Palmer Amaranth Control Research Articles

Optimizing Pyroxasulfone-Coated Fertilizer in Cotton

Abstract Two studies were conducted in 2022 and 2023 near Rocky Mount and Clayton, NC, to determine the optimal granular ammonium sulfate (AMS) rate and application timing for pyroxasulfone-coated AMS. In the rate study, AMS rates included 161, 214, 267, 321, 374, 428, and 481 kg ha-1, equivalent to 34, 45, 56, 67, 79, 90, and 101 kg N ha-1, respectively. All rates were coated with pyroxasulfone at 118 g ai ha-1 and top-dressed onto 5- to 7-leaf cotton. In the timing study, pyroxasulfone (118 g ai ha-1) was coated on AMS and top-dressed at 321 kg ha-1 (67 kg N ha-1) onto 5- to 7-leaf, 9- to 11-leaf, and first bloom cotton. In both studies, weed control and cotton tolerance to pyroxasulfone-coated AMS was compared to pyroxasulfone applied postemergence (POST) and postemergence-directed (POST-directed). The check in both studies received non-herbicide-treated AMS (321 kg ha-1). Before treatment applications, all plots (including the check) were maintained weed-free with glyphosate and glufosinate. In both studies, pyroxasulfone applied POST was most injurious (8 to 16%), while pyroxasulfone-coated AMS resulted in ≤ 4% injury. Additionally, no differences in cotton lint yield were observed in both studies. With the exception of the lowest rate of AMS (161 kg ha-1; 79%), all AMS rates coated with pyroxasulfone controlled Palmer amaranth ≥ 83%, comparable to pyroxasulfone applied POST (92%) and POST-directed (89%). In the timing study, the application method did not affect Palmer amaranth control; however, applications made at the mid- and late timings outperformed early applications. These results indicate pyroxasulfone-coated AMS can control Palmer amaranth comparable to pyroxasulfone applied POST and POST-directed, with minimal risk of cotton injury. However, the application timing could warrant additional treatment to achieve adequate late-season weed control.

Sensitivity of difficult‐to‐control Palmer amaranth accessions to fluridone and diflufenican

AbstractPalmer amaranth (Amaranthus palmeri S. Watson) has evolved resistance to herbicides targeting nine sites of action. Phytoene desaturase‐inhibiting herbicides remain among the few sites of action that are still effective in controlling Palmer amaranth. Greenhouse experiments were conducted to evaluate the effectiveness of fluridone and diflufenican in controlling difficult‐to‐control Palmer amaranth accessions collected in Arkansas from 2016 to 2022, to quantify the response of three accessions that exhibited low sensitivity to both herbicides, and to evaluate the effectiveness of photosystem II inhibitors alone and with fluridone or diflufenican on three accessions. At 14 days after a preemergence application, Palmer amaranth control with fluridone and diflufenican ranged from 42% to 100% and 33% to 99% across 23 accessions, respectively. Three accessions required 10.2 to 26.7 times more fluridone than the susceptible Palmer amaranth standard based on LD50 values (where LD50 is the lethal dose to kill 50% of the population), with less than 50% mortality achieved with a 1x herbicide rate for two of the three accessions. For diflufenican, the LD50 values for the three accessions were 3.9–18.5 times greater than the susceptible standard. An additive response for mortality and biomass reduction resulted from adding fluometuron to fluridone at all rates tested. An additive or sometimes a synergistic response occurred with the combinations of metribuzin plus diflufenican. Overall, fluridone exhibited higher efficacy than diflufenican for most accessions tested. While phytoene desaturase‐inhibiting herbicides remain an effective control option for most Palmer amaranth accessions, herbicide mixtures targeting multiple sites of action remain essential for delaying resistance and obtaining effective control of this problematic weed.

Residual Weed Control in Cotton Utilizing Herbicide-Coated Fertilizer

Abstract An experiment was conducted in 2022 and 2023 near Rocky Mount and Clayton, NC, to evaluate residual herbicide-coated fertilizer for cotton tolerance and Palmer amaranth control. Treatments included acetochlor; atrazine; dimethenamid-P; diuron; flumioxazin; fluometuron; fluridone; fomesafen; linuron; metribuzin; pendimethalin; pyroxasulfone; pyroxasulfone + carfentrazone; S-metolachlor; and sulfentrazone. Each herbicide was individually coated on granular ammonium sulfate (AMS) and top-dressed at 321 kg ha-1 (67 kg N ha-1) onto 5- to 7-leaf cotton. The check received the equivalent rate of non-herbicide-treated AMS. Before top-dress, all plots (including the check) were treated with glyphosate and glufosinate to control previously emerged weeds. All herbicides resulted in transient cotton injury, except metribuzin. Cotton response to metribuzin varied by year and location. In 2022, metribuzin caused 11 to 39% and 8 to 17% injury at Clayton and Rocky Mount, respectively. In 2023, metribuzin caused 13 to 32% injury at Clayton and 73 to 84% injury at Rocky Mount. Pyroxasulfone (91%), pyroxasulfone + carfentrazone (89%), fomesafen (87%), fluridone (86%), flumioxazin (86%), and atrazine (85%) controlled Palmer amaranth ≥ 85%. Pendimethalin and fluometuron were the least effective treatments, resulting in 58% and 62% control, respectively. As anticipated, early season metribuzin injury translated into yield loss; plots treated with metribuzin yielded 640 kg ha-1 and were only comparable to linuron (790 kg ha-1). These findings research suggest, with the exception of metribuzin, residual herbicides coated on AMS may be suitable and effective in cotton production, providing growers with additional modes of action for late-season control of multiple herbicide-resistant Palmer amaranth.

Palmer amaranth (Amaranthus palmeri) Control in Furrow-Irrigated Rice with Fluridone

Abstract Herbicide-resistant Palmer amaranth is creating additional challenges for producers choosing to adopt a furrow-irrigated rice production system due to the absence of a sustained flood, enabling extended weed emergence. Fluridone has been shown to control Palmer amaranth effectively in cotton production systems and was recently registered for use in rice. Experiments were initiated in 2022 and 2023 to evaluate 1) Palmer amaranth control and rice tolerance to preemergence- and postemergence-applied fluridone at a 0.5x (84 g ai ha-1) and 1x rate (168 g ai ha-1) on a silt loam soil and 2) the effect of various herbicide programs containing fluridone on Palmer amaranth biomass, seed production, and rough rice grain yield. Preemergence applications of fluridone at a 1x rate in combination with clomazone resulted in 84% control of Palmer amaranth 21 d after treatment (DAT). Fluridone, in combination with clomazone preemergence, caused up to 36% rice injury 21 DAT; however, early season injury did not negatively affect rice yields. Palmer amaranth biomass and fecundity were reduced with herbicide programs that included fluridone plus florpyrauxifen-benzyl, and, in some instances, there was no Palmer amaranth biomass or seed production following multiple applications of both herbicides. Fluridone- and florpyrauxifen-benzyl-based herbicide programs achieved effective control of Palmer amaranth when applied timely, but injury to hybrid rice is enhanced with preemergence applications of fluridone that are not permitted with the current label.

Influence of Application Timing and Cultivar on Soybean Tolerance and Weed Control from Diflufenican or a Diflufenican-Containing Premixture

Abstract Bayer CropScience anticipates launching several premixtures for use in soybean, targeted at control of Palmer amaranth. One of the premixtures will contain diflufenican (WSSA Group 12), metribuzin (Group 5), and flufenacet (Group 15) (DFF-containing premixture), offering an alternative site of action for soybean producers. Field experiments were conducted in Arkansas and Michigan to evaluate application timings of the DFF-containing premixture for soybean tolerance and weed control and possible cultivar tolerance differences to diflufenican and the DFF-containing premixture. Soybean injury from the 1X and 2X rates of the DFF-containing premixture ranged from 0% to 60% 14 d after planting (DAP), with injury increasing the closer the herbicide was applied to soybean emergence. Excluding the 2X rate applied 3 DAP in Arkansas in 2023, soybean injury was <20% regardless of location, site year, application timing, and rate. For weed control experiments, only a 1X rate of the DFF-containing premixture was applied at the various application timings. Control of five weed species, encompassing broadleaves and grasses, ranged from 81% to 98%, regardless of application timing by 28 DAP. By 42 DAP, weed control ranged from 71% to 97%, with the 14-d preplant application timing typically being the least effective. The DFF-containing premixture and diflufenican alone were applied PRE at 1X and 2X rates for the soybean cultivar study. Soybean metribuzin sensitivity did not affect the degree of crop response, even on a high pH soil, and injury to soybean never exceeded 20%. Overall, the DFF-containing premixture will be a tool that soybean producers can integrate into a season-long herbicide program for use across the United States regardless of the soybean cultivar.

Evaluation of residual palmer amaranth control with herbicides coated on fertilizer

Evaluation of residual palmer amaranth control with herbicides coated on fertilizer

Crop safety and control of acetolactate synthase inhibitor-resistant Palmer amaranth (Amaranthus palmeri) with very long-chain fatty acid-inhibiting herbicides in dry edible bean

Palmer amaranth is a relatively recent arrival in Western Nebraska, where acetolactate synthase (ALS)-inhibitor-resistant biotypes are common in the region. With limited effective postemergence (POST) herbicides for controlling ALS-inhibitor-resistant Palmer amaranth in dry edible bean, a sequential preemergence (PRE) followed by (fb) POST program of very long-chain fatty acid (VLCFA)-inhibiting herbicides shows promise. Currently, dimethenamid-P is the only VLCFA-inhibiting herbicide registered for POST use in dry edible bean in Nebraska. The objective of this study was to assess the crop safety and effectiveness in weed control of sequential PRE fb POST programs, including pendimethalin + dimethenamid-P applied PRE fb dimethenamid-P POST, pendimethalin + S-metolachlor PRE fb S-metolachlor POST, and pendimethalin + pyroxasulfone PRE fb POST, in comparison with pendimethalin + dimethenamid-P applied PRE fb imazamox + bentazon + fomesafen applied POST in dry edible bean. Results showed that sequential PRE fb POST programs were more effective in reducing both the density and biomass of Palmer amaranth compared to PRE-alone programs. Pendimethalin + dimethenamid-P applied PRE fb dimethenamid-P POST, along with pendimethalin + S-metolachlor PRE fb S-metolachlor POST, resulted in over 85% control of Palmer amaranth, similar to the effectiveness of pendimethalin + dimethenamid-P PRE fb fomesafen + imazamox + bentazon applied POST. Pendimethalin + pyroxasulfone applied PRE-alone and the application of pendimethalin + pyroxasulfone PRE fb pyroxasulfone POST showed inconsistent control of Palmer amaranth, causing high crop injury, stand loss, and delayed maturity, ultimately leading to yield loss. Dimethenamid-P and S-metolachlor demonstrated excellent crop safety when applied either PRE-only or sequentially. Pendimethalin + S-metolachlor PRE fb S-metolachlor POST provided control of Palmer amaranth comparable to that achieved with POST applications of dimethenamid-P and imazamox + bentazon + fomesafen.

Integrating fall‐planted cereal rye cover crop with herbicides for reducing Palmer amaranth seed production in soybean under planting green conditions

AbstractCover crops are usually terminated prior to planting the cash crop; however, “planting green” is an alternative approach that allows growers to plant cash crop into an actively growing, green cover crop, which is then terminated after the establishment of the cash crop. The objectives of this study were (1) to determine whether planting soybean (Glycine max L. Merr.) into a standing cereal rye (Secale cereale L.) cover crop provides superior weed suppression compared to terminating cereal rye 2 weeks before soybean planting and (2) to evaluate an integrated effect of herbicide programs and cereal rye termination timing on Palmer amaranth (Amaranthus palmeri S. Watson) control, biomass, seed production, soybean grain yield, and benefit/cost ratio. Field experiments were conducted in southcentral Nebraska from 2020 to 2022. Preemergence (PRE) herbicide with 2 weeks after planting (WAP) termination of cereal rye provided >95% Palmer amaranth control in 2021 and varied from 88% to 98% in 2022 at 28 days after PRE. A PRE herbicide followed by (fb) late‐postemergence (LPOST) herbicide with 2 WAP termination of cereal rye controlled Palmer amaranth 85%–92% in 2021 compared with 97%–99% control 28 days after LPOST herbicide application in 2022. Palmer amaranth density was higher with 2 WBP cereal rye termination compared with 2 WAP termination regardless of the herbicide program. PRE fb LPOST herbicide programs integrated with 2 WAP termination of cereal rye reduced Palmer amaranth seed production to less than 9100 seeds plant−1 in 2021 and no seed production in 2022. In 2021, terminating cereal rye 2 WAP played an integral role in controlling and reducing the density of Palmer amaranth; however, it had noticeable impact on soybean yield compared to terminating 2 WBP. In 2022, hail and windstorm had a confounding effect on soybean stand and yield.

Weed control and rice response to microencapsulated acetochlor and a fenclorim seed treatment on a clay soil

AbstractRecent research has demonstrated the ability of a fenclorim seed treatment to reduce rice (Oryza sativa L.) injury to acetochlor. However, all studies were conducted on silt loam soils and have not evaluated rice tolerance or weed control on clay soils. Experiments were initiated in 2021 and 2022 at the Northeast Research and Extension Center near Keiser, AR, to determine rice response and the effectiveness of delayed‐preemergence (DPRE)‐applied microencapsulated (ME) acetochlor (1.1, 1.7, and 2.3 lb ai acre−1) when applied to a clay soil with and without a fenclorim seed treatment at 0 or 2.5 lb ai 1000‐lb−1 of seed. Averaged over the fenclorim seed treatment, acetochlor at 1.1 and 1.7 lb ai acre−1 caused similar injury levels to rice; however, barnyardgrass [Echinochloa crus‐galli (L.) P. Beauv.] control increased at 1.7 lb ai acre−1, eliciting 19% injury to rice and 82% barnyardgrass control at 28 days after emergence (DAE). Palmer amaranth (Amaranthus palmeri S. Watson) control ranged from 82% to 93%. Additionally, fenclorim did not influence barnyardgrass or Palmer amaranth control, but it did reduce rice injury and increase shoot density, plant height, and rough rice grain yield. At 14 and 28 DAE, fenclorim at 2.5 lb ai 1000‐lb−1 of seed (averaged over acetochlor rates) reduced visible rice injury from 61% to 13% and 40% to 8%, respectively. Results from this study indicate ME acetochlor could be successfully applied to rice grown on a clay soil when a fenclorim seed treatment is used, providing producers a new site of action for use in U.S. rice production.

Multiple resistance to EPSPS and ALS inhibitors in Palmer amaranth (Amaranthus palmeri) identified in Turkey

AbstractAmaranthus palmeri was first reported in Turkey in 2016, and an immediate heavy infestation of the weed was found in fruit orchards and summer crops such as maize, cotton, and sunflower. There have been farmers' complaints about the ineffective control of Palmer amaranth through the use of glyphosate and some sulfonylureas herbicides. Hence, this study aimed to determine the possible herbicide resistance evolution in Palmer amaranth against glyphosate and acetolactate synthase (ALS) herbicides. Seeds of 21 Palmer amaranth populations were collected from five provinces of Turkey where control problems with glyphosate and ALS inhibitors were reported in maize fields. Seeds of certain biotypes categorized as resistant or susceptible were grown to obtain the F2 generation. A single‐dose experiment determined the possible resistance to ALS inhibitors and glyphosate among the 21 populations. Of this, 18 populations were included in the subsequent dose–response experiments due to evident survival. Based on ED50 values from the dose–response experiment, SNU‐04 and ADN‐21 biotypes had the highest resistance index for glyphosate which was more than 7. The biotypes ADN‐21, OSM‐15, and DIR‐09 recorded the highest ED50 value with a resistance index of 9.21–10.35 after nicosulfuron application. Whereas, the biotypes SNU‐04, OSM‐15, and ADN‐21 were with the highest ED50 value and resistance index of 6.41–7.44, after the application of foramsulfuron + iodosulfuron methyl‐sodium. The increase in genomic 5‐enolpyruvylshikimate‐3‐phosphate synthase (EPSPS) copy number has been observed in suspected cases that have been accepted as the molecular basis for the development of resistance against glyphosate. The sequence alignment results for the ALS gene contained Ala122Val and Pro197Arg mutations related to target‐site resistance against ALS herbicides.

Comparison of residual activity of pre‐emergence herbicides for control and seed production of multiple herbicide‐resistant Palmer amaranth in food‐grade white corn

AbstractNebraska is the number‐one producer of food‐grade white corn (Zea mays L.) in the United States. Food‐grade white corn has not been genetically engineered; therefore, non‐selective herbicides such as glyphosate or glufosinate cannot be used. Multiple herbicide‐resistant (MHR) Palmer amaranth (Amaranthus palmeri S. Watson) populations have been reported in multiple counties in Nebraska, and their management is a challenge, particularly for white corn producers. The objectives of this study were to evaluate the residual activity of pre‐emergence (PRE) herbicides for acetolactate synthase inhibitor (halosulfuron‐methyl)/atrazine/glyphosate‐resistant Palmer amaranth control, density, biomass, seed production, and grain yield in food‐grade white corn. Field experiments were conducted during summer 2020 and 2021 in a grower's field infested with MHR Palmer amaranth near Carleton, NE. All the PRE herbicides resulted in similar control (>90%) 30 days after PRE application (DAPRE) apart from atrazine (64%). At 45 DAPRE, acetochlor/mesotrione, atrazine/bicyclopyrone/mesotrione/S‐metolachlor, and acetochlor/clopyralid/mesotrione controlled 90%–95% Palmer amaranth. Acetochlor/clopyralid/mesotrione and atrazine/bicyclopyrone/mesotrione/S‐metolachlor provided 96%–99% MHR Palmer amaranth control and reduced Palmer amaranth density and biomass to 2–4 plants m−2 and 5–12 g m−2 60 DAPRE. The highest corn yields of 12,139 kg ha−1 and 12,093 kg ha−1 in 2020 and 2021, respectively, were obtained with acetochlor/clopyralid/mesotrione. Palmer amaranth seed production was least with acetochlor/clopyralid/mesotrione (32,894 seeds m−2). Tested residual PRE herbicides did not show corn injury and were safe to use in food‐grade white corn. It is concluded that acetochlor/clopyralid/mesotrione and atrazine/bicyclopyone/mesotrione/S‐metolachlor are the best options for early season control of MHR Palmer amaranth in food‐grade white corn.

Spray interval, application order, and plant height influences control of dicamba-resistant Palmer amaranth

Abstract Palmer amaranth, which is resistant to glyphosate and protoporphyrinogen oxidase inhibitors, remains a threat to cotton and soybean production in Tennessee. This is partly due to the recent evolution of dicamba-resistant Palmer amaranth in western Tennessee, which further complicates weed management. Experiments were conducted in 2021 and 2022 to determine the best timing between sequential applications and the order in which 2,4-D or dicamba should be used with glufosinate to control resistant Palmer amaranth. Palmer amaranth control increased when the interval between postemergence herbicide applications decreased from 21 to 7 d. At the 7-d interval in a dicamba-based system, the order of herbicides did not affect Palmer amaranth control. However, in a 2,4-D-based system, the greatest control was achieved when 2,4-D was applied first, followed by either 2,4-D or glufosinate. While weed height at the time of application had a significant effect on Palmer amaranth control with auxin herbicides, control was still unacceptable in the field at the labeled rates of dicamba or 2,4-D when applied to weeds that were <10 cm tall (48% and 53%, respectively). Neither dicamba nor 2,4-D provided acceptable control of the Palmer amaranth populations evaluated in this study. Sequential applications separated by 7 d provided better weed control than those separated by 21 d. Given that the better 7-d sequential treatments provided less than 90% control and resulted in more than 64,000 surviving Palmer amaranth plants per hectare suggests that relying solely on these herbicides for Palmer amaranth control is not a sustainable weed management strategy.

Response of stevia to reduced-risk synthetic and nonsynthetic herbicides applied post-transplant

Abstract Greenhouse trials were conducted to determine the response of stevia to reduced-risk synthetic and nonsynthetic herbicides applied over-the-top post-transplant. In addition, field trials were conducted with stevia grown in a polyethylene mulch production system to determine crop response and weed control in planting holes to reduced-risk synthetic and nonsynthetic herbicides applied post-transplant directed. Treatments included caprylic acid plus capric acid, clove oil plus cinnamon oil, d-limonene, acetic acid (200 grain), citric acid, pelargonic acid, eugenol, ammonium nonanoate, and ammoniated soap of fatty acids. Stevia yield (dry aboveground biomass) in the greenhouse was reduced by all herbicide treatments. Citric acid and clove oil plus cinnamon oil were the least injurious, reducing yield by 16% to 20%, respectively. In field studies, d-limonene, pelargonic acid, ammonium nonanoate, and ammoniated soap of fatty acids controlled Palmer amaranth (>90% 1 wk after treatment (WAT). In field studies caprylic acid plus capric acid, pelargonic acid, and ammonium nonanoate caused >30% injury to stevia plants at 2 WAT, and d-limonene, citric acid, acetic acid, and ammoniated soap of fatty acids caused 18% to 25% injury 2 WAT. Clove oil plus cinnamon oil and eugenol caused <10% injury. Despite being injurious, herbicides applied in the field did not reduce yield compared to the nontreated check. Based upon yield data, these herbicides have potential for use in stevia; however, these products could delay harvest if applied to established stevia. In particular, clove oil plus cinnamon oil has potential for use for early-season weed management for organic production systems. The application of clove oil plus cinnamon oil over-the-top resulted in <10% injury 28 d after treatment (DAT) in the greenhouse and 3% injury 6 WAT postemergence-directed in the field. In addition, this treatment provided 95% control of Palmer amaranth 4 WAT.

Effect of herbicide programs on control and seed production of multiple herbicide–resistant Palmer amaranth (Amaranthus palmeri) in corn resistant to 2,4-D/glufosinate/glyphosate

Abstract Multiple herbicide–resistant (MHR) Palmer amaranth is among the most problematic summer annual broadleaf weeds in Nebraska and several other states. A new MHR corn cultivar (resistant to 2,4-D/glufosinate/glyphosate, also known as Enlist corn) has been commercially available in the United States since 2018. Growers are searching for herbicide programs for control and reduce seed production of MHR Palmer amaranth among Enlist corn crops. The objectives of this study were to evaluate herbicides applied preemergence, early postemergence, or preemergence followed by (fb) late postemergence for the management of MHR Palmer amaranth in Enlist corn fields and to assess their effect on Palmer amaranth biomass, density, seed production, and corn yield. Field experiments were conducted near Carleton, NE, in 2020 and 2021, in a grower’s field of Enlist corn infested with acetolactate synthase–inhibitor/atrazine/glyphosate–resistant Palmer amaranth. Herbicides applied preemergence, such as flufenacet/isoxaflutole/thiencarbazone-methyl, acetochlor/clopyralid/flumetsulam, or acetochlor/clopyralid/mesotrione, provided 75% to 99% control of Palmer amaranth 30 d after preemergence. Preemergence fb late postemergence herbicides resulted in 94% Palmer amaranth control 90 d after late postemergence, reduced weed density to 0 to 8 plants m−2 30 d after late postemergence, and reduced biomass to 2 to 14 g m−2 15 d after late postemergence compared to preemergence-only (59% control, 0 to 15 plants m−2, and 4 to 123 g m−2) and early postemergence–only herbicides (78% control, 6 to 30 plants m−2, and 8 to 25 g m−2). Based on contrast analysis, Palmer amaranth seed production was reduced to 14,050 seeds m–2 in preemergence fb late postemergence herbicide programs compared with 325,490 seed m–2 in preemergence-only and 376,750 seed m–2 in early postemergence–only programs. Based on orthogonal contrast, higher corn yield of 12,340 and 11,730 kg ha−1 was obtained with preemergence fb late postemergence herbicide programs compared with preemergence-only (10,840 and 11,510 kg ha−1) and early postemergence–only programs (10,850 and 10,030 kg ha−1) in 2020 and 2021, respectively.

Efficacy of dicamba and dicamba/tembotrione with and without ammonium sulfate for broadleaf weed control

Abstract Mixing ammonium sulfate (AMS) can increase dicamba volatility. Therefore, AMS cannot be used with dicamba products in dicamba-resistant soybean. However, most dicamba products applied in corn are labeled to mix with AMS. The objectives of this study were to evaluate broadleaf weed control with dicamba (DiFlexx®) and dicamba/tembotrione (DiFlexx® DUO) applied alone or with AMS or AMS substitute and their effect on broadleaf weed density and biomass. Field experiments were conducted in Illinois, Missouri, and Nebraska in 2018 and 2019. In Illinois and Nebraska, mixing AMS + crop oil concentrate (COC) with dicamba applied at 1,120 g ae ha−1 increased the control of Palmer amaranth and waterhemp (Amaranthus species) from 78% to 92% and velvetleaf from 73% to 96% compared with dicamba applied alone 14 d after application (DAA); however, Missouri data showed no difference. Mixing AMS + COC with dicamba/tembotrione at 597 and 746 g ai ha−1 did not improve broadleaf weed control 14 DAA at any site compared with dicamba/tembotrione applied alone. Control of Amaranthus species was increased from 82% with dicamba applied at 840 g ae ha−1 to 96% when mixed with AMS + COC 28 DAA in Illinois; however, control was similar to dicamba applied at 1,120 g ae ha−1. Broadleaf weed control did not differ among dicamba or dicamba/tembotrione 28 and 56 DAA in Missouri and Nebraska. Broadleaf weed density decreased from 64 plants m−2 to 24 plants m−2 with dicamba at 1,120 g ae ha−1 with AMS + COC 14 DAA in Nebraska; however, no differences were observed in broadleaf weed density or biomass 56 DAA in any state. The results suggest that dicamba or dicamba/tembotrione can be applied without AMS or AMS substitute, especially at higher rates, without losing broadleaf weed control efficacy.

Growth and fecundity of Palmer amaranth escaping glufosinate in cotton with and without grass competition

Abstract Field experiments were conducted at Clayton and Rocky Mount, NC, during summer 2020 to determine the growth and fecundity of Palmer amaranth plants that survived glufosinate with and without grass competition in cotton. Glufosinate (590 g ai ha−1) was applied to Palmer amaranth early postemergence (5 cm tall), mid-postemergence (7 to 10 cm tall), and late postemergence (>10 cm tall) and at orthogonal combinations of those timings. Nontreated Palmer amaranth was grown in weedy, weed-free in-crop (WFIC) and weed-free fallow (WFNC) conditions for comparisons. Palmer amaranth control decreased as larger plants were treated; no plants survived the sequential glufosinate applications in both experiments. The apical and circumferential growth of Palmer amaranth surviving glufosinate treatments was reduced by more than 44% compared to the WFIC and WFNC Palmer amaranth in both experiments. The biomass of Palmer amaranth plants surviving glufosinate was reduced by more than 62% when compared with the WFIC and WFNC in all experiments. The fecundity of Palmer amaranth surviving glufosinate treatments was reduced by more than 73% compared to WFNC Palmer amaranth in all experiments. Remarkably, the plants that survived glufosinate were fecund as WFIC plants only in the Grass Competition experiment. The results prove that despite decreased vegetative growth of Palmer amaranth surviving glufosinate treatment, plants remain fecund and can be fecund as nontreated plants in cotton. These results suggest that a glufosinate-treated grass weed may not have a significant interspecific competition effect on Palmer amaranth that survives glufosinate. Glufosinate should be applied to 5 to 7 cm Palmer amaranth to cease vegetative and reproductive capacities.

Palmer amaranth (Amaranthus palmeri) control affected by weed size and herbicide spray solution with nozzle type pairings

Abstract Palmer amaranth can grow 4.2 mm in height per degree day; hence, delays of a few days in weed control deployment can result in applications of herbicides to weeds that are larger than those for which the herbicide label recommends. Therefore, it is critically necessary to understand the effect of plant size at the time of herbicide application in conjunction with herbicide spray solution and nozzle type pairings on the effectiveness of weed management programs in the Enlist E3 and XtendFlex production systems. Field experiments were conducted in 2020, in no-crop conditions, at two locations in Arkansas, to evaluate the influence of Palmer amaranth size on its control with glufosinate, dicamba, and 2,4-D applied alone and in mixture with specific nozzle pairings as mandated by label requirements. Also, a laboratory experiment was conducted to evaluate the droplet size and velocity of the spray solutions and nozzles used in the field experiments. A 5- and 10-percentage point reduction in control was observed when dicamba (66%) and 2,4-D (63%) were applied alone, respectively, compared with those herbicides mixed with glufosinate (71% and 73%, respectively). Palmer amaranth density increased to 55, 73, 100, 115, and 140 plants m−2 when plants were sprayed at heights of 15, 25, 41, 61, and 76 cm, respectively, compared with plants that were sprayed when they were 5 cm tall (9 plants m−2). Nozzle type did not affect weed control or density. The percentage of driftable fines increased when a mixture of glufosinate and 2,4-D were used compared with 2,4-D alone. Effective short-term and long-term chemical control of Palmer amaranth will require growers to correctly time their weed management practices and overlay residuals, and expect the need for sequential applications.

Effect of plant height on control of multiple herbicide-resistant Palmer amaranth (Amaranthus palmeri) in glufosinate/glyphosate-resistant corn

Multiple herbicide-resistant (MHR) Palmer amaranth is a troublesome weed in several crops across the USA, including corn. Due to unavoidable weather conditions, it is sometimes not possible for growers to apply pre-emergence herbicide; therefore, post-emergence (POST) herbicide is needed for effective control of MHR Palmer amaranth. The objectives of this study were to evaluate the effect of POST herbicides applied at two heights (10-15 cm and 20-30 cm) for MHR Palmer amaranth control and their effect on Palmer amaranth biomass, density, and seed production as well as yield of glufosinate/glyphosate-resistant corn. Field experiments were conducted at a grower’s field near Carleton, Nebraska, USA in 2020 and 2021. Control of MHR Palmer amaranth was affected by the plant height when herbicides were applied. Glufosinate, dicamba, dicamba/diflufenzopyr, and dicamba/tembotrione applied to 10-15 cm tall Palmer amaranth provided ≥ 94% control 30 d after EPOST (DAEPOST), whereas atrazine/bicyclopyone/mesotrione/S-metolachlor applied to 20-30 cm tall MHR Palmer amaranth provided 85% control in 2021. Glufosinate provided 85% to 90% control when applied to 20-30 cm tall Palmer amaranth in both years. At 90 DALPOST, dicamba, dicamba/diflufenzopyr, and dicamba/tembotrione applied to 10-15 cm tall Palmer amaranth provided ≥ 88% control. Dicamba/tembotrione, atrazine/bicyclopyone/mesotrione/S–metolachlor, and dicamba applied to 20-30 cm tall Palmer amaranth provided 85% to 92% control. Glufosinate, dicamba, and atrazine/bicyclopyone/mesotrione/S–metolachlor were the most effective for reducing Palmer amaranth density 2 to 19 plants m−2 when applied to 10-15 cm Palmer amaranth 30 DAEPOST compared with the nontreated control (137 plants m−2) in 2021; however, when applied to 20-30 cm Palmer amaranth, glufosinate, and atrazine/bicyclopyone/mesotrione/S–metolachlor reduced density 5 to 19 plants m−2. At 30 DAEPOST, glufosinate and atrazine/bicyclopyone/mesotrione/S–metolachlor had the lowest Palmer amaranth biomass (3-17 g m−2). Corn yield in 2020 was higher than 2021 due to more rain in 2020. All herbicides resulted in a similar yield in 2020. Lower seed production of 6,269 and 1,953 seeds plant-1 for 10-15 cm and 20-30 cm MHR Palmer amaranth were recorded with dicamba and atrazine/bicyclopyone/mesotrione/S–metolachlor.

Palmer Amaranth (Amaranthus palmeri) Control in Dicamba-Tolerant Cotton (Gossypium hirsutum) when Applied with Various Dicamba + Insecticide Tank-Mixtures

Postemergence herbicide application timings for Palmer amaranth (Amaranthus palmeri) often coincide with foliar insecticide applications in cotton (Gossypium hirsutum). Combining multiple pesticides into a single application increases efficiency while reducing overall application cost. Multiple foliar insecticides are now labeled for dicamba tank-mixes. However, there is limited literature with respect to efficacy of dicamba on Palmer amaranth control when applied with insecticides. Field experiments were conducted from 2018 to 2020 to evaluate the effect of carrier volume and insecticide on the efficacy of dicamba (XtendiMaxTM with VaporGripTM) to control Palmer amaranth in XtendFlex™ cotton production systems. Dicamba was tank-mixed with acephate and dimethoate to four-leaf cotton and with thiamethoxam and sulfoxaflor just prior to bloom. Applications were made at carrier volumes of 140 and 280 L ha-1 and with a single spray droplet size of 800 µm. Palmer amaranth control 7 d after treatment was negatively impacted when applications included dimethoate; however, no corresponding response in seedcotton yield was observed relative to dicamba applied alone. However, when pooled over carrier volume, applications containing dicamba + acephate resulted in higher seedcotton yield than applications of dicamba + dimethoate. Additionally, pre-bloom applications of dicamba + thiamethoxam or sulfoxaflor increased Palmer amaranth efficacy relative to dicamba applied alone. Although certain insecticides are known to increase herbicidal crop response, the impact of insecticides on herbicide efficacy to control specific weed species is largely unknown. Therefore, we conclude that multiple dicamba + acephate, thiamethoxam, or sulfoxaflor tank-mixtures provide similar control of Palmer amaranth compared to dicamba alone.

Palmer amaranth identification using hyperspectral imaging and machine learning technologies in soybean field

Weed infestation and their management are a critical production challenge in agricultural fields. Palmer amaranth has created management challenges because it has multiple emergence pattern and has evolved resistant to nine unique herbicide sites of action. Effective Palmer amaranth detection and positive identification in field conditions will help to improve Palmer amaranth control. A field-based hyperspectral imaging system was developed to record Palmer amaranth in soybean fields. The data were pre-processed applying Savitzky-Golay 2nd derivative, Multiplicative Scatter Correction, and Standard Normal Variate in a forward feed manner. Recursive feature elimination, SelectFromModel, sequential forward selection, and backward elimination were used to select significant wavebands from the available 224 bands. Later, supervised machine-learning models were generated to classify soybean and Palmer amaranth using the selected wavebands. Matthew’s correlation coefficient (MCC), F1 score, precision, and recall were considered as the most significant parameters to evaluate the models’ performance. The highest result was obtained by quadratic discriminant analysis with a prediction accuracy of 93.95%, a precision of 90.30%, a recall of 90.29%, an F1 score of 0.95, and an MCC score of 0.85. The findings of this study showed that the combination of hyperspectral imaging and machine-learning is a potential technique for real-time weed detection in the open field condition.