Field dodder is an obligatory stem and leaf plant parasite that causes significant damage in field and vegetable crops in all agricultural regions of the globe. Selective and effective measures to control the parasite are extremely limited. In recent studies, we have shown that granular formulations of dinitroaniline cell division–inhibiting herbicides applied after crop establishment and before dodder germination fit our dodder control strategy and kill the parasite effectively and selectively. The aim of our study conducted from 2014 to 2018 was to evaluate the efficacy and selectivity of granular pendimethalin for dodder control in chickpea under laboratory, greenhouse, and field conditions. Petri dish experiments revealed that the herbicide reduces dodder seed germination while its main effect is a restriction of shoot elongation. Greenhouse experiments demonstrated that the inhibition and distortion of dodder shoot growth impede shoot twining and prevent attachment to the host plant. In dose–response experiments conducted in the greenhouse, we observed that half the recommended rate of granular pendimethalin provides efficient dodder control with no damage to chickpea seedlings. In 3 yr of chickpea field trials, GPM applied across the seeding bed at the recommended rate resulted in high crop yields that were not significantly different from those observed for the untreated no-dodder control, while half of the recommended dose efficiently controlled dodder and other weeds with no damage to the crop, resulting in significantly increased chickpea yields and profitability. These studies indicate that GPM can provide efficient and selective dodder control in chickpea.

The adoption of chemical fallow rotations in Pacific Northwest dryland winter wheat production has caused a weed species composition shift in which scouringrush has established in production fields. Thus, there has been interest in identifying herbicides that effectively control scouringrush in winter wheat–chemical fallow cropping systems. Field experiments were established in growers’ fields near Reardan, WA, in 2014, and The Dalles, OR, in 2015. Ten herbicide treatments were applied to mowed and nonmowed plots during chemical fallow rotations. Scouringrush stem densities were quantified the following spring and after wheat harvest at both locations. Chlorsulfuron plus MCPA-ester resulted in nearly 100% control of scouringrush through wheat harvest. Before herbicide application, mowing had no effect on herbicide efficacy. We conclude chlorsulfuron plus MCPA-ester is a commercially acceptable treatment for smooth and intermediate scouringrush control in winter wheat–chemical fallow cropping systems; however, the lack of a positive yield response when scouringrushes were controlled should factor into management decisions.

Blessed milkthistle is considered to be a noxious weed in irrigated and rainfed areas of Pakistan due to its strong allelopathic effects on food crops. For sustainable wheat production, it is necessary to know the critical time for weed removal (CTWR) for blessed milkthistle to allow wheat growers to get maximum benefit from control of this weed. A field study was conducted in 2014 and 2015 at the College of Agriculture, University of Sargodha, Pakistan, to investigate the CTWR of blessed milkthistle in wheat. The field experiments were designed with seven treatments; weed free (control); 2, 3, 4, 5, and 6 wk after emergence (WAE); and weedy check. At 6 WAE, a significant reduction was noted in plant height (8% and 17%), number of productive tillers per square meter (16% and 16%), spike length (23% and 54%), grains per spike (13% and 34%), 1,000-grain weight (14% and 37%), grain yield (20% and 21%), and biological yield (24% and 50%) compared with control (weed-free plots) during 2014 and 2015, respectively. The logistic model supports the field study results and suggests that blessed milkthistle’s CTWR for wheat is 1 to 5 WAE based on acceptable yield losses of 5% to 15% during both years. The experimental results and logistic model indicate that blessed milkthistle should be controlled within 1 to 5 WAE to get better wheat crop harvests without compromising farmers’ profits. To our knowledge, this is the first study ever in Pakistan regarding the CTWR in terms of WAE of blessed milkthistle and could help other scientists create weed control strategies for other areas of the country.

Pigweed is difficult to manage in grain sorghum because of widespread herbicide resistance, a limited number of registered effective herbicides, and the synchronous emergence of pigweed with grain sorghum in Kansas. The combination of cultural and mechanical control tactics with an herbicide program are commonly recognized as best management strategies; however, limited information is available to adapt these strategies to dryland systems. Our objective for this research was to assess the influence of four components, including a winter wheat cover crop (CC), row-crop cultivation, three row widths, with and without a herbicide program, on pigweed control in a dryland system. Field trials were implemented during 2017 and 2018 at three locations for a total of 6 site-years. The herbicide program component resulted in excellent control (>97%) in all treatments at 3 and 8 weeks after planting (WAP). CC provided approximately 50% reductions in pigweed density and biomass for both timings in half of the site-years; however, mixed results were observed in the remaining site-years, ranging from no attributable difference to a 170% increase in weed density at 8 WAP in 1 site-year. Treatments including row-crop cultivation reduced pigweed biomass and density in most site-years 3 and 8 WAP. An herbicide program is required to achieve pigweed control and should be integrated with row-crop cultivation or narrow row widths to reduce the risk of herbicide resistance. Additional research is required to optimize the use of CC as an integrated pigweed management strategy in dryland grain sorghum.

Tomato is injured by low doses of 2,4-D, dicamba, quinclorac, and glyphosate. New crop varieties resistant to 2,4-D and dicamba are likely to increase use of these herbicides and may increase drift problems. There is a diverse germplasm of tomato available that includes wild relatives known to be tolerant to numerous biotic and abiotic stresses. A greenhouse and field study was conducted to investigate auxin tolerance in three wild tomato accessions (TOM199, TOM198, and TOM300) and compare them with two commercial tomato cultivars (‘Money Maker’ and ‘Better Boy’). Auxin herbicides, which included 2,4-D, dicamba, and quinclorac, were applied at doses of 11, 3, and 39 g ae ha−1, respectively. Visible injury ratings of each accession for each herbicide treatment were recorded at 7, 14, 21, and 28 d after treatment (DAT) on a 0% to 100% scale. Results indicate that all three wild tomato accessions exhibited less than 15% injury compared with 100% injury for two commercial cultivars after application of dicamba. The three wild accessions (TOM199, TOM198, and TOM300) did not show any significant reduction in plant height compared with nontreated plants. At 28 DAT, plant heights of TOM199, TOM198, and TOM300 were 25, 25, and 28 cm when treated with dicamba and 31, 30, and 31 cm nontreated, respectively. Based on these results, the identified lines can serve as a genetic resource for developing herbicide-tolerant tomato, thus minimizing or eliminating the negative impacts of drift from nonlabeled herbicides tested in this project.

Off-target movement of dicamba and 2,4-D may injure and reduce the yield of many fruit and vegetable crops, impacting specialty crop producers and herbicide applicators alike. Two field experiments were established, using plant growth regulator–resistant soybean herbicide technologies, to evaluate drift and carryover risks to horseradish production. The drift experiment was conducted in 2015 and 2016 to evaluate impact of dicamba and 2,4-D simulated drift on horseradish production with a mid-POST application in soybean. Simulated drift rates were 1/10,000X, 1/1,000X, and 1/100X, with 1/2X, 1X, and 2X of standard application rates. Injury and yield loss was greater following application of 2,4-D than with dicamba. Yield reductions were observed beginning at the 1/1,000X rate of 2,4-D, with complete crop loss occurring when rates exceed 1/2X. In comparison, dicamba only reduced yields when applied at the 1X and 2X rates. Only horseradish roots from plants treated with dicamba at the 2X rate had greater dicamba residue than the nontreated control, and the amount detected, 0.32 parts per billion (ppb), was lower than the EPA tolerance of 100 ppb in root crops. There was little to no harvestable tissue for 2,4-D residue analysis for plants treated with 2,4-D at rates above 1/2X. The carryover experiment was a 2-yr rotational evaluation conducted in 2014, 2015, and 2016 to assess dicamba carryover to horseradish following application to dicamba-resistant soybean the previous season. Observations taken at 4, 6, and 8 wk after planting indicated no significant horseradish injury, nor was height, stand, or root weight reduced. These results suggest that horseradish growers should have few concerns about injury from dicamba drift or carryover. While 2,4-D applicators may need to be cautious when making applications near horseradish fields, 2,4-D may be an effective tool for controlling volunteer horseradish in 2,4-D–resistant soybean.

Effective POST herbicides and herbicide mixtures are key components of integrated weed management in corn; however, herbicides vary in their efficacy based on application timing. Six field experiments were conducted over 2 yr (2017–2018) in southwestern Ontario, Canada, to determine the effects of herbicide application timing and rate on the efficacy of tolpyralate, a new 4-hydroxyphenyl pyruvate dioxygenase inhibitor. Tolpyralate at 15, 30, or 40 g ai ha−1 in combination with atrazine at 500 or 1,000 g ai ha−1 was applied PRE, early POST, mid-POST, or late POST. Tolpyralate + atrazine at rates ≥30 + 1,000 g ha−1 provided equivalent control of common lambsquarters and Powell amaranth applied PRE or POST, whereas no rate applied PRE controlled common ragweed, velvetleaf, barnyardgrass, or green foxtail. Common ragweed, common lambsquarters, velvetleaf, and Powell amaranth were controlled equally regardless of POST timing. In contrast, control of barnyardgrass and green foxtail declined when herbicide application was delayed to the late-POST timing, irrespective of herbicide rate. Similarly, corn grain yield declined within each tolpyralate + atrazine rate when herbicide applications were delayed to late-POST timing. Overall, the results of this study indicate that several monocot and dicot weed species can be controlled with tolpyralate + atrazine with an early to mid-POST herbicide application timing, before weeds reach 30 cm in height, and Powell amaranth and common lambsquarters can also be controlled PRE. Additionally, this study provides further evidence highlighting the importance of effective, early-season weed control in corn.

Research was conducted from 2013 to 2015 across three sites in Mississippi to evaluate corn response to sublethal paraquat or fomesafen (105 and 35 g ai ha−1, respectively) applied PRE, or to corn at the V1, V3, V5, V7, or V9 growth stages. Fomesafen injury to corn at three d after treatment (DAT) ranged from 0% to 38%, and declined over time. Compared with the nontreated control (NTC), corn height 14 DAT was reduced approximately 15% due to fomesafen exposure at V5 or V7. Exposure at V1 or V7 resulted in 1,220 and 1,110 kg ha−1 yield losses, respectively, compared with the NTC, but yield losses were not observed at any other growth stage. Fomesafen exposure at any growth stage did not affect corn ear length or number of kernel rows relative to the NTC. Paraquat injury to corn ranged from 26% to 65%, depending on growth stage and evaluation interval. Corn exposure to paraquat at V3 or V5 consistently caused greater injury across evaluation intervals, compared with other growth stages. POST timings of paraquat exposure resulted in corn height reductions of 13% to 50%, except at V7, which was most likely due to rapid internode elongation at that stage. Likewise, yield loss occurred after all exposure times of paraquat except PRE, compared with the NTC. Corn yield was reduced 1,740 to 5,120 kg ha−1 compared with the NTC, generally worsening as exposure time was delayed. Paraquat exposure did not reduce corn ear length, compared with the NTC, at any growth stage. However, paraquat exposure at V3 or V5 was associated with reduction of kernel rows by 1.1 and 1.7, respectively, relative to the NTC. Paraquat and fomesafen applications near corn should be avoided if conditions are conducive for off-target movement, because significant injury and yield loss can result.

All herbicides will move off-target to sensitive crops when not applied correctly. Therefore, low-dose applications of flumioxazin and metribuzin were evaluated in soybean at the unifoliate, V2, and V4 growth stages. Rates evaluated were 12.5%, 25%, and 50% of the labeled use rates of 72 and 316 g ai ha−1 of flumioxazin and metribuzin, respectively. Flumioxazin injury was characterized by necrosis and visible height and width reduction. Injury increased with rate 3 d after treatment (DAT), with unifoliate, V2, and V4 soybean injured 15% to 30%, 18% to 27%, and 5% to 8%, respectively. Unifoliate and V4 soybean were injured more than V4 soybean 3 to 14 DAT, but injury decreased to <5% by 42 DAT. Soybean yields in the flumioxazin study were 92% to 96% of the nontreated, resulting in a yield loss of 196 to 393 kg ha−1 and a revenue loss of 71 to 141 US$ ha−1. Metribuzin injury was primarily chlorosis with necrosis and a visible reduction in soybean height and width. Soybean at the V2 growth stage was injured 14% more than V4 soybean 3 DAT, regardless of metribuzin rate. Injury to V2 and V4 soybean was similar 14 DAT, with injury of 21% to 40% across rates. Soybean injury when treated at the V2 and V4 growth stages was 6% to 29% 42 DAT compared to unifoliate soybean at 0 to 17%. Soybean yields in the metribuzin study yields were 96% to 98% of the nontreated. However, a 2% to 4% reduction equates to a loss of 90 to 180 kg ha−1 and a revenue loss of 32 to 65 US$ ha−1. Unifoliate and V2 soybean are more sensitive to a low dose of flumioxazin POST, and V2 and V4 soybean are more sensitive to a low dose of metribuzin POST. Injury and the impact on soybean growth could potentially cause economic loss for a soybean producer.

Successful pigweed management requires an integrated strategy to delay the development of resistance to any single control tactic. Field trials were implemented during 2017 and 2018 in three counties in Kansas on dryland (limited rainfall, nonirrigated), glufosinate-resistant soybean. The objective was to assess pigweed control with combinations of a winter wheat cover crop (CC), three soybean row widths (76, 38, and 19 cm), row-crop cultivation 2.5 weeks after planting (WAP), and an herbicide program to develop integrated pigweed management recommendations. All combinations of the four components were assessed by 16 treatments. All treatments with the herbicide program resulted in excellent (>97%) pigweed control and were analyzed separately from the other components. Treatments containing row-crop cultivation reduced pigweed density and biomass 3 and 8 WAP in all locations compared with the 76-cm row width plus no CC treatment. CC impacts were mixed. In Riley County, Palmer amaranth density and biomass were reduced; in Reno County, no additional Palmer amaranth control was observed; in Franklin County, the CC had greater waterhemp density and biomass compared with the treatments containing no CC. Narrow row widths achieved the most consistent results of all cultural components when data were pooled across locations: Decreasing row widths from 76 to 38 cm resulted in a 23% reduction in pigweed biomass 8 WAP and decreasing row width from 38 to 19 cm achieved a 15% reduction. Row-crop cultivation should be incorporated where possible as a mechanical option to manage pigweed, and narrow row widths should be used to suppress late-season pigweed growth when feasible. Inconsistent pigweed control from CC was achieved and should be given special consideration before implementation. The integral use of these components with an herbicide program as a system should be recommended to achieve the best pigweed control and reduce the risk of developing herbicide resistance.

Although dicamba-resistant crops can provide an effective weed management option, risk of dicamba off-site movement to sensitive crops is a concern. Previous research with indeterminate soybean identified 14 injury criteria associated with dicamba applied at V3/V4 or R1/R2 at 0.6 to 280 g ae ha−1. Injury criteria rated on a 0 to 5 scale (none to severe), along with percent visible injury and plant height reduction, and canopy height collected 7 and 15 d after treatment (DAT) were analyzed using multiple regression with a forward-selection procedure to develop yield prediction models. Variables included in the 15 DAT models (in order of selection) for V3/V4 were lower stem base lesions/cracking, plant height reduction, terminal leaf epinasty, leaf petiole droop, leaf petiole base swelling, and stem epinasty, whereas for R1/R2 variables were lower stem base lesions/cracking, terminal leaf chlorosis, leaf petiole base swelling, stem epinasty, terminal leaf necrosis, and terminal leaf cupping. To validate the models, experiments including the same dicamba rates and application timings used in previous research were conducted at two locations. For the variables specific to each model, data collected for the dicamba rates were used to predict yield. For the V3/V4 15 DAT model, predicted yield reduction (compared with the nontreated control for dicamba at 0.6 to 4.4 g ha−1) underestimated or overestimated observed yield reduction by an average of 1 and 3 percentage points. For 8.8 g ha−1, predicted yield reduction overestimated observed yield reduction by 8 points and for 17.5 g ha−1 by 20 points. For the R1/R2 15 DAT model, predicted yield reduction for 0.6 to 4.4 g ha−1 overestimated observed yield reduction by an average of 3 to 5 percentage points. For dicamba at 8.8 g ha−1, predicted yield reduction underestimated observed yield reduction by 8 points and for 17.5 g ha−1 overestimated by 6 points.

Crop plants have been used as mimic weeds to substitute for real weeds in competition studies. These mimic weeds have the advantages of availability of seed, uniform germination and growth, and the potential to confer better experimental controllability and repeatability. However, the underlying assumption that the competitive effects of mimic weeds are similar to real weeds has not been tested. We compared a range of morphological traits (plant height, node and leaf number, leaf area, leaf size, and dry weight) between the mimic weeds and real weeds: Japanese millet vs. junglerice, mungbean vs. bladder ketmia, and common sunflower vs. fierce thornapple. The impact of these mimic and real weeds on cotton was also assessed. There were similarities and differences between the mimic and real weeds, but impact on cotton lint yield was most closely associated with weed height and dry weight at mid-season. Mimic weeds may be satisfactorily substituted for real weeds in competition experiments where seasonal and environmental conditions are not limiting, such as with fully irrigated cotton, provided the plants have similar dry weight and height at mid-season. Alternatively, one can account for the differences in dry weight and height. We define here a generalized relationship estimating the yield loss of high-yielding, irrigated cotton from weed competition over a range of weed dry weights and heights, allowing extrapolation from the results with mimic weeds to the competitive effects of a range of weeds.

Data from surveys are used to help quantitatively diagnose the relative importance of chemical and nonchemical management practices, identify weed problems, and provide potential solutions. However, to our knowledge, such surveys have not been conducted in Argentina. In 2016, advisors and crop producers from cropping areas across Argentina were surveyed through email with the objectives to identify the main weed species problems and assess the use of chemical and nonchemical weed management practices in different crop production areas in Argentina. Fleabane, pigweed, johnsongrass, fingergrass, goosegrass, barnyardgrass, and ryegrass were considered the most important weeds. More than 53% of the producers used only chemical options; 86% used chemical fallow (i.e., keeping weed free with chemical application); 62% used full herbicide rates; 46% used proper herbicide timing; 41% used multiple modes of action; and 32% used rotation of herbicide modes of action. The main nonchemical practices used were crop rotation (45%); avoiding seed production during (31%) and after (25%) the crop cycle; narrow row spacing (19%); and cultivars with greater competitive ability (18%). Less than 15% of the people surveyed used increased crop densities or altered date of sowing. There is a high dependence on chemical control in the main crops grown in Argentina. Extension efforts are needed to emphasize the importance of integrated weed management.

Commercial mushroom producers grow several varieties of mushrooms on compost. Upon completion of the growing cycle, the spent mushroom compost is often sold as a soil amendment for both agricultural and homeowner use. Mushroom compost ingredients often come from fields infested with weeds, and in turn compost may spread unwanted weed seed. We conducted studies to assess the viability of weed seed following specific stages of the commercial mushroom production process. Weed seed was more likely to survive if the entire production process was not completed. However, no viable hairy vetch, Italian ryegrass, ivyleaf morningglory, Palmer amaranth, or velvetleaf remained at the end of the study. Although the seeds of most species were eliminated earlier in the composting process, ivyleaf morningglory required the complete process to eliminate 100% of the seed. These results indicate that spent mushroom compost is free of many weed species upon removal from mushroom houses and is unlikely to spread weed seed.

Integrating multiple weed management (cultural, physical, chemical) strategies is often recommended to combat herbicide resistance. With the increased use of interseeded cover crops, the effects of PRE herbicides on their establishment and growth require study. An investigation was conducted in Lexington, KY, in 2016 through 2018 to assess the extent to which commonly used PRE corn herbicide combinations influenced interseeded red clover and annual ryegrass establishment and growth. Annual ryegrass density was reduced 29% at 3 wk after interseeding by the combination of residual dimethenamid-P and atrazine; however, biomass the following spring was not affected by herbicide combinations. Neither density of interseeded red clover at 2 to 3 wk after interseeding nor biomass prior to termination the following spring were influenced by herbicide combinations. However, red clover density was affected by herbicide treatment 5 wk after interseeding in 2016. These results could have been influenced by low summer survival, particularly in 2016. The environmental factors may have influenced the survival of the interseeded cover crops more than the PRE herbicides. This study suggests that multiple PRE herbicides can be used with minimal risk to interseeded red clover or annual ryegrass. However, the influence of the environment on establishment and survival of interseeded cover crops following the use of PRE herbicides requires further study.

Experiments were conducted to evaluate the impact of spray volume, nozzle type, adjuvants, the presence of dew, and their interactions on foliar retention of creeping bentgrass. Tartrazine, a common food dye, was used as a tracer in this study. Increasing spray volume from 95 L ha−1 to 1,500 L ha−1 decreased foliar retention efficiency from 98% to approximately 85%. Compared with flat-fan nozzles, air-induction nozzles delivered similar retention efficiency at all spray volumes evaluated. However, flat-fan nozzles provided higher uniformity and more thorough coverage. Adding nonionic surfactants, organosilicone adjuvants, or methylated seed oils at typical concentrations yielded retention efficiency of approximately 90% to 93% regardless of spray volumes. In contrast, with water alone, increasing spray volume reduced retention efficiency from 95.9% to 87.3%. Simulated dew applied at 1,950 L ha−1 increased retention efficiency by approximately 3% when spray application volume was 190 L ha−1, while no difference was observed at 750 L ha−1. The presence of dew reduced the impact of adjuvants on retention efficiency. Large quantities of dew, 3,800 L ha−1, did reduce retention efficiency.

Due to depressed corn and soybean prices over the last few years in the United States, growers in Nebraska are showing interest in no-tillage (hereafter referred to as no-till) conventional (non–genetically engineered [non-GE]) soybean production. Due to the increasing number of herbicide-resistant weeds in the United States, weed control in no-till non-GE soybean using POST herbicides is a challenge. The objectives of this study were to compare PRE-only, PRE followed by (fb) POST, and PRE fb POST with residual (POST-WR) herbicide programs for Palmer amaranth and velvetleaf control and soybean injury and yield, as well as to estimate the gross profit margins and benefit–cost ratio of herbicide programs. A field experiment was conducted in 2016 and 2017 at Clay Center, NE. The PRE herbicides tested in this study resulted in ≥95% Palmer amaranth and velvetleaf control at 28 d after PRE (DAPRE). Averaged across the programs, the PRE-only program controlled Palmer amaranth 66%, whereas 86% and 97% control was obtained with the PRE fb POST and PRE fb POST-WR programs, respectively, at 28 d after POST (DAPOST). At 28 DAPOST, the PRE fb POST herbicide programs controlled velvetleaf 94%, whereas the PRE-only program resulted in 85% control. Mixing soil-residual herbicides with foliar-active POST programs did not improve velvetleaf control. Averaged across herbicide programs, PRE fb POST programs increased soybean yield by 10% and 41% in 2016 and 2017, respectively, over the PRE-only programs. Moreover, PRE fb POST-WR programs produced 7% and 40% higher soybean yield in 2016 and 2017, respectively, compared with the PRE fb POST programs. The gross profit margin (US$1,184.3 ha−1) was highest under flumioxazin/pyroxasulfone (PRE) fb fluthiacet-methyl plus S-metolachlor/fomesafen (POST-WR) treatment; however, the benefit–cost ratio was highest (6.1) with the PRE-only program of flumioxazin/chlorimuron-ethyl.

S-Metolachlor is commonly used by soybean and cotton growers, especially with POST treatments for overlapping residuals, to obtain season-long control of glyphosate- and acetolactate synthase (ALS)–resistant Palmer amaranth. In Crittenden County, AR, reports of Palmer amaranth escapes following S-metolachlor treatment were first noted at field sites near Crawfordsville and Marion in 2016. Field and greenhouse experiments were conducted to confirm S-metolachlor resistance and to test for cross-resistance to other very-long-chain fatty acid (VLCFA)–inhibiting herbicides in Palmer amaranth accessions from Crawfordsville and Marion. Palmer amaranth control in the field (soil <3% organic matter) 14 d after treatment (DAT) was ≥94% with a 1× rate of acetochlor (1,472 g ai ha–1; emulsifiable concentrate formulation) and dimethenamid-P (631 g ai ha–1). However, S-metolachlor at 1,064 g ai ha–1 provided only 76% control, which was not significantly different from the 1/2× and 1/4× rates of dimethenamid-P and acetochlor (66% to 85%). In the greenhouse, Palmer amaranth accessions from Marion and Crawfordsville were 9.8 and 8.3 times more resistant to S-metolachlor compared with two susceptible accessions based on LD50 values obtained from dose–response experiments. Two-thirds and 1.5 times S-metolachlor at 1,064 g ha–1 were the estimated rates required to obtain 90% mortality of the Crawfordsville and Marion accessions, respectively. Data collected from the field and greenhouse confirm that these accessions have evolved a low level of resistance to S-metolachlor. In an agar-based assay, the level of resistance in the Marion accession was significantly reduced in the presence of a glutathione S-transferase (GST) inhibitor, suggesting that GSTs are the probable resistance mechanism. With respect to other VLCFA-inhibiting herbicides, Marion and Crawfordsville accessions were not cross-resistant to acetochlor, dimethenamid-P, or pyroxasulfone. However, both accessions, based on LD50 values obtained from greenhouse dose–response experiments, exhibited reduced sensitivity (1.5- to 3.6-fold) to the tested VLCFA-inhibiting herbicides.

The development of a linuron-free weed management strategy for carrot production is essential as a result of the herbicide reevaluation programs launched by the Pest Management Regulatory Agency in Canada for herbicides registered before 1995 and the discovery of linuron-resistant pigweed species in Ontario. Field trials were conducted in one of Ontario’s main carrot-growing regions on high organic soils in 2016 and 2017. Pigweed species seedlings were effectively controlled with PRE treatments of prometryn, pendimethalin, S-metolachlor, or glufosinate. POST treatments of pyroxasulfone and metribuzin followed by predetermined biologically effective dose (≥90% control of pigweed seedlings) of acifluorfen, oxyfluorfen, fluthiacet-methyl, and fomesafen achieved excellent crop selectivity and commercially acceptable pigweed species seedling control under field conditions. Carfentrazone-ethyl or fomesafen applied PRE severely reduced seedling emergence and yield in the wet growing season of 2017. This study demonstrated clearly that an alternative linuron-free strategy can be developed for carrots. The strategy of exploring the potential to use the biologically effective dose of selected herbicides to achieve crop selectivity and control of pigweed species seedlings was verified.

Palmer amaranth accessions resistant to protoporphyrinogen oxidase (PPO), 5-enolpyruvyl-shikimate-3-phosphate synthase, and acetolactate synthase (ALS)-inhibitor herbicides are widespread in the Midsouth, making control difficult. Field experiments were conducted in Marion and Crawfordsville, AR, in 2016 and 2017 to assess PRE and POST herbicides labeled for use in corn, cotton, or soybean for control of multiresistant Palmer amaranth. Accessions at both locations were resistant to glyphosate and ALS inhibitors and segregating for both the R128 and ΔG210 PPO resistance mechanisms. Of the 15 herbicide treatments tested, only atrazine (1,120 g ai ha−1), pyroxasulfone (149 g ha−1), and flumioxazin (144 g ha−1) provided 85% or greater Palmer amaranth control 14 days after treatment (DAT). Visible control ratings at 35 DAT declined sharply, with no treatment providing more than 84% control, suggesting POST applications should be made no later than 28 DAT. Glufosinate (594 and 818 g ha−1), dicamba (560 g ae ha−1), 2,4-D plus glyphosate (784 g ae ha−1 plus 834 g ae ha−1), and paraquat (700 g ha−1) applied POST to 7- to 10-cm plants reduced Palmer amaranth density 83% or more 14 DAT. Both glyphosate (1,266 g ha−1) and pyrithiobac sodium (73 g ha−1) provided less than 7% Palmer amaranth control. Although flumioxazin alone at a labeled rate controlled Palmer amaranth 82% in the PRE experiment, PPO inhibitors by themselves applied POST provided no more than 37% control at 14 DAT. Effective foliar herbicides applied POST, including residual herbicides, should be made when Palmer amaranth are less than 10-cm tall for optimal control of these multiresistant Palmer amaranth accessions.