Herbicides that inhibit 4-hydroxyphenylpyruvate dioxygenase (HPPD) can be mixed with herbicides that generate reactive oxygen species (ROS) to enhance the spectrum, level, speed, and consistency of weed control efficacy; however, mixtures of these herbicides can increase corn injury. A total of five field trials were conducted from 2021 to 2023 in Ridgetown, Ontario, to determine the sensitivity of two corn hybrids (‘DKC39-97’ and ‘B79N56PWE’) to tolpyralate plus ROS-generating herbicides (atrazine, bromoxynil, bentazon, or glufosinate) applied postemergence at the recommended rate (1×) and sequentially to represent a spray overlap (2×) in the field. Tolpyralate plus atrazine, bromoxynil, bentazon, or glufosinate (2× rates) caused greater corn injury to DKC39-97 than B79N56PWE corn at 1, 2, and 4 wk after treatment (WAT). Tolpyralate plus atrazine, bromoxynil, bentazon, or glufosinate (2× rates) caused 38%, 36%, 29%, and 18% injury to DKC39-97 corn, but only 5%, 20%, 9%, and 2% injury to B79N56PWE corn, respectively at 1 WAT. Corn injury to both hybrids decreased over time with ≤2% injury at 8 WAT. Tolpyralate + atrazine, bromoxynil, or bentazon (2× rates) caused a 17%, 16%, and 13% height reduction, respectively, of DKC39-97 corn at 2 WAT; however, tolpyralate + glufosinate did not reduce DKC39-97 corn height. Tolpyralate + bromoxynil or bentazon (2× rates) caused a 12% and 10% height reduction of B79N56PWE corn, respectively, at 2 WAT; however, tolpyralate + atrazine or glufosinate did not reduce B79N56PWE corn height. Tolpyralate + atrazine or glufosinate (2× rates) caused a greater corn height reduction of DKC39-97 corn than B79N56PWE corn at 2 WAT. Grain yield was on average 2% lower from DKC39-97 than B79N56PWE corn. Tolpyralate + bromoxynil or bentazon (2× rates) caused 7% and 6% corn grain yield reduction compared to tolpyralate plus glufosinate (2× rate). Results indicate that tolpyralate + ROS-generating herbicides can cause corn injury, which is influenced by corn hybrid and ROS-generating herbicide. Corn producers need to consider the differential sensitivity of corn hybrids and ROS-generating herbicides when using an HPPD-inhibiting herbicide for weed management.

The sustainable management of herbicides is critical to modern agriculture and the environment. This article examines the evolution and environmental implications of herbicide use in Saskatchewan, Canada, agriculture. It quantifies changes in herbicide use and their environmental impacts by analyzing farm-level herbicide use data from 1991 to 1994 and from 2016 to 2019 through the environmental impact quotient. Results confirm significant reductions in both environmental and toxicological impacts of herbicides used, underlining the pivotal shift from tillage-based weed control to herbicide-resistant cropping systems. The environmental impact of the top five herbicides (glufosinate, glyphosate, clethodim, imazamox, and 2,4-D) used from 2016 to 2019 is 65% lower than that for those herbicides (MCPA, 2,4-D, bromoxynil, diclofop-methyl, and trifluralin) used from 1991 to 1994, with a 45% reduction in the active ingredient applied per acre. Despite increased herbicide use due to more crop acres being seeded, the findings highlight a marked improvement in the sustainability of herbicide use, affirming the importance of technological advancements in agriculture. This research contributes valuable insights into long-term trends in herbicide use, offering a practical framework for informed decisions aligning with sustainable agricultural practices as well as reduced biodiversity impacts.

Russian thistle is one of the most important broadleaf weeds in the semiarid U.S. Pacific Northwest. It consumes soil water after wheat harvest, compromising the yield of the following crop. The objectives of this work were to determine the impact of post–wheat harvest herbicide application timing on Russian thistle control and of stubble height on Russian thistle postharvest control and plant dispersal. For the first objective, experiments were conducted at the Columbia Basin Agricultural Research Center, Adams, OR (CBARC), and the Lind Dryland Research Station, Lind, WA (LDRS), in 2020 and 2021. Herbicides evaluated included paraquat, glyphosate, and either bromoxynil + pyrasulfotole (CBARC) or bromoxynil + metribuzin (LDRS). The different post–wheat harvest application timings were 24 h and 1, 2, and 3 wk after harvest. For the second objective, two stubble heights (short and tall) were compared for their impact on control at CBARC and in a production field near Ione, OR. Paraquat provided the greatest control in all scenarios, with no differences in application timings or stubble height. Impacts of application timings were not clear for glyphosate or bromoxynil mixtures. For glyphosate treatments, control in short stubble was 11% greater than in tall stubble in both years. Control was also greater in short stubble for the bromoxynil + pyrasulfotole application in 2020. However, Russian thistle plant dispersal was greater in short stubble at both locations. At CBARC, plant dispersal in short stubble was 58%, compared to 18% in tall stubble. Near Ione, plant dispersal in flattened stubble was 88%, compared to 43% in nonflattened short stubble. Leaving tall stubble at harvest should be considered to reduce Russian thistle plant dispersal if the infestation is going to be left untreated after harvest; otherwise, short stubble might result in better Russian thistle control when using systemic herbicides, such as glyphosate.

The complementary modes of action of 4-hydroxyphenylpyruvate dioxygenase (HPPD) and photosystem II (PSII) inhibitors have been credited for the synergistic weed control improvement of several species. Recent research discovered that reactive oxygen species (ROS) generation and subsequent lipid peroxidation is the cause of cell death by the glutamine synthetase inhibitor glufosinate. Therefore, a basis for synergy exists between glufosinate and HPPD inhibitors, but the interaction has not been well reported. Four field experiments were conducted in Ontario, Canada, in 2020 and 2021 to determine the interaction between HPPD-inhibiting (mesotrione and tolpyralate) and ROS-generating (atrazine, bromoxynil, bentazon, and glufosinate) herbicides on control of annual weed species in corn (Zea mays L.). The ROS generators were synergistic with the HPPD inhibitors and provided ≥95% control of velvetleaf (Abutilon theophrasti Medik.), except for tolpyralate + glufosinate, which was additive at 8 wk after application (WAA) and provided 87% control. Tank mixes of HPPD inhibitors plus ROS generators were synergistic for the control of common ragweed (Ambrosia artemisiifolia L.), except for tolpyralate + glufosinate, which was antagonistic at 8 WAA. Tolpyralate + glufosinate was antagonistic for the control of barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] and Setaria spp. at 8 WAA. Common lambsquarters (Chenopodium album L.) control at 8 WAA was synergistic and ≥95% with mesotrione plus atrazine, bromoxynil, or glufosinate and with tolpyralate plus bromoxynil or bentazon. Herbicide tank mixes were generally additive for the control of wild mustard (Sinapis arvensis L.) at 8 WAA, except for the synergistic tank mixes of tolpyralate plus atrazine or bromoxynil; however, each tank mix provided 97% to 100% control of S. arvensis. Results from this study demonstrate that co-application of ROS generators with mesotrione or tolpyralate controlled all broadleaf weed species >90% at 8 WAA, with the exceptions of A. artemisiifolia and C. album control with tolpyralate + glufosinate. Mesotrione plus PSII inhibitors controlled E. crus-galli and Setaria spp. 48 to 68 percentage points less than tolpyralate plus the respective PSII inhibitor at 8 WAA; however, mesotrione + glufosinate and tolpyralate + glufosinate controlled the grass weed species similarly.

Glyphosate-resistant (GR) horseweed [Conyza canadensis (L.) Cronquist; syn.: Erigeron canadensis L.] interference can substantially reduce corn (Zea mays L.) yield. The complementary activity of 4-hydroxyphenylpyruvate dioxygenase (HPPD) and photosystem II (PSII) inhibitors has been investigated for the control of several weed species, and in many cases has been synergistic; however, there is little information on the interaction of HPPD- and PSII-inhibiting herbicides for postemergence control of GR C. canadensis in corn. Four field trials were studied over 2 yr (2019, 2020) in Ontario, Canada, in commercial corn fields with natural infestations of GR C. canadensis to evaluate the level of GR C. canadensis control with three HPPD-inhibiting herbicides (mesotrione, tolpyralate, and topramezone) and three PSII-inhibiting herbicides (atrazine, bromoxynil, and bentazon) applied individually and in tank-mix combinations, and to document the interaction of the three HPPD inhibitors tank mixed with the three PSII inhibitors. Mesotrione, tolpyralate, and topramezone controlled GR C. canadensis 83%, 84%, and 72%, respectively, at 8 wk after application (WAA). Bromoxynil and bentazon controlled GR C. canadensis 71% and 79%, respectively, while atrazine provided only 31% control at 8 WAA. The joint application of atrazine, bromoxynil, or bentazon with mesotrione increased GR C. canadensis control from 83% to 100% at 8 WAA. Tolpyralate tank mixed with atrazine, bromoxynil, or bentazon controlled GR C. canadensis 96%, 98%, and 98%, respectively, which was comparable to the mesotrione tank mixes at 8 WAA. Topramezone plus atrazine, bromoxynil, or bentazon controlled GR C. canadensis 91%, 93%, and 95%, respectively, at 8 WAA. Interactions between HPPD and PSII inhibitors were synergistic for all combinations of mesotrione or tolpyralate with atrazine, bromoxynil, or bentazon. The interaction between topramezone and PSII inhibitors was additive. All nine tank mixes controlled GR C. canadensis >90%. This study concludes that bromoxynil or bentazon, instead of atrazine, can be co-applied with mesotrione, tolpyralate, or topramezone without compromising GR C. canadensis control in corn.

Wild radish is the most problematic broadleaf weed in Australian grain production. The propensity of wild radish to evolve resistance to herbicides has led to high frequencies of multiple herbicide–resistant populations present in these grain production regions. The objective of this study was to evaluate the potential of mesotrione to selectively control wild radish in wheat. The initial dose response pot trials determined that at the highest mesotrione rate of 50 g ha−1 applied preemergence (PRE) was 30% more effective than when applied postemergence (POST) on wild radish. This same rate of mesotrione applied POST resulted in a 30% reduction in wheat biomass compared to 0% for the PRE application. Subsequent mesotrione PRE dose response trials identified a wheat selective rate range of >100 and <300 g ai ha−1 that provided greater than 85% wild radish control with less than 15% reduction in wheat growth. Field evaluations confirmed the efficacy of mesotrione at 100 to 150 g ai ha−1 in reducing wild radish populations by greater than 85% following PRE application and incorporation by wheat planting. Additionally, these field trials demonstrated the opportunity for season-long control of wild radish when mesotrione applied PRE was followed by bromoxynil applied POST. The sequential PRE application of mesotrione, a herbicide that inhibits p-hydroxyphenylpyruvate dioxygenase, followed by POST application of bromoxynil, a herbicide that inhibits photosystem II, has the potential to provide 100% wild radish control with no effect on wheat growth.

Herbicide management information is lacking for recently developed turf-type bahiagrass germplasm. The objective of this study was to evaluate the herbicide tolerance of nine experimental bahiagrass genotypes compared to the industry standard ‘Argentine’. The experimental entries included Argentine and ‘Wilmington’ mutants, and wild-type breeding lines. Plants were grown under greenhouse conditions, and 12 herbicides were applied at 1× and 2× labeled rates. Bentazon, bromoxynil, carfentrazone + 2,4-D + MCPP + dicamba, and carfentrazone were classified as safe. Fluroxypyr, halosulfuron, and triclopyr + clopyralid reduced growth >50% when applied at twice the label rate. Fenoxaprop, sulfentrazone + imazethapyr, and thiencarbazone + iodosulfuron + dicamba reduced growth and caused turfgrass injury above an acceptable threshold (≥20%). In general, the Argentine mutants showed greater herbicide injury compared to the Wilmington mutants. However, metsulfuron exceeded the acceptable injury threshold and stopped growth in all the genotypes, Argentine and genotype WT6 were the least injured by this herbicide. The experimental genotype WT6 consistently showed the greatest herbicide tolerance. Except for one genotype (WT4), the experimental genotypes responded similarly or better than Argentine to the tested herbicides, except for metsulfuron.

Late-season control of Palmer amaranth in postharvest wheat stubble is important for reducing the seedbank. Our objectives were to evaluate the efficacy of late-season postemergence herbicides for Palmer amaranth control, shoot dry biomass, and seed production in postharvest wheat stubble. Field experiments were conducted at Kansas State University Agricultural Research Center near Hays, KS, during 2019 and 2020 growing seasons. The study site had a natural seedbank of Palmer amaranth. Herbicide treatments were applied 3 wk after wheat harvest when Palmer amaranth plants had reached the inflorescence initiation stage. Palmer amaranth was controlled by 96% to 98% 8 wk after treatment and shoot biomass as well as seed production was prevented when paraquat was applied alone or when mixed with atrazine, metribuzin, flumioxazin, 2,4-D, sulfentrazone, pyroxasulfone + sulfentrazone, or flumioxazin + metribuzin, and with glyphosate + dicamba, glyphosate + 2,4-D, saflufenacil + 2,4-D, glufosinate + dicamba + glyphosate, and glufosinate + 2,4-D + glyphosate. Palmer amaranth was controlled by 89% to 93% with application of glyphosate, glufosinate, dicamba + 2,4-D, saflufenacil + atrazine, and saflufenacil + metribuzin resulting in Palmer amaranth shoot biomass of 15 to 56 g m−2 and production of 1,080 to 7,040 seeds m−2. Palmer amaranth control was less than 86% with application of dicamba, 2,4-D, dicamba + atrazine, and saflufenacil resulting in Palmer amaranth shoot biomass of 38 to 47 g m−2 and production of 3,110 to 6,190 seeds m−2. Palmer amaranth was controlled 63% and 72%, shoot biomass was 178 and 161 g m−2, and seed production was 35,180 and 39,510 seeds m−2, respectively, with application of 2,4-D + bromoxynil + fluroxypyr, and bromoxynil + pyrasulfotole + atrazine. Growers should use these effective postemergence herbicide mixes for Palmer amaranth control to prevent seed prevention postharvest in wheat stubble.

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.

Four field experiments were completed in commercial corn fields during 2019 and 2020 to determine glyphosate-resistant (GR) horseweed control in corn with tiafenacil alone or in combination with bromoxynil, dicamba, or tolpyralate applied preplant (PP). Corn planted 1 to 10 d after herbicide application was not injured with any of the herbicides tested. GR horseweed interference reduced corn grain yield 32% when left uncontrolled. Herbicides reduced GR horseweed interference and resulted in corn grain yield that was similar to the weed-free control. Glyphosate (900 g ae ha−1) + tiafenacil at 12.5, 25, and 37.5 g ha−1 controlled GR horseweed 63%, 68%, and 72% at 4 wk after treatment (WAT) and decreased GR horseweed density 64%, 43%, and 83% and dry biomass 69%, 55%, and 83%, respectively. Glyphosate + tiafenacil at 12.5, 25, and 37.5 g ha−1 plus bromoxynil (280 g ai ha−1) controlled GR horseweed 81%, 88%, and 87% at 4 WAT and reduced GR horseweed density 82%, 94%, and 93% and dry biomass 93%, 93%, and 98%, respectively. Glyphosate + tiafenacil at 12.5, 25, and 37.5 g ha−1 plus dicamba (300 g ai ha−1) controlled GR horseweed 86%, 88%, and 88% at 4 WAT and decreased GR horseweed density 76%, 89%, and 86% and dry biomass 94%, 98%, and 98%, respectively. Glyphosate + tiafenacil at 12.5, 25, and 37.5 g ha−1 plus tolpyralate (30 g ai ha−1) controlled GR horseweed 90%, 90%, and 91% at 4 WAT and decreased GR horseweed density 93%, 91%, and 95% and dry biomass 98%, 97%, and 97%, respectively. The industry standards in Ontario, glyphosate + dicamba/atrazine and glyphosate + saflufenacil/dimethenamid-p controlled GR horseweed 95% and 100% at 4, 8, and 12 WAT and caused 99% and 100% density or biomass reduction, respectively.

Glyphosate-resistant (GR) horseweed was first confirmed in Ontario in 2010. GR horseweed interference can reduce soybean yield by up to 97%. Bromoxynil is a photosystem II–inhibiting herbicide that is primarily used for annual broadleaf weed control in monocot crops. The objective of this study was to determine the biologically effective dose (BED) of bromoxynil applied alone and when mixed with metribuzin applied preplant for control of GR horseweed in soybean in Ontario. Five field experiments were conducted over a 2-yr period (2019–2020) to determine the predicted dose of bromoxynil with or without metribuzin that would control GR horseweed 50%, 80%, and 95%. No soybean injury was observed. The predicted doses of bromoxynil to achieve 50% and 80% control of GR horseweed were 98 and 277 g ai ha−1, respectively, at 8 wk after application (WAA). When mixed with metribuzin (400 g ai ha−1), the predicted doses of bromoxynil for 50%, 80%, and 95% control of GR horseweed were 10, 25, and 54 g ai ha−1, respectively. Bromoxynil (280 g ai ha−1) plus metribuzin (400 g ai ha−1) controlled GR horseweed 97%, a finding that was similar to the industry standards of saflufenacil + metribuzin (99% control) and glyphosate/dicamba + saflufenacil (100% control) at 8 WAA. This study concludes that bromoxynil + metribuzin applied before planting provides excellent control of GR horseweed in soybean.

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.

In southern Australia, annual sowthistle and prickly lettuce have become more prevalent following the adoption of reduced tillage cropping systems. They are especially problematic in lentil and other pulse crops, which are weakly competitive and have few herbicide options available for POST control of broadleaf weeds. This study aimed to evaluate the influence of management in a previous cereal crop on weed densities in a subsequent crop. At two field sites, crop seeding density and POST herbicide treatments (a conventional choice that included metsulfuron-methyl and MCPA; and a proactive choice that included bromoxynil, picolinafen, and MCPA) were applied to a wheat crop, and weed density was assessed at the beginning of the following season to measure for a legacy effect of the treatments. Study site populations were also screened for herbicide resistance and were found to have high (≥90% survival) ALS inhibitor resistance. Crop competition treatments had no effect on weed populations, and effects of herbicide treatment were significant at only one of the sites. At this site, both herbicide treatments had lower weed densities than the nontreated in the first year, but the legacy effect was only significant for annual sowthistle density in the proactive treatment. At both sites, even where weeds were extremely sparse or completely controlled following herbicide treatment in the first year, moderate densities were observed the following year, indicating that colonization from the seedbank or adjacent areas could be contributing to weed numbers. Weed density assessments and accurate knowledge of the herbicide resistance status of target weeds should guide herbicide selection to maximize control.

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

Horseweed is a competitive summer or winter annual weed that produces up to 230,000 small seeds per plant that are capable of traveling more than 500 km via wind. Giant ragweed is a tall, highly competitive summer annual weed. Glyphosate-resistant (GR) horseweed and GR giant ragweed pose significant challenges for producers in the United States and Ontario, Canada. It is thought that an integrated weed management (IWM) system involving herbicide rotation is required to control GR biotypes. Halauxifen-methyl is a new selective broadleaf POST herbicide registered for use in cereal crops; there is limited information on its efficacy on horseweed and giant ragweed. The purpose of this research was to determine the efficacy of halauxifen-methyl applied POST, alone and in a tank mix, for the control of GR horseweed and GR giant ragweed in wheat across southwestern Ontario. For each weed species, an efficacy study consisting of six field experiments was conducted over a 2-yr period (2018, 2019). At 8 wk after application (WAA), halauxifen-methyl, fluroxypyr/halauxifen-methyl, fluroxypyr/halauxifen-methyl + MCPA EHE, fluroxypyr + MCPA ester, 2,4-D ester, clopyralid, and pyrasulfotole/bromoxynil + ammonium sulfate controlled GR horseweed >95%. Fluroxypyr and MCPA provided only 86% and 37% control of GR horseweed, respectively. At 8 WAA, fluroxypyr, fluroxypyr/halauxifen-methyl, fluroxypyr/halauxifen-methyl + MCPA EHE, fluroxypyr + MCPA ester, fluroxypyr/halauxifen-methyl + MCPA EHE + pyroxsulam, 2,4-D ester, clopyralid, and thifensulfuron/tribenuron + fluroxypyr + MCPA ester controlled GR giant ragweed 87%, 88%, 90%, 94%, 96%, 96%, 98%, and 93%, respectively. Halauxifen-methyl and pyroxsulam provided only 45% and 28% control of GR giant ragweed, respectively. Halauxifen-methyl applied alone POST in the spring controlled GR horseweed but not GR giant ragweed in winter wheat.

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.

Horseweed biotypes resistant to glyphosate and ALS-inhibiting herbicides are becoming more prevalent in Canada and the United States and present a significant management challenge in field crops. Tolpyralate is a recently commercialized herbicide for use in corn that inhibits 4-hydroxyphenylpyruvate dioxygenase (HPPD), and there is little information regarding its efficacy on horseweed. Six field experiments were conducted in 2017 and 2018 at four locations in Ontario, Canada, to determine the biologically effective dose of tolpyralate and tolpyralate + atrazine and to compare label rates of tolpyralate and tolpyralate + atrazine to currently accepted herbicide standards for POST control of glyphosate and cloransulam-methyl resistant (MR) horseweed. At 8 wk after application (WAA), tolpyralate at 4.8 and 22.6 g ha–1 provided 50% and 80% control, respectively. When applied with atrazine at a 1:33.3 tank-mix ratio, 22.3 + 741.7 g ha–1 provided 95% control of MR horseweed. The addition of atrazine to tolpyralate at label rates improved control of MR horseweed to 98%, which was similar to the control provided by dicamba:atrazine and bromoxynil + atrazine. The results of this study indicate that tolpyralate + atrazine provides excellent control of MR horseweed POST in corn.

Flax yield can be severely reduced by weeds. The combination of limited herbicide options and the spread of herbicide-resistant weeds across the prairies has resulted in a need for more weed control options for flax producers. The objective of this research was to evaluate the tolerance of flax to topramezone, pyroxasulfone, flumioxazin, and fluthiacet-methyl applied alone as well as in a mix with currently registered herbicides. These herbicides were applied alone and in mixtures at the 1X and 2X rates and compared with three industry standards and one nontreated control. This experiment was conducted at Carman, MB, and Saskatoon, SK, as a randomized complete block with four replications. Data were collected for crop population, crop height, yield, and thousand-seed weight. Ratings for crop damage (phytotoxicity) were also taken at three separate time intervals: 7 to 14, 21 to 28, and 56+ d after treatment. Crop tolerance to these herbicides varied between site-years. This was largely attributed to differences in spring moisture conditions and the differences in soil characteristics between sites. Herbicide injury was transient. Hence, no herbicide or combination of herbicides significantly impacted crop yield consistently. Flumioxazin was the least promising herbicide evaluated, as it caused severe crop damage (>90%) when conditions were conducive. Overall, flax had excellent tolerance to fluthiacet-methyl, pyroxasulfone, and topramezone. Flax had excellent crop safety to the combination of pyroxasulfone + sulfentrazone. However, mixing fluthiacet-methyl and topramezone with MCPA and bromoxynil, respectively, increased crop damage and would not be recommended.

Doveweed is a problematic weed of lawns and sod production, as well as golf course roughs, fairways, and tees. End-user reports of selective POST control options are inconsistent and control is often short-lived. In addition, inconsistent control with non-selective herbicides such as glyphosate is common. The goals of this research were: (1) evaluate selective POST doveweed control options in ‘Tifway’ hybrid bermudagrass turf; (2) compare efficacy of single vs. sequential applications of selective POST herbicides; (3) quantify doveweed tolerance to glyphosate; and (4) quantify recovery of foliar applied glyphosate following treatment with a C14-glyphosate solution. A single application of sulfentrazone+metsulfuron; thiencarbazone+iodosulfuron+dicamba or 2,4-D+MCPP+dicamba+carfentrazone; or thiencarbazone+foramsulfuron+halosulfuron provided >60% control 2 weeks after initial treatment (WAIT). A second application of these treatments 3 WAIT improved control 6 WAIT. Two applications of 2,4-D+MCPP+dicamba+carfentrazone or thiencarbazone+foramsulfuron+halosulfuron provided ~80% control 6 WAIT. Doveweed was tolerant to glyphosate application up to 5.68 kgaeha-1. Absorption of 14C-glyphosate was compared between doveweed with cuticle intact, doveweed with a disturbed cuticle, and smooth crabgrass. 14C-glyphosate recovery from the leaf surface of doveweed plants with an intact cuticle was 93.6%. In comparison, 14C-glyphosate recovery from the leaf surface of doveweed plants with a disrupted cuticle and the leaf surface of crabgrass plants was 79.1 and 70.5%, respectively.