Palmer amaranth is the most common and troublesome weed in North Carolina sweetpotato. Field studies were conducted in Clinton, NC, in 2016 and 2017 to determine the critical timing of Palmer amaranth removal in ‘Covington’ sweetpotato. Palmer amaranth was grown with sweetpotato from transplanting to 2, 3, 4, 5, 6, 7, 8, and 9 wk after transplanting (WAP) and maintained weed-free for the remainder of the season. Palmer amaranth height and shoot dry biomass increased as Palmer amaranth removal was delayed. Season-long competition by Palmer amaranth interference reduced marketable yields by 85% and 95% in 2016 and 2017, respectively. Sweetpotato yield loss displayed a strong inverse linear relationship with Palmer amaranth height. A 0.6% and 0.4% decrease in yield was observed for every centimeter of Palmer amaranth growth in 2016 and 2017, respectively. The critical timing for Palmer amaranth removal, based on 5% loss of marketable yield, was determined by fitting a log-logistic model to the relative yield data and was determined to be 2 WAP. These results show that Palmer amaranth is highly competitive with sweetpotato and should be managed as early as possible in the season. The requirement of an early critical timing of weed removal to prevent yield loss emphasizes the importance of early-season scouting and Palmer amaranth removal in sweetpotato fields. Any delay in removal can result in substantial yield reductions and fewer premium quality roots.

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

Field trials were conducted near Lubbock, TX, in 2013, 2014, and 2015 to evaluate non–2,4-D–resistant cotton response to low rates of glyphosate plus 2,4-D choline. Cotton was treated with five rates of glyphosate plus 2,4-D choline (0.0183, 0.183, 1.83, 18.3, and 183 g ae ha−1) at two application timings (nine leaf and first bloom). These rates correspond to contamination rates of 0.0008%, 0.008%, 0.08%, 0.8%, and 8%, respectively. Visual cotton injury, boll retention, lint yield, and fiber properties were recorded. When averaged over contamination rates, visual injury after applications made to nine-leaf cotton was greater than for first-bloom cotton in three of 3 yr and yield loss was greater when applications were made to nine-leaf cotton when compared with first-bloom cotton in two of 3 yr. Averaged over application timing, lint yield in 2013, 2014, and 2015 after glyphosate plus 2,4-D choline contamination rates of 0.0008% and 0.008% were not different than that of the nontreated control, whereas contamination rates of 0.08%, 0.8%, and 8% decreased yield by 3% to 20%, 45% to 58%, and 80% to 96%, respectively. Contamination rates of 0.0008%, 0.008%, 0.08%, and 0.8% rarely affected fiber quality; however, a contamination rate of 8% frequently decreased micronaire, fiber length, fiber length uniformity, and fiber strength. This decrease in fiber quality also resulted in a reduction in cotton loan value and potential financial return. Although decreases in fiber quality parameters were not observed with the 0.8% contamination rate, significant reductions in financial return occurred due to yield loss caused by injury from glyphosate plus 2,4-D choline.

In recent years, soybean acreage has increased significantly in western Canada. One of the challenges associated with growing soybean in western Canada is the control of volunteer glyphosate-resistant (GR) canola, because most soybean cultivars are also glyphosate resistant. The objective of this research was to determine the impact of soybean seeding rate and planting date on competition with volunteer canola. We also attempted to determine how high seeding rate could be raised while still being economically feasible for producers. Soybean was seeded at five different seeding rates (targeted 10, 20, 40, 80, and 160 plants m−2) and three planting dates (targeted mid-May, late May, and early June) at four sites across western Canada in 2014 and 2015. Soybean yield consistently increased with higher seeding rates, whereas volunteer canola biomass decreased. Planting date generally produced variable results across site-years. An economic analysis determined that the optimal rate was 40 to 60 plants m−2, depending on market price, and the optimal planting date range was from May 20 to June 1.

Currently, there are seven herbicides labeled for U.S. tobacco production; however, additional modes of action are greatly needed in order to reduce the risk of herbicide resistance. Field experiments were conducted at five locations during the 2017 and 2018 growing seasons to evaluate flue-cured tobacco tolerance to S-metolachlor applied pretransplanting incorporated (PTI) and pretransplanting (PRETR) at 1.07 (1×) and 2.14 (2×) kg ai ha−1. Severe injury was observed 6 wk after transplanting at the Whiteville environment in 2017 when S-metolachlor was applied PTI. End-of-season plant heights from PTI treatments at Whiteville were likewise reduced by 9% to 29% compared with nontreated controls, although cured leaf yield and value were reduced only when S-metolachlor was applied PTI at the 2× rate. Severe growth reduction was also observed at the Kinston location in 2018 where S-metolachlor was applied at the 2× rate. End-of-season plant heights were reduced 11% (PTI, 2×) and 20% (PRETR, 2×) compared with nontreated control plants. Cured leaf yield was reduced in Kinston when S-metolachlor was applied PRETR at the 2× rate; however, treatments did not impact cured leaf quality or value. Visual injury and reductions in stalk height, yield, quality, and value were not observed at the other three locations. Ultimately, it appears that injury potential from S-metolachlor is promoted by coarse soil texture and high early-season precipitation close to transplanting, both of which were documented at the Whiteville and Kinston locations. To reduce plant injury and the negative impacts to leaf yield and value, application rates lower than 1.07 kg ha−1 may be required in these scenarios.

Management of volunteer glyphosate-resistant (GR) corn may be problematic in soybean resistant to glyphosate and 2,4-D or dicamba, as auxinic herbicides often antagonize graminicide efficacy. Field and greenhouse trials were conducted using mixtures of 2,4-D or dicamba in combination with glyphosate and clethodim-A (formulated without an adjuvant) or clethodim-SM (adjuvant-inclusive formulation) to determine the effect on volunteer GR corn control. Neither auxinic herbicide reduced clethodim efficacy, regardless of clethodim rate or formulation in field trials. However, the addition of glyphosate to these mixtures at the 35 g ai ha−1 clethodim dose reduced control from clethodim-A and clethodim-SM by 62% to 75% and 27% to 47%, respectively. Increasing the clethodim dose to 105 g ha−1 or greater in combination with glyphosate and either auxinic herbicide generally restored clethodim efficacy (74% to 98% control); in one site-year, the addition of glyphosate plus dicamba to clethodim-A at 140 g ha−1 still reduced control by 34%. In greenhouse experiments, clethodim-A efficacy was reduced by 17% and 28% when applied with glyphosate plus 420 and 1,680 g ae ha−1 2,4-D, respectively, in the absence of crop oil concentrate (COC). Increasing the dose of dicamba in a similar mixture had a negligible effect. Irrespective of auxinic herbicide dose, the inclusion of COC to clethodim-A mixtures with glyphosate plus 2,4-D or dicamba resulted in ≥ 90% control. These results specify an enhanced risk of reduced clethodim efficacy on volunteer GR corn when glyphosate is added to mixtures containing 2,4-D or dicamba. To optimize control from these mixtures, clethodim should be applied at ≥ 105 g ha−1 and should include an activator adjuvant in the form of COC and/or an adjuvant-inclusive clethodim formulation. This recommendation contrasts with several labels of clethodim that do not require COC when applied with adjuvant-loaded glyphosate products.

Residual herbicides remain the primary tool for efficient weed control in cucurbit crops because of the lack of crop tolerance to many POST herbicide options. Field experiments were conducted in New Jersey in 2018 and 2019 to determine weed control efficacy and tolerance of direct-seeded cucumber ‘Python’ and summer squash ‘Gold Prize’ to S-metolachlor applied at 0.7 or 1.4 kg ai ha−1 at planting (PRE) or when crops reached the second- to third-leaf stage (EPOST). Regardless of applied rate, S-metolachlor PRE or EPOST provided 96% to 100% control 3 wk after planting (WAP) of smooth pigweed, large crabgrass, and giant foxtail. S-metolachlor PRE significantly improved American black nightshade and carpetweed control 3 WAP with respect to bensulide, and smooth pigweed with respect to clomazone + ethalfluralin. Summer squash showed excellent tolerance, regardless of S-metolachlor rate or timing of application, with stunting not exceeding 17% 4 WAP and 3% 7 WAP at the 1.4 kg ha−1 rate. Marketable yield decreased by 15% with S-metolachlor PRE or POST at 1.4 kg ha−1 with respect to clomazone + ethalfluralin, a reduction not noted when comparing with bensulide or the handweeded control. Marketable fruit number plant−1 and individual fruit weight were not affected by S-metolachlor applications. Conversely, cucumber was more sensitive to S-metolachlor than summer squash was with 30% seedling emergence reduction and 36% to 43% stunting 4 WAP when S-metolachlor was applied PRE at 1.4 kg ha−1. EPOST application resulted in 15% to 26% cucumber injury 1 wk after treatment. Marketable yield declined by 21% and 39% with the 0.7 and 1.4 kg ha−1 rates of S-metolachlor, respectively, compared with clomazone + ethalfluralin. Therefore, S-metolachlor may be a novel alternative to already labeled residual herbicides for summer squash, but unacceptable injury and yield reduction do not support its registration on cucumber.

Halauxifen plus florasulam, thifensulfuron plus fluroxypyr, and bromoxynil plus bicyclopyrone are three, relatively new POST premix herbicides developed for control of broadleaf weeds in winter wheat. These herbicides, along with older products, were evaluated for their control of horseweed in Altus, Perkins, and Ponca City, Oklahoma, during the spring of 2017 and 2018. Horseweed has become a critical weed in Oklahoma because of its extensive germination window, changes in tillage practices, and increase in herbicide-resistant horseweed biotypes. Visual weed control was estimated every 2 wk throughout the growing season and wheat yield was collected from three of the six site-years. Horseweed size ranged from 5 to 20 cm at time of application. The halauxifen plus florasulam, and thifensulfuron plus fluroxypyr combinations were effective at controlling a wide range of horseweed rosette sizes across all locations, whereas control with other treatments varied depending on presence of herbicide resistance, weed size at time of application, and mix partner.

Management options are needed to limit sweetpotato yield loss due to weeds. Greenhouse studies were conducted in 2018 in Greensboro, NC, and in the field from 2016 to 2018 in Clinton, NC, to evaluate the effect of bicyclopyrone on sweetpotato and Palmer amaranth (field only). In greenhouse studies, Covington and NC04-531 clones were treated with bicyclopyrone (0, 25, 50, 100, or 150 g ai ha−1) either preplant (PP; i.e., immediately before transplanting) or post-transplant (PT; i.e., on the same day after transplanting). Sweetpotato plant injury and stunting increased, and vine length and shoot dry weight decreased with increasing rate of bicyclopyrone regardless of clone or application timing. In field studies, Beauregard (2016) or Covington (2017 and 2018) sweetpotato clones were treated with bicyclopyrone at 50 g ha−1 PP, flumioxazin at 107 g ai ha−1 PP, bicyclopyrone at 50 or 100 g ha−1 PP followed by (fb) S-metolachlor at 800 g ai ha−1 PT, flumioxazin at 107 g ha−1 PP fb S-metolachlor at 800 g ha−1 PT, flumioxazin at 107 g ha−1 PP fb S-metolachlor at 800 g ha−1 PT fb bicyclopyrone at 50 g ha−1 PT-directed, and clomazone at 420 g ai ha−1 PP fb S-metolachlor at 800 g ha−1 PT. Bicyclopyrone PP at 100 g ha−1 fb S-metolachlor PT caused 33% or greater crop stunting and 44% or greater marketable yield reduction compared with the weed-free check in 2016 (Beauregard) and 2017 (Covington). Bicyclopyrone PP at 50 g ha−1 alone or fb S-metolachlor PT resulted in 12% or less injury and similar no. 1 and jumbo yields as the weed-free check in 2 of 3 yr. Injury to Covington from bicyclopyrone PT-directed was 4% or less at 4 or 5 wk after transplanting and marketable yield was similar to that of the weed-free check in 2017 and 2018.

Australian conservation cropping systems are practiced on very large farms (approximately 3,000 ha) where herbicides are relied on for effective and timely weed control. In many fields, though, there are low weed densities (e.g., <1.0 plant 10 m−2) and whole-field herbicide treatments are wasteful. For fallow weed control, commercially available weed detection systems provide the opportunity for site-specific herbicide treatments, removing the need for whole-field treatment of fallow fields with low weed densities. Concern about the sustainability of herbicide-reliant weed management systems remain and there has not been interest in the use of weed detection systems for alternative weed control technologies, such as targeted tillage. In this paper, we discuss the use of a targeted tillage technique for site-specific weed control in large-scale crop production systems. Three small-scale prototypes were used for engineering and weed control efficacy testing across a range of species and growth stages. With confidence established in the design approach and a demonstrated 100% weed-control potential, a 6-m wide pre-commercial prototype, the “Weed Chipper,” was built incorporating commercially available weed-detection cameras for practical field-scale evaluation. This testing confirmed very high (90%) weed control efficacies and associated low levels (1.8%) of soil disturbance where the weed density was fewer than 1.0 plant 10 m−2 in a commercial fallow. These data established the suitability of this mechanical approach to weed control for conservation cropping systems. The development of targeted tillage for fallow weed control represents the introduction of site-specific, nonchemical weed control for conservation cropping systems.

The pyridine carboxylic acid (PCA) herbicide family can exhibit differential activity within and among plant species, despite molecular resemblances. Aminocyclopyrachlor (AMCP), a pyrimidine carboxylic acid, is a recently discovered compound with similar use patterns to those of the PCA family; however, relative activity among PCAs and AMCP is not well understood. Therefore, the objective of this study was to quantify relative activity among aminopyralid, picloram, clopyralid, triclopyr, and AMCP in canola, squash, and okra using dose-response whole-plant bioassays. Clopyralid was less active than all other herbicides in all species and did not fit dose-response models. Aminopyralid and picloram performed similarly in squash (ED50 = 21.1 and 23.3 g ae ha−1, respectively). Aminopyralid was 3.8 times and 1.7 times more active than picloram in canola (ED50 = 60.3 and 227.7 g ha−1, respectively) and okra (ED50 = 10.3 and 17.3 g ha−1, respectively). Triclopyr (ED50 = 37.3 g ha−1) was more active than AMCP (ED50 = 112.9 g ha−1) and picloram in canola. Aminocyclopyrachlor (ED50 = 6.6 g ha−1) and triclopyr (ED50 = 7.8 g ha−1) were more active in squash than aminopyralid and picloram. In okra, AMCP (ED50 = 14.6 g ha−1) and aminopyralid (ED50 = 10.3 g ha−1) performed similarly but were more active than triclopyr (ED50 = 88.2 g ha−1). Herbicidal activity among AMCP and PCAs was vastly different despite molecular similarities that could be due to variable target-site sensitivity among species.

The occurrence of herbicide tank contamination with dicamba or 2,4-D will likely increase with the recent commercialization of dicamba- and 2,4-D-resistant soybean. High-value sensitive crops, including dry bean, will be at higher risks for exposure. In 2017 and 2018, two separate field experiments were conducted in Michigan to understand how multiple factors may influence dry bean response to dicamba and 2,4-D herbicides, including 1) the interaction between herbicides applied POST to dry bean and dicamba or 2,4-D, and 2) the impact of low rates of glyphosate with dicamba or 2,4-D. Dry bean injury was 20% and 2% from POST applications of dicamba (5.6 h ae ha−1) and 2,4-D (11.2 g ae ha−1), respectively, 14 days after treatment (DAT). The addition of glyphosate (8.4 g ae ha−1) did not increase dry bean injury from dicamba or 2,4-D. Over 2 site-years the addition of dry bean herbicides to dicamba or dicamba + glyphosate (8.4 g ae ha−1) increased dry bean injury and reduced yield by 6% to 10% more than when dicamba or dicamba + glyphosate was applied alone. The interaction between 2,4-D (11.2 g ae ha−1) and dry bean herbicides was determined to be synergistic. However, 2,4-D (11.2 g ae ha−1) had little effect on dry bean with or without the addition of a dry bean herbicide program. These studies document that synergy also occurs between dicamba and dicamba + glyphosate and both common dry bean herbicide programs tested: 1) imazamox (35 g ha−1) + bentazon (560 g ha−1), and 2) fomesafen (280 g ha−1). The synergy between dry bean herbicide and dicamba and dicamba + glyphosate can increase plant injury, delay maturity, and reduce yield to a greater extent than dicamba or dicamba + glyphosate alone. This work emphasizes the need to properly clean out sprayers after applications of dicamba to reduce the risk of exposure to other crops.

In recent years, unmanned aerial vehicle (UAV) technology has expanded to include UAV sprayers capable of applying pesticides. Very little research has been conducted to optimize application parameters and measure the potential of off-target movement from UAV-based pesticide applications. Field experiments were conducted in Raleigh, NC during spring 2018 to characterize the effect of different application speeds and nozzle types on target area coverage and uniformity of UAV applications. The highest coverage was achieved with an application speed of 1 m s−1 and ranged from 30% to 60%, whereas applications at 7 m s−1 yielded 13% to 22% coverage. Coverage consistently decreased as application speed increased across all nozzles, with extended-range flat-spray nozzles declining at a faster rate than air-induction nozzles, likely due to higher drift. Experiments measuring the drift potential of UAV-applied pesticides using extended-range flat spray, air-induction flat-spray, turbo air–induction flat-spray, and hollow-cone nozzles under 0, 2, 4, 7, and 9 m s−1 perpendicular wind conditions in the immediate 1.75 m above the target were conducted in the absence of natural wind. Off-target movement was observed under all perpendicular wind conditions with all nozzles tested but was nondetectable beyond 5 m away from the target. Coverage from all nozzles exhibited a concave-shaped curve in response to the increasing perpendicular wind speed due to turbulence. The maximum target coverage in drift studies was observed when the perpendicular wind was 0 and 8.94 m s−1, but higher turbulence at the two highest perpendicular wind speeds (6.71 and 8.94 m s−1) increased coverage variability, whereas the lowest variability was observed at 2.24 m s−1 wind speed. Results suggested that air-induction flat-spray and turbo air–induction flat-spray nozzles and an application speed of 3 m s−1 provided an adequate coverage of target areas while minimizing off-target movement risk.

During the 2015, 2016, and 2017 growing seasons, a survey of 63 pastures in Missouri was conducted to determine the effects of selected soil and forage parameters on the density of common annual, biennial, and perennial weed species. Permanent sampling areas were established in each pasture at a frequency of one representative 20-m2 area per 4 ha of pasture, and weed species and density in each area were determined at 14-d intervals for a period from mid-April until late September. The parameters evaluated included soil pH, phosphorus (P), potassium (K), magnesium (Mg), calcium (Ca), sulfur (S), zinc (Zn), manganese (Mn), and copper (Cu) concentrations, as well as tall fescue density, forage groundcover density, and stocking rate. An increase of 1 unit in soil pH was associated with 146 fewer weeds per hectare, the largest reduction in weed density in response to any soil parameter. Increased soil pH was associated with the greatest reduction in perennial grass weed density, along with an average reduction of 1,410 brush weeds per hectare for each 1-unit increase in soil pH. Common ragweed, a widespread weed of pastures, could be reduced by 3,056 weeds ha−1 when soil pH was 1 unit greater. A 1-ppm increase in soil P was correlated with a decrease of 206 biennial broadleaf weeds per hectare. Perennial broadleaf weed density was reduced in soils with greater concentrations of P, K, and Ca. Additionally, for every 1% increase of tall fescue and forage groundcover, there was a decrease of 18 and 38 perennial broadleaf weeds per hectare. The results from this research indicate that the density of many common weed species can be reduced with higher soil pH and adjustments to soil macro- and micronutrient concentrations, especially P.

Sensitive cotton varieties planted into soil treated with 2,4-D or dicamba utilized in burndowns can result in stunting and stand loss if use rate is too high and the plant-back interval is too short. The objective of this study was to evaluate cotton stunting and yield responses resulting from 2,4-D or dicamba residues in soil after preplant burndown applications at three locations in 2016 and 2017. Treatments with 2,4-D included 532 and 1,063 g ae ha−1 applied 3 wk before planting (WBP) and 53, 160, 266, 532, 1,063 g ae ha−1 applied at planting. Dicamba treatments included 560 and 1,120 g ae h−1 applied 3 WBP and 56, 168, 280, 560, 1,120 g ae ha−1 applied at planting. Dicamba or 2,4-D treatments applied 3 WBP resulted in no adverse effects on cotton stand, plant height, or yield. Dicamba 560 g ae h−1 applied at planting reduced cotton stand by 36% at 21 to 24 d after planting (DAP) over all locations in 2016. In 2017, stands were reduced by dicamba at 168, 280, 560, and 1,120 g ae ha−1 by 17% to 25% at 20 to 23 DAP. Moreover, cotton stands were not affected by 2,4-D in 2016, and only 266, 532, and 1,063 g ae ha−1 of 2,4-D caused stand reductions of 26% to 36% at 20 to 23 DAP over all locations in 2017. Dicamba at 560 g ae ha−1 at planting was the only treatment in this study that reduced plant height. Although stand losses were observed in both years, no yield loss occurred. The data suggest that stunting and stand reduction may occur if susceptible varieties are planted soon after burndown applications with 2,4-D or dicamba, but yield may not be affected after a full growing season. Dicamba showed greater potential to cause stunting and stand reduction than 2,4-D.

Rush skeletonweed is an aggressive perennial weed that establishes itself on land in the Conservation Reserve Program (CRP), and persists during cropping following contract expiration. It depletes critical soil moisture required for yield potential of winter wheat. In a winter wheat/fallow cropping system, weed control is maintained with glyphosate and tillage during conventional fallow, and with herbicides only in no-till fallow. Research was conducted for control of rush skeletonweed at two sites in eastern Washington, Lacrosse and Hay, to compare the effectiveness of a weed-sensing sprayer and broadcast applications of four herbicides (aminopyralid, chlorsulfuron + metsulfuron, clopyralid, and glyphosate). Experimental design was a split-plot with herbicide and application type as main and subplot factors, respectively. Herbicides were applied in the fall at either broadcast or spot-spraying rates depending on sprayer type. Rush skeletonweed density in May was reduced with use of aminopyralid (1.1 plants m−2), glyphosate (1.4 plants m−2), clopyralid (1.7 plants m−2), and chlorsulfuron + metsulfuron (1.8 plants m−2) compared with the nontreated check (2.6 plants m−2). No treatment differences were observed after May 2019. There was no interaction between herbicide and application system. Area covered using the weed-sensing sprayer was, on average, 52% (P < 0.001) less than the broadcast application at the Lacrosse location but only 20% (P = 0.01) at the Hay location. Spray reduction is dependent on foliar cover in relation to weed density and size. At Lacrosse, the weed-sensing sprayer reduced costs for all herbicide treatments except aminopyralid, with savings up to US$6.80 per hectare. At Hay, the weed-sensing sprayer resulted in economic loss for all products because of higher rush skeletonweed density. The weed-sensing sprayer is a viable fallow weed control tool when weed densities are low or patchy.

Rapid crop canopy formation is important to reduce weed emergence and selection for herbicide resistance. Field experiments were conducted in 2017 and 2018 in Fayetteville, AR, to evaluate the impacts of PRE applications of flumioxazin on soybean injury, soybean density, canopy formation, and incidence of soil-borne pathogens. Flumioxazin was applied at 0, 70, and 105 g ai ha−1 to predetermined flumioxazin-tolerant and -sensitive soybean varieties. Flumioxazin at 70 g ha−1 injured the tolerant and sensitive varieties from 0% to 4% and 14% to 15%, respectively. When averaged over flumioxazin rates, density of the sensitive variety was only reduced in 2017 when activation of flumioxazin was delayed 7 d. Compared to the tolerant soybean variety, flumioxazin at 70 g ha−1 delayed the sensitive variety from reaching 20%, 40%, 60%, and 80% groundcover by 15, 16, 11, and 5 d, respectively. No delay in canopy closure (95% groundcover) was observed with either variety. Consequently, no yield loss occurred for either variety following a flumioxazin application. Flumioxazin did not impact root colonization of Didymella, Fusarium, Macrophomina, or Rhizoctonia. Pythium colonization of the soybean stem was increased by flumioxazin in 2017, but not in 2018. Increased injury, delays in percent groundcover, and an increase in Pythium colonization of soybean following a flumioxazin application may warrant the need for other soil-applied herbicides at soybean planting. Alternatively, soybean injury and delays in percent groundcover following flumioxazin applications can be mitigated through appropriate variety selection; however, comprehensive screening is needed to determine which varieties are most tolerant to flumioxazin.

Velvetleaf is an economically important weed in popcorn production fields in Nebraska. Many PRE herbicides in popcorn have limited residual activity or provide partial velvetleaf control. There are a limited number of herbicides applied POST in popcorn compared with field corn, necessitating the evaluation of POST herbicides for control of velvetleaf. The objectives of this study were to (1) evaluate the efficacy and crop safety of labeled POST herbicides for controlling velvetleaf that survived S-metolachlor/atrazine applied PRE and (2) determine the effect of velvetleaf height on POST herbicide efficacy, popcorn injury, and yield. Field experiments were conducted in 2018 and 2019 near Clay Center, Nebraska. The experiments were arranged in a split-plot design with four replications. The main plot treatments were velvetleaf height (≤15 cm and ≤30 cm) and subplot treatments included a no-POST herbicide control, and 11 POST herbicide programs. Fluthiacet-methyl, fluthiacet-methyl/mesotrione, carfentrazone-ethyl, dicamba, and dicamba/diflufenzopyr provided greater than 96% velvetleaf control 28 d after treatment (DAT), reduced velvetleaf density to fewer than 7 plants m−2, achieved 99% to 100% biomass reduction, and had no effect on popcorn yield. Herbicide programs tested in this study provided greater than 98% control of velvetleaf 28 DAT in 2019. Most POST herbicide programs in this study provided greater than 90% control of up to 15 cm and up to 30 cm velvetleaf and no differences between velvetleaf heights in density, biomass reduction, or popcorn yield were observed, except with topramezone and nicosulfuron/mesotrione 28 DAT in 2018. On the basis of contrast analysis, herbicide programs with fluthiacet-methyl or dicamba provided better control than herbicide programs without them at 28 DAT in 2018. It is concluded that POST herbicides are available for control of velvetleaf up to 30-cm tall in popcorn production fields.

Field studies were conducted on organic soils in Belle Glade, FL, in 2016 to 2017 to evaluate sugarcane tolerance and fall panicum control with topramezone applied alone or in combination with triazine herbicides (atrazine, metribuzin, ametryn). Treatments included topramezone (25 and 50 g ai ha−1) applied alone or in combination with atrazine (2,240 g ai ha−1), metribuzin (2,240 g ai ha−1), and ametryn (440 g ha−1) on four plant cane varieties to evaluate tolerance, and on second ratoon fields to determine efficacy on fall panicum control. Topramezone applied alone had no effect on sugarcane chlorophyll fluorescence (i.e., the ratio of variable fluorescence to maximum fluorescence), total chlorophyll, and carotenoid 7 to 28 d after treatment (DAT), suggesting sugarcane tolerance. Significant reduction of these parameters occured 7 to 14 DAT when topramezone (50 g ai ha−1) was applied with ametryn or metribuzin; however, reductions were not detected thereafter, indicating recovery. Sugarcane yield was not affected by topramezone applied alone or in combination with the triazine herbicides. Topramezone (50 g ai ha−1) plus metribuzin resulted in acceptable control of fall panicum (84%) with limited to no regrowth of meristematic tissue at sugarcane canopy closure, equivalent to 56 to 70 DAT. These results indicate that when sequential applications of topramezone, applied alone or in combination with these triazine herbicides, are required for efficacious weed control, topramezone applications alone can be made after 7 d, whereas the combinations can be made after 14 or 21 d, depending on sugarcane sensitivity.

Evolution of kochia resistance to glyphosate and dicamba is a concern for growers in the US Great Plains. An increasing use of glyphosate and dicamba with the widespread adoption of glyphosate/dicamba-resistant (GDR) soybean in recent years may warrant greater attention. Long-term stewardship of this new stacked-trait technology will require the implementation of diverse weed control strategies, such as the use of soil-residual herbicides (PRE) aimed at effective control of GDR kochia. Field experiments were conducted in Huntley, MT, in 2017 and 2018, and Hays, KS, in 2018 to determine the effectiveness of various PRE herbicides applied alone or followed by (fb) a POST treatment of glyphosate plus dicamba for controlling GDR kochia in GDR soybean. Among PRE herbicides tested, sulfentrazone provided complete (100%), season-long control of GDR kochia at both sites. In addition, PRE fb POST programs tested in this study brought 71% to 100% control of GDR kochia throughout the season at both sites. Pyroxasulfone applied PRE resulted in 57% to 70% control across sites at 9 to 10 wk after PRE (WAPRE). However, mixing dicamba with pyroxasulfone improved control up to 25% at both sites. Kochia plants surviving pyroxasulfone applied PRE alone produced 2,530 seeds m−2 compared with pyroxasulfone + dicamba (230 seeds m−2) at the Montana site. No differences in soybean grain yields were observed with PRE alone or PRE fb POST treatments at the Montana site; however, dicamba, pyroxasulfone, and pendimethalin + dimethenamid-P applied PRE brought lower grain yield (1,150 kg ha−1) compared to all other tested programs at the Kansas site. In conclusion, effective PRE or PRE fb POST (two-pass) programs tested in this research should be proactively utilized by the growers to manage GDR kochia in GDR soybean.