Sulfuryl fluoride (SF) is currently used as a fumigant for control of drywood termites and insects in building structures, vehicles, wood products, postharvest commodities, and food processing facilities. This research investigated the feasibility of using SF as a preplant soil fumigant for purple nutsedge control in plastic-mulched tomato production. SF treatments included SF injected through drip tapes or SF injected through drip tapes a few hours following shank injection of chloropicrin (Pic). Results revealed that SF alone at 224, 336, or 448 kg ha−1 was generally less effective compared with when it was applied in conjunction with Pic at 168 kg ha−1. SF alone provided inconsistent control of purple nutsedge. In contrast, SF + Pic was as efficacious or more efficacious on purple nutsedge than the industry standards, including 1,3-dichloropropene (1,3-D) plus Pic and metam potassium. None of the fumigant treatments visually injured tomato plants, stunted growth, or adversely affected tomato yield. In one of the four tomato seasons, tomato plants growing in plots fumigated with SF + Pic resulted in taller tomato plants and higher markable yields. Results indicate that soil fumigation with SF + Pic is safe on plastic-mulched tomato and effectively controls purple nutsedge.

Control of waterhemp is becoming more difficult in Ontario because biotypes have evolved resistance to four herbicide sites of action (SOA), including groups 2, 5, 9, and 14. The objective of this study was to compare PRE, POST, and PRE followed by (fb) POST herbicide programs for their effect on control, density, and biomass of multiple-herbicide–resistant (MHR) waterhemp as well as corn injury and grain yield. Two separate field studies, each consisting of five field trials, were conducted over a 2-yr period (2018 and 2019) in fields where corn was grown in Ontario, Canada. The first experiment evaluated MHR waterhemp control with an inhibitor of 4-hydroxyphenyl-pyruvate dioxygenase (HPPD) applied PRE, PRE fb glufosinate applied POST, and glufosinate applied POST. The second experiment evaluated MHR waterhemp control with a non-HPPD inhibitor applied PRE, then PRE fb a POST application of atrazine + mesotrione, and then atrazine + mesotrione applied POST. Atrazine + isoxaflutole caused 3% to 5% corn injury at environment 1 (E1); no corn injury was observed with PRE and POST herbicide programs at environments E2, E3, E4, and E5. In general, atrazine/bicyclopyrone/mesotrione/S-metolachlor and dimethenamid-P/saflufenacil applied PRE controlled MHR waterhemp ≥95% 12 wk after POST application (WAA). A POST application of glufosinate following atrazine + tolpyralate PRE, and a POST application of atrazine + mesotrione following atrazine/dicamba or atrazine/S-metolachlor PRE, improved control at 4, 8, and 12 WAA in most environments. In general, PRE fb POST applications resulted in better control of MHR waterhemp throughout the growing season than single PRE and POST applications (P < 0.05). We conclude that herbicide programs based on multiple effective SOAs may offer effective control of MHR waterhemp where field corn is grown. It is advisable that when choosing an herbicide application program that excellent control of MHR waterhemp should be the goal given its high fecundity and competitive ability.

Information on performance of sequential treatments of quizalofop-P-ethyl with florpyrauxifen-benzyl on rice is lacking. Field studies were conducted in 2017 and 2018 in Stoneville, MS, to evaluate sequential timings of quizalofop-P-ethyl with florpyrauxifen-benzyl included in preflood treatments of rice. Quizalofop-P-ethyl treatments were no quizalofop-P-ethyl; sequential applications of quizalofop-P-ethyl at 120 g ha−1 followed by (fb) 120 g ai ha−1 applied to rice in the 2- to 3-leaf (EPOST) fb the 4-leaf to 1-tiller (LPOST) growth stages or LPOST fb 10 d after flooding (PTFLD); quizalofop-P-ethyl at 100 g ha−1 fb 139 g ha−1 EPOST fb LPOST or LPOST fb PTFLD; quizalofop-P-ethyl at 139 g ha−1 fb 100 g ha−1 EPOST fb LPOST and LPOST fb PTFLD; and quizalofop-P-ethyl at 85 g ha−1 fb 77 g ha−1 fb 77 g ha−1 EPOST fb LPOST fb PTFLD. Quizalofop-P-ethyl was applied alone and in mixture with florpyrauxifen-benzyl at 29 g ai ha−1 LPOST. Visible rice injury 14 d after PTFLD (DA-PTFLD) was no more than 3%. Visible control of volunteer rice (‘CL151’ and ‘Rex’) 7 DA-PTFLD was similar and at least 95% for each quizalofop-P-ethyl treatment. Barnyardgrass control with quizalofop-P-ethyl at 120 fb 120 g ha−1 LPOST fb PTFLD was greater (88%) in mixture with florpyrauxifen-benzyl. The addition of florpyrauxifen-benzyl to quizalofop-P-ethyl increased rough rice yield when quizalofop-P-ethyl was applied at 100 g ha−1 fb 139 g ha−1 EPOST fb LPOST. Sequential applications of quizalofop-P-ethyl at 120 g ha−1 fb 120 g ha−1 EPOST fb LPOST, 100 g ha−1 fb 139 g ha−1 EPOST fb LPOST, or 139 g ha−1 fb 100 g ha−1 EPOST fb LPOST controlled grass weed species. The addition of florpyrauxifen-benzyl was not beneficial for grass weed control. However, because quizalofop-P-ethyl does not control broadleaf weeds, florpyrauxifen-benzyl could provide broad-spectrum weed control in acetyl coenzyme A carboxylase–resistant rice.

Palmer amaranth is an extremely troublesome weed for soybean growers because of its aggressive growth, adaptability, prolific seed production, and widespread resistance to many herbicides. Studies were initiated to determine the effects of herbicide application at first female inflorescence on Palmer amaranth control, biomass, seed production, cumulative germination, and seed viability. Enlist (2,4-D–resistant) soybean and Xtend (dicamba-resistant) soybean were planted and various combinations of either 2,4-D or dicamba with and without glufosinate and/or glyphosate were applied at first visible female Palmer amaranth inflorescence. Mixtures of 2,4-D + glufosinate and 2,4-D + glufosinate + glyphosate provided the greatest control at 4 wk after treatment in Enlist soybean. Similarly, in Xtend soybean, combinations of dicamba + glufosinate and dicamba + glufosinate + glyphosate provided the greatest control. The greatest reductions in biomass were from combinations of auxin herbicides (2,4-D or dicamba) plus glufosinate with and without glyphosate. Seed production was reduced most by treatments containing at least one effective site of action: an auxin herbicide (2,4-D or dicamba) or glufosinate. In contrast to previous research, cumulative germination and seed viability were not affected by herbicide treatments. This research indicates the efficacy of auxin herbicides or glufosinate alone and in combination to reduce the seed production of Palmer amaranth when applied at first female inflorescence. More research is needed to evaluate the full potential for applications of these herbicides at flower initiation to mitigate the evolution of herbicide resistance.

Topramezone and carfentrazone + 2,4-D + mecoprop-p + dicamba (SpeedZone®) are herbicides labeled for POST goosegrass (Eleusine indica L. Gaertn.) control in hybrid bermudagrass (Cynodon dactylon × C. transvaalensis Burtt Davy). Field research was conducted in Knoxville, TN, during 2019 and 2020 to evaluate goosegrass control and hybrid bermudagrass tolerance to these herbicides applied alone and in mixture. Treatments included topramezone (12.2 g ha–1), SpeedZone® [carfentrazone (33.6 g ha–1) + 2,4-D (1,029 g ha–1) + mecoprop-p (322 g ha–1) + dicamba (91 g ha–1)] and SpeedZone® + topramezone at 12.2, 6.1, 3.6, or 2.4 g ha–1. A nontreated control was included for comparison. Hybrid bermudagrass tolerance was assessed on four cultivars (‘Northbridge’, ‘Tifway’, ‘Tahoma 31’, and ‘TifTuf’) via visual ratings of turfgrass injury and assessments of normalized difference vegetation index (NDVI). At the termination of the experiment, SpeedZone® alone and in mixture with topramezone controlled goosegrass better than or equal to topramezone alone. Mixtures of SpeedZone® + topramezone reduced injury on all cultivars compared to topramezone alone, particularly when mixtures delivered ≤6.1 g ha–1 topramezone. Injury subsided on all cultivars by 28 d after treatment regardless of herbicide. Findings suggest that SpeedZone® can be mixed with topramezone at the rates tested herein to minimize hybrid bermudagrass injury from topramezone applications for goosegrass control.

Many factors such as environment, herbicide rate, growth stage at application, and days between sequential applications can influence the response of a crop to herbicides. Florpyrauxifen-benzyl is a new broad-spectrum, POST herbicide that was commercialized for use in U.S. rice production in 2018. Field experiments were conducted in 2018 at the Pine Tree Research Station (PTRS) near Colt, AR, and the Rice Research and Extension Center (RREC), near Stuttgart, AR, to evaluate crop injury and yield response of three rice cultivars to sequential applications of florpyrauxifen-benzyl. Greenhouse and growth chamber experiments were conducted at the Altheimer Laboratory in Fayetteville, AR, to evaluate cultivar responses when florpyrauxifen-benzyl was applied at 30 or 60 g ae ha−1 to rice exposed to different temperature regimes or at various growth stages. Three rice cultivars were used in all experiments: long-grain variety ‘CL111’, medium-grain variety ‘CL272’, and long-grain hybrid cultivar ‘CLXL745’. CL111 exhibited sufficient tolerance to florpyrauxifen-benzyl with only 10% visible injury and no effect on yield. CL272 showed 15% injury 3 wk after the second application in the field experiment when applications were made 14 d apart. Additionally, 12% injury was observed in greenhouse studies when florpyrauxifen-benzyl was applied at 30 g ae ha−1, averaged over various growth stages at application. Florpyrauxifen-benzyl did not reduce the yield of CL272 in field experiments, indicating that CL272 can recover from florpyrauxifen-benzyl injury. As much as 64% injury was observed for CLXL745 at 3 wk after application (WAA) when sequential herbicide applications were made 4 d apart. High levels of injury occurred in the growth chamber and greenhouse studies for this cultivar as well. Sequential applications of florpyrauxifen-benzyl reduced yields of CLXL745 in nearly all treatments. Data from these experiments suggest that CL272 and CLXL745 are sensitive to sequential applications of florpyrauxifen-benzyl. Growers must follow the prescribed guidelines for using florpyrauxifen-benzyl in these cultivars and others like it.

Field experiments were conducted from 2017 to 2019 to determine the tolerance of carinata to several preemergence and postemergence herbicides. Preliminary screenings identified herbicides that caused large variation on carinata injury, indicating the potential for selectivity. Dose-response field studies were conducted to quantify the tolerance of carinata to select herbicides. Diuron applied preemergence at rates of 280 g ai ha−1 or higher reduced carinata population density 54% to 84% compared to the nontreated control. In certain locations, clomazone applied preemergence caused minor injury with an acceptable level of carinata tolerance and only doses above 105 g ai ha−1 caused yield reductions. Napropamide doses of 2,856 g ai ha−1 or higher applied preemergence caused at least 25% injury to carinata; however, the damage was not severe enough to reduce yields. Simazine applied postemergence at rates above 1,594 g ai ha−1 caused 50% or more injury, resulting in yield losses ranging from 0% to 95% depending on location. Clopyralid applied postemergence at 2,512 g ai ha−1 caused 25% injury with relative yield reductions, which varied across locations. The present study identified clomazone and napropamide applied preemergence, and clopyralid applied postemergence as potential herbicides for weed control in carinata. In contrast, diuron, simazine, metribuzin, imazethapyr, and chlorimuron caused high levels of carinata mortality and can be used to control volunteer carinata plants in rotational crops.

The evolution of multiple herbicide-resistant weeds, including Palmer amaranth, has necessitated the implementation of an integrated weed management (IWM) program. Understanding weed emergence patterns is critical for developing effective IWM strategies. The objective of this study was to evaluate the effect of tillage timings and residual herbicides on cumulative emergence and emergence pattern of Palmer amaranth. Field experiments were conducted in 2015 and 2016 in a field naturally infested with photosystem (PS) II and 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor-resistant Palmer amaranth near Shickley, Nebraska, in a bare ground study, with no crop planted in the plots, although residues from the preceding corn crop were present on the soil surface. Treatments consisted of shallow tillage timings (early, mid, and late), three premix corn or soybean residual herbicides, and a nontreated control. The Weibull function was fitted to cumulative Palmer amaranth emergence with day of year (DOY) and thermal time (TT) as independent variables. Year by treatment interaction was significant for time to 10%, 25%, 50%, 75%, and 90% Palmer amaranth emergence and cumulative emergence. The majority of Palmer amaranth seedlings emerged early, following early tillage with 90% cumulative emergence occurring on DOY 172 compared with DOY 210 to 212 for mid- and late-tillage, and DOY 194 for the nontreated control in 2015. In 2016, 90% of cumulative emergence following early-, mid-, and late-tillage (DOYs 201 to 211) were similar, and that of the nontreated control (DOY 188) was similar to that of early tillage. Nontreated control and PRE herbicide treatments had similar DOY values for 90% emergence in both years. The number of emerged Palmer amaranth seedlings over the season was higher with shallow tillage than no tillage or with the use of PRE herbicides.

Only four herbicides are registered for smooth crabgrass or goosegrass control on creeping bentgrass golf putting greens. None of the four herbicides control weedy grasses for the entire season or control weeds postemergence when applied once at labeled rates. Three of the product labels prohibit repeated use or application during stressful summer conditions. We hypothesized frequently applying herbicides at low doses could provide season-long control of summer grasses while minimizing turf injury. Seven field experiments were conducted on creeping bentgrass putting greens to evaluate various herbicides applied monthly, biweekly, or weekly for postemergence and residual control of goosegrass and smooth crabgrass as well as creeping bentgrass putting green tolerance. Metamifop applied twice monthly at 200 g ai ha−1, topramezone applied eight times weekly at 1.5 g ae ha−1, and siduron applied weekly at 5.6 kg ai ha−1 or four times biweekly at 11 kg ha−1 did not injure creeping bentgrass greater than 10% and maintained creeping bentgrass quality and cover equivalent to nontreated turf. Weekly or biweekly programs of fenoxaprop or quinclorac caused unacceptable injury and quality decline. Metamifop applied monthly and either fenoxaprop program controlled both smooth crabgrass and goosegrass by 97% to 99% throughout the growing season. Programs containing either quinclorac or siduron controlled smooth crabgrass by 99% to 100% but did not control goosegrass greater than 39%. All topramezone programs controlled smooth crabgrass by 69% to 77% and goosegrass by 93% to 98%. In additional studies, siduron applied five times biweekly did not injure creeping bentgrass on putting greens and controlled smooth crabgrass by more than 90% at seasonal, cumulative rates between 17 and 65 kg ai ha−1. This method of frequent, low-dose herbicide treatment to control smooth crabgrass and goosegrass on golf putting greens is novel and currently could be legally implemented with siduron.

Establishment of alfalfa by interseeding it with corn planted for silage can enhance crop productivity but weed management is a challenge to adoption of the practice. Although a simple and effective approach to weed management would be to apply a glyphosate-based herbicide, concerns about herbicide resistance and limitations in available alfalfa varieties exist. Field experiments were conducted to compare the efficacy and selectivity of PRE, POST, and PRE followed by POST herbicide programs to a glyphosate-only strategy when interseeding alfalfa with corn. Experiment 1 compared PRE applications of acetochlor, mesotrione, S-metalochlor, metribuzin, and flumetsulam. Results indicate that acetochlor and metribuzin, and S-metalochlor used at a rate of 1.1 kg ai ha−1 were the most effective and selective PRE herbicides 4 wk after treatment (WAT), but each resulted in greater overall weed cover than glyphosate by 8 WAT. Experiment 2 evaluated applications of bentazon, bromoxynil, 2,4-DB, and mesotrione at early and late POST times. Several herbicides used POST exhibited similar effectiveness and selectivity as glyphosate, including early applications of bromoxynil (0.14 kg ai ha−1) and 2,4-DB (0.84 or 1.68 kg ai ha−1), as well as late applications of bromoxynil (0.42 kg ai ha−1), 2,4-DB (0.84 kg ai ha−1), and mesotrione (0.05 or 0.11 kg ai ha−1). A third experiment compared applications of acetochlor PRE, bromoxynil POST, and a combination of acetochlor PRE with bromoxynil POST. All treatments were effective and safe for use in this interseeded system, although interseeded alfalfa provided 65% to 70% weed suppression in corn planted for silage without any herbicide. Herbicide treatments had no observable impacts on corn and alfalfa yields, so weed management was likely of limited economic importance. However, weed competitiveness can vary based on several different factors including weed species, density, and site-specific factors, and so further investigations under different environments and conditions are needed.

A field study was conducted in 2017 and 2018 to determine foliar efficacy of halauxifen-methyl, 2,4-D, or dicamba applied alone and in combination with glyphosate at preplant burndown timing. Experiments were conducted near Painter, VA; Rocky Mount, NC; Jackson, NC; and Gates, NC. Control of horseweed, henbit, purple deadnettle, cutleaf evening primrose, curly dock, purple cudweed, and common chickweed were evaluated. Halauxifen-methyl applied at 5 g ae ha−1 controlled small and large horseweed 89% and 79%, respectively, and was similar to control by dicamba applied at 280 g ae ha−1. Both rates of 2,4-D—533 g ae ha−1(low rate [LR]) or 1,066 g ae ha−1 (high rate [HR])—were less effective than halauxifen-methyl and dicamba for controlling horseweed. Halauxifen-methyl was the only auxin herbicide to control henbit (90%) and purple deadnettle (99%). Cutleaf evening primrose was controlled 74% to 85%, 51%, and 4% by 2,4-D, dicamba, and halauxifen-methyl, respectively. Dicamba and 2,4-D controlled curly dock 59% to 70% and were more effective than halauxifen-methyl (5%). Auxin herbicides applied alone controlled purple cudweed and common chickweed 21% or less. With the exception of cutleaf evening primrose (35%) and curly dock (37%), glyphosate alone provided 95% or greater control of all weeds evaluated. These experiments demonstrate halauxifen-methyl effectively (≥79%) controls horseweed, henbit, and purple deadnettle, whereas common chickweed, curly dock, cutleaf evening primrose, and purple cudweed control by the herbicide is inadequate (≤7%).

Velvetleaf is an economically important weed in agronomic crops in Nebraska and the United States. Dicamba applied alone usually does not provide complete velvetleaf control, particularly when velvetleaf is taller than 15 cm. The objectives of this experiment were to evaluate the interaction of dicamba, fluthiacet-methyl, and glyphosate applied alone or in a mixture in two- or three-way combinations for velvetleaf control in dicamba/glyphosate–resistant (DGR) soybean and to evaluate whether velvetleaf height (≤12 cm or ≤20 cm) at the time of herbicide application influences herbicide efficacy, velvetleaf density, biomass, and soybean yield. Field experiments were conducted near Clay Center, NE in 2019 and 2020. The experiment was arranged in a split-plot with velvetleaf height (≤12 cm or ≤20 cm) as the main plot treatment and herbicides as subplot treatment. Fluthiacet provided ≥94% velvetleaf control 28 d after treatment (DAT) and ≥96% biomass reduction regardless of application rate or velvetleaf height. Velvetleaf control was 31% to 74% at 28 DAT when dicamba or glyphosate was applied alone to velvetleaf ≤20 cm tall compared with 47% to 100% control applied to ≤12-cm-tall plants. Dicamba applied alone to ≤20-cm-tall velvetleaf provided <75% control and <87% biomass reduction 28 DAT compared with ≥90% control with dicamba at 560 g ae ha−1 + fluthiacet at 7.2 g ai ha−1 or glyphosate at 1,260 g ae ha−1. Dicamba at 280 g ae ha−1 + glyphosate at 630 g ae ha−1 applied to ≤20-cm-tall velvetleaf resulted in 86% control 28 DAT compared with the expected 99% control. The interaction of dicamba + fluthiacet + glyphosate was additive for velvetleaf control and biomass reduction regardless of application rate and velvetleaf height.

XtendFlexTM cotton with resistance to glyphosate, glufosinate, and dicamba may become available in Australia. Resistance to these herbicides enables two additional modes of action to be applied in crop. The double-knock strategy, typically glyphosate followed by paraquat, has been a successful tactic for control of glyphosate-resistant cotton in fallow situations in Australia. Glufosinate is a contact herbicide and may be useful as the second herbicide in a double knock for use in XtendFlexTM cotton crops. We tested the effectiveness of glufosinate applied at intervals of 1, 3, 7, and 10 d after initial applications of glyphosate, dicamba, clethodim, and glyphosate mixtures with dicamba or clethodim on glyphosate-resistant and glyphosate-susceptible populations of flaxleaf fleabane, common sowthistle, feather fingergrass, windmill grass, and junglerice. Effective treatments for flaxleaf fleabane with 100% control were dicamba and glyphosate+dicamba followed by glufosinate independent of the interval between applications. Common sowthistle was effectively controlled in Experiment 1 by all treatments. However, in Experiment 2, effective treatments were dicamba and glyphosate+dicamba followed by glufosinate (99.3% to 100% control). Timing of the follow-up glufosinate did not affect the control achieved. Consistent control of feather fingergrass was achieved with glyphosate, clethodim, or glyphosate+clethodim followed by glufosinate at 7-d and 10-d intervals (99.7% to 100% control). Control of feather fingergrass was inconsistent. The best treatment for windmill grass was glyphosate+clethodim followed by glufosinate 10 d later (99.8% to 100% control). Junglerice was effectively controlled with all treatments except for glyphosate on the glyphosate-resistant population. Additional in-crop use of glufosinate and dicamba should be beneficial for weed management in XtendFlexTM cotton crops, when using the double knock tactic with glufosinate. For effective herbicide resistance management, it is important that these herbicides be used in addition to, rather than substitution for, existing weed management tactics.

During the 2017 to 2019 growing seasons, samples of waterhemp and Palmer amaranth that had reportedly survived field-rate applications of protoporphyrinogen oxidase (PPO)–inhibiting herbicides were collected from the American Midwest and tested for target-site mutations known at the time to confer resistance. Target-site resistance was identified in nearly all (135 of 145) tested common waterhemp populations but in only 8 of 13 Palmer amaranth populations. Follow-up research on one population of Palmer amaranth (W-8), which tested negative for all such mutations, confirmed it was resistant to lactofen, with a magnitude of resistance comparable to that conferred by the ΔG210 PPO2 mutation. Gene sequences from both isoforms of PPO (PPO1 and PPO2) were compared between W-8 and known PPO inhibitor–sensitive sequence. A glycine-to-alanine substitution at the 399th amino acid position (G399A) of PPO2, recently identified to reduce target-site herbicide sensitivity, was observed in a subset of resistant W-8 plants. Because no missense mutation completely delimited resistant and sensitive sequences, we initially suspected the presence of a secondary, non-target-site resistance mechanism in this population. To isolate G399A, a segregating F2 population was produced and screened with a delimiting rate of lactofen. χ2 goodness-of-fit analysis of dead/alive ratings indicated single-locus inheritance of resistance in the F2 population, and molecular markers for the W-8 parental PPO2 coding region co-segregated tightly, but not perfectly, with resistance. More research is needed to fully characterize Palmer amaranth PPO inhibitor–resistance mechanisms, which appear to be more diverse than those found in common waterhemp.

In current and next-generation weed control technologies, sequential applications of contact and systemic herbicides for postemergence control of troublesome weeds are needed to mitigate the evolution of herbicide resistance. A clear understanding of the impact auxin herbicide symptomology has on Palmer amaranth groundcover will aid optimization of sequential herbicide applications. Field and greenhouse experiments were conducted in Fayetteville, AR, and a laboratory experiment was conducted in Lonoke, AR, in 2020 to evaluate changes in Palmer amaranth groundcover following an application of 2,4-D and dicamba with various nozzles, droplet sizes, and velocities. Field experiments utilized three nozzles: Extended Range (XR), Air Induction Extended Range (AIXR), and Turbo TeeJet® Induction (TTI), to assess the effect of spray droplet size on changes in Palmer amaranth groundcover. Nozzle did not affect Palmer amaranth groundcover when dicamba was applied. However, nozzle selection did impact groundcover when 2,4-D was applied; the following nozzle order XR > AIXR > TTI reduced Palmer amaranth groundcover the most in both site-years of the field experiment. This result (XR > AIXR > TTI) matches percent spray coverage data for 2,4-D and is inversely related to spray droplet size data. Rapid reductions of Palmer amaranth groundcover from 100% at time zero to 39.4% to 64.1% and 60.0% to 85.8% were observed 180 min after application in greenhouse and field experiments, respectively, regardless of herbicide or nozzle. In one site-year of the greenhouse and field experiments, regrowth of Palmer amaranth occurred 10,080 min (14 d) after an application of either 2,4-D or dicamba to larger than labeled weeds. In all experiments, complete reduction of live Palmer amaranth tissue was not observed 21 d after application with any herbicide or nozzle combination. Control of Palmer amaranth escapes with reduced groundcover may potentially lead to increased selection pressure on sequentially applied herbicides due to a reduction in spray solution contact with the targeted pest.

Residual herbicides applied PRE provide early season weed control, potentially avoid the need for multiple POST herbicides, and can provide additional control of herbicide-resistant weeds. Thus, field studies were conducted in 2017 and 2018 at Concord, NE, to evaluate the influence of PRE herbicides on critical time for postemergence weed removal (CTWR) in corn. The studies were arranged in a split-plot design that consisted of three herbicide regimes as main plot treatments and seven weed removal timings as subplot treatments in four replications. The herbicide regimes included no PRE herbicide, atrazine, and a premix of saflufenacil/dimethenamid-P mixed with pyroxasulfone. The weed removal timings were at V3, V6, V9, V12, and V15 corn growth stages and then plots were kept weed-free until harvest. A weed-free and nontreated control were included for comparison. The relationship between corn growth or yield, and weed removal timings in growing degree days (GDD) was described by a four-parameter log-logistic model. This model was used to estimate the critical time for weed removal based on 5% crop yield loss threshold. A delay in weed removal until the V2 to V3 corn growth stage (91 to 126 GDD) reduced corn biomass by 5% without PRE herbicide application. The CTWR started at V3 without PRE herbicide in both years. Atrazine delayed the CTWR up to V5 in both years, whereas saflufenacil/dimethenamid-P plus pyroxasulfone further delayed the CTWR up to the V10 and V8 corn growth stages in 2017 and 2018, respectively. Herbicide applied PRE particularly with multiple sites of action can delay the CTWR in corn up to a maximum growth stage of V10, and delay or reduce the need for POST weed management.

Carfentrazone-ethyl is one of few herbicides labeled for control of silvery-thread moss (STM) in golf course putting greens, but common use rates are up to three times higher than for broadleaf weeds. Our objective was to determine the efficacy of a single POST application of carfentrazone-ethyl for STM control in greenhouse and field dose response studies. In the greenhouse, carfentrazone-ethyl was applied at 0, 14, 28, 56, 112, and 224 g ai ha–1 to pots containing established STM and creeping bentgrass. Percent gametophyte injury was visually estimated at 14, 28, 49, and 77 d after treatment (DAT). Shoot viability was determined by excising shoots from treated pots and plating them in Petri dishes containing sand. The 28- and 49-DAT ED90 (doses required to cause 90% gametophyte injury) were 26.8 and 54.3 g ha–1, respectively; both of these doses are substantially lower than the label rates for long- and short-term control, respectively. All doses reduced the viability of transplanted shoots at 10 DAT compared to untreated STM; however, regrowth occurred in all Petri dishes by 17 DAT. Field studies were initiated in Manhattan, KS and San Luis Obispo, CA to corroborate greenhouse results. Averaged across locations, carfentrazone-ethyl applied at 56 and 112 g ha–1 caused 76% and 84% STM injury at 14 DAT, but STM injury quickly lessened to 45% and 48% by 28 DAT, respectively. In greenhouse and field studies, STM recovery did not occur until 2 wk after treatment (WAT), which indicates the label-stipulated application interval of 2 wk is too short. Our research suggests that 56 g ha–1 can provide similar burndown control of STM as compared to the highest label rate (112 g ha–1), and turfgrass managers should consider extending the reapplication interval to 3 or 4 wk when moss recovery is observed.

A prepackaged mixture of desmedipham + phenmedipham was previously labeled for control of Amaranthus spp. in sugarbeet. Currently, there are no effective POST herbicide options to control glyphosate-resistant Palmer amaranth in sugarbeet. Sugarbeet growers are interested in using desmedipham + phenmedipham to control escaped Palmer amaranth. In 2019, a greenhouse experiment was initiated near Scottsbluff, NE, to determine the selectivity of desmedipham and phenmedipham between Palmer amaranth and sugarbeet. Three populations of Palmer amaranth and four sugarbeet hybrids were evaluated. Herbicide treatments consisted of desmedipham and phenmedipham applied singly or as mixtures at an equivalent rate. Herbicides were applied when Palmer amaranth and sugarbeet were at the cotyledon stage, or two true-leaf sugarbeet stage and when Palmer amaranth was 7 cm tall. The selectivity indices for desmedipham, phenmedipham, and desmedipham + phenmedipham were 1.61, 2.47, and 3.05, respectively, at the cotyledon stage. At the two true-leaf application stage, the highest rates of desmedipham and phenmedipham were associated with low mortality rates in sugarbeet, resulting in a failed response of death. The highest rates of desmedipham + phenmedipham caused a death response of sugarbeet; the selectivity index was 2.15. Desmedipham treatments resulted in lower LD50 estimates for Palmer amaranth compared to phenmedipham, indicating that desmedipham can provide greater levels of control for Palmer amaranth. However, desmedipham also caused greater injury in sugarbeet, producing lower LD50 estimates compared to phenmedipham. Desmedipham + phenmedipham provided 90% or greater control of cotyledon-size Palmer amaranth at a labeled rate but also caused high levels of sugarbeet injury. Neither desmedipham, phenmedipham, nor desmedipham + phenmedipham was able to control 7-cm tall Palmer amaranth at previously labeled rates. Results indicate that desmedipham + phenmedipham can only control Palmer amaranth if applied at the cotyledon stage and a high level of sugarbeet injury is acceptable.

Weed control of paraquat can be erratic and may be attributable to differing species sensitivity and/or environmental factors for which minor guidance is available on commercial labels. Therefore, the objectives of this research were to quantify selectivity of paraquat across select weed species and the influence of environmental factors. Experiments were performed under controlled conditions in the greenhouse and growth chamber. Compared with purple deadnettle (dose necessary to reduce shoot biomass by 50% = 39 g ai ha−1), waterhemp, Palmer amaranth, giant ragweed, and horseweed were 4.9, 3.3, 1.9, and 1.3 times more sensitive to paraquat, respectively. The injury progression rate over 3 d after treatment (DAT) was a more accurate predictor of final efficacy at 14 DAT than the lag phase until symptoms first appeared. For example, at the 17.5 g ha−1 dose, the injury rate of waterhemp and Palmer amaranth was, on average, 3.6 times greater than that of horseweed and purple deadnettle. The influence of various environmental factors on paraquat efficacy was weed specific. Applications made at sunrise improved control of purple deadnettle over applications at solar noon or sunset. Lower light intensities (200 or 600 μmol m−2 s−1) surrounding the time of application improved control of waterhemp and horseweed more than 1,000 μmol m−2 s−1. Day/night temperatures of 27/16 C improved horseweed and purple deadnettle control compared with day/night temperatures of 18/13 C. Though control was positively associated with injury rates in the application time of day and temperature experiments, a negative relationship was observed for waterhemp in the light-intensity experiment. Thus, although there are conditions that enhance paraquat efficacy, the specific target species must also be considered. These results advocate paraquat dose recommendations, currently based on weed height, be expanded to address sensitivity differences among weeds. Moreover, these findings contrast with paraquat labels stating temperatures of 13 C or lower do not reduce paraquat efficacy.

Palmer amaranth is the most problematic and troublesome weed in agronomic cropping systems in the United States. Acetolactate synthase (ALS) inhibitor and glyphosate-resistant (GR) Palmer amaranth has been confirmed in Nebraska and it is widespread in several counties. Soybean resistant to isoxaflutole/glufosinate/glyphosate has been developed that provides additional herbicide site of action for control of herbicide-resistant weeds. The objectives of this study were to evaluate herbicide programs for control of ALS inhibitor/GR Palmer amaranth and their effect on Palmer amaranth density and biomass, as well as soybean injury and yield in isoxaflutole/glufosinate/glyphosate–resistant soybean. Field experiments were conducted in a grower’s field infested with ALS inhibitor and GR Palmer amaranth near Carleton, Nebraska, in 2018 and 2019. Isoxaflutole applied alone or mixed with sulfentrazone/pyroxasulfone, flumioxazin/pyroxasulfone, or imazethapyr/saflufenacil/pyroxasulfone provided similar control (86%–99%) of Palmer amaranth 21 d after PRE (DAPRE). At 14 d after early-POST (DAEPOST), isoxaflutole applied PRE and PRE followed by (fb) POST controlled Palmer amaranth by 10% to 63% compared to 75% to 96% control with glufosinate applied EPOST in both years. A PRE herbicide fb glufosinate controlled Palmer amaranth 80% to 99% 21 d after late-POST (DALPOST) in 2018, and reduced density 89% to 100% in 2018 and 58% to 100% in 2019 at 14 DAEPOST. No soybean injury was observed from any of the herbicide programs tested in this study. Soybean yield in 2019 was relatively higher due to higher precipitation compared with 2018 with generally no differences between herbicide programs. This research indicates that herbicide programs are available for effective control of ALS inhibitor/GR Palmer amaranth in isoxaflutole/glufosinate/glyphosate-resistant soybean.