Target-site mutations Ile1781Leu and Ile2041Asn in the ACCase2 gene confer resistance to fluazifop-p-butyl and pinoxaden herbicides in a johnsongrass accession from Arkansas, USA.

Johnsongrass [Sorghum halepense (L.) Pers.] is a troublesome weed species in different agricultural and non-agricultural areas. Because of its biology, reproductive system, and seed production, effective management is challenging. An accession with low susceptibility to the acetyl-CoA carboxylase (ACCase)-inhibiting herbicides fluazifop-p-butyl (fluazifop) and pinoxaden was collected in eastern Arkansas. In this research, the molecular mechanisms responsible for ACCase resistance were investigated. Dose-response experiments showed a resistance factor of 181 and 133 for fluazifop and pinoxaden, respectively. Molecular analysis of both ACCase1 and ACCase2 genes was researched. Nucleotide comparison of ACCase1 between resistant and susceptible accessions showed no single nucleotide polymorphisms. Nonetheless, analysis of ACCase2 in fluazifop-resistant johnsongrass plants revealed the Ile1781Leu target-site mutation was dominant (nearly 75%), whereas the majority of pinoxaden-resistant johnsongrass plants had the Ile2041Asn (60%). Not all sequenced johnsongrass plants displayed a target-site mutation, suggesting the presence of additional resistance mechanisms. Amplification of ACCase1 and ACCase2 was not responsible for resistance because of the similar values obtained in both resistant and susceptible accessions. Experiments with malathion and NBD-Cl suggest the presence of herbicide metabolism. Outcomes of this research demonstrated that fluazifop- and pinoxaden-resistant johnsongrass plants displayed a target-site mutation in ACCase2, but also that non-target-site resistance mechanisms would be involved and require a detailed study.

U.S. cereal rye winter cover crop growth database.

Winter cover crop performance metrics (i.e., vegetative biomass quantity and quality) affect ecosystem services provisions, but they vary widely due to differences in agronomic practices, soil properties, and climate. Cereal rye (Secale cereale) is the most common winter cover crop in the United States due to its winter hardiness, low seed cost, and high biomass production. We compiled data on cereal rye winter cover crop performance metrics, agronomic practices, and soil properties across the eastern half of the United States. The dataset includes a total of 5,695 cereal rye biomass observations across 208 site-years between 2001-2022 and encompasses a wide range of agronomic, soils, and climate conditions. Cereal rye biomass values had a mean of 3,428 kg ha-1, a median of 2,458 kg ha-1, and a standard deviation of 3,163 kg ha-1. The data can be used for empirical analyses, to calibrate, validate, and evaluate process-based models, and to develop decision support tools for management and policy decisions.

Quizalofop resistance in weedy rice (Oryza sativa L.) is mainly conferred by an Ile1781Leu mutation.

Weedy rice (Oryza sativa L.) is an economically important weed species in rice (Oryza sativa L.) cropping systems. Two weedy rice samples (acc7 and acc8) suspected to be resistant to quizalofop-ethyl (quizalofop) were collected in Arkansas. In this research, susceptibility to quizalofop and resistance mechanisms have been explored. Dose-response assays displayed a resistance index of 42- and 58-fold for the acc7 and acc8, respectively. Experiments with metabolism inhibitors demonstrated that NBD-Cl (4-chloro-7-nitrobenzofurazan) increased quizalofop efficacy slightly in acc8, whereas malathion did not improve effectiveness in resistant samples. Sequencing of the ACCase gene displayed an Ile1781Leu substitution in the resistant samples, like the mutation present in Provisia™ rice. In addition, an allele-specific PCR was developed to genotype the Ile1781Leu mutation. The gene copy number of ACCase showed similar values among samples. In the resistant plants, a KASP (Kompetitive Allele Specific PCR) assay to detect the ALSS653D (acetolactate synthase) and HIS1 (HPPD Inhibitor Sensitive 1) traits revealed that 37.5% of plants carried the ALSS653D trait, whereas 25% showed the HIS1 allele. In summary, a target-site mutation is the main resistance mechanism to quizalofop in weedy rice. Results also suggest the presence of herbicide metabolism (a non-target site resistance mechanism) mediated by glutathione-S-transferases (GSTs) in one resistant sample.

Evaluation of secondary sexual dimorphism of the dioecious Amaranthus palmeri under abiotic stress.

The evolution of secondary sex-specific traits of dioecious species under abiotic stress conditions has received limited research, especially in the case of Amaranthus palmeri, a fast adapting and highly competing plant. Here, we have examined the interactive effects of abiotic stress on mineral accumulation, chlorophyll a and b content, and the operating capacity of Photosystem II (PSII) in both male and female A. palmeri plants grown under three different intensities of white light, and under N, K or P deficiency. Mineral profiling of the leaves and stems (with inflorescence) highlighted intra- and intersexual differences in their accumulation pattern and mineral associations. Chlorophyll a and chlorophyll b were different between the male and the female plants, being slightly lower in the latter, at high light intensity towards maturity, or under K or P deficiency. Further, slight, although statistically significant differences were recorded in the chlorophyll a/b ratio, which was lower at the higher light intensity in the female, over that in the male, plants towards maturity. Chlorophyll fluorescence parameters, i.e., steady state and maximum fluorescence increased under high light intensity, whereas the PSII operating efficiency decreased in the female plants, indicating reduced PSII capacity. Sex-specific differences in A. palmeri showed a differential response to stressful conditions because of differences in their ontogeny and physiology, and possibly due to the cost of reproduction. We suggest that the breeding system of dioecious species has weaknesses that can be used for the ecological management of dioecious weeds without relying on the use of herbicides.

Metabolic Exploration for Cyhalofop-Butyl Antagonism in Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] Following Pretreatment of Malathion.

The present study investigated the effects of broad-spectrum metabolic inhibitors malathion (cytochrome P450 inhibitor) and/or 4-chloro-7-nitrobenzofurazan (NBD-Cl; glutathione S-transferase inhibitor) on the metabolism of cyhalofop-butyl (CyB) in barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] biotypes confirmed previously with multiple resistance to two herbicides CyB and florpyrauxifen-benzyl. The metabolic inhibitors were not effective at recovering the sensitivity of resistant barnyardgrass biotypes to CyB treated at the labeled rate (313 g ai ha-1). Rather, treatment with malathion followed by CyB caused antagonism, reducing the efficacy of CyB and promoting the growth of resistant biotypes. Pretreatment with malathion did not influence absorption/translocation of the applied form CyB and its conversion to the active herbicide form cyhalofop-acid (CyA), in both susceptible and resistant biotypes. In contrast, metabolism of the applied form (CyB) decreased 1.5 to 10.5 times by the malathion pretreatment. Taken together, the maintained CyA production against the reduced CyB metabolism could be the mechanism to account for the cause of CyB antagonism observed in barnyardgrass following malathion pretreatment. Additionally, the evolution of CyB resistance in barnyardgrass might be associated with reduced production of CyA in resistant biotypes, independent of activities of cytochrome P450 or GST enzymes.

Exploratory Analysis on Herbicide Metabolism and Very-Long-Chain Fatty Acid Production in Metolachlor-Resistant Palmer Amaranth (Amaranthus palmeri S. Wats.).

A Palmer amaranth (Amaranthus palmeri S. Wats.) biotype resistant to S-metolachlor was confirmed from crop fields in Arkansas, USA. This study investigated the metabolic effects of malathion (cytochrome P450 inhibitor) and 4-chloro-7-nitrobenzofurazan [NBD-Cl; glutathione S-transferase inhibitor] on the S-metolachlor-resistant A. palmeri biotype. Root elongation of the resistant biotype was 20% more inhibited by treatment of NBD-Cl (50 nM) and S-metolachlor (2 µM) in mixture than by treatment of S-metolachlor alone. Metabolites of S-metolachlor were 1.4-12.1 times greater produced in the resistant biotype for 7 d than in the susceptible standard. Production of cerotic acid, one of the very-long-chain fatty acids containing 26 carbons, was more reduced in the susceptible standard (3.8-fold) than in the resistant biotype (1.8-fold) by S-metolachlor treatment. Conclusively, evolution of S-metolachlor resistance observed in this study was likely associated with improved activity of glutathione S-transferases. Further studies are needed to genetically evaluate plant endogenous enzymes involving cerotic acid production.

Overexpression of Acetyl CoA Carboxylase 1 and 3 (ACCase1 and ACCase3), and CYP81A21 were related to cyhalofop resistance in a barnyardgrass accession from Arkansas.

Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] is the most difficult-to-control weed species of rice production systems worldwide. It has evolved resistance to different herbicide sites of action, including the acetyl-CoA carboxylase (ACCase)-inhibiting herbicides. Target-site mutations conferring resistance to ACCase-inhibiting herbicides are well documented; however, the role of the different ACCase genes in conferring resistance to cyhalofop-p-butyl (cyhalofop), an ACCase-inhibiting herbicide, remains poorly understood. This research assessed the contribution of gene amplification and expression of ACCase genes in a cyhalofop-resistant barnyardgrass accession. Additionally, the expression of glutathione-S-transferases (GSTs) and cytochrome P450 monooxygenases (P450s) genes as possible contributors to resistance to cyhalofop were investigated. Results demonstrated that ACCase gene amplification does not contribute to cyhalofop resistance. However, ACCase1 and ACCase3 were found to be overexpressed in the cyhalofop-resistant barnyardgrass accession. At 24 h after cyhalofop treatment, an overexpression of 2.0- and 2.8-fold was detected in ACCase1 and ACCase3, respectively. In addition, CYP81A21 (a P450 gene) was found to be 2.5-fold overexpressed compared to the susceptible accession in the same time period. These results suggest that ACCase1, ACCase3, and CYP81A21 are crucial genes in contributing cyhalofop resistance in this barnyardgrass accession.

Herbicide spray drift from ground and aerial applications: Implications for potential pollinator foraging sources.

A field spray drift experiment using florpyrauxifen-benzyl was conducted to measure drift from commercial ground and aerial applications, evaluate soybean [Glycine max (L.) Merr.] impacts, and compare to United States Environmental Protection Agency (US EPA) drift models. Collected field data were consistent with US EPA model predictions. Generally, with both systems applying a Coarse spray in a 13-kph average wind speed, the aerial application had a 5.0- to 8.6-fold increase in drift compared to the ground application, and subsequently, a 1.7- to 3.6-fold increase in downwind soybean injury. Soybean reproductive structures were severely reduced following herbicide exposure, potentially negatively impacting pollinator foraging sources. Approximately a 25% reduction of reproductive structures up to 30.5-m downwind and nearly a 100% reduction at 61-m downwind were observed for ground and aerial applications, respectively. Aerial applications would require three to five swath width adjustments upwind to reduce drift potential similar to ground applications.

Cross-resistance of barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] to aryloxyphenoxypropionate herbicides.

Managing emerged weeds that have evolved resistance to acetyl CoA carboxylase (ACCase)-inhibiting herbicides is a challenging task. A dose-response experiment was conducted on barnyardgrass biotypes resistant (R) and susceptible (S) to three aryloxyphenoxypropionate herbicides cyhalofop-butyl (CyB), fenoxaprop-ethyl (FeE), and quizalofop-ethyl (QuE) along with investigations into the potential resistance mechanism of these biotypes. The tested R barnyardgrass biotypes had strong resistance to CyB and FeE (resistant/susceptible ratio: 7.9-14.4) but weak resistance to QuE (resistant/susceptible ratio: 2.4-3.1). Absorption, translocation, and total metabolism of CyB and QuE were not associated with differences among S and R barnyardgrass biotypes. However, differences between S and R barnyardgrass were observed in production of active acid forms of each herbicide (cyhalofop-acid and quizalofop-acid). Production of cyhalofop-acid was >1.6-fold less in R barnyardgrass (3-8%) for 24 h after herbicide application than in the S barnyardgrass (8-16%). Meanwhile, production of quizalofop-acid was less in R barnyardgrass (< 14%) throughout the study period than in the S barnyardgrass (< 22%). Sequencing results of ACCase gene showed no difference between S and R barnyardgrass. Overall results show that a non-target-site resistance mechanism altering metabolism of CyB and QuE likely contributes to resistance of the barnyardgrass biotypes to these herbicides.

Evaluation of dicamba volatilization when mixed with glyphosate using imazethapyr as a tracer.

Expansion of dicamba-resistant crops increased the frequency of off-target movement issues, especially in the midsouthern United States. Six field trials were conducted over two growing seasons with the purpose to determine the contribution of volatilization and physical suspension of particles to the off-target movement of dicamba when applied with glyphosate and imazethapyr - a non-volatile herbicide used as a tracer for physical off-target movement. Applications included dicamba at 560 g ha-1, glyphosate at 1260 g ha-1, and imazethapyr at 105 g ha-1. Applicators include glyphosate with dicamba to increase the spectrum of weed control from these applications; however, this addition increases potential for dicamba volatilization. Following application of the mixture, air samplers were placed in the field to collect dicamba and imazethapyr. Results showed there was at least 50 times more dicamba than imazethapyr detected even though the dicamba:imazethapyr ratio applied was 5.3:1. Dicamba was detected in the treated area and the off-site locations and all intervals of air sampling, ranging from 126 to 5990 ng. No more than 37.5 ng of imazethapyr was detected during the first 24-h after application (HAA) inside the treated area. Imazethapyr was only detected in 9 of the 20 sampling combinations during these experiments, and most of these detections (6) occurred during the first 24 HAA and inside the treated area. While some movement from the suspension of particles occurred based on the detection of imazethapyr in air samples, results show that most dicamba detection was due to the volatilization of the herbicide.

Large-scale evaluation of physical drift and volatility of 2,4-D choline in cotton: a four-year field study.

BACKGROUND: Cotton with resistance to 2,4-d choline, glufosinate and glyphosate allows over-the-top use of these herbicides for postemergence weed control. Nontransgenic cotton is highly responsive to low rates of 2,4-d, causing concern among farmers when the herbicide is applied near the crop. Injury to nearby cotton following application of 2,4-d choline is sometimes blamed on volatilization of the herbicide. RESULTS: A large-scale experiment was conducted in 2018-2021 to better understand causes of damage to nearby sensitive cotton following an application of 2,4-d choline plus glufosinate-ammonium. The herbicides were applied to 0.4 ha in the center of a 4-ha non-Enlist cotton field. At 30 min after application, air samplers were established in the treated center and outside the treated area in cardinal directions. The 72-h cumulative air concentration of 2,4-d in the swath ranged from 3.3 to 7.1 ng m-3 , and most volatile residues (5.0-25.5 ng m-3 ) were detected in samplers established in the downwind direction directly adjacent to the treated field. Cotton plants in three downwind transects that were covered for 30 min after application were not damaged by 2,4-d, whereas noncovered plants along the downwind transects were injured. No cotton injury occurred outside the treated area, except in the downwind direction during application even though wind direction changed after application. CONCLUSION: 2,4-d choline volatilizes, but findings show that the volatilization is not sufficient to damage cotton in the neighboring area following applications, pointing to the importance of applicators understanding wind direction/shift during the application along with proximity of sensitive crops in the downwind direction. © 2022 Society of Chemical Industry.

Utility of roller wiper applications of dicamba for Palmer amaranth control in soybean.

BACKGROUND: The commercialization of dicamba-resistant soybean has resulted in increased concern for off-target movement of dicamba onto sensitive vegetation. To mitigate the off-target movement through physical drift, one might consider use of rope wicks and other wiper applicators. Although wiper-type application methods have been efficacious in pasture settings, the utility of dicamba using wiper applicators in agronomic crops is not available in scientific literature. To determine the utility of roller wipers for dicamba applications in dicamba-resistant soybean, two separate experiments were conducted in the summer of 2020 and replicated in both Keiser and Fayetteville, AR, USA. RESULTS: Utilizing opposing application directions and a 2:1:1 ratio of water: formulated glyphosate: formulated dicamba were the most efficacious practices for controlling Palmer amaranth. The high herbicide concentrations and wiping in opposing directions increased dicamba-resistant soybean injury when the wiper contacted the crop, but no yield loss was observed because of this injury. Broadcast applications resulted in greater Palmer amaranth mortality than roller wiper applications, and the most effective roller wiper treatments were when two sequential applications were made inside the crop canopy. CONCLUSIONS: Dicamba applications require adequate coverage for optimum weed control. While efforts can be made to increase roller wiper efficacy by optimizing coverage and timing of applications, broadcast applications are superior to roller wiper applicators for weed control. Roller wiper applications did not reduce soybean yield, thus wiper-type applications may be safely used in dicamba-resistant soybean, albeit the likelihood for off-target damage caused by volatilization of these treatments would need to be investigated. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Non-target-site resistance mechanism of barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] to florpyrauxifen-benzyl.

BACKGROUND: Florpyrauxifen-benzyl (FPB) is an arylpicolinate herbicide (Group IV) for barnyardgrass control in rice. One susceptible (Sus) and three putative FPB-resistant (R1, R2, and R3) barnyardgrass biotypes were selected based on resistant/susceptible (R/S) ratios obtained from dose-response tests and used to investigate the potential resistance mechanisms. RESULTS: Based on visual control results, the R/S ratios of barnyardgrass biotypes R1, R2, and R3 were 60-, 33-, and 16-fold greater than the Sus standard, respectively. Sequencing results of TIR1 and AFB genes in the tested barnyardgrass revealed no difference between Sus and R barnyardgrass biotypes. Absorption of [14 C]-FPB in Sus barnyardgrass increased over time and reached 90%, which was >10 percentage points greater than that in R biotypes. The [14 C]-FPB absorption in all R barnyardgrass equilibrated after 48 h. For both Sus and R barnyardgrass, most [14 C]-FPB absorbed was present in the treated leaf (79.8-88.8%), followed by untreated aboveground (9.5-18.6%) and belowground tissues (1.3-2.2%). No differences in translocation were observed. Differences between Sus and R barnyardgrass biotypes were found for FPB metabolism. Production of the active metabolite, florpyrauxifen-acid, was greater in Sus barnyardgrass (21.5-52.1%) than in R barnyardgrass (5.5-34.9%). CONCLUSION: In conclusion, reductions in FPB absorption and florpyrauxifen-acid production may contribute to the inability to control barnyardgrass with FPB. © 2021 Society of Chemical Industry.

Absorption, translocation, and metabolism of florpyrauxifen-benzyl and cyhalofop-butyl in cyhalofop-butyl-resistant barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.].

Dose-response experiments were conducted to assess the sensitivity of one susceptible and three putative resistant (R1, R2, and R3) barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] biotypes to florpyrauxifen-benzyl and cyhalofop-butyl alone and as a formulated premix. Subsequently, potential resistance mechanisms of the barnyardgrass were evaluated. Based on biomass reduction results, resistant/susceptible ratios were calculated for R1 (7.0-50), R2 (7.0-150), and R3 (18-214) biotypes. Absorption and translocation of [14C]-florpyrauxifen-benzyl decreased in R1 and R3 biotypes, but not for [14C]-cyhalofop-butyl. The metabolism of [14C]-florpyrauxifen-benzyl to [14C]-florpyrauxifen-acid was >2-fold less in resistant biotypes (9-11%) than in the susceptible biotype (23%). Moreover, the production of [14C]-florpyrauxifen-acid in susceptible barnyardgrass (not in the R biotypes) increased 3-fold when florpyrauxifen-benzyl and cyhalofop-butyl were applied in mixture compared to florpyrauxifen-benzyl applied alone. The tested barnyardgrass biotypes had no mutation in the Transport Inhibitor Response1, auxin-signaling F-box, and acetyl coenzyme A carboxylase genes. Although further studies on cyhalofop-butyl resistance with respect to analysis of specific metabolites are needed, our findings in this study demonstrates that the evolution of florpyrauxifen-benzyl resistance in multiple resistant barnyardgrass can be related to non-target-site resistance mechanisms reducing absorption and translocation of the herbicide and causing reduced conversion or rapid degradation of florpyrauxifen-acid.

Understanding Resistance Mechanisms to Trifluralin in an Arkansas Palmer Amaranth Population.

Amaranthus palmeri S. Watson (Palmer amaranth) is considered a problematic and troublesome weed species in many crops in the USA, partly because of its ability to evolve resistance to herbicides. In this study, we explored the mechanism of resistance in a trifluralin-resistant A. palmeri accession collected from Arkansas, USA. Dose-response assays using agar plates demonstrated an EC50 (effective concentration that reduces root length by 50%) of 1.02 µM trifluralin compared to 0.39 µM obtained in the susceptible accession. Thus, under these conditions, the resistant accession required 2.6 times more trifluralin to inhibit root length by 50%. Seeds in the presence or absence of the cytochrome P450-inhibitior malathion displayed a differential response with no significant influence on root length, suggesting that resistance is not P450-mediated. In addition, application of 4-chloro-7-nitrobenzofurazan (NBD-Cl), a glutathione S-transferase (GST) inhibitor, showed significant differences in root length, indicating that GSTs are most likely involved in the resistance mechanism. Sequencing of α- and ß-tubulin genes revealed no single nucleotide polymorphisms (SNPs) previously described between accessions. In addition, relative gene copy number of α- and ß-tubulin genes were estimated; however, both resistant and susceptible accessions displayed similar gene copy numbers. Overall, our results revealed that GST-mediated metabolism contributes to trifluralin resistance in this A. palmeri accession from Arkansas.

Recurrent Selection with Sub-Lethal Doses of Mesotrione Reduces Sensitivity in Amaranthus palmeri.

Amaranthus palmeri, ranked as the most prolific and troublesome weed in North America, has evolved resistance to several herbicide sites of action. Repeated use of any one herbicide, especially at lower than recommended doses, can lead to evolution of weed resistance, and, therefore, a better understanding of the process of resistance evolution is essential for the management of A. palmeri and other difficult-to-control weed species. Amaranthus palmeri rapidly developed resistance to 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors such as mesotrione. The objective of this study was to test the potential for low-dose applications of mesotrione to select for reduced susceptibility over multiple generations in an A. palmeri population collected from an agricultural field in 2001. F0 plants from the population were initially treated with sub-lethal mesotrione rates and evaluated for survival three weeks after treatment. All F0 plants were controlled at the 1× rate (x = 105 g ai ha-1). However, 2.5% of the F0 plants survived the 0.5× treatment. The recurrent selection process using plants surviving various mesotrione rates was continued until the F4 generation was reached. Based on the GR50 values, the sensitivity index was determined to be 1.7 for the F4 generation. Compared to F0, HPPD gene expression level in the F3 population increased. Results indicate that after several rounds of recurrent selection, the successive generations of A. palmeri became less responsive to mesotrione, which may explain the reduced sensitivity of this weed to HPPD-inhibiting herbicides. The results have significance in light of the recently released soybean and soon to be released cotton varieties with resistance to HPPD inhibitors.

Presence of the HPPD Inhibitor Sensitive 1 Gene and ALSS653N Mutation in Weedy Oryza sativa Sensitive to Benzobicyclon.

Benzobicyclon has shown varying results in controlling weedy rice, including those with imidazolinone (IMI) resistance. Tolerance to benzobicyclon in cultivated japonica rice, but not indica or aus-like cultivars, is conferred by a fully functional HPPD Inhibitor Sensitive 1 (HIS1) gene. Herein, a diagnostic Kompetitive Allele Specific PCR (KASP) assay was developed to predict the HIS1 genotype of weedy rice plants from 37 accessions and correlated to their response to benzobicyclon in the field. Two-thirds of the 693 weedy rice plants screened were tolerant to benzobicyclon (371 g ai ha-1, SC formulation) at 30 days after treatment (DAT). Thirty-four percent of plants were homozygous for the HIS1 allele and 98% of these plants exhibited field tolerance. However, the his1 genotype did not always correlate with field data. Only 52% of his1 plants were considered sensitive, indicating that the single nucleotide polymorphisms (SNPs) chosen in the KASP assay are not a reliable tool in predicting his1 homozygous plants. In an additional experiment, 86% of the 344 plants with at least one copy of the ALSS653N trait harbored a HIS1 allele, suggesting fields infested with IMI herbicide-resistant weedy rice are unlikely to be controlled with benzobicyclon.

Current outlook and future research needs for harvest weed seed control in North American cropping systems.

Harvest weed seed control (HWSC) comprises a set of tools and tactics that prevents the addition of weed seed to the soil seed bank, attenuating weed infestations and providing a method to combat the development and spread of herbicide-resistant weed populations. Initial HWSC research efforts in North America are summarized and, combined with the vast area of crops suitable for HWSC, clearly indicate strong potential for this technology. However, potential limitations exist that are not present in Australian cropping systems where HWSC was developed. These include rotations with crops that are not currently amenable to HWSC (e.g. corn), high moisture content at harvest, untimely harvest, and others. Concerns about weeds becoming resistant to HWSC (i.e. adapting) exist, as do shifts in weed species composition, particularly with the diversity of weeds in North America. Currently the potential of HWSC vastly outweighs any drawbacks, necessitating further research. Such expanded efforts should foremost include chaff lining and impact mill commercial scale evaluation, as this will address potential limitations as well as economics. Growers must be integrated into large-scale, on-farm research and development activities aimed at alleviating the problems of using HWSC systems in North America and drive greater adoption subsequently. © 2020 Society of Chemical Industry.

Uptake, translocation, and metabolism of glyphosate, glufosinate, and dicamba mixtures in Echinochloa crus-galli and Amaranthus palmeri.

BACKGROUND: Echinochloa crus-galli (L.) Beauv. and Amaranthus palmeri S. Wats are two common and problematic weeds prevalent across the Midsouth of the USA. Herbicide absorption, translocation, and metabolism were investigated as potential sources of herbicide antagonism on A. palmeri and E. crus-galli using 14 C-labeled herbicides. Three 14 C-labeled herbicides, glyphosate, glufosinate, and dicamba, were utilized individually in separate experiments. RESULTS: Uptake of 14 C-glyphosate in E. crus-galli was 15% of the total applied radioactivity for glyphosate/glufosinate (897 + 595 g a.i./a.e. ha-1 ) compared to 25% for glyphosate alone. Similarly, uptake of 14 C-glyphosate in A. palmeri reduced by 10% when applied with glufosinate. Applying glyphosate/dicamba (897/560 g a.e. ha-1 ) reduced 14 C-glyphosate uptake in both species. In the 14 C-glufosinate experiment, both species absorbed less 14 C-glufosinate when mixed with glyphosate compared to glufosinate alone. No metabolic degradation of glyphosate was observed in either species. E. crus-galli metabolized dicamba 23 times faster than A. palmeri. When glufosinate was applied with dicamba, metabolic degradation of 14 C-dicamba was limited in both species. For example, 99.9% of the applied radioactivity was recovered in A. palmeri as the parent compound when 14 C-glufosinate dicamba was applied with glufosinate, compared to 95.7% for dicamba alone. CONCLUSION: These findings demonstrate absorption, translocation, or metabolism of dicamba, glufosinate, and glyphosate can be affected by mixing with another herbicide. As mixing two herbicides is often a critical component of resistance management, careful investigation into the performance of these mixtures in the field is needed. © 2020 Society of Chemical Industry.