Residual herbicides applied to summer cash crops have the potential to injure subsequent winter annual cover crops, yet little information is available to guide growers’ choices. Field studies were conducted in 2016 and 2017 in Blacksburg and Suffolk, Virginia, to determine carryover of 30 herbicides commonly used in corn, soybean, or cotton on wheat, barley, cereal rye, oats, annual ryegrass, forage radish, Austrian winter pea, crimson clover, hairy vetch, and rapeseed cover crops. Herbicides were applied to bare ground either 14 wk before cover crop planting for a PRE timing or 10 wk for a POST timing. Visible injury was recorded 3 and 6 wk after planting (WAP), and cover crop biomass was collected 6 WAP. There were no differences observed in cover crop biomass among herbicide treatments, despite visible injury that suggested some residual herbicides have the potential to effect cover crop establishment. Visible injury on grass cover crop species did not exceed 20% from any herbicide. Fomesafen resulted in the greatest injury recorded on forage radish, with greater than 50% injury in 1 site-year. Trifloxysulfuron and atrazine resulted in greater than 20% visible injury on forage radish. Trifloxysulfuron resulted in the greatest injury (30%) observed on crimson clover in 1 site-year. Prosulfuron and isoxaflutole significantly injured rapeseed (17% to 21%). Results indicate that commonly used residual herbicides applied in the previous cash crop growing season result in little injury on grass cover crop species, and only a few residual herbicides could potentially affect the establishment of a forage radish, crimson clover, or rapeseed cover crop.

Metam potassium (metam-K) is a soil fumigant used commonly in Florida at the end of the tomato and pepper production season. The fumigant essentially cleans a field by killing the established weeds and crops after harvest. The goal of this project was to determine the optimal rate of metam-K for the effective termination of tomato, pepper, and established weeds such as purple nutsedge, goosegrass, and dogfennel. Tomato, pepper, and purple nutsedge at bed center were effectively terminated with the metam-K rate of 65 kg ha−1. Optimal rates required for the termination of goosegrass and dogfennel were 91 and 156 kg ha−1, respectively. In contrast, metam-K at 500 to 680 kg ha−1 was required to terminate purple nutsedge on bed edges. The reduced efficacy of metam-K at bed edge might be related to the limited movement of metam-K in soil.

Acetyl co-enzyme A carboxylase (ACCase)-resistant rice allows quizlaofop-p-ethyl to be applied as a POST control of troublesome grass weeds. A field study was conducted in 2017 and 2018 at the H. Rouse Caffey Rice Research Station near Crowley, LA, to evaluate the influence of a crop oil concentrate (COC), a silicon-based surfactant plus a nitrogen source (SNS), or a high-concentrate COC (HCOC) in overcoming the grass weed control antagonism of quizalofop-p-ethyl when mixed with bispyribac-Na. Quizalofop-p-ethyl was applied at 120 g ai ha−1, bispyribac-Na was applied at 34 g ai ha−1, and all adjuvants were applied at 1% vol/vol. Antagonistic interactions were observed at 14 d after treatment (DAT) when quizalofop-p-ethyl was mixed with bispyribac-Na with no adjuvant for control of barnyardgrass, the non–ACCase-tolerant rice cultivars ‘CL-111’ and ‘CLXL-745’, and red rice. At 14 DAT, antagonism of quizalofop-p-ethyl for control of barnyardgrass was observed when mixed with bispyribac-Na plus COC, SNS, or HCOC, with an observed control of 43%, 63%, and 86%, respectively, compared with an expected control of 95% for quizalofop-p-ethyl alone. However, the antagonism of quizalofop-p-ethyl when mixed with bispyribac-Na plus HCOC for barnyardgrass control at 14 DAT was overcome by 28 DAT, with an observed control of 91%, compared with an expected control of 97%. Synergistic or neutral interactions were observed at 14 and 28 DAT when COC, SNS, or HCOC was added to a mixture of quizalofop-p-ethyl plus bispyribac-Na for CL-111, CLXL-745, and red rice control. According to the results of this study, HCOC is the most effective adjuvant for quizalofop-p-ethyl and bispyribac-Na mixtures for control of weedy rice and barnyardgrass.

Safeners have been widely used to reduce phytotoxicity to crops, thus serving as an alternative weed control strategy. Benoxacor and fenclorim safeners have the potential to protect plants from herbicide phytotoxicity by increasing glutathione S-transferase (GST) activity within the plant. The study aimed to evaluate the safening effect of benoxacor and fenclorim on tomato against selected herbicides applied POST. The experiment was conducted in a greenhouse in a completely randomized designed with four replications in a 9 × 3 factorial scheme, where Factor A consisted of eight herbicides including a nontreated control, and Factor B consisted of two safeners including a nontreated control. The herbicide treatments were sulfentrazone (0.220 kg ai ha−1), fomesafen (0.280 kg ai ha−1), flumioxazin (0.070 kg ai ha−1), linuron (1.200 kg ai ha−1), metribuzin (0.840 kg ai ha−1), pyroxasulfone (0.220 kg ai ha−1), and bicyclopyrone (0.040 kg ai ha−1). Safener treatments consisted of benoxacor (0.67 g L−1) and fenclorim (10 µM). Tomato seeds were immersed in safener solution before sowing and herbicides were applied when tomato plants were at the 3-leaf stage, or 25 days after sowing. Visible injury was scored at 3, 7, 14, and 21 d after application (DAA), and shoot biomass was recorded 21 DAA. Seed treatment with fenclorim reduced injury caused by imazamox and bicyclopyrone by 5.5 and 1.3 times, respectively, whereas benoxacor reduced the injury from bicyclopyrone 1.3 times. In addition, tomato plants pretreated with fenclorim showed a lesser reduction in biomass after application of imazamox, fomesafen, and metribuzin, whereas plants pretreated with benoxacor showed lesser biomass reduction after metribuzin application. Thus, the use of safeners promotes greater crop selectivity, allowing the application of herbicides with different mechanisms of action on the crop.

Field surveys were conducted across the Blacklands region of Texas during 2016 and 2017 to document the distribution of herbicide-resistant Lolium spp. infesting winter wheat production fields in the region. A total of 68 populations (64 Italian ryegrass, four perennial ryegrass) were evaluated in a greenhouse for sensitivity to herbicides of three different modes of action: an acetolactate synthase (ALS) inhibitor (mesosulfuron-methyl), two acetyl-coenzyme-A carboxylase (ACCase) inhibitors (diclofop-methyl and pinoxaden), and a 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) inhibitor (glyphosate). Herbicides were applied at twice the label-recommended rates for mesosulfuron-methyl (29 g ai ha−1), diclofop-methyl (750 g ai ha−1), and pinoxaden (118 g ai ha−1); and at the recommended rate for glyphosate (868 g ae ha−1). The herbicide screenings were followed by dose-response assays of the most-resistant ryegrass population for each herbicide at eight rates (0.5, 1, 2, 4, 8, 16, 32, and 64×), compared with a susceptible population at six rates (0.0625, 0.125, 0.25, 0.5, 1, and 2×). The initial screening and dose-response experiments were conducted in a completely randomized design with three replications and two experimental runs. Survivors (<80% injury) were characterized as highly resistant (0% to 20% injury) or moderately resistant (21% to 79%). Results showed that 97%, 92%, 39%, and 3% of the Italian ryegrass populations had survivors to diclofop-methyl, mesosulfuron-methyl, pinoxaden, and glyphosate treatments, respectively. Of the four perennial ryegrass populations, three were resistant to diclofop-methyl and mesosulfuron-methyl, and one was resistant to pinoxaden as well. Perennial ryegrass populations did not exhibit any resistance to glyphosate. Dose-response assays revealed 37-, 196-, and 23-fold resistance in Italian ryegrass to mesosulfuron-methyl, diclofop-methyl, and pinoxaden, respectively, compared with a susceptible standard. One Italian ryegrass population exhibited three-way multiple resistance to ACCase-, ALS-, and EPSPS-inhibitors. The proliferation of multiple herbicide–resistant ryegrass is a challenge to sustainable wheat production in Texas Blacklands and warrants diversified management strategies.

Cover cropping is limited by seasonal constraints following corn harvest in the Upper Midwest of the United States. Grass, clover, and brassica cover crops can be interseeded in corn; however, this is problematic because cover crops must tolerate herbicide applications to manage weeds. The objective of this research was to determine the tolerance of broadcast interseeded annual ryegrass, oilseed radish, and crimson clover to PRE and POST residual herbicide applications in corn. From 2016 to 2018 field trials were conducted in Michigan to determine the tolerance of annual ryegrass, oilseed radish, and crimson clover to 13 PRE and 14 POST (applied to V2 corn) herbicides. Cover crops were interseeded into corn at the V3 and V6 stages. Greenhouse experiments to evaluate these species were also conducted from 2016 to 2018; PRE and POST herbicides were applied at 1×, 0.5×, and 0.25× (0.25× was PRE only) of field-application rates. Based on these results, annual ryegrass can be interseeded into V3 or V6 corn following a PRE application of atrazine, clopyralid, saflufenacil, bicyclopyrone, isoxaflutole, or mesotrione, or a POST application of atrazine, bromoxynil, or mesotrione. Oilseed radish can be interseeded into V3 or V6 corn following a PRE application of clopyralid, atrazine, S-metolachlor, bicyclopyrone, or isoxaflutole or at V6 following application of acetochlor, dimethenamid-P, or mesotrione. Oilseed radish can also be interseeded following POST application of atrazine (571 g ai ha−1), bromoxynil, fluthiacet, acetochlor, mesotrione, dicamba + diflufenzopyr, or dimethenamid-P + topramezone. In greenhouse trials, crimson clover was tolerant to rimsulfuron, saflufenacil, and pyroxasulfone applied PRE. Annual ryegrass and oilseed radish can be interseeded into corn at the V3 and V6 stages, but special attention must be given to cover crop species selection and herbicide label restrictions when following herbicide applications in corn.

A field study was conducted during the 2016 and 2017 crop seasons at the LSU AgCenter H. Rouse Caffey Rice Research Station to evaluate weed control and rice yield after quizalofop-p-ethyl applications in water-seeded coenzyme A carboxylase (ACCase)–resistant ‘PVLO1’ long-grain rice production utilizing different flood systems, application timings, and quizalofop rates. The initial application of quizalofop was applied at five timings beginning when ‘PVLO1’ rice was at the coleoptile stage (PEG) through the one- to two-tiller stage. A total quizalofop rate of 240 g ai ha–1 was split into two applications: 97 followed by 143 g ha–1 or 120 followed by 120 g ai ha–1 in both pinpoint and delayed flood water-seeded management systems. A second quizalofop application was applied 14 d after initial treatment (DAIT). At 14 DAIT, a reduction in control of barnyardgrass and red rice was observed by delaying the initial quizalofop application to the two- to four-tiller stage compared with rice treated at earlier growth stages. At 42 DAIT, control of barnyardgrass was 94% to 96%, and red rice was 98% following the second application of quizalofop, regardless of initial application timing. Rice treated with quizalofop at the PEG and two- and three-leaf stage resulted in a rice height of 104 cm at harvest compared with 96 to 100 cm when the initial application of quizalofop was delayed to later growth stages. Applying the initial application of quizalofop to rice at the PEG timing in the pinpoint or the delayed flood system resulted in a total gross value per hectare of $450 and $590, respectively. Within each flood system, delaying the initial application of quizalofop to the one- to two-tiller stage resulted in a gross per-hectare value reduction of $100 ha-1 in the pinpoint flood and $110 ha-1 in the delayed flood.

Identification of common weeds is fundamental in determining adequate recommendations for management practices. The aim of this study was to identify the patterns of weed management adopted by rice farmers and the perspectives of consultants who work in flooded rice areas in Rio Grande do Sul (RS) State, Brazil. Fifty-three public and 50 private consultants who worked with rice in RS in 2017 and 2018 were interviewed. Data were analyzed by descriptive statistics. Both weedy rice and Echinochloa sp. occurred and escaped more often from chemical control because they remained in the field until harvest in 59% of the area. According to consultants, the main reasons for reduced weed control were related to herbicide resistance and late herbicide application. Fifty-six percent of farmers used imidazolinone herbicides at rates that were greater than those indicated on the label for POST application. The consultants’ main challenges were weed escapes, resistance management, and guidelines on herbicide rates. Survey results show that the use of herbicide rates above label recommendations and consultants’ work on control of weed escapes are directly related to the high occurrence of herbicide resistance.

Off-target paraquat movement to rice has become a major problem in recent years for rice producers in the midsouthern United States. Nitrogen (N) fertilizer is applied to rice in greater quantity and frequency than all other nutrients to optimize rice yield. Two separate field studies were conducted from 2015 to 2018 in Stoneville, MS, to assess whether starter N fertilizer can aid rice recovery from exposure to a sub-lethal concentration of paraquat and to evaluate rice response to different N fertilizer management strategies following exposure to a sub-lethal concentration of paraquat. In both studies, paraquat treatments consisted of paraquat at 0 and 84 g ai ha–1 applied to rice in the two- to three-leaf (EPOST) growth stage. In the starter fertilizer study, N fertilizer at 24 kg ha–1 as ammonium sulfate (AMS) was applied to rice at spiking- to one-leaf (VEPOST), two- to three-leaf (EPOST), or three- to four-leaf (MPOST) growth stages before and after paraquat treatment. In the N fertilizer timing study, N fertilizer at 168 kg N ha–1 was applied in a single four-leaf to one-tiller (LPOST) application or two-, three-, and two four-way split applications. Despite starter N fertilizer applications, paraquat injured rice ≥41%, reduced height 57%, reduced dry weight prior to flooding 77%, delayed maturity 10 d, reduced dry weight at maturity 33%, and reduced rough rice yield 35% in the starter fertilizer study. Similarly, in the N fertilizer timing study, paraquat injured rice ≥45%, reduced height 14%, delayed maturity 10 d, reduced dry weight at maturity 44%, and reduced rough rice yield 50% for all N fertilizer management strategies. Both studies indicate that severe complications in growth and development can occur from rice exposure to a sub-lethal concentration of paraquat. In both studies, manipulation of N fertilizer management did not facilitate rice recovery from early-season exposure to paraquat.

A field study was conducted in 2015 and 2016 at the H. Rouse Caffey Rice Research Station near Crowley, Louisiana, to evaluate the interactions of quizalofop and a mixture of propanil plus thiobencarb applied sequentially or mixed to control weedy rice and barnyardgrass. Visual weed control evaluations occurred at 14, 28, and 42 d after treatment (DAT). Quizalofop was applied at 120 g ai ha−1 at 7, 3, and 1 d before and after propanil plus thiobencarb were each applied at 3,360 g ai ha−1. In addition, quizalofop was applied alone and in a mixture with propanil plus thiobencarb at day 0. Control of red rice ‘CL-111’ and ‘CLXL-745’ was greater than 91% when quizalofop was applied alone at day 0, similar to control for quizalofop applied 7, 3, and 1 d prior to propanil plus thiobencarb at all evaluation dates. Control of the same weeds treated with quizalofop plus propanil plus thiobencarb applied in a mixture at day 0 was 70% to 76% at each evaluation date, similar to quizalofop applied 1 or 3 d after propanil plus thiobencarb. A similar trend in control of barnyardgrass by 88% to 97% occurred when quizalofop was applied alone and by 48% to 53% at 14, 28, and 42 DAT when the mixture was used. ‘PVL01’ rough rice yield was 4,060 kg ha−1 when treated with quizalofop alone; however, yield was reduced to 3,180 kg ha−1 when it was treated with quizalofop mixed with propanil plus thiobencarb at day 0, similar to PVL01 rice treated with quizalofop 1 or 3 d following the propanil plus thiobencarb application.

Acifluorfen is a nonsystemic PPO-inhibiting herbicide commonly used for POST Palmer amaranth control in soybean, peanut, and rice across the southern United States. Concerns have been raised regarding herbicide selection pressure and particle drift, increasing the need for application practices that optimize herbicide efficacy while mitigating spray drift. Field research was conducted in 2016, 2017, and 2018 in Mississippi and Nebraska to evaluate the influence of a range of spray droplet sizes [150 μm (Fine) to 900 μm (Ultra Coarse)], using acifluorfen to create a novel Palmer amaranth management recommendation using pulse width modulation (PWM) technology. A pooled site-year generalized additive model (GAM) analysis suggested that 150-μm (Fine) droplets should be used to obtain the greatest Palmer amaranth control and dry biomass reduction. Nevertheless, GAM models indicated that only 7.2% of the variability observed in Palmer amaranth control was due to differences in spray droplet size. Therefore, location-specific GAM analyses were performed to account for geographical differences to increase the accuracy of prediction models. GAM models suggested that 250-μm (Medium) droplets optimize acifluorfen efficacy on Palmer amaranth in Dundee, MS, and 310-μm (Medium) droplets could sustain 90% of maximum weed control. Specific models for Beaver City, NE, indicated that 150-μm (Fine) droplets provide maximum Palmer amaranth control, and 340-μm (Medium) droplets could maintain 90% of greatest weed control. For Robinsonville, MS, optimal Palmer amaranth control could be obtained with 370-μm (Coarse) droplets, and 90% maximum control could be sustained with 680 μm (Ultra Coarse) droplets. Differences in optimal droplet size across location could be a result of convoluted interactions between droplet size, weather conditions, population density, plant morphology, and soil fertility levels. Future research should adopt a holistic approach to identify and investigate the influence of environmental and application parameters to optimize droplet size recommendations.

Bearded sprangletop is a problematic weed in California rice production. The objective of this research was to determine the response of two bearded sprangletop biotypes (clomazone-susceptible [S] and -resistant [R]) to flooding depth. A study was conducted in 2017 and 2018 at the California Rice Experiment Station in Biggs, CA, to evaluate the flooding tolerance of the two biotypes against 5-, 10-, and 20-cm continuous flooding depths. Plant emergence, plant height, panicles per plant, seed per panicle, 100-seed weight, and seed per plant data were collected. At the 5-cm flood depth, neither biotype was controlled, and the R biotype had 260% more emergence, produced 475% more panicles per plant, and 455% more seed per plant than the S biotype. With a 10-cm flood, only the R biotype survived flooding and produced more panicles per plant and seed per plant than any other flood depth–biotype combination evaluated. There was no emergence of either bearded sprangletop biotype at the 20-cm flood depth. Continuous flooding can still be used as a management tool to control bearded sprangletop; however, the depth of flooding appears to limit emergence of S biotypes at 5 cm and R biotypes at 10 cm, and completely inhibits growth of both biotypes at 20 cm. The results of this study indicate that clomazone-resistant bearded sprangletop is more likely to spread throughout the Sacramento Valley because this biotype can survive clomazone applications and can tolerate a standard 10-cm flood.

Florpyrauxifen-benzyl and quizalofop were available for POST applications in 2018; however, little is known about the response of acetyl-CoA carboxylase (ACCase)–resistant rice cultivars and advanced lines to POST herbicides. A field study was conducted in 2017 and 2018 at Stoneville, MS, to characterize the response of ACCase-resistant rice cultivars and advanced lines to POST applications of florpyrauxifen-benzyl. The imidazolinone-resistant (IR) rice cultivars ‘CL163’ and ‘CLXL 745’, and ACCase-resistant rice cultivars ‘PVL01’, ‘PVL013’, ‘PVL024-B’, ‘PVL038’, ‘PVL080’, and ‘PVL081’were treated with florpyrauxifen-benzyl at 0 (nontreated control for each cultivar) and 58 g ai ha–1 at the four-leaf to one-tiller (LPOST) growth stage. At 14 d after treatment (DAT), PVL01 was injured 5% to 6% greater than CLXL 745, PVL013, and PVL081; however, injury was ≤10% at that evaluation for all cultivars. Similarly, injury was ≤13% for all cultivars 28 DAT. Mature heights were reduced for all cultivars except PVL013 and PVL081. Rough rice yield was ≥100% of the control for all cultivars except PVL081, PVL013, and CL163. Results suggest that florpyrauxifen-benzyl can safely be applied POST to rice cultivars grown in Mississippi as well as ACCase-resistant cultivars that are currently under development.

Weeds can cause significant yield loss in watermelon production systems. Commercially acceptable weed control is difficult to achieve, even with heavy reliance on herbicides. A study was conducted to evaluate a spring-seeded cereal rye cover crop with different herbicide application timings for weed management between row middles in watermelon production systems. Common lambsquarters and pigweed species (namely, Palmer amaranth and smooth pigweed) densities and biomasses were often lower with cereal rye compared with no cereal rye, regardless of herbicide treatment. The presence of cereal rye did not negatively influence the number of marketable watermelon fruit, but average marketable fruit weight in cereal rye versus no cereal rye treatments varied by location. These results demonstrate that a spring-seeded cereal rye cover crop can help reduce weed density and weed biomass, and potentially enhance overall weed control. Cereal rye alone did not provide full-season weed control, so additional research is needed to determine the best methods to integrate spring cover cropping with other weed management tactics in watermelon for effective, full-season control.

Provisia™ rice was developed recently by the BASF Corporation for control of grass weeds and is complementary to existing Clearfield® technology. Our previous research showed that resistance of Provisia™ rice to the acetyl coenzyme-A carboxylase herbicide quizalofop-p-ethyl (QPE) in laboratory and greenhouse environments is governed by a single dominant Mendelian gene. However, these results may not be consistent in different populations or field environments. Therefore, the first objective of the current research is to determine the inheritance of resistance to QPE in rice using different segregating populations evaluated under U.S. field environments. The second objective is to evaluate the response of QPE-resistant breeding lines to various herbicide concentrations at two U.S. locations. Chi-square tests of 12 F2 populations evaluated in Louisiana during 2014 and 2015 indicated that QPE seedling resistance at 240 g ai ha−1 was governed by a single dominant Mendelian gene with no observable maternal effects. Similar results were obtained in five F3 populations derived from the aforementioned F2 populations. Allele-specific SNP markers for QPE resistance also followed Mendelian segregation in the five F2 populations. For the second objective, six QPE-resistant inbred lines showed transient leaf injury at 1× (120 g ai ha−1) or 2× (240 g ai ha−1) field rates 7 and 21 d after treatment (DAT). However, a trend of reduced injury (recovery) from 7 through 33 DAT was observed for all breeding material. No differences in grain yield were found between untreated QPE-resistant lines and those treated with 1× or 2× QPE field rate. Single gene inheritance and good levels of QPE herbicide field resistance in different genetic populations suggest feasibility for rapid and effective development of new QPE-resistant varieties and effective stewardship of the Provisia™ technology.

Bearded sprangletop is a problematic weed in California rice production and few herbicides provide effective control. As control of bearded sprangletop has declined, grower suspicion of resistance to clomazone has increased, because of the continuous rice cropping system and herbicide dependence in the region. The objectives of this research were to confirm clomazone resistance in bearded sprangletop populations and determine the level of resistance. Seed from 21 suspected clomazone-resistant populations was collected from the California rice growing region. A greenhouse experiment was conducted to determine population sensitivity to clomazone. Clomazone was applied into the water to emerging seedlings. Plant ht and control of bearded sprangletop were recorded weekly for 3 wk, plants were then harvested, and dry weight was measured. Of the populations tested, 17 were susceptible and four (5%) were resistant to clomazone. A dose-response assay was conducted using eight doses ranging from an eighth of the full rate to 12 times the full rate. The three most resistant populations had resistant-to susceptible ratios of 1.25×, 2×, and 5× the labeled rate of clomazone. The use of clomazone in California rice production is beneficial; however, it should be used at the appropriate timing and as part of an herbicide program to prevent further development of clomazone resistance.

Dissipation of S-metolachlor, a soil-applied herbicide, on organic and mineral soils used for sugarcane production in Florida was evaluated using field studies in 2013 to 2016. S-metolachlor was applied PRE at 2,270 g ha−1 on organic and mineral soils with 75% and 1.6% organic matter, respectively. The rate of dissipation of S-metolachlor was rapid on mineral soils compared with organic soils. Dissipation of S-metolachlor on organic soils followed a negative linear trend resulting in half-lives (DT50) ranging from 50 to 126 d. S-metolachlor loss on organic soils was more rapid under high soil-moisture conditions than in corresponding low soil-moisture conditions. On mineral soils, dissipation of S-metolachlor followed an exponential decline. The DT50 of S-metolachlor on mineral soils ranged from 12 to 24 d. The short persistence of S-metolachlor on mineral soils was likely attributed to low organic matter content with limited adsorptive capability. The results indicate that organic matter content and soil moisture are important for persistence of S-metolachlor on organic and mineral soils used for sugarcane production in Florida.

The introduction of 2,4-D–resistant soybean and cotton provided growers a new POST active ingredient to include in weed management programs. The technology raises concerns regarding potential 2,4-D off-target movement to sensitive vegetation, and spray droplet size is the primary management factor focused on to reduce spray particle drift. The objective of this study was to investigate the droplet size distribution, droplet velocity, and particle drift potential of glyphosate plus 2,4-D choline pre-mixture (Enlist Duo®) applications with two commonly used venturi nozzles in a low-speed wind tunnel. Applications with the TDXL11004 nozzle had larger DV0.1 (291 µm), DV0.5 (544 µm), and DV0.9 (825 µm) values compared with the AIXR11004 nozzle (250, 464, and 709 µm, respectively), and slower average droplet velocity (8.1 m s−1) compared with the AIXR11004 nozzle (9.1 m s−1). Nozzle type had no influence on drift deposition (P = 0.65), drift coverage (P = 0.84), and soybean biomass reduction (P = 0.76). Although the TDXL11004 nozzle had larger spray droplet size, the slower spray droplet velocity could have influenced the nozzle particle drift potential. As a result, both TDXL11004 and AIXR11004 nozzles had similar spray drift potential. Further studies are necessary to understand the impact of droplet velocity on drift potential at field scale and test how different tank solutions, sprayer configurations, and environmental conditions could influence the droplet size and velocity dynamics and consequent drift potential in pesticide applications.

Weedy rice is an emerging problem of cultivated rice in California. Infestations of weedy rice in cultivated rice result in yield loss and reduced grain quality. In this study, we aimed to evaluate growth and yield components of a widely grown cultivated rice variety in California in response to weedy rice competition. Greenhouse competition experiments in an additive design were conducted in 2017 and 2018 to determine the growth and yield components of ‘M-206’ rice and five weedy rice biotypes found in California at varying weed densities. M-206 rice initially grew at a faster relative growth rate of 0.53 cm−1 wk−1 under competitive conditions compared with 0.47 cm−1 wk−1 in the absence of weedy rice, but absolute and relative growth rates declined more rapidly under competitive conditions as plants approached maturity. At harvest, M-206 plant height was reduced 13% under competitive conditions, and M-206 tiller number was reduced 23% to 49%, depending on the weedy rice biotype it was competing with. Except for 100-grain weight, the growth traits and grain yield components of M-206 rice were reduced with increasing density of weedy rice. At the highest weed density measured, 40 plants m−2, M-206 rice had yield losses of 69% grain yield plant−1, 69% panicle weight, 59% fresh and dry biomass, 55% grain yield panicle−1, and 54% panicle number. The five evaluated weedy rice biotypes varied widely in early growth rates, height, biomass production, and grain yield, indicating differing competitive strategies. Most weedy rice biotypes produce plants with greater plant height, tiller number, panicle number, and above- and below-ground biomass compared with cultivated rice. Weedy rice biotypes produced 45% to 57% higher grain yield per plant than M-206 rice under competitive conditions.

Cover crops provide a number of agronomic benefits, including weed suppression, which is important as cases of herbicide resistance continue to rise. To effectively suppress weeds, high cover crop biomass is needed, which necessitates later termination timing. Cover crop termination is important to mitigate potential planting issues and prevent surviving cover crop competition with cash crops. Field studies were conducted in Virginia to determine the most effective herbicide options alone or combined with glyphosate or paraquat to terminate a range of cover crop species. Results revealed that grass cover crop species were controlled (94% to 98%) by glyphosate alone 4 wk after application (WAA). Overall, legume species varied in response to the single active-ingredient treatments, and control increased with the addition of glyphosate or paraquat. Mixes with glyphosate provided better control of crimson clover and hairy vetch by 7% to 8% compared with mixes containing paraquat 4 WAA. Mix partner did not influence control of Austrian winter pea. No treatment adequately controlled rapeseed in this study, with a maximum of 58% control observed with single active-ingredient treatments and 62% control with mixes. Height reduction for all cover crop species supports visible rating data. Rapeseed should be terminated when smaller, which could negate weed suppressive benefits from this cover crop species. Growers should consider herbicide selection and termination timing in their cover crop plan to ensure effective termination.