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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.
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.
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.
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.
In current and next-generation weed control technologies, sequential applications of contact and systemic herbicides for postemergence control of troublesome weeds are needed to mitigate the evolution of herbicide resistance. A clear understanding of the impact auxin herbicide symptomology has on Palmer amaranth groundcover will aid optimization of sequential herbicide applications. Field and greenhouse experiments were conducted in Fayetteville, AR, and a laboratory experiment was conducted in Lonoke, AR, in 2020 to evaluate changes in Palmer amaranth groundcover following an application of 2,4-D and dicamba with various nozzles, droplet sizes, and velocities. Field experiments utilized three nozzles: Extended Range (XR), Air Induction Extended Range (AIXR), and Turbo TeeJet® Induction (TTI), to assess the effect of spray droplet size on changes in Palmer amaranth groundcover. Nozzle did not affect Palmer amaranth groundcover when dicamba was applied. However, nozzle selection did impact groundcover when 2,4-D was applied; the following nozzle order XR > AIXR > TTI reduced Palmer amaranth groundcover the most in both site-years of the field experiment. This result (XR > AIXR > TTI) matches percent spray coverage data for 2,4-D and is inversely related to spray droplet size data. Rapid reductions of Palmer amaranth groundcover from 100% at time zero to 39.4% to 64.1% and 60.0% to 85.8% were observed 180 min after application in greenhouse and field experiments, respectively, regardless of herbicide or nozzle. In one site-year of the greenhouse and field experiments, regrowth of Palmer amaranth occurred 10,080 min (14 d) after an application of either 2,4-D or dicamba to larger than labeled weeds. In all experiments, complete reduction of live Palmer amaranth tissue was not observed 21 d after application with any herbicide or nozzle combination. Control of Palmer amaranth escapes with reduced groundcover may potentially lead to increased selection pressure on sequentially applied herbicides due to a reduction in spray solution contact with the targeted pest.
Weedy rice (Oryza sativa L.) is among the most problematic weeds in rice (Oryza sativa L.) production. The commercialization of herbicide-resistant (HR) rice nearly two decades ago provided an effective tool to manage weedy rice; however, resistance evolution and volunteer HR hybrid rice kept weedy rice at the forefront of rice weed control needs. This research aimed to assess the prevalence and severity of weedy rice infestations, identify production practices that may have contributed to an increase in weedy rice, and determine control strategies that may still be effective on weedy rice across Arkansas and adjacent U.S. Midsouth locales. Two questionnaires, one for rice growers and consultants and one for County Extension agents (CEAs), were distributed through email and physical copies in 2020. Thirty-three respondents returned the rice grower (25) and consultant (8) survey, representing 26 and 7 counties in Arkansas and the Missouri Bootheel area, respectively, as well as four parishes in northeast Louisiana. Eighteen respondents returned the CEA survey. Respondents ranked weedy rice the third most problematic weed in rice, behind Echinochloa spp. and Cyperus spp. The most common infestation levels reported in 78% of fields was less than 12 m−2. Crop rotation (64% growers/consultants, 50% CEAs) and HR rice technology (27% growers/consultants, 50% CEAs) were the top two most-effective methods for weedy rice management, respectively. Tillage and crop rotation practices significantly influenced weedy rice infestation. Rice–soybean [Glycine max (L.) Merr.] rotation had the lowest weedy rice infestation compared with rice monoculture and other crop rotation practices. Crop rotation was not practiced on 26% of reported fields, primarily due to poor drainage. The imidazolinone (IMI)-resistant rice technology was still effective (>70% control) in 60% of fields, but quizalofop-resistant rice is needed to control IMI-resistant weedy rice. Overall, weedy rice remains a challenging weed in rice production.
Knowledge of crop–weed interference effects on weed biology along with yield penalties can be used for the development of integrated weed management (IWM) tactics. Nevertheless, little is known about the beneficial effects of soybean [Glycine max (L.) Merr.] density, an important aspect of IWM, on late Palmer amaranth (Amaranthus palmeri S. Watson) establishment time. Two field experiments were conducted in 2014 and 2015 to investigate how various soybean densities and A. palmeri establishment timings in weeks after crop emergence (WAE) affect height, biomass, and seed production of the weed but also crop yield in drill-seeded soybean. Soybean density had a significant impact on dry weight and seed production of A. palmeri that established within the first 2 wk of crop emergence, but not for establishment timings of the weed 4 wk and later in relation to crop emergence. Differential performance of A. palmeri gender was observed, regarding greater biomass production of female than male plants under crop presence, and merits further investigation. Grain yield reductions were recorded at earlier A. palmeri establishment timings (i.e., 0 and 1 WAE) compared with 8 WAE establishment timing in 2014 and 2015. High soybean densities resulted in greater soybean yields compared with low soybean density, but no grain yield benefits were observed between medium and high soybean densities. Crop budget analysis revealed the benefits of moderate seeding rate (i.e., 250, 000 seeds ha−1) increases in comparison to lower (i.e., 125,000 seeds ha−1) or high (i.e., 400,000 seeds ha−1) on crop revenue, net income returns, and breakeven price. Earlier A. palmeri establishment timings (i.e., 0, 1, and 2 WAE) resulted in lower crop revenue and net income returns compared with later establishment timings of the weed.
Information about weed biology and weed population dynamics is critical for the development of efficient weed management programs. A field experiment was conducted in Fayetteville, AR, during 2014 and 2015 to examine the effects of Palmer amaranth (Amaranthus palmeri S. Watson) establishment time in relation to soybean [Glycine max (L.) Merr.] emergence and the effects of A. palmeri distance from the soybean row on the weed’s height, biomass, seed production, and flowering time and on soybean yield. The establishment time factor, in weeks after crop emergence (WAE), was composed of six treatment levels (0, 1, 2, 4, 6, and 8 WAE), whereas the distance from the crop consisted of three treatment levels (0, 24, and 48 cm). Differences in A. palmeri biomass and seed production averaged across distance from the crop were found at 0 and 1 WAE in both years. Establishment time had a significant effect on A. palmeri seed production through greater biomass production and height increases at earlier dates. Amaranthus palmeri that was established with the crop (0 WAE) overtopped soybean at about 7 and 10 WAE in 2014 and 2015, respectively. Distance from the crop affected A. palmeri height, biomass, and seed production. The greater the distance from the crop, the higher A. palmeri height, biomass, and seed production at 0 and 1 WAE compared with other dates (i.e., 2, 4, 6, and 8 WAE). Amaranthus palmeri establishment time had a significant impact on soybean yield, but distance from the crop did not. The earlier A. palmeri interfered with soybean (0 and 1 WAE), the greater the crop yield reduction; after that period no significant yield reductions were recorded compared with the rest of the weed establishment times. Knowledge of A. palmeri response, especially at early stages of its life cycle, is important for designing efficient weed management strategies and cropping systems that can enhance crop competitiveness. Control of A. palmeri within the first week after crop emergence or reduced distance between crop and weed are important factors for an effective implementation of weed management measures against A. palmeri and reduced soybean yield losses due to weed interference.
The widespread occurrence of Palmer amaranth resistant to acetolactate synthase inhibitors and/or glyphosate led to the increased use of protoporphyrinogen oxidase (PPO)-inhibiting herbicides. This research aimed to: (1) evaluate the efficacy of foliar-applied fomesafen to Palmer amaranth, (2) evaluate cross-resistance to foliar PPO inhibitors and efficacy of foliar herbicides with different mechanisms of action, (3) survey the occurrence of the PPO Gly-210 deletion mutation among PPO inhibitor–resistant Palmer amaranth, (4) identify other PPO target-site mutations in resistant individuals, and (5) determine the resistance level in resistant accessions with or without the PPO Gly-210 deletion. Seedlings were sprayed with fomesafen (263 gaiha−1), dicamba (280 gaiha−1), glyphosate (870 gaiha−1), glufosinate (549 g ai ha−1), and trifloxysulfuron (7.84 gaiha−1). Selected fomesafen-resistant accessions were sprayed with other foliar-applied PPO herbicides. Mortality and injury were evaluated 21 d after treatment (DAT). The PPX2L gene of resistant and susceptible plants from a selected accession was sequenced. The majority (70%) of samples from putative PPO-resistant populations in 2015 were confirmed resistant to foliar-applied fomesafen. The efficacy of other foliar PPO herbicides on fomesafen-resistant accessions was saflufenacil>acifluorfen=flumioxazin>carfentrazone=lactofen>pyraflufen-ethyl>fomesafen>fluthiacet-methyl. With small seedlings, cross-resistance occurred with all foliar-applied PPO herbicides except saflufenacil (i.e., 25% with acifluorfen, 42% with flumioxazin). Thirty-two percent of PPO-resistant accessions were multiple resistant to glyphosate and trifloxysulfuron. Resistance to PPO herbicides in Palmer amaranth occurred in at least 13 counties in Arkansas. Of 316 fomesafen survivors tested, 55% carried the PPO Gly-210 deletion reported previously in common waterhemp. The PPO gene (PPX2L) in one accession (15CRI-B), which did not encode the Gly-210 deletion, encoded an Arg-128-Gly substitution. The 50% growth reduction values for fomesafen in accessions with Gly-210 deletion were 8- to 15-fold higher than that of a susceptible population, and 3- to 10-fold higher in accessions without the Gly-210 deletion.
The commercialization of imazethapyr-resistant (Clearfield™, CL) rice in the southern United States has raised serious concerns about gene flow to red rice, producing imazethapyr-resistant red rice populations. Our objectives were to determine the impact of planting date, CL cultivars, and red rice biotypes on outcrossing rate; and to investigate the relative contribution of flowering time of CL rice and red rice biotypes, together with air temperature and relative humidity (RH), on outcrossing rate. Field experiments were conducted at Stuttgart, Rohwer, and Kibler, AR, from 2005 to 2007, at three or four planting times from mid-April to late May. ‘CL161’ (inbred cultivar) and ‘CLXL8’ (hybrid) rice were planted in nine-row plots, with red rice planted in the middle row. Twelve red rice biotypes were used. The flowering of red rice and CL rice, air temperature, and RH were recorded. Red rice seeds were collected at maturity. To estimate outcrossing rate, resistance to imazethapyr was evaluated in subsequent years and confirmed using rice microsatellite markers. CLXL8 rice flowered 2 to 4 d earlier than CL161 rice, and flowering was completed within 1 wk in all plantings. The flowering duration of most red rice biotypes ranged from 4 to 17 d. Flowering synchrony of red rice biotypes and CL rice ranged from 0 to 100% at different plantings. In general, CLXL8 had greater flowering overlap and higher outcrossing rate with red rice than did CL161 rice. The outcrossing rate of red rice biotypes ranged from 0 to 0.21% and 0 to 1.26% with CL161 and CLXL8 rice, respectively. The outcrossing rate differed within each planting date (P < 0.05). Outcrossing was generally lower in mid-May and late May than in mid-April and late April planting times. Flowering synchrony and outcrossing rate were not correlated (r2 < 0.01). Outcrossing with CL161 was primarily influenced by red rice biotype. A minimum air temperature of > 24 C in the evening also favors outcrossing with CL161. With CLXL8 rice, outcrossing was most affected by RH. When RH was < 54%, outcrossing was less (0.12%) than when RH was ≥ 54% (0.38%). With CLXL8 rice, a minimum RH of ≥ 54%, from mid-morning to noon, increased outcrossing with red rice. To fully understand the interaction effects of these factors on outcrossing with red rice, controlled experiments are needed.
Cultivated rice yield losses due to red rice infestation vary by cultivar, red rice density, and duration of interference. The competition effects of red rice could be influenced further by emergence characteristics, red rice biotype, and planting time of cultivated rice. We aimed to characterize the emergence of red rice biotypes at different planting dates and evaluate the effect of red rice biotype, rice cultivar, and planting date on cultivated rice yield loss. Field experiments were conducted at the Southeast Research and Extension Center, Rohwer, AR, and at the Arkansas Rice Research and Extension Center, Stuttgart, AR, in the summer of 2005 and 2006. The experimental design was a split-split plot with three or four replications. Planting time, ClearfieldTM (CL) rice cultivar, and red rice biotype were the main plot, subplot, and sub-subplot factors, respectively. There were three planting times from mid-April to mid-May at 2-wk intervals. CL rice cultivars, CL161 and hybrid CLXL8, and 12 red rice biotypes were planted. The emergence rate and coefficient of uniformity of germination differed among some red rice biotypes within a planting time. Planting date affected the emergence characteristics of red rice biotypes. The mean emergence rate of red rice was 0.043 d−1 in the mid-April planting and 0.058 d−1 in the late April planting. For the mid-April planting, 50% of red rice biotypes emerged in 20 ± 2 d compared with 15 ± 2 d for CL rice cultivars. Yield losses due to red rice biotypes generally increased in later planting dates, up to 49%. Yield losses due to interference from red rice biotypes ranged from 14 to 45% and 6 to 35% in CL161 and CLXL8, respectively. Cultivated rice became less competitive with red rice in later plantings, resulting in higher yield losses.
The occurrence of 36 arable weed species across 13 counties in the eastern Arkansas–Mississippi Delta area on 489 randomly selected road sites was surveyed in 2012. Palmer amaranth, johnsongrass, large crabgrass, barnyardgrass, prickly sida, and broadleaf signalgrass were the top six weed species, with occurrence noted at 313, 294, 261, 238, 176, and 136 sites, respectively. Factors found to affect weed occurrence along Mississippi Delta roadsides included topographical characteristics, weed species, ditch slope, road type, and nearby land use. Among roadside topographical characteristics, road shoulder was found to strongly affect weed occurrence. In addition, paved and gravel road types with moderate roadside slope explained most of the variability of weed occurrence at each sampling site. Additionally, nearby arable land use affected weed occurrence more so than natural, residential, and pastoral land. Barnyardgrass, johnsongrass, and Palmer amaranth were 3.6 to 4.3 times more likely to occur than all other species identified. An effective weed management plan along eastern Arkansas–Mississippi Delta roadsides should focus on road shoulder, adjacent arable land use, road type, and specific weed species (e.g., Palmer amaranth, johnsongrass, and barnyardgrass). The inclusion of these parameters in future weed control programs can prove invaluable for preventing the spread of the herbicide-resistant Palmer amaranth, barnyardgrass, and johnsongrass.
A survey was conducted in 2012 across 13 counties in the eastern Arkansas–Mississippi Delta area on 489 randomly selected road sites to assess the distribution of the most commonly occurring arable weeds. Among the 36 species recorded, Palmer amaranth, johnsongrass, large crabgrass, barnyardgrass, prickly sida, and broadleaf signalgrass were the top six weed species, occurring at 313, 294, 261, 238, 176, and 136 sites, respectively. Barnyardgrass, johnsongrass, and Palmer amaranth were present at 34, 32, and 31% of all sampling occasions (site by roadside topographical characteristic). Habitat preferences varied between weed species. Palmer amaranth, large crabgrass, and johnsongrass exhibited a preference for disturbed habitats as well as field shoulders. Conversely, barnyardgrass, yellow nutsedge, hemp sesbania, and giant ragweed exhibit a preference for moist environments similar to these found in roadside ditches. Herbicide use on roadsides is subject to many environmental regulations and public concerns that, in combination with the evolution of herbicide resistance, necessitate an effective plan for managing agronomically important weed species on eastern Arkansas–Mississippi Delta roadsides.
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