Industrial hemp is a multipurpose crop cultivated for fiber, seed, human food, and animal feed. Hemp legalization in Texas creates a considerable potential to increase its acreage in semi-arid conditions; however, knowledge is limited on growing hemp optimally in Texas. Best management practices, including weed control, require evaluation for profitable hemp production. As little is known about the herbicide tolerance of hemp, field studies were conducted to test several soil-residual herbicides with different modes of action for phytotoxicity to two hemp cultivars, ‘Yuma’ and ‘Jinma’. The experimental units were randomized three times in a blocked split-plot design with hemp cultivars in the main plots and soil-residual herbicides in the subplots. Ethalfluralin, the mixture of sulfentrazone and S-metolachlor, prometryn, and S-metolachlor, resulted in 60% to 90% and 73% to 100% weed control as compared to the nontreated control in 2021 and 2022, respectively. The highest hemp germination, stand count, and plant height were observed with ethalfluralin and S-metolachlor herbicides; however, no significant differences were observed for hemp germination and plant height compared to the nontreated control. S-metolachlor, ethalfluralin, fomesafen, and prometryn resulted in similar hemp biomass compared to the nontreated control. Overall, the results indicate that hemp is tolerant to ethalfluralin, prometryn, and S-metolachlor, and these soil-residual herbicides were effective for weed control in hemp. The mixture of bicyclopyrone plus S-metolachlor, metribuzin plus S-metolachlor, and mesotrione should be avoided, as they caused significant injury to hemp plants. Future research is needed to test the efficacy of different preemergence and postemergence herbicides that can be used in industrial hemp grown under different environments, making sure the delta-9-tetrahydrocannabinol content of the hemp is below the legal content restrictions.

The herbicides that inhibit 4-hydroxyphenylpyruvate dioxygenase (HPPD) are primarily used for weed control in corn, barley, oat, rice, sorghum, sugarcane, and wheat production fields in the United States. The objectives of this review were to summarize 1) the history of HPPD-inhibitor herbicides and their use in the United States; 2) HPPD-inhibitor resistant weeds, their mechanism of resistance, and management; 3) interaction of HPPD-inhibitor herbicides with other herbicides; and 4) the future of HPPD-inhibitor-resistant crops. As of 2022, three broadleaf weeds (Palmer amaranth, waterhemp, and wild radish) have evolved resistance to the HPPD inhibitor. The predominance of metabolic resistance to HPPD inhibitor was found in aforementioned three weed species. Management of HPPD-inhibitor-resistant weeds can be accomplished using alternate herbicides such as glyphosate, glufosinate, 2,4-D, or dicamba; however, metabolic resistance poses a serious challenge, because the weeds may be cross-resistant to other herbicide sites of action, leading to limited herbicide options. An HPPD-inhibitor herbicide is commonly applied with a photosystem II (PS II) inhibitor to increase efficacy and weed control spectrum. The synergism with an HPPD inhibitor arises from depletion of plastoquinones, which allows increased binding of a PS II inhibitor to the D1 protein. New HPPD inhibitors from the azole carboxamides class are in development and expected to be available in the near future. HPPD-inhibitor-resistant crops have been developed through overexpression of a resistant bacterial HPPD enzyme in plants and the overexpression of transgenes for HPPD and a microbial gene that enhances the production of the HPPD substrate. Isoxaflutole-resistant soybean is commercially available, and it is expected that soybean resistant to other HPPD inhibitor herbicides such as mesotrione, stacked with resistance to other herbicides, will be available in the near future.

Three herbicide premixes have recently been introduced for weed control in wheat: halauxifen + florasulam, thifensulfuron + fluroxypyr, and bromoxynil + bicyclopyrone. The objective of this study was to evaluate these herbicides along with older products for their control of small-seeded false flax in winter wheat in Oklahoma. Studies took place during the 2017, 2018, and 2020 winter wheat growing seasons. Weed control was visually estimated every 2 wk throughout the growing season, and wheat yield was collected in all 3 yr. Small-seeded false flax diameter was approximately 6 cm at the time of application in all years. Control ranged from 96% to 99% following all treatments with the exception of bicyclopyrone + bromoxynil and dicamba alone, which controlled false flax 90%. All treatments containing an acetolactate synthase (ALS)–inhibiting herbicide achieved adequate control; therefore, resistance is not suspected in this population. Halauxifen + florasulam and thifensulfuron + fluroxypyr effectively controlled small-seeded false flax similarly to other standards recommended for broadleaf weed control in wheat in Oklahoma. Rotational use of these products allows producers flexibility in controlling small-seeded false flax and reduces the potential for development of herbicide resistance in this species.

Kochia accessions (designated as KS-4A and KS-4H) collected from a corn field near Garden City, KS, have previously shown multiple resistance to glyphosate, dicamba, and fluroxypyr. These accessions were also suspected as being resistant to photosystem II (PS II) inhibitors. The main objectives of this research were to 1) confirm the coexistence of cross-resistance to PS II inhibitors (atrazine and metribuzin) applied PRE and POST, 2) investigate the underlying mechanism of PS II-inhibitor resistance, and 3) determine the effectiveness of alternative POST herbicides for control of these multiple herbicide–resistant (MHR) kochia accessions. Results from dose-response experiments revealed that the KS-4A and KS-4H kochia accessions were 23-fold to 48-fold resistant to PRE- and POST-applied atrazine and 13-fold to 18-fold resistant to POST-applied metribuzin compared to a known susceptible kochia accession (KS-SUS). Both accessions also showed putative resistance to PRE-applied metribuzin that needs to be confirmed. Sequence analyses of the psbA gene further revealed that all samples from the KS-4A and KS-4H kochia accessions had a Ser264Gly point mutation. A pretreatment with malathion followed by a POST application of atrazine at 1,120 g ha−1 or metribuzin at 630 g ha−1 did not reverse the resistance phenotypes of these MHR accessions. In a separate greenhouse study, alternative POST herbicides, including bicyclopyrone + bromoxynil; bromoxynil + pyrasulfotole; paraquat alone or in combination with atrazine, metribuzin, 2,4-D, or saflufenacil; and saflufenacil alone or in combination with 2,4-D effectively controlled the KS-4H accession (≥97% injury). To our knowledge, this research reports the first case of kochia accessions with cross-resistance to PRE-applied atrazine and POST-applied metribuzin. Growers should adopt diversified weed control strategies, including the use of competitive crops, cover crops, targeted tillage, and harvest weed seed control along with effective alternative PRE and POST herbicides with multiple sites of action to control MHR kochia seedbanks on their production fields.

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.

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.

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

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.

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.

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

Goldenrods are common perennial weeds in lowbush blueberry fields in Nova Scotia. Management options are limited to mowing and suppression with POST mesotrione applications. The objectives of this research were to (1) compare efficacy of single versus sequential nonbearing-year POST mesotrione applications on goldenrod (2) identify the optimal interval between sequential POST mesotrione applications (3) evaluate nonbearing-year POST bicyclopyrone applications on goldenrod, and (4) evaluate nonbearing-year summer and fall herbicide spot treatments on goldenrod. POST mesotrione applications at 144 g ai ha−1 caused 39% to 77% injury but did not reduce goldenrod shoot density. In contrast, mesotrione applications at 144 g ai ha−1 followed by sequential mesotrione application at 14, 21, or 28 days after initial treatment caused greater than 90% injury to goldenrod and reduced both nonbearing- and bearing-year shoot density. POST bicyclopyrone applications at 50 g ai ha−1 caused 69% to 80% injury to goldenrod but did not reduce shoot density. A bicyclopyrone plus mesotrione tank mixture did not improve goldenrod control relative to mesotrione or bicyclopyrone alone. Summer spot applications of glyphosate (7.24 g ae L water−1), glufosinate (0.75 g ai L water−1), and mesotrione (0.72 g ai L water−1) consistently injured goldenrod and reduced both nonbearing- and bearing-year shoot density. Summer spot applications of bicyclopyrone (0.25 g ai L water−1), flazasulfuron (0.31 g ai L water−1), dicamba (1 g ae L water−1), dicamba plus diflufenzopyr (0.7 g ae L water−1 plus 0.3 g ai L water−1), triclopyr (1.68 g ai L water−1), clopyralid (0.08 g ai L water−1), tribenuron methyl (0.2 g ai L water−1), and foramsulfuron (0.2 g ai L water−1) injured goldenrod but did not consistently reduce shoot density. When these herbicides were evaluated as fall spot applications, only glyphosate reduced goldenrod shoot density in the year after application.

Field and greenhouse studies were conducted in 2016 and 2017 to determine sweetpotato tolerance to herbicides applied to plant propagation beds. Herbicide treatments included PRE application of flumioxazin (107 g ai ha−1), S-metolachlor (800 g ai ha−1), fomesafen (280 g ai ha−1), flumioxazin plus S-metolachlor (107 g ai ha−1 + 800 g ai ha−1), fomesafen plus S-metolachlor (280 g ai ha−1 + 800 g ai ha−1), fluridone (1,120 or 2,240 g ai ha−1), fluridone plus S-metolachlor (1,120 g ai ha−1 + 800 g ai ha−1), napropamide (1,120 g ai ha−1), clomazone (420 g ai ha−1), linuron (560 g ai ha−1), linuron plus S-metolachlor (560 g ai ha−1 + 800 g ai ha−1), bicyclopyrone (38 or 49.7 g ai ha−1), pyroxasulfone (149 g ai ha−1), pre-mix of flumioxazin plus pyroxasulfone (81.8 g ai ha−1 + 104.2 g ai ha−1), or metribuzin (294 g ai ha−1). Paraquat plus non-ionic surfactant (280 g ai ha−1 + 0.25% v/v) POST was also included. After plants in the propagation bed were cut and sweetpotato slip number, length, and weight had been determined, the slips were then transplanted to containers and placed either in the greenhouse or on an outdoor pad to determine any effects from the herbicide treatments on initial sweetpotato growth. Sweetpotato slip number, length, and/or weight were affected by flumioxazin with or without S-metolachlor, S-metolachlor with or without fomesafen, clomazone, and all fluridone treatments. In the greenhouse studies, initial root growth of plants after transplanting was inhibited by fluridone (1,120 g ai ha−1) and fluridone plus S-metolachlor. However, by 5 wk after transplanting few differences were observed between treatments. Fomesafen, linuron with or without S-metolachlor, bicyclopyrone (38 or 49.7 g ai ha−1), pyroxasulfone with or without flumioxazin, metribuzin, and paraquat did not cause injury to sweetpotato slips in any of the studies conducted.

Weed control in vegetable production is especially challenging, because few registered herbicides simultaneously offer excellent crop tolerance and broad-spectrum weed control. We report here the response of several vegetables and weeds to 37.5 and 50 g ai ha−1 of the new herbicide bicyclopyrone (BCP). Vegetable crops showed good tolerance to BCP PRE and post-directed (POST-DIR) in high organic matter content muck soil. POST BCP severely injured all crops. Soil type and the rate of BCP PRE significantly affected response of vegetable crops, and variety of onion was significant. POST BCP controlled hairy galinsoga and small common purslane plants (>80% injury). Hairy galinsoga was not controlled by BCP PRE application in muck soil but was controlled in a 2:3 (vol/vol) blend of Wooster silt loam and a commercial potting mix. Common purslane was slightly injured in the muck soil and was well controlled in the soil and potting mix blend by PRE BCP. The herbicide did not control prostrate pigweed in either soil type or at any growth stage.

Field studies were conducted to determine watermelon tolerance and yield response when treated with bicyclopyrone preplant (PREPLANT), POST, and POST-directed (POST-DIR). Treatments consisted of two rates of bicyclopyrone (37.5 and 50 g ai ha–1), fomesafen (175 g ai ha–1), S-metolachlor (802 g ai ha–1), and a nontreated check. Preplant treatments were applied to formed beds 1 d prior to transplanting and included bicyclopyrone (37.5 and 50 g ha–1) and fomesafen (175 g ha–1), and new polyethylene mulch was subsequently laid above treated beds. POST and POST-DIR treatments were applied 14 ± 1 d after watermelon transplanting and included bicyclopyrone (37.5 and 50 g ha–1) POST and POST-DIR, and S-metolachlor (802 g ai ha–1) POST-DIR. POST-DIR treatments were applied to row middles, ensuring that no herbicide contacted watermelon vines or polyethylene mulch. At 2 wk after transplanting (WAT), 15% foliar bleaching was observed in watermelon treated with bicyclopyrone (50 g ha–1) PRE. At 3 WAT, bicyclopyrone (37.5 and 50 g ha–1) POST caused 16% and 17% foliar bleaching and 8% and 9% crop stunting, respectively. At 4 WAT, initial injury had subsided and bicyclopyrone (37.5 and 50 g ha–1) POST caused 4% and 4% foliar bleaching and 4% and 8% crop stunting, respectively. No symptoms of bleaching or stunting were observed at 6- and 8-WAT ratings. Watermelon total yield, marketable yield, total fruit number, marketable fruit number, and average fruit size were unaffected by herbicide treatments. Therefore, registration of bicyclopyrone (37.5 and 50 g ha–1) PREPLANT, POST, and POST-DIR would offer watermelon producers a safe herbicide option and a novel mode of action for weed management.

Two putative glyphosate-resistant (GR) Russian-thistle accessions were collected from fallow fields (wheat-fallow rotation): one from Choteau County, MT (MT-R), and a second from Columbia County, WA (WA-R) in summer/fall of 2015. Greenhouse and outdoor/field whole-plant dose-response studies were conducted to confirm and characterize the levels of glyphosate resistance in these GR accessions relative to known glyphosate-susceptible accessions (MT-S and WA-S from MT and WA, respectively). Based on GR50 values of the progeny plants, the MT-R accession exhibited 4.5-fold and 5.9-fold resistance to glyphosate relative to the MT-S accession under greenhouse and outdoor conditions, respectively. The WA-R accession showed 3.0- to 5.0-fold resistance relative to the WA-S accession in greenhouse experiments, and 1.9- to 7.5-fold resistance in multi-site field experiments. In a separate greenhouse study on alternative POST herbicides to control GR Russian-thistle, bicyclopyrone plus bromoxynil, bromoxynil plus fluroxypyr, bromoxynil plus pyrasulfotole, bromoxynil plus MCPA, paraquat alone, paraquat plus metribuzin, saflufenacil alone, saflufenacil plus 2,4-D, and 2,4-D plus bromoxynil plus fluroxypyr provided effective control (≥95%) and shoot dry weight reduction (up to 98%) of GR accessions. This research confirms the first global case of field-evolved GR Russian-thistle. Best management practices (BMPs); including alternative, effective herbicide programs (based on multiple mechanisms of action highlighted in this study) need immediate implementation to prevent further spread of GR or evolution of multiple HR Russian-thistle populations in this region.