Investigations of the relevance of low tunnel methodology and air sampling concerning the off-target movement of dicamba were conducted from 2018 to 2022, focused primarily on volatility. This research, divided into three experiments, evaluated the impact of herbicides and adjuvants added to dicamba and the type of surface treated on dicamba volatility. Treatment combinations included glyphosate and glufosinate, the presence of a simulated contamination rate of ammonium sulfate (AMS), the benefit of a volatility reduction agent (VRA), and a vegetated (dicamba-resistant cotton) or soil surface treated with dicamba. Volatility assessments included air sampling collected over 48 h. Dicamba treatments were applied four times to each of two bare soil or cotton trays and placed inside the tunnels. The extraction and quantification of dicamba from air samples were conducted. Field assessments included the maximum and average visible injury in bioindicator soybean and the lateral movement of dicamba damage expressed by the furthest distance from the center of the plots to the position in which plants had 5% injury. Adding glufosinate and glyphosate to dicamba increased the dicamba amount in air samples. A simulated tank contamination rate of AMS (0.005% v/v) did not impact dicamba emissions compared to a treatment lacking AMS. Adding a VRA reduced dicamba in air samples by 70% compared to treatment without the adjuvant. Dicamba treatments applied on vegetation generally produced greater amounts of dicamba detected than treatments applied to bare soil. Field assessment results usually followed differences in dicamba concentration by treatments tested. Results showed that low tunnel methodology allowed simultaneous comparisons of several treatment combinations concerning dicamba volatility.

Off-target movement of 2,4-D and dicamba is sometimes to blame as the cause of symptoms observed in weeds growing in production fields. Pesticide regulatory authorities routinely sample tissues 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 1-mo 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 1,065 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 to 15 cm, 20 to 30 cm, and 35 to 50 cm in height). Analytical results show 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. Nondetections were frequent, even as early as 2 to 3 d after treatment (DAT), with 0.01X and 0.001X rates of 2,4-D or dicamba. Residues declined rapidly on Xtend® soybean treated with dicamba and on Enlist® soybean treated with 2,4-D. The severity of auxin symptomology generally agreed with the ability to detect dicamba or 2,4-D residue in plant tissue for Palmer amaranth, whereas 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.

Only a limited number of herbicides are available to provide postemergence (POST) control of selective monocot weeds in grain sorghum crops. The herbicides currently labeled for use with 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 use herbicides that inhibit either acetolactate synthase (ALS) or acetyl coenzyme A carboxylase (ACCase) for POST 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 TamArk™ 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 TamArk grain sorghum reached the 2- to 3-leaf stage. TamArk grain sorghum was tolerant of 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 that were evaluated. Additionally, all ACCase inhibitors except diclofop and pinoxaden controlled all monocots tested by >91% at 28 d after application (DAA). Conversely, the two ALS inhibitors, imazamox and nicosulfuron, provided ≤81% control of broadleaf signalgrass 28 DAA but still controlled all other monocots by >95%. TamArk grain sorghum has low sensitivity to multiple ACCase-inhibiting herbicides and thus provides an effective POST option for monocot weed control. In addition, unwanted volunteer TamArk plants can be controlled with cledthodim, sethoxydim, nicosulfuron, or imazamox. Although the ALS-inhibiting herbicides imazamox and nicosulfuron were not useful on TamArk grain sorghum, they are effective options for monocot control on Igrowth™ and Inzen™ grain sorghum crops, 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 R 2 = 0.82) than height reduction (generalized R 2 = 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.

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.

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.

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.

Palmer amaranth has developed resistance to at least seven herbicide sites of action in the Cotton Belt of the United States, leaving producers with fewer options to manage this weed. Previous research with corn and newly commercially released soybean systems have found the use of 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides such as isoxaflutole (IFT) to be effective at managing Palmer amaranth. Consequently, a new transgenic cultivar of cotton is being developed with tolerance to IFT, allowing for in-crop applications of the herbicide. Two separate studies were conducted near Marianna, AR, in 2019 and replicated in 2020, to investigate the crop safety and utility of IFT when added to cotton herbicide programs. Herbicide programs featured IFT as a preemergence or early-postemergence option, residual herbicides in subsequent postemergence applications, and the presence or absence of a layby application. The use of IFT did not significantly impact cotton injury or yield, whereas the use of layered residual herbicides, including IFT, increased Palmer amaranth control compared to those without. Regardless of earlier use of IFT, layby applications were needed for season-long control of Palmer amaranth, entireleaf morningglory, broadleaf signalgrass, and johnsongrass, as evidenced by greater than a 20 percentage point improvement in control of all weeds when a layby application was made. Overall, findings from these studies indicate IFT to be a suitable tool for managing Palmer amaranth and will provide an additional site of action for cotton herbicide programs. Sequential herbicide applications and overlaying residuals were found to be paramount for managing Palmer amaranth throughout the season.

Dicamba residues in sprayers are difficult to remove and may interact with subsequent herbicides, including contact herbicides labeled for use in soybean. Without proper tank cleanout, applicators treating dicamba-resistant and non–dicamba resistant crops are at risk of contaminating the spray solution with dicamba residue from previous applications. Experiments were conducted in Fayetteville, AR, in 2018 and 2019, with the first experiment evaluating consequences of dicamba tank contamination with contact herbicides and the second experiment addressing the impact of dicamba exposure on a glufosinate-resistant soybean cultivar relative to a contact herbicide application. Experiments for tank contamination and timing of dicamba exposure were designed as a three-factor and a two-factor randomized complete block with four replications, respectively, considering site-year as a fixed effect in each experiment. Dicamba at 0, 0.056, 0.56, and 5.6 g ae ha−1 was applied alone, with glufosinate, with acifluorfen, or with glufosinate plus acifluorfen to V3 soybean. Dicamba applied in combination with contact herbicides exacerbated visible auxin symptomology over dicamba alone at 21 and 28 d after treatment (DAT), while dicamba at 5.6 g ae ha−1 reduced soybean height. Injury and height reductions caused by dicamba mixtures with contact herbicides did not reduce grain yield. In the second experiment, dicamba was applied at 2.8 g ae ha−1 at VC, V1, V2, and V3 and at 3, 7, and 10 d after a glufosinate application to V3 soybean (DATV3). Greater soybean injury was observed when dicamba exposure followed a glufosinate application than when dicamba preceded glufosinate or was applied in a mixture with glufosinate, with yield reductions resulting from 7 and 10 DATV3 dicamba applications. Dicamba exposure in the presence of contact herbicides resulted in increased auxin symptomology and can be intensified if soybean are exposed to dicamba following a contact herbicide application.

Previous research has shown that glufosinate and nicosulfuron at low rates can cause yield loss to grain sorghum. However, research has not been conducted to pinpoint the growth stage at which these herbicides are most injurious to grain sorghum. Therefore, field tests were conducted in 2016 and 2017 to determine the most sensitive growth stage for grain sorghum exposure to both glufosinate and nicosulfuron. Field test were designed with factor A being the herbicide applied (glufosinate or nicosulfuron). Factor B consisted of timing of herbicide application including V3, V8, flagleaf, heading, and soft dough stages. Factor C was glufosinate or nicosulfuron rate where a proportional rate of 656 g ai ha−1 of glufosinate and 35 g ai ha−1 of nicosulfuron was applied at 1/10×, 1/50×, and 1/250×. Visible injury, crop canopy heights (cm), and yield were reported as a percent of the nontreated. At the V3 growth stage visible injury of 32% from the 1/10× rate of glufosinate and 51% from the 1/10× rate of nicosulfuron was observed. This injury was reduced by 4 wk after application (WAA) and no yield loss occurred. Nicosulfuron was more injurious than glufosinate at a 1/10× and 1/50× rate when applied at the V8 and flagleaf growth stages resulting in death of the shoot, reduced heading, and yield. Yield losses from the 1/10× rate of nicosulfuron were observed from V8 through early heading and ranged from 41% to 96%. Yield losses from the 1/50× rate of nicosulfuron were 14% to 16% at the flagleaf and V8 growth stages respectively. The 1/10× rate of glufosinate caused 36% visible injury 2 WAA when applied at the flagleaf stage, which resulted in a 16% yield reduction. By 4 WAA visible injury from either herbicide at less than the 1/10× rate was not greater than 4%. Results indicate that injury can occur, but yield losses are more probable from low rates of nicosulfuron at V8 and flagleaf growth stages.

A thorough understanding of commonly used herbicide application practices and technologies is needed to provide recommendations and determine necessary application education efforts. An online survey to assess ground and aerial herbicide application practices in Arkansas was made available online in spring 2019. The survey was direct-emailed to 272 agricultural aviators and 831 certified commercial pesticide applicators, as well as made publicly available online through multiple media sources. A total of 124 responses were received, of which 75 responses were specific to herbicide applications in Arkansas agronomic crops, accounting for approximately 49% of Arkansas’ planted agronomic crop hectares in 2019. Ground and aerial application equipment were used for 49% and 51% of the herbicide applications on reported hectares, respectively. Rate controllers were commonly used application technologies for both ground and aerial application equipment. In contrast, global positioning system-driven automatic nozzle and boom shut-offs were much more common on ground spray equipment than aerial equipment. Applicator knowledge of nozzles and usage was limited, regardless of ground or aerial applicators, as only 28% of respondents provided a specific nozzle type used, indicating a need for educational efforts on nozzles and their importance in herbicide applications. Of the reported nozzle types, venturi nozzles and straight-stream nozzles were the most commonly used for ground and aerial spray equipment, respectively. Spray carrier volumes of 96.3 and 118.8 L ha−1 for ground spray equipment and 49.6 and 59.9 L ha−1 for aerial application equipment were the means of reported spray volumes for systemic and contact herbicides, respectively. Respondents indicated application optimization was a major benefit of utilizing newer application technologies, herbicide drift was a primary challenge, and research needs expressed by respondents included adjuvants, spray volume efficacy, and herbicide drift. Findings from this survey provided insight into current practices, technologies, and needs of Arkansas herbicide applicators. Research and education efforts can be implemented as a result to address aforementioned needs while providing applied research-based information to applicators based on current practices.

A non-GMO trait called Inzen™ was recently commercialized in grain sorghum to combat weedy grasses, allowing the use of nicosulfuron POST in the crop. Inzen™ grain sorghum carries a double mutation in the acetolactate synthase (ALS) gene Val560Ile and Trp574Leu, which potentially results in cross-resistance to a wide assortment of ALS-inhibiting herbicides. To evaluate the scope of cross-resistance to Weed Science Society of America Group 2 herbicides in addition to nicosulfuron, tests were conducted in 2016 and 2017 at the Lon Mann Cotton Research Station near Marianna, AR, the Arkansas Agricultural Research and Extension Center in Fayetteville, AR, and in 2016 at the Pine Tree Research Station near Colt, AR. The tests included ALS-inhibiting herbicides from all five families: sulfonylureas, imidazolinones, pyrimidinylthiobenzoics, triazolinones, and triazolopyrimidines. Treatments were made PRE or POST to grain sorghum at a 1× rate for crops in which each herbicide is labeled. Grain sorghum planted in the PRE trial were Inzen™ and a conventional cultivar. Visible estimates of injury and sorghum heights were recorded at 2 and 4 wk after herbicide application, and yield data were collected at crop maturity. In the PRE trial, no visible injury, sorghum height reduction, or yield loss were observed in plots containing the Inzen™ cultivar. Applications made POST to the Inzen™ grain sorghum caused visible injury, sorghum height reduction, and yield loss of 20%, 13%, and 35%, respectively, only in plots where bispyribac-Na was applied. There was no impact on the crop from other POST-applied ALS-inhibiting herbicides. These results demonstrate that the Inzen™ trait confers cross-resistance to most ALS-inhibiting herbicides and could offer promising new alternatives for weed control and protection from carryover of residual ALS-inhibiting herbicides in grain sorghum.

Throughout eastern Arkansas, Palmer amaranth resistant to protoporphyrinogen oxidase (PPO)-inhibiting herbicides (Group 14 herbicides) has become widespread. Most PPO-resistant Palmer amaranth biotypes possess a target-site mutation, but a metabolic resistance mechanism to fomesafen (Group 14) has also been identified. Once metabolic resistance manifests, plants may also be tolerant to other herbicides and sites of action. To evaluate whether varying spray parameters affected control of PPO-resistant Palmer amaranth in dicamba-tolerant crops, field trials were conducted in 2017 and 2018 at the Lon Mann Cotton Research Station near Marianna, AR, and on-farm in Marion, AR. The experiment included split plot factors of dicamba rate, nozzle type, and carrier volume, with a whole plot factor of population. Dicamba was applied at 560 or 1120 g ae ha−1 through 110015 TTI or AirMix nozzles at 70 or 140 L ha−1 to PPO-resistant or PPO-susceptible Palmer amaranth. Palmer amaranth control 14 d after treatment (DAT) was influenced by an interaction between population and carrier volume. PPO-resistant Palmer amaranth control 14 DAT was 81% regardless of carrier volume, compared with 90% and 95% control at 70 and 140 L ha−1, respectively, of the PPO-susceptible population. An interaction between nozzle type and carrier volume influenced Palmer amaranth control 21 DAT, whereas AirMix nozzles at 140 L ha−1 controlled Palmer amaranth at a greater level (94%) than any other nozzle and carrier volume combination (≤90%). An interaction between population and dicamba rate influenced the relative density of Palmer amaranth 21 DAT. PPO-resistant Palmer amaranth density was less affected by dicamba at either rate than PPO-susceptible Palmer amaranth, relative to the nontreated check. Results concur with those of other research that suggest PPO-resistant Palmer amaranth is harder to control with dicamba. Otherwise, increasing carrier volume affected overall Palmer amaranth control to a greater degree than any other factor.

Palmer amaranth is one of the most difficult-to-control weeds in row crop systems and has evolved resistance to several herbicide sites of action (SOAs). A late-season weed-escape survey had been conducted earlier to determine the distribution of protoporphyrinogen oxidase–inhibitor resistant Palmer Amaranth in Arkansas. The objective of this study was to evaluate the susceptibility of Arkansas Palmer amaranth accessions to commonly used herbicide SOAs. The SOAs evaluated were group 2 + 9, 3, 4, 5, 10, 14, 15, and 27, and the representative herbicide from each group was imazethapyr + glyphosate (79 + 860 g ha−1), trifluralin (1,120 g ha−1), dicamba (280 and 560 g ha−1), atrazine (560 g ha−1), glufosinate (594 g ha−1), fomesafen (395 g ha−1), S-metolachlor (1,064 g ha−1), and tembotrione (92 g ha−1), respectively. Palmer amaranth mortality varied among accessions across SOAs. Averaged across accessions, the mortality rates, by treatment in order from lowest to highest, were as follows: glyphosate + imazethapyr (16%), tembotrione (51%), dicamba at 280 g ha−1 (51%), fomesafen (76%), dicamba at 560 g ha−1 (82%), atrazine (85%), trifluralin (87%), S-metolachlor (96%), and glufosinate (99.5%). This study provides evidence that Palmer amaranth accessions with low susceptibility to glyphosate + imazethapyr, fomesafen, and tembotrione are widespread throughout Arkansas. Of the remaining SOAs, most Palmer amaranth accessions were sensitive; however, within each herbicide SOA, except glufosinate, control of some accessions was less than expected and resistance is suspected.

Atrazine offers growers a reliable option to control a broad spectrum of weeds in grain sorghum production systems when applied PRE or POST. However, because of the extensive use of atrazine in grain sorghum and corn, it has been found in groundwater in the United States. Given this issue, field experiments were conducted in 2017 and 2018 in Fayetteville and Marianna, Arkansas, to explore the tolerance of grain sorghum to applications of assorted photosystem II (PSII)-inhibiting herbicides in combination with S-metolachlor (PRE and POST) or mesotrione (POST only) as atrazine replacements. All experiments were designed as a factorial, randomized complete block; the two factors were (1) PSII herbicide and (2) the herbicide added to create the mixture. The PSII herbicides were prometryn, ametryn, simazine, fluometuron, metribuzin, linuron, diuron, atrazine, and propazine. The second factor consisted of either no additional herbicide, S-metolachlor, or mesotrione; however, mesotrione was excluded in the PRE experiments. Crop injury estimates, height, and yield data were collected or calculated in both studies. In the PRE study, injury was less than 10% for all treatments except those containing simazine, which caused 11% injury 28 d after application (DAA). Averaged over PSII herbicide, S-metolachlor–containing treatments caused 7% injury at 14 and 28 DAA. Grain sorghum in atrazine-containing treatments yielded 97% of the nontreated. Grain sorghum receiving other herbicide treatments had significant yield loss due to crop injury, compared with atrazine-containing treatments. In the POST study, ametryn- and prometryn-containing treatments were more injurious than all other treatments 14 DAA. Grain sorghum yield in all POST treatments was comparable to atrazine, except prometryn plus mesotrione, which was 65% of the nontreated. More herbicides should be evaluated to find a comparable fit to atrazine when applied PRE in grain sorghum. However, when applied POST, diuron, fluometuron, linuron, metribuzin, propazine, and simazine have some potential to replace atrazine in terms of crop tolerance and should be further tested as part of a weed control program across a greater range of environments.

Narrow-windrow burning has been a successful form of harvest weed seed control in Australian cropping systems, but little is known about the efficacy of narrow-windrow burning on weed seeds infesting U.S. cropping systems. An experiment was conducted using a high-fire kiln that exposed various grass and broadleaf weed seeds to temperatures of 200, 300, 400, 500, and 600 C for 20, 40, 60, and 80 s to determine the temperature and time needed to kill weed seeds. Weeds evaluated included Italian ryegrass, barnyardgrass, johnsongrass, sicklepod, Palmer amaranth, prickly sida, velvetleaf, pitted morningglory, and hemp sesbania. Two field experiments were also conducted over consecutive growing seasons, with the first experiment aimed at determining the amount of heat produced during burning of narrow windrows of soybean harvest residues (chaff and straw) and the effect of this heat on weed seed mortality. The second field experiment aimed to determine the effect of wind speed on the duration and intensity of burning narrow windrows of soybean harvest residues. Following exposure to the highest temperature and longest duration in the kiln, only sicklepod showed any survival (<1% average); however, in most cases, the seeds were completely destroyed (ash). A heat index of only 22,600 was needed to kill all seeds of Palmer amaranth, barnyardgrass, and Italian ryegrass. In the field, all seeds of the evaluated weed species were completely destroyed by narrow-windrow burning of 1.08 to 1.95 kg m−2 of soybean residues. The burn duration of the soybean harvest residues declined as wind speed increased. Findings from the kiln and field experiments show that complete kill is likely for weed seeds concentrated into narrow windrows of burned soybean residues. Given the low cost of implementation of narrow-windrow burning and the seed kill efficacy on various weed species, this strategy may be an attractive option for destroying weed seed.

Weed control in corn traditionally has relied on atrazine as a foundational tool to control problematic weeds. However, the recent discovery of atrazine in aquifers and other water sources increases the likelihood of more strict restrictions on its use. Field-based research trials to find atrazine alternatives were conducted in 2017 and 2018 in Fayetteville, AR, by testing the tolerance of corn to PRE and POST applications of different photosystem II (PSII) inhibitors alone or in combination with mesotrione or S-metolachlor. All experiments were designed as a two-factor factorial, randomized complete block, with the two factors being (1) PSII-inhibiting herbicide and (2) the herbicide added to create the mixture. The PSII-inhibiting herbicides were prometryn, ametryn, simazine, fluometuron, metribuzin, linuron, diuron, atrazine, and propazine. The second factor consisted of either no additional herbicide, S-metolachlor, or mesotrione. Treatments were applied immediately after planting in the PRE experiments and to 30-cm–tall corn for the POST experiments. For the PRE study, low levels of injury (<15%) were observed at 14 and 28 d after application and corn height was negatively affected by the PSII-inhibiting herbicide applied. PRE-applied fluometuron- and ametryn-containing treatments consistently caused injury to corn, often exceeding 5%. Because of low injury levels caused by all treatments, crop density and yield did not differ from that of the nontreated plants. For the POST study, crop injury, relative height, and relative yield were affected by PSII-inhibiting herbicide and the herbicide added. Ametryn-, diuron-, linuron-, propazine-, and prometryn-containing treatments caused at least 25% injury to corn in at least 1 site-year. All PSII-inhibiting herbicides, except metribuzin and simazine when applied alone, caused yield loss in corn when compared with atrazine alone. Diuron-, linuron-, metribuzin-, and simazine-containing treatments applied PRE and metribuzin- and simazine-containing treatments applied POST should be investigated further as atrazine replacements.