer amaranth in isoxaflutole/glufosinate/glyphosate-resistant soybean.

Mechanism of Resistance to S-metolachlor in Palmer amaranth.

Herbicides are major tools for effective weed management. The evolution of resistance to herbicides in weedy species, especially contributed by non-target-site-based resistance (NTSR) is a worrisome issue in crop production globally. Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) is one of the extremely difficult weeds in southern US crop production. In this study, we present the level and molecular basis of resistance to the chloroacetamide herbicide, S-metolachlor, in six field-evolved A. palmeri populations that had survivors at the recommended field-dose (1.1 kg ai ha−1). These samples were collected in 2014 and 2015. The level of resistance was determined in dose-response assays. The effective dose for 50% control (ED50) of the susceptible population was 27 g ai ha−1, whereas the ED50 of the resistant populations ranged from 88 to 785 g ai ha−1. Therefore, A. palmeri resistance to S-metolachlor evolved in Arkansas as early as 2014. Metabolic-inhibitor and molecular assays indicated NTSR in these populations, mainly driven by GSTs. To understand the mechanism of resistance, selected candidate genes were analyzed in leaves and roots of survivors (with 1 × S-metolachlor). Expression analysis of the candidate genes showed that the primary site of S-metolachlor detoxification in A. palmeri is in the roots. Two GST genes, ApGSTU19 and ApGSTF8 were constitutively highly expressed in roots of all plants across all resistant populations tested. The expression of both GSTs increased further in survivors after treatment with S-metolachlor. The induction level of ApGSTF2 and ApGSTF2like by S-metolachlor differed among resistant populations. Overall, higher expression of ApGSTU19, ApGSTF8, ApGSTF2, and ApGSTF2like, which would lead to higher GST activity in roots, was strongly associated with the resistant phenotype. Phylogenetic relationship and analysis of substrate binding site of candidate genes suggested functional similarities with known metolachlor-detoxifying GSTs, effecting metabolic resistance to S-metolachlor in A. palmeri. Resistance is achieved by elevated baseline expression of these genes and further induction by S-metolachlor in resistant plants.

Effect of single or sequential POST herbicide applications on seed production and viability of glyphosate-resistant Palmer amaranth (Amaranthus palmeri) in dicamba- and glyphosate-resistant soybean

AbstractGlyphosate-resistant (GR) Palmer amaranth is a troublesome weed that can emerge throughout the soybean growing season in Nebraska and several other regions of the United States. Late-emerging Palmer amaranth plants can produce seeds, thus replenishing the soil seedbank. The objectives of this study were to evaluate single or sequential applications of labeled POST herbicides such as acifluorfen, dicamba, a fomesafen and fluthiacet-methyl premix, glyphosate, and lactofen on GR Palmer amaranth control, density, biomass, seed production, and seed viability, as well as grain yield of dicamba- and glyphosate-resistant (DGR) soybean. Field experiments were conducted in a grower’s field infested with GR Palmer amaranth near Carleton, NE, in 2018 and 2019, with no PRE herbicide applied. Acifluorfen, dicamba, a premix of fomesafen and fluthiacet-methyl, glyphosate, or lactofen were applied POST in single or sequential applications between the V4 and R6 soybean growth stages, with timings based on product labels. Dicamba applied at V4 or in sequential applications at V4 followed by R1 or R3 controlled GR Palmer amaranth 91% to 100% at soybean harvest, reduced Palmer amaranth density to as low as 2 or fewer plants m−2, reduced seed production to 557 to 2,911 seeds per female plant, and resulted in the highest soybean yield during both years of the study. Sequential applications of acifluorfen, fomesafen and fluthiacet premix, or lactofen were not as effective as dicamba for GR Palmer amaranth control; however, they reduced seed production similar to dicamba. On the basis of the results of this study, we conclude that dicamba was effective for controlling GR Palmer amaranth and reduced density, biomass, and seed production without DGR soybean injury. Herbicides evaluated in this study had no effect on Palmer amaranth seed viability.

Response of Palmer amaranth (Amaranthus palmeri S. Watson) and sugarbeet to desmedipham and phenmedipham

AbstractA prepackaged mixture of desmedipham + phenmedipham was previously labeled for control of Amaranthus spp. in sugarbeet. Currently, there are no effective POST herbicide options to control glyphosate-resistant Palmer amaranth in sugarbeet. Sugarbeet growers are interested in using desmedipham + phenmedipham to control escaped Palmer amaranth. In 2019, a greenhouse experiment was initiated near Scottsbluff, NE, to determine the selectivity of desmedipham and phenmedipham between Palmer amaranth and sugarbeet. Three populations of Palmer amaranth and four sugarbeet hybrids were evaluated. Herbicide treatments consisted of desmedipham and phenmedipham applied singly or as mixtures at an equivalent rate. Herbicides were applied when Palmer amaranth and sugarbeet were at the cotyledon stage, or two true-leaf sugarbeet stage and when Palmer amaranth was 7 cm tall. The selectivity indices for desmedipham, phenmedipham, and desmedipham + phenmedipham were 1.61, 2.47, and 3.05, respectively, at the cotyledon stage. At the two true-leaf application stage, the highest rates of desmedipham and phenmedipham were associated with low mortality rates in sugarbeet, resulting in a failed response of death. The highest rates of desmedipham + phenmedipham caused a death response of sugarbeet; the selectivity index was 2.15. Desmedipham treatments resulted in lower LD50 estimates for Palmer amaranth compared to phenmedipham, indicating that desmedipham can provide greater levels of control for Palmer amaranth. However, desmedipham also caused greater injury in sugarbeet, producing lower LD50 estimates compared to phenmedipham. Desmedipham + phenmedipham provided 90% or greater control of cotyledon-size Palmer amaranth at a labeled rate but also caused high levels of sugarbeet injury. Neither desmedipham, phenmedipham, nor desmedipham + phenmedipham was able to control 7-cm tall Palmer amaranth at previously labeled rates. Results indicate that desmedipham + phenmedipham can only control Palmer amaranth if applied at the cotyledon stage and a high level of sugarbeet injury is acceptable.

Palmer Amaranth Control by Glufosinate Depends on Application Time of Day

Glufosinate is used widely to control glyphosate-resistant Palmer amaranth (Amaranthus palmeri S. Wats.) in cotton (Gossypium hirsutum L.). Previous research has shown weed control with several herbicides, including glufosinate, can be affected by application time of day. The response sometimes has been attributed to diurnal leaf movement (leaf orientation) in the weeds. The objectives of our research were to determine the influence of time of day of glufosinate application on Palmer amaranth control and to determine if the response was related to diurnal leaf orientation. Field experiments in five states evaluated Palmer amaranth control with glufosinate applied at nine intervals ranging from 1 h before sunrise to 6 h after sunrise and nine intervals ranging from 6 h before sunset to 1 h after sunset. Greatest Palmer amaranth control was achieved with glufosinate applied 2 h after sunrise to 1 h before sunset. Ammonium sulfate, added only to glufosinate 1 h before sunrise or 1 h after sunset treatments, did not improve control. Leaf angles of Palmer amaranth and velvetleaf (Abutilon theophrasti L.) were determined in a greenhouse at 1 h before light, 2 h after light, mid-day, 2 h before dark, and 1 h after dark. Leaves of velvetleaf oriented downward during the dark periods but time of day had no effect on leaf orientation of Palmer amaranth. These results demonstrate the need for sunlight for optimum glufosinate efficacy on Palmer amaranth.

Effective two-pass herbicide programs to control glyphosate-resistant Palmer amaranth (Amaranthus palmeri) in glyphosate/dicamba-resistant soybean

AbstractField experiments were conducted in 2018 and 2019 at Kansas State University Ashland Bottoms (KSU-AB) research farm near Manhattan, KS, and Kansas State University Agricultural Research Center (KSU-ARC) near Hays, KS, to determine the effectiveness of various PRE-applied herbicide premixes and tank mixtures alone or followed by (fb) an early POST (EPOST) treatment of glyphosate + dicamba for controlling glyphosate-resistant (GR) Palmer amaranth in glyphosate/dicamba-resistant (GDR) soybean. In experiment 1, PRE-applied sulfentrazone + S-metolachlor, saflufenacil + imazethapyr + pyroxasulfone, chlorimuron + flumioxazin + pyroxasulfone, and metribuzin + flumioxazin + imazethapyr provided 85% to 94% end-of-season control of GR Palmer amaranth across both sites. In comparison, Palmer amaranth control ranged from 63% to 87% at final evaluation with PRE-applied pyroxasulfone + sulfentrazone, pyroxasulfone + sulfentrazone plus metribuzin, pyroxasulfone + sulfentrazone plus carfentrazone + sulfentrazone, and sulfentrazone + metribuzin at the KSU-ARC site in experiment 2. All PRE fb EPOST (i.e., two-pass) programs provided near-complete (98% to 100%) control of GR Palmer amaranth at both sites. PRE-alone programs reduced Palmer amaranth shoot biomass by 35% to 76% in experiment 1 at both sites, whereas all two-pass programs prevented Palmer amaranth biomass production. No differences in soybean yields were observed among tested programs in experiment 1 at KSU-ARC site; however, PRE-alone sulfentrazone + S-metolachlor, saflufenacil + imazethapyr + pyroxasulfone, and chlorimuron + flumioxazin + pyroxasulfone had lower grain yield (average, 4,342 kg ha−1) compared with the top yielding (4,832 kg ha−1) treatment at the KSU-AB site. PRE-applied sulfentrazone + metribuzin had a lower soybean yield (1,776 kg ha−1) compared with all other programs in experiment 2 at the KSU-ARC site. These results suggest growers should proactively adopt effective PRE-applied premixes fb EPOST programs evaluated in this study to reduce selection pressure from multiple POST dicamba applications for GR Palmer amaranth control in GDR soybean.

Dicamba effects on fruiting in sensitive cotton

AbstractSince the release of dicamba-tolerant cotton in 2016, preplant and POST applications of dicamba to control glyphosate-resistant Palmer amaranth have increased. With the increase in area treated with dicamba, the risk of off-target movement to nontarget crops has increased. A field study was conducted at the Texas Tech University New Deal Research Farm equipped with subsurface drip irrigation in 2017 and 2018 to evaluate non-dicamba tolerant cotton response to dicamba when applied at four crop growth stages [first square (FS) + 2 wk, first bloom (FB), FB + 2 wk, and FB + 5 wk]. Dicamba at 0.56 (1×), 0.056 (1/10×), 0.0112 (1/50×), 0.0056 (1/100×), and 0.00112 (1/500×) kg ae ha−1 was applied to ‘FM 1830GLT’ cotton. When applications were made at FS + 2 wk, a shift in boll nodal position was apparent following dicamba at the 1/50× rate in 2017 and at 1/10× in 2018 compared to the nontreated control (NTC). A shift in boll distribution from the 1/50× rate of dicamba was apparent at FB in 2017, but not in 2018. Dicamba applied at the 1× rate at FB + 2 wk resulted in reduced boll numbers. No change in boll number or boll position was apparent following any dicamba rate when applied at FB + 5 wk in both years. Dicamba applied at 1/500×, 1/100×, and 1/50× rates at all timings did not affect yield relative to the NTC. When dicamba was applied at the 1/10× rate, the greatest yield loss was observed at FS + 2 wk followed by FB and FB + 2 wk. Micronaire increased following dicamba applied at 1/10× at FS + 2 wk, FB, and FB + 2 wk in 2017. In 2018, micronaire decreased following dicamba applied at 1/10× at FB + 5 wk.

Effects of <i>Myrothecium verrucaria</i> on Two Glyphosate-Resistant <i>Amaranthus palmeri</i> Biotypes Differing in Betacyanin Content

Previously we found two biotypes of Amaranthus palmeri (Palmer amaranth) in a population of this economically important weed that were resistant to glyphosate but differed with respect to pigmentation. One biotype was typically red-pigmented (betacyanin) while the other was green, with no visual appearance of red hue on any plant part at any growth stage. We have also reported that a strain of Myrothecium verrucaria (MV) exhibited bioherbicidal activity against several important weeds including glyphosate-resistant Palmer amaranth. In greenhouse tests, MV was applied to these two biotypes (red and green) at two ages (3-week- and 6-week-old) and effects of this fungus monitored over a 5-day time course. Initial symptoms of MV (16 to 24 h after inoculation) were: epinastic curvature, wilting and development of lesions on leaves and stems. Generally, the younger plants tended to be more sensitive to MV than older plants. Bioherbicidal damage increased with time leading to necrosis and plant mortality and increasing disease progress. Severe loss of fresh weight occurred in both biotypes as compared to untreated plants. Results indicated that MV was effective on both biotypes, but effects on growth reduction and disease progression were more rapid and generally greater in the green biotype, suggesting that compounds responsible for red pigmentation may be more potent as defense against pathogen attack.

Herbicide Resistance Traits in Maize and Soybean: Current Status and Future Outlook.

This article reviews, focusing on maize and soybean, previous efforts to develop nontransgenic herbicide-resistant crops (HRCs), currently available transgenic HRC traits and technologies, as well as future chemical weed management options over the horizon. Since the mid twentieth century, herbicides rapidly replaced all other means of weed management. Overreliance on ‘herbicide-only’ weed control strategies hastened evolution of HR weed species. Glyphosate-resistant (GR) crop technology revolutionized weed management in agronomic crops, but GR weeds, led by Palmer amaranth, severely reduced returns from various cropping systems and affected the bottom line of growers across the world. An additional problem was the lack of commercialization of a new herbicide mode of action since the 1990s. Auxinic HRCs offer a short-term alternative for management of GR Palmer amaranth and other weed species. New HRCs stacked with multiple herbicide resistance traits and at least two new herbicide modes of action expected to be available in the mid-2020s provide new chemical options for weed management in row crops in the next decade.

Cereal rye cover crop and herbicide application method affect cotton stand, Palmer amaranth (Amaranthus palmeri) control, and cotton yield

AbstractSix on-farm studies determined the effects of a rolled rye cover crop, herbicide program, and planting technique on cotton stand, weed control, and cotton yield in Georgia. Treatments included: (1) rye drilled broadcast with 19-cm row spacing and a broadcast-herbicide program (2) rye drilled with a 25-cm rye-free zone in the cotton row and a broadcast-herbicide program (3) rye drilled with a 25-cm rye-free zone in the cotton row with PPI and PRE herbicides banded in the cotton planting row, and (4) no cover crop (i.e., weedy cover) with broadcast herbicides. At two locations, cotton stand was lowest with rye drilled broadcast; at these sites the rye-free zone maximized stand equal to the no-cover system. At a third location, cover crop systems resulted in greater stand, due to enhanced soil moisture preservation compared with the no-cover system. Treatments did not influence cotton stand at the other three locations and did not differ in the control of weeds other than Palmer amaranth at any location. Treatments controlled Palmer amaranth equally at three locations; however, differences were observed at the three locations having the greatest glyphosate-resistant plant densities. For these locations, when broadcasting herbicides, Palmer amaranth populations were reduced 82% to 86% in the broadcast rye and rye-free zone systems compared with the no-cover system at harvest. The system with banded herbicides was nearly 21 times less effective than the similar system broadcasting herbicides. At these locations, yields in the rye broadcast and rye-free zone systems with broadcast herbicides were increased 9% to 16% compared with systems with no cover or a rye-free zone with PPI and PRE herbicides banded. A rolled rye cover crop can lessen weed emergence and selection pressure while improving weed control and cotton yield, but herbicides should be broadcast in fields heavily infested with glyphosate-resistant Palmer amaranth.

Impact of Cereal Rye Seeding Rate and Planting Method on Weed Control in Cotton

Costs related to herbicide use have increased greatly due to evolution and proliferation of glyphosate-resistant Palmer amaranth (Amaranthus palmeri S.). The use of cover crops in conservation tillage offers advantages such as weed suppression through physical and allelopathic effects. A field study was initiated in fall 2013 and 2014 in Fayetteville, AR to determine the impact of cereal rye (Secale cereal L.) seeding rate and planting method on weed control and cotton (Gossypium hirsutum L.) yield. Cereal rye seeding rates were 56, 112, and 168 kg ha-1 in absence or presence of a herbicide program. Planting methods consisted of drilled and broadcast. No differences were observed between planting methods in any parameter evaluated. In both years, cereal rye biomass production increased as seeding rate increased. When herbicides were not applied, cereal rye at 56 kg ha-1 provided the least weed control. Cereal rye at 112 and 168 kg ha-1 provided comparable levels of Palmer amaranth control. At 8 wk after cotton planting, all plots treated with a commonly used herbicide program had 99% or greater grass control, regardless of the seeding rate. Yields from plots with a standard herbicide program were greater than from plots without herbicide. Yield improvement was observed due to use of cereal cover crop compared to no cover crop in 2014, whereas no differences were observed in 2015.

Evaluation of 2,4-D–based Herbicide Mixtures for Control of Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri)

AbstractUnderstanding control of glyphosate-resistant (GR) Palmer amaranth with multiple herbicide sites of action, including synthetic auxins, is crucial for growers to minimize GR Palmer amaranth interference with crops. Field studies in 2013 and 2014 and a greenhouse study in 2014 were conducted in Stoneville, MS, to evaluate POST control of GR Palmer amaranth with 2,4-D alone and in mixtures with glyphosate and/or glufosinate. In the greenhouse study, control of 5- and 10-cm GR Palmer amaranth was 87% with 2,4-D at 0.84 kg ae ha−1. Dry weight reduction of GR Palmer amaranth was ≥81% with 2,4-D at 0.84 kg ha−1. In field studies, mixtures of glufosinate at 0.59 kg ai ha−1and 2,4-D at 0.56 or 1.12 kg ae ha−1controlled 5- to 10-cm GR Palmer amaranth 87% at 28 d after treatment (DAT). Averaged across glyphosate treatments, glufosinate applied alone applied to 5- to 10-cm GR Palmer amaranth reduced dry weight at 28 DAT to 20 g m−2from 82 g m−2and was comparable with that following 2,4-D applied alone at 1.12 kg ae ha−1and mixtures of glufosinate plus 2,4-D at 0.56 and 1.12 kg ae ha−1. Mixtures of 2,4-D plus glufosinate provided ≥92% control of 15- to 20-cm GR Palmer amaranth at 28 DAT. When applied to 15- to 20-cm plants, mixtures of 2,4-D plus glufosinate reduced GR Palmer amaranth density to ≤5 plants m−2compared with 65 plants m−2where no 2,4-D or glufosinate was applied. Glufosinate and 2,4-D are viable control options for 5- to 10-cm or 15- to 20-cm GR Palmer amaranth. However, 2,4-D did not improve GR Palmer amaranth control when added to any herbicide mixture except glyphosate and glufosinate applied to 15- to 20-cm plants at the 28 DAT evaluation.