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Off-target movement of 2,4-D and dicamba are sometimes to blame as the cause of symptoms observed in weeds growing in production fields. Pesticide regulatory authorities routinely sample tissue of weeds or crops from fields under investigation for potential illegal use of auxin herbicides. This research aimed to determine if analytical tests of herbicide residue on soybean or Palmer amaranth vegetation treated with dicamba or 2,4-D could be used to differentiate between rates applied and how the residue levels decay over a one-month interval. Four rates of each herbicide (1X, 0.1X, 0.01X, and 0.001X) were applied, with a 1X rate of dicamba and 2,4-D assumed to be 560 and 1065 g ae ha-1, respectively. Experiments included dicamba- and 2,4-D-resistant soybean (Xtend and Enlist traits, respectively) and Palmer amaranth categorized by size (8-15 cm, 20-30 cm, and 35-50 cm in height). Analytical results showed that herbicide residues were detected above detection limits of 0.04 µg g-1 for dicamba and 0.004 µg g-1 for 2,4-D, respectively, particularly for samples treated with a 1X and 0.1X rate of dicamba or 2,4-D. Non-detections were frequent, even as early as 2 to 3 days after treatment (DAT), with 0.01X and 0.001X rates of 2,4-D or dicamba. Dicamba residues declined rapidly on Xtend soybean treated with dicamba, and 2,4-D residue in Enlist soybean. The severity of auxin symptomology generally agreed with the ability to detect dicamba or 2,4-D residue in plant tissue for Palmer amaranth, while, for soybean, this was not always the case. Hence, detecting dicamba or 2,4-D residues in both Palmer amaranth and soybean vegetation, along with visible symptoms on both plants during investigations, would generally indicate an earlier direct application of the auxin herbicide rather than off-target movement being the cause of detection.
Several Echinochloa P. Beauv. species, introduced at multiple events, have established themselves as a persistent concern for US rice production. In this review, we highlight the key biological characteristics of economically relevant Echinochloa in US rice, revisit their historical trajectory, and put forward research directions for their management with special reference to barnyardgrass. Ecologically-differentiated Echinochloa species have a distinct association with rice culture methods that have been practiced in a region, barnyardgrass being historically predominant in drill-seeded rice in the mid-South, and early watergrass and late watergrass in water-seeded California rice for the last few decades. However, the emerging evidence challenges the dogma that other Echinochloa species for the specific regions are of less importance. Primarily managed by the water-seeding method of rice culture in the early years, Echinochloa species have persisted in the sophisticated US rice culture through the evolution of resistance to herbicides in the later course. Accumulating knowledge, including those of recent genomic insights, suggests the rapid adaptability of Echinochloa. The last decade has seen a (re)emergence of non-chemical methods as a key component of a sustainable management approach, among which use of harvest weed seed control (HWSC) methods and cover crops in the mid-South and stale-drill seeding in California are being considered as potential tools for management of Echinochloa. In recent years, furrow-irrigated rice has rapidly supplanted a significant proportion of conventionally flooded rice in the mid-South, whereas the propensity for compromised continuous submergence is increasing in California rice. On the cusp of this shift, the question at the forefront is how this will affect Echinochloa interference in rice and how this change will dictate the management efforts. Future research will develop a clear understanding of the impact of the changing agroecosystems on Echinochloa species as well as their response to the prospective integrated control interventions.
Postemergence selective monocot control in grain sorghum is an issue due to the limited number of herbicides available. The herbicides currently labeled in grain sorghum have strict use restrictions, low efficacy on johnsongrass, or weed resistance issues. To introduce a new effective herbicide mode of action for monocot control, multiple companies and universities have been developing herbicide-resistant grain sorghum that would allow producers to utilize either acetolactate synthase (ALS) or acetyl coenzyme A carboxylase (ACCase) inhibitors for postemergence monocot control. An experiment was conducted in Fayetteville, AR, in 2020 and 2021 to determine the effectiveness of two ALS-inhibiting herbicides and nine ACCase-inhibiting herbicides on TamArkTM grain sorghum, conventional grain sorghum, and problematic monocot weed species. Grain sorghum and monocot weeds (johnsongrass, broadleaf signalgrass, barnyardgrass, and Texas panicum) were sprayed when TamArkTM grain sorghum reached the 2- to 3-leaf stage. TamArkTM grain sorghum was tolorant to all ACCase-inhibiting herbicides tested exhibiting ≤10% injury at all evaluation timings, except clethodim and sethoxydim, and had no resistance to the ALS-inhibiting herbicides evaluated. Additionally, all ACCase inhibitors except diclofop and pinoxaden controlled all monocots tested >91% by 28 days after application (DAA). Conversely, the two ALS inhibitors, imazamox and nicosulfuron had ≤81% control of broadleaf signalgrass 28 DAA but still controlled all other monocots >95%. TamArkTM grain sorghum 'has low sensitivity to multiple ACCase-inhibiting herbicides thus providing an effective POST option for monocot weed control and unwanted volunteer TamArkTM plants can be controlled with cledthodim, sethoxydim, nicosulfuron, or imazamox has low sensitivity to multiple ACCase-inhibiting herbicides. Imazamox and nicosulfuron, both ALS-inhibiting herbicides, while not useful on TamArkTM grain sorghum, are effective options for monocot control in IgrowthTM and InzenTM grain sorghum, respectively.
Genetic similarities between johnsongrass and grain sorghum leave producers with limited herbicide options for postemergence johnsongrass control. TamArkTM grain sorghum with resistance to acetyl CoA carboxylase-inhibiting herbicides was developed through a collaboration between the University of Arkansas System Division of Agriculture and Texas A&M AgriLife Research. Two field experiments were conducted in 2021 in two locations each Keiser and Marianna, AR or Fayetteville and Marianna, AR. The objective of the first was to determine the optimal rate and application timing of fluazifop-butyl for control of natural johnsongrass populations in a non-crop setting, and of the second was to evaluate johnsongrass control and TamArkTM grain sorghum tolerance in response to fluazifop-butyl applied at different timings and rates based on crop growth stage. The highest levels of johnsongrass control occurred when sequential applications of fluazifop-butyl were utilized. All sequential treatments provided at least 80% johnsongrass control at any rate or application timing tested. A single application of fluazifop-butyl provided greater than 90% johnsongrass control when applied at 210 g ai ha-1 to johnsongrass with less than 6 leaves. Weed size played a role in achieving high levels of johnsongrass control. Greater than 90% control was achieved when johnsongrass had 6-leaves or less at the initial application for the sequential application treatments. A single application of fluazifop-butyl at 105 g ai ha-1 resulted in no more than 82% johnsongrass mortality at any application timing. TamArk TM grain sorghum injury did not exceed 6% at any application timing or rate. It was, therefore, considered to be safe even if the initial application was made before the 6-leaf crop stage. Since no unacceptable levels of injury were observed with TamArkTM grain sorghum for fluazifop-butyl, johnsongrass size at the time of application should be the most critical aspect for control with this herbicide.
Damage to non–dicamba resistant (non-DR) soybean [Glycine max (L.) Merr.] has been frequent in geographies where dicamba-resistant (DR) soybean and cotton (Gossypium hirsutum L.) have been grown and sprayed with the herbicide in recent years. Off-target movement field trials were conducted in northwest Arkansas to determine the relationship between dicamba concentration in the air and the extent of symptomology on non-DR soybean. Additionally, the frequency and concentration of dicamba in air samples at two locations in eastern Arkansas and environmental conditions that impacted the detection of the herbicide in air samples were evaluated. Treatment applications included dicamba at 560 g ae ha−1 (1X rate), glyphosate at 860 g ae ha−1, and particle drift retardant at 1% v/v applied to 0.37-ha fields with varying degrees of vegetation. The relationship between dicamba concentration in air samples and non-DR soybean response to the herbicide was more predictive with visible injury (generalized R2 = 0.82) than height reduction (generalized R2 = 0.43). The predicted dicamba air concentration resulting in 10% injury to soybean was 1.60 ng m−3 d−1 for a single exposure. The predicted concentration from a single exposure to dicamba resulting in a 10% height reduction was 3.78 ng m−3 d−1. Dicamba was frequently detected in eastern Arkansas, and daily detections above 1.60 ng m−3 occurred 17 times in the period sampled. The maximum concentration of dicamba recorded was 7.96 ng m−3 d−1, while dicamba concentrations at Marianna and Keiser, AR, were ≥1 ng m−3 d−1 in six samples collected in 2020 and 22 samples in 2021. Dicamba was detected consistently in air samples collected, indicating high usage in the region and the potential for soybean damage over an extended period. More research is needed to quantify the plant absorption rate of volatile dicamba and to evaluate the impact of multiple exposures of gaseous dicamba on non-targeted plant species.
Allowing the use of two additional modes of action (MOAs), Enlist™ corn is a novelty in the continuum of herbicide-resistant crop development efforts that have occurred since the 1990s. Knowledge of Enlist corn tolerance to labeled herbicides and other herbicides within the same MOA for various use and/or exposure scenarios is not well established. Four site-year field experiments for preemergence (PRE) and postemergence (POST) applications were conducted at sites in Fayetteville (2021 and 2022) and Tillar (2020 and 2021), Arkansas, to evaluate Enlist corn response following PRE or POST applications of synthetic auxin herbicides or those that inhibit acetyl-CoA carboxylase (ACCase). A non-Enlist and an Enlist corn hybrid were used for each herbicide treatment to establish differential tolerance. Injury response to PRE application varied among site-years; clethodim was the only herbicide that occasionally caused significant (7% to 17%) injury to Enlist corn. None of the PRE treatments affected plant height, stand, or yield of Enlist corn; these responses were generally similar or better for Enlist corn compared to non-Enlist corn. Enlist corn showed significant injury to POST applications of florpyrauxifen-benzyl (>10%), fluazifop-P-butyl and quizalofop-P-ethyl (>5%), and clethodim and sethoxydim (>75%) 1 wk after application (WAA). These initial injury responses to clethodim and sethoxydim were generally reflected in Enlist corn yield; however, the minimal injury from fluazifop-P-butyl and quizalofop-P-ethyl did not affect yield. Injury to non-Enlist corn with POST-applied ACCase-inhibiting herbicides 2 WAA was >80%, resulting in a proportionate yield reduction. Even though florpyrauxifen-benzyl caused more initial injury to non-Enlist corn, yield reduction in non-Enlist corn was occasionally less than of Enlist corn, with both hybrids experiencing >75% yield reduction. In summary, Enlist corn may occasionally show transient injury even to labeled herbicides when applied POST, and even though the injury from florpyrauxifen-benzyl is initially mild, it nonetheless results in substantial yield loss.
Rice producers in the United States need effective herbicides to control problematic weeds. Previous research has demonstrated that acetochlor can provide in-season weed control in rice; however, undesirable injury is common. Thus, trials were initiated in 2020 and 2021 to evaluate 1) rice cultivar tolerance to microencapsulated (ME) acetochlor with the use of a fenclorim seed treatment at 2.5 g ai kg−1 of seed; 2) a dose-response of a fenclorim seed treatment with ME acetochlor; and 3) rice tolerance to fenclorim and ME acetochlor under cool, wet conditions. For all trials, acetochlor was applied delayed-preemergence (4 to 7 d after planting). In the dose-response trials and in the presence of acetochlor, the fenclorim seed treatment rate of 2.5 g ai kg−1 reduced rice injury and increased rice plant heights and shoot numbers relative to acetochlor without fenclorim, and plant heights and shoot numbers were comparable to those of the nontreated control in all evaluations. In the cultivar screening, 14 of 16 cultivars exhibited <20% injury with acetochlor at 1,260 g ai ha−1 and fenclorim at 2.5 g ai kg−1 2 wk after emergence (WAE) at the Pine Tree Research Station (PTRS). At the Rice Research and Extension Center (RREC) 2 and 4 WAE and at PTRS 4 WAE, all cultivars exhibited <20% injury with acetochlor and fenclorim. The fenclorim seed treatment in the presence of acetochlor provided comparable rice plant height, shoot numbers, groundcover, and rough rice yield to that of the nontreated control. Under cool, wet conditions, rice injury without fenclorim ranged from 15% to 60% with acetochlor at 1,050 g ai ha−1, whereas injury from acetochlor with the fenclorim seed treatment ranged from 0% to 20%. Based on the results of these experiments, the fenclorim seed treatment appears to safen an assortment of rice cultivars from injury caused by ME acetochlor.
Many problematic weeds have evolved resistance to herbicides in mid-southern U.S. rice fields. With the lack of new effective herbicides, rice producers seek alternatives that are currently not labeled for rice production. Inhibitors of very-long chain fatty acid elongase (VLCFA) are currently not labeled for use with U.S. rice crops but are labeled for use in other U.S. row cropping systems and rice production in Asia. Previous research has demonstrated the utility of VLCFA inhibitors for weed control in rice; however, these herbicides induce variable amounts of injury to the crop when applied early in the growing season. Experiments were initiated in 2020 and 2021 at the Rice Research and Extension Center near Stuttgart, AR, to evaluate rice tolerance and weed control with acetochlor and seed treatment with a herbicide safener, fenclorim. Three rates of a microencapsulated formulation of acetochlor (630, 1,260, and 1,890 g ai ha−1), four application timings (preemergence, PRE; delayed-preemergence, DPRE; spiking; and 1-leaf), and without or with the fenclorim seed treatment (2.5 g kg−1 of seed) were used to evaluate rice tolerance, weedy rice control, and barnyardgrass control. Acetochlor applied DPRE at 1,260 g ai ha−1 provided better weedy rice and barnyardgrass control than applications at the 1-leaf stage at the same rate. Acetochlor rates of 1,260 and 1,890 g ai ha−1 reduced barnyardgrass and weedy rice densities by more greater than the 630 g ai ha−1 rate. The fenclorim seed treatment did not influence weedy rice or barnyardgrass control but did reduce injury for DPRE acetochlor applications. Based on these results, acetochlor can be safely applied to rice DPRE (≤19% injury) at 1,260 g ai ha−1 when the seed is treated with fenclorim, leading to ≥88% barnyardgrass and ≥45% weedy rice control 28 d after treatment.
Quizalofop-resistant rice allows for over-the-top applications of quizalofop, a herbicide that inhibits acetyl-coenzyme A carboxylase. However, previous reports have indicated that quizalofop applied postemergence may cause significant injury to quizalofop-resistant rice. Therefore, field experiments were conducted to evaluate the response of quizalofop-resistant rice cultivars to quizalofop applications across different planting dates. Under controlled conditions, the effects of soil moisture content, air temperature, and light intensity on quizalofop-resistant rice sensitivity to quizalofop were investigated. In the planting date experiment, injury of more than 11 percentage points was observed on early-planted rice compared with late-planted rice at the 5-leaf stage, with higher injury observed under saturated soil conditions. However, quizalofop applications at the labeled rate caused ≤16% reduction in yield regardless of planting environment. Quizalofop-resistant cultivars exhibited more injury by at least 25 percentage points when soil was maintained at 90% or 100% of field capacity because rice cultivars ‘PVL01’, ‘PVL02’, and ‘RTv7231 MA’ exhibited ≥42%, 30%, and ≥54% injury, respectively, compared with ≤10%, ≤5%, and ≤22% injury, respectively, at 40% or 50% of field capacity, pooled over rating timing. Greater injury ranging from 18% to 31% was observed on quizalofop-resistant rice grown under low light intensity (600 µmol m−2s−1) compared with 5% to 14% injury under high light intensity (1,150 µmol m−2s−1). The injury persisted from 7 to 28 d after 5-leaf stage application (DAFT), averaged over quizalofop-resistant cultivars and air temperatures (20/15 C and 30/25 C day/night, respectively). At 7 DAFT, greater injury (by 5 to 21 percentage points) was observed on quizalofop-resistant cultivars; PVL01, PVL02, and RTv7231 MA exhibited 33%, 9%, and 58% injury, respectively, under 20/15 C temperature conditions compared with 13%, 4%, and 37% injury, respectively, under 30/25 C day/night conditions averaged over light intensities. Overall, quizalofop is likely to cause a greater risk for injury to quizalofop-resistant rice if it is applied under cool, cloudy, and moist soil conditions.
The threat of herbicide-resistant weed species, such as Palmer amaranth, has driven the development of robust weed management programs that rely on more than chemicals for weed control. Previous research has shown that zero-tolerance weed thresholds, cover crops, deep tillage, and diverse herbicide programs are effective strategies for controlling Palmer amaranth. Unfortunately, research investigating the integration of all four of these weed management strategies in a system is lacking. To better leverage these integrated weed management strategies in cotton production systems, a long-term study was initiated in fall 2018 near Marianna, AR, with zero tolerance, deep tillage, a cereal rye cover crop, and either a dicamba or non-dicamba in-crop herbicide program as factors. Results found that total Palmer amaranth emergence was reduced 76% as the result of deep tillage in 2019 and, in the absence of a zero-tolerance strategy, 73% in 2020. In the absence of a zero-tolerance strategy, the combination of a non–cover crop strategy and dicamba herbicide program decreased total Palmer amaranth emergence by 73%, while the combination of a cover crop strategy and dicamba herbicide program decreased total Palmer amaranth emergence by 78% compared to the combination of a cover crop and non-dicamba herbicide program. Under a zero-tolerance strategy in 2019, tillage reduced cotton yield by 12% and partial returns by US$370 ha−1. In 2020, tillage reduced cotton yield by 14% and partial returns of US$371 ha−1 under a non-zero-tolerance strategy, while a 12% yield reduction and a US$260 ha−1 decrease in partial returns were observed under a zero-tolerance strategy. In 2019, the non-dicamba program resulted in greater partial returns than the dicamba in-crop program because of greater yield and lower program costs. However, in 2020, partial returns were greater for the dicamba in-crop herbicide program owing to greater yields achieved by this program.
The herbicides that inhibit 4-hydroxyphenylpyruvate dioxygenase (HPPD) are primarily used for weed control in corn, barley, oat, rice, sorghum, sugarcane, and wheat production fields in the United States. The objectives of this review were to summarize 1) the history of HPPD-inhibitor herbicides and their use in the United States; 2) HPPD-inhibitor resistant weeds, their mechanism of resistance, and management; 3) interaction of HPPD-inhibitor herbicides with other herbicides; and 4) the future of HPPD-inhibitor-resistant crops. As of 2022, three broadleaf weeds (Palmer amaranth, waterhemp, and wild radish) have evolved resistance to the HPPD inhibitor. The predominance of metabolic resistance to HPPD inhibitor was found in aforementioned three weed species. Management of HPPD-inhibitor-resistant weeds can be accomplished using alternate herbicides such as glyphosate, glufosinate, 2,4-D, or dicamba; however, metabolic resistance poses a serious challenge, because the weeds may be cross-resistant to other herbicide sites of action, leading to limited herbicide options. An HPPD-inhibitor herbicide is commonly applied with a photosystem II (PS II) inhibitor to increase efficacy and weed control spectrum. The synergism with an HPPD inhibitor arises from depletion of plastoquinones, which allows increased binding of a PS II inhibitor to the D1 protein. New HPPD inhibitors from the azole carboxamides class are in development and expected to be available in the near future. HPPD-inhibitor-resistant crops have been developed through overexpression of a resistant bacterial HPPD enzyme in plants and the overexpression of transgenes for HPPD and a microbial gene that enhances the production of the HPPD substrate. Isoxaflutole-resistant soybean is commercially available, and it is expected that soybean resistant to other HPPD inhibitor herbicides such as mesotrione, stacked with resistance to other herbicides, will be available in the near future.
Injury to quizalofop-resistant rice was reported in some fields following postemergence applications of quizalofop. Glyphosate-resistant (GR) corn, cotton, and soybean, and imidazolinone-resistant rice are grown near quizalofop-resistant rice. Herbicide drift from glyphosate and imazethapyr and the resulting crop injury and potential yield loss is a cause of concern for producers. Field experiments conducted near Colt, and Keiser, AR, in 2021 evaluated whether low rates of glyphosate or imazethapyr interact with sequential quizalofop applications to exacerbate injury to quizalofop-resistant rice compared to quizalofop applications alone. Herbicide treatments consisted of a low rate of glyphosate (90 g ae ha−1) or imazethapyr (10.7 g ai ha−1) applied 10, 7, 4, and 0 d before the 2-leaf growth stage of rice, and glyphosate or imazethapyr, at the same rate and timings, followed by quizalofop at 120 g ai ha−1 applied to 2-leaf rice. All plots treated with quizalofop received a subsequent application of the same herbicide and rate at the 5-leaf rice stage. At 28 d after final treatment (DAFT), glyphosate followed by quizalofop the same day to 2-leaf rice caused 77% injury compared with 58% when glyphosate was applied alone, regardless of location. Glyphosate followed by quizalofop the same day reduced rough rice grain yield by 67% compared with 33% when glyphosate was applied alone to 2-leaf rice at the Colt location. Application of imazethapyr followed by quizalofop the same day to 2-leaf rice caused more injury (63% and 19% injury at the Colt and Keiser locations, respectively) than imazethapyr alone (42% and 7% injury at the Colt and Keiser locations, respectively) at 35 DAFT. Overall, glyphosate and imazethapyr followed by quizalofop applications worsened injury compared to glyphosate, imazethapyr, and quizalofop applications alone. As the interval between exposure to a low rate of glyphosate or imazethapyr and quizalofop decreases, the detrimental effect of herbicide on rice likewise increases.
Dicamba was labeled in dicamba-resistant cotton (Gossypium hirsutum L.) and soybean [Glycine max (L.) Merr.] in 2017, resulting in a record number of off-target complaints. To address off-target movement via volatilization, experiments were conducted to evaluate the effectiveness of potassium tetraborate tetrahydrate (KBo) as a volatility-reducing agent (VRA) with dicamba. Low-tunnel experiments examined: (1) whether KBo functions as a dicamba VRA, (2) the relationship between KBo concentration and dicamba volatilization, (3) the effectiveness of KBo compared with potassium acetate as a VRA, and (4) the impact of KBo on dicamba volatilization with and without glufosinate. In a large-scale trial (0.4-ha plots), the effectiveness of KBo in reducing dicamba volatilization was quantified relative to a commercial dicamba application labeled for use in 2020. The addition of KBo to dicamba reduced volatility over dicamba alone and a dicamba plus potassium acetate premix. As KBo concentration increased in the dicamba spray solution, volatilization was exponentially reduced. Dicamba volatilization with the addition of KBo at 0.01 M was comparable to dicamba plus potassium acetate at 0.05 M. Potassium tetraborate tetrahydrate was more effective than potassium acetate at reducing volatility of a dicamba plus glufosinate mixture. In large-scale experiments over a 30-h period, the addition of KBo to a diglycolamine plus glyphosate mixture lowered dicamba volatilization 82% to 89% over the herbicide mixture alone. Overall, the addition of KBo to dicamba appears promising as a VRA compared with what is commercially available.
Gowan Company recently registered benzobicyclon, a WSSA Group 27 herbicide, as a postflood option in rice. It is the first 4-hydroxyphenylpyruvate dioxygenase-inhibiting herbicide commercially available in mid-southern U.S. rice production. In 2018 and 2019, field experiments were conducted across multiple sites in Arkansas to determine if the addition of benzobicyclon to quizalofop- or imidazolinone-resistant rice herbicide programs would improve weedy rice control. Across site-years, one application of quizalofop, either at the 1- or 3-leaf rice stage, followed by benzobicyclon applied postflood, provided comparable weedy rice control to two sequential applications of quizalofop, which is a standard herbicide program in quizalofop-resistant rice. Additionally, treatments containing quizalofop or quizalofop followed by benzobicyclon injured rice ≤5% at 28 d after the postflood application. Across site-years, at 28 d after the postflood application of benzobicyclon, all treatments containing a full-season herbicide program followed by benzobicyclon postflood provided comparable or improved weedy rice control when compared to two sequential early postemergence applications of imazethapyr. In both experiments, rice treated with benzobicyclon yielded comparably or better than treatments containing the standard herbicide program for each system. Findings from this research suggest that the use of benzobicyclon in quizalofop- and imidazolinone-resistant rice systems could be an additional and viable weedy rice control option for rice producers.
A field experiment was conducted in 2019 and 2020 that included six site-years and four locations in Arkansas to determine the optimal sequence and timing of dicamba and glufosinate applications when applied alone, sequentially, or in combination to control Palmer amaranth by size: labeled (<10 cm height) and non-labeled (13 to 25 cm height). Single applications of dicamba, glufosinate, and dicamba plus glufosinate (not labeled) resulted in less than 80% Palmer amaranth control, regardless of weed size. The mixture of dicamba plus glufosinate was antagonistic for Palmer amaranth control and percent mortality. Sequential applications, averaged over all time intervals and herbicides, improved the percentage of Palmer amaranth control 11 to 17 percentage points over a single application, regardless of weed size at application 28 d after final application (DAFA). Palmer amaranth control with glufosinate followed by (fb) glufosinate and dicamba fb dicamba, pending weed size, were optimized at intervals of 7 d, and 14 to 21 d, respectively. Because single site of action (SOA) postemergence herbicide systems increase the likelihood of the development of resistant biotypes and are not a best management practice (BMP) in that regard; sequential applications involving both dicamba and glufosinate were more effective. Furthermore, the sequence of application mattered with a preference for applying dicamba first. Dicamba fb glufosinate at a 14-d interval was profit-maximizing and the only herbicide treatment that resulted in 100% weed control when size was <10 cm. For larger weed sizes, economic analysis revealed that dicamba fb dicamba performed better than dicamba fb glufosinate when no penalty was assigned for using a single SOA. This resulted in greater yield loss risk and soil weed seed bank in comparison to timelier weed control with the smaller weed size. Hence, timely weed control and two SOAs to control Palmer amaranth are recommended as BMPs that reduce producer risk.
Glufosinate resistance in Palmer amaranth (Amaranthus palmeri S. Watson) was recently detected in three accessions from Arkansas, USA. Amaranthus palmeri is the first and only broadleaf weed species resistant to this herbicide, and the resistance mechanism is still unclear. A previous study characterized the glufosinate resistance level in the accessions from Arkansas. A highly glufosinate-resistant accession was further used to investigate the mechanism conferring glufosinate resistance in A. palmeri. Experiments were designed to sequence the herbicide target enzyme cytosolic and chloroplastic glutamine synthetase isoforms (GS1 and GS2, respectively) and quantify copy number and expression. Absorption, translocation, and metabolism of glufosinate using the 14C-labeled herbicide were also evaluated in the resistant and susceptible accessions. The glufosinate-resistant accession had an increase in copy number and expression of GS2 compared with susceptible plants. All accessions showed only one GS1 copy and no differences in expression. No mutations were identified in GS1 or GS2. Absorption (54% to 60%) and metabolism (13% to 21%) were not different between the glufosinate-resistant and glufosinate-susceptible accessions. Most residues of glufosinate (94% to 98%) were present in the treated leaf. Glufosinate translocation to tissues above the treated leaf and in the roots was not different among accessions. However, glufosinate translocation to tissues below the treated leaf (not including roots) was greater in the resistant A. palmeri (2%) compared with the susceptible (less than 1%) accessions. The findings of this paper strongly indicate that gene amplification and increased expression of the chloroplastic glutamine synthetase enzyme are the mechanisms conferring glufosinate resistance in the A. palmeri accession investigated. Thus far, no additional resistance mechanism was observed, but further investigations are ongoing.
Palmer amaranth is a common weed on levees in rice fields but has become increasingly problematic with the adoption of furrow-irrigated rice and lack of an established flood. Florpyrauxifen-benzyl previously has been found effective for controlling Palmer amaranth in rice, but the efficacy of low rates of florpyrauxifen-benzyl and the effect of Palmer amaranth size on controlling it is unknown. The objective of this research was to determine the level of Palmer amaranth control expected with single and sequential applications of florpyrauxifen-benzyl at varying weed heights. The first study was conducted near Marianna, AR, in 2019 and 2020, to determine the effect of florpyrauxifen-benzyl rate on control of <10 cm (labeled size) and 28- to 32-cm-tall (larger-than-labeled size) Palmer amaranth. The second experiment was conducted in 2020 at two locations in Arkansas to compare single applications of florpyrauxifen-benzyl at low rates to sequential applications at the same rates with a 14-d interval on 20- and 40-cm-tall Palmer amaranth. Results revealed that florpyrauxifen-benzyl at 15 g ae ha−1 was as effective as 30 g ae ha−1 in controlling <10-cm-tall Palmer amaranth (92% and 95% mortality in 2019). Sequential applications of florpyrauxifen-benzyl at 8 g ae ha−1 were as effective as single or sequential applications at 30 g ae ha−1. However, no rate of florpyrauxifen-benzyl applied to 20- or 40-cm-tall Palmer amaranth was sufficient to provide season-long control of the weed, with the escaping female plants producing as many as 6,120 seed per plant following a single application.
A 3-yr field study was conducted in Keiser, AR, to investigate the response of the naturally occurring weed flora, dominated by Palmer amaranth, under various combinations of 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide-based programs and crop rotation sequences. In the first year, corn plots were established with three corn HPPD-based herbicide programs designed to represent a range of efficacies and selection pressures for resistance. In the following two years, corn as monoculture or with soybean and/or cotton crops was included in the rotation sequence for selected herbicide programs. Weed emergence, weed biomass, and soil seedbank were assessed through the entire experimental period. The results show that crop rotation, especially a rotation sequence with corn followed by (fb) soybean fb cotton, and the lowest-risk herbicide program involving seven sites of action over the course of the entire crop rotation was effective in reducing the emergence of naturally occurring weeds, including Palmer amaranth, prickly sida, morningglory species, and grass weeds (broadleaf signalgrass, large crabgrass, barnyardgrass, and johnsongrass) by 88.3%, 57.5%, 28.7%, and 76.3%, respectively. Treatments without crop rotation (corn as monoculture for 3 consecutive years) and poor herbicide programs, with one site of action, increased weed emergence, notably of Palmer amaranth and prickly sida, by 73.5% and 74.1%, respectively. The soil seedbank showed a similar trend to weed emergence. This study highlights the fact that reducing the weed seedbank cannot rely on one management practice but requires a multitactic approach with various control methods. HPPD-inhibiting herbicide programs seem to be effective on Palmer amaranth when coupled with crop rotation and should be used with other best management practices.
The ability of weed populations to evolve resistance to herbicides affects management strategies and the profitability of crop production. The objective of this research was to screen Palmer amaranth accessions from Arkansas for glufosinate resistance. Additional efforts focused on the effectiveness of various herbicides, across multiple sites of action (SOAs), on each putative-resistant accession. The three putative accessions were selected from 60 Palmer amaranth accessions collected in 2019 and 2020 and screened with to 0.5× and 1× rates of glufosinate. A dose-response experiment was conducted for glufosinate on accessions A2019, A2020, and B2020. The effectiveness of various preemergence- and postemergence-applied herbicides were evaluated on each accession. Resistance ratios of A2019, A2020, and B2020 to glufosinate ranged from 5.1 to 27.4 when comparing LD50 values to two susceptible accessions, thus all three accessions were resistant to glufosinate. All three accessions (A2019, A2020, and B2020) were found to have a reduction equal to or greater than 20 percentage points in mortality to at least one herbicide from five different SOAs equal to or greater than five sites of action. Herbicides from nine different SOAs controlled A2019 at least 20 percentage points less than the susceptible accessions, which points to a need for additional research to characterize the response of this accession.
Italian ryegrass is a major weed in winter cereals in the south-central United States. Harvest weed seed control (HWSC) tactics that aim to remove weed seed from crop fields are a potential avenue to reduce Italian ryegrass seedbank inputs. To this effect, a 4-yr, large-plot field study was conducted in College Station, Texas, and Newport, Arkansas, from 2016 to 2019. The treatments were arranged in a split-plot design. The main-plot treatments were (1) no narrow-windrow burning (a HWSC strategy) + disk tillage immediately after harvest, (2) HWSC + disk tillage immediately after harvest, and (3) HWSC + disk tillage 1 mo after harvest. The subplot treatments were (1) pendimethalin (1,065 g ai ha−1; Prowl H2O®) as a delayed preemergence application (herbicide program #1), and (2) a premix of flufenacet (305 g ai ha−1) + metribuzin (76 g ai ha−1; Axiom®) mixed with pyroxasulfone (89 g ai ha−1; Zidua® WG) as an early postemergence application followed by pinoxaden (59 g ai ha−1; Axial® XL) in spring (herbicide program #2). After 4 yr, HWSC alone was significantly better than no HWSC. Herbicide program #2 was superior to herbicide program #1. Herbicide program #2 combined with HWSC was the most effective treatment. The combination of herbicide program #1 and standard harvest practice (no HWSC; check) led to an increase in fall Italian ryegrass densities from 4 plants m−2 in 2017 to 58 plants m−2 in 2019 at College Station. At wheat harvest, Italian ryegrass densities were 58 and 59 shoots m−2 in check plots at College Station and Newport, respectively, whereas the densities were near zero in plots with herbicide program #2 and HWSC at both locations. These results will be useful for developing an improved Italian ryegrass management strategy in this region.