Six field experiments were established in southwestern Ontario in 2021 and 2022 to evaluate whether the addition of a grass herbicide (acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor) to tolpyralate + atrazine improves late-season weed control in corn. Tolpyralate + atrazine caused 12% and 5% corn injury at 1 and 4 wk after herbicide application (WAA); corn injury was not increased with the addition of a grass herbicide. Weed interference reduced corn yield 60%. The addition of a grass herbicide to tolpyralate + atrazine did not enhance velvetleaf control. The addition of acetochlor or dimethenamid-p to tolpyralate + atrazine enhanced pigweed species control 4% 4 WAA; the addition of other grass herbicides tested did not increase pigweed species control. The addition of acetochlor enhanced common ragweed control 5% at 4 WAA, and the addition of acetochlor or dimethenamid-p enhanced common ragweed control 8% at 8 WAA; the addition of other grass herbicides did not improve common ragweed control. The addition of acetochlor to tolpyralate + atrazine enhanced common lambsquarters control up to 4%; there was no enhancement in common lambsquarters control with the addition of the other grass herbicides. Tolpyralate + atrazine controlled barnyardgrass 90% and 78% at 4 and 8 WAA, respectively; the addition of a grass herbicide enhanced barnyardgrass control 9% to 10% and 21% at 4 and 8 WAA, respectively. Tolpyralate + atrazine controlled green or giant foxtail 80% and 69% at 4 and 8 WAA, respectively; the addition of a grass herbicide enhanced foxtail species control 15% to 19% and 24% to 29% at 4 and 8 WAA, respectively. This research shows that adding a grass herbicide to tolpyralate + atrazine mixture can improve weed control efficacy, especially increased annual grass control in corn production.

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.

Many studies have documented the interaction between 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting and photosystem II (PSII)-inhibiting herbicides. Most have focused on the interaction between mesotrione and atrazine, with only a few studies characterizing the nature of the interaction between tolpyralate and atrazine. Therefore, five field experiments were conducted in Ontario, Canada, over a 3-yr period (2019 to 2021) to characterize the interaction between three rates of tolpyralate (15, 30, and 45 g ai ha−1) and three rates of atrazine (140, 280, and 560 g ai ha−1) for the control of seven annual weed species in corn (Zea mays L.). Tolpyralate at 30 or 45 g ha−1 applied with atrazine at 280 or 560 g ha−1 controlled velvetleaf (Abutilon theophrasti Medik.), redroot pigweed (Amaranthus retroflexus L.), common ragweed (Ambrosia artemisiifolia L.), common lambsquarters (Chenopodium album L.), and wild mustard (Sinapis arvensis L.) >90% at 8 wk after application (WAA). Tolpyralate and atrazine were synergistic at each rate combination for the control of A. theophrasti at 8 WAA. In contrast, A. retroflexus and S. arvensis control at 8 WAA was additive with each rate combination. At 8 WAA, C. album control was generally additive, but one rate combination was synergistic. Ambrosia artemisiifolia control at 8 WAA was synergistic with five rate combinations and additive with the other four. Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] control at 8 WAA was additive with seven of the rate combinations and synergistic with two. Setaria spp. control at 8 WAA was synergistic with one more rate combination compared with E. crus-galli, but the two weed species shared the same synergistic rate combinations. This study concludes that extrapolation or broad classifications of the interaction between tolpyralate and atrazine would be inappropriate, as the interaction can vary due to herbicide rate, weed species, and the response parameter analyzed.

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

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

Previous reports have underscored antagonism that may result from mixing glyphosate and glufosinate across a wide range of application rates and for various broadleaf and grass weed species, but no investigation has been conducted to characterize glyphosate absorption and translocation when combined with glufosinate. The objectives of this study were to evaluate herbicide efficacy and assess herbicide interaction and physiological response with combinations of glyphosate and glufosinate on common lambsquarters, velvetleaf, and giant foxtail. Greenhouse studies to determine interaction resulted in strong and persistent antagonism between glyphosate at 110 and 220 g ae ha−1 and glufosinate at 20 or 40 g ae ha−1 in giant foxtail, whereas only transient and reduced antagonism was noted for velvetleaf and common lambsquarters. Combining glyphosate and glufosinate increased the maximum absorption of glyphosate by 9% and 23% in velvetleaf and giant foxtail, respectively, compared with glyphosate alone. In velvetleaf, averaged over time, only 2.6% of the applied radioactivity translocated out of the treated leaf when glufosinate was mixed with glyphosate compared with 9.9% when glyphosate was applied alone. In giant foxtail, 21.6% of the [14C]glyphosate translocated out of the treated leaf when glufosinate was mixed with glyphosate compared with 52.4% when glyphosate was applied alone. Conversely, no change in the radioactive pattern of translocation was noted for common lambsquarters. These results suggest that reduced translocation of glyphosate is the physiological mechanism responsible for the antagonism observed between glyphosate and glufosinate in giant foxtail and, to a lesser extent, in velvetleaf.

Translocation of 14C-haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} in quackgrass [Agropyron repens (L.) Beauv. # AGRRE] from the treated area (the middle 2.5 cm of the second of three leaves) to the lower leaves was reduced in plants in which 0.56 kg/ha of acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} or bentazon [3-1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one-2,2-dioxide] was added in tank mixture to a 0.07 kg ai/ha application of haloxyfop. Reduced translocation did not occur when haloxyfop rates were increased to 0.28 and 1.1 kg/ha. Bentazon reduced absorption of 14C-DPX-Y6202 {2-[4-[(6-chloro-2-quinoxalinyl)oxy] phenoxy] propionic acid} when applied in tank mixtures with 0.28 and 1.1 kg ai/ha DPX-Y6202. There was less 14C-DPX-Y6202 in the leaf tips of plants treated with 0.07 kg/ha DPX-Y6202 plus 0.56 kg/ha acifluorfen in tank mixture. Relatively small amounts of both 14C-haloxyfop and DPX-Y6202 accumulated in the rhizome and adjacent shoots and acifluorfen did not influence translocation to these plant parts. Considerably more 14C-haloxyfop than 14C-DPX-Y6202 was absorbed by quackgrass. Treatment of quackgrass with acifluorfen or bentazon 24 h prior to application of 14C-haloxyfop or 14C-DPX-Y6202 did not influence absorption or translocation.

Field experiments were conducted to determine the effect of CGA184927 rate, weed growth stage, and tank mixes with various broadleaf herbicides on the control of green foxtail and wild oat in spring wheat. CGA 184927 controlled green foxtail and wild oat equally well when applied at the 2- to 3- or 4- to 5-leaf stages. Green foxtail and wild oat were controlled at similar rates of CGA 184927 but the application rate giving > 90% control ranged from 22 to 90 g/ha over locations and years, indicating that CGA 184927 efficacy is sensitive to environmental conditions. CGA 184927 in tank mixtures was compatible with bromoxynil, clopyralid, and 2,4-D ester. However, tank mixing with metsulfuron or dicamba reduced activity on green foxtail and wild oat. Broadleaf herbicide activity on kochia and redroot pigweed was not reduced when such herbicides were tank-mixed with CGA 184927. Spring wheat tolerated 120 g/ha of CGA 184927. CGA 184927 provides growers with another herbicide option to control green foxtail and wild oat in wheat.

Experiments were conducted in the field, greenhouse, and controlled environment chambers to determine the extent to which MCPA {[(4-chloro-o-tolyl)oxy] acetic acid} antagonizes wild oat (Avena fatua L.) control with diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid}. Wild oat control with diclofop at 1 kg/ha was reduced from 96% when used alone to 76, 48, 31, and 14% by tank mixture with IAA (3-indole acetic acid), MCPA, 2,4-D [(2,4-dichlorophenoxy)acetic acid], or dicamba (3,6-dichloro-o-anisic acid), respectively. Wild oat control with diclofop applied alone at 1.1 kg/ha was similar to that of diclofop at 2.2 kg/ha applied as a tank mixture with MCPA at 0.15 or 0.3 kg/ha. MCPA antagonism of wild oat control with diclofop increased as the post-treatment temperature increased from 10 to 30 C. MCPA antagonism of wild oat control with diclofop was the same whether the herbicides were applied to the foliage only or to the foliage and soil. Approximately 20% of the wild oat root inhibition with diclofop applied postemergence, however, was from diclofop uptake from the soil. MCPA at 0.6 kg/ha did not reduce wild oat control when applied as a sequential treatment 2 days before or 1 day after diclofop at 1.1 kg/ha.

The interaction between diclofop-methyl {2-[4-(2,4-dichlorophenoxy)phenoxy] methyl propanoate} and 2,4-D [(2,4-dichlorophenoxy)acetic acid] was evaluated. Foliar applications of the two herbicides in a tank mixture were antagonistic with respect to wild oat (Avena fatua L.) control. Application of mixtures of the two herbicides to wild oat roots resulted in an additive herbicidal effect. The acid form of 2,4-D was determined to be the component of the 2,4-D formulation that reduced the toxicity of foliar-applied diclofop-methyl to wild oat. Analysis of diclofop-methyl solutions with and without added 2,4-D revealed no degradation products of dichlofop-methyl, nor any evidence of complexing between diclofop-methyl and 2,4-D. Addition of 2,4-D to the diclofop-methyl spray solution did not affect diclofop-methyl spray retention by, or penetration into, wild oat leaves. Movement of radioactivity to roots and to shoot apices following application of 14C-diclofop-methyl to wild oat leaves was reduced by addition of 2,4-D to the treatment solution. Deesterification of diclofop-methyl was inhibited by 2,4-D. The accumulation of diclofop-methyl in these wild oat leaves resulted in membrane damage and leaf necrosis. Symplastic movement of the des-methyl acid, diclofop, to sensitive meristematic areas was reduced. As insufficient toxicant reached meristematic areas to permanently interrupt meristematic activity, the wild oat plants were able to outgrow the contact damage to their leaves.

Greenhouse and field studies were conducted to examine the interaction of sethoxydim or fluazifop-P tank mixed with chlorsulfuron or thiameturon for selective weed control in safflower. Under greenhouse conditions, the addition of chlorsulfuron to sethoxydim or fluazifop-P slightly improved the control of green foxtail above that achieved with either herbicide alone. Control of wild oat and wild mustard was similar with the tank mixes and each herbicide alone. In the field, control of wild oat with sethoxydim or fluazifop-P was not altered by adding chlorsulfuron or thiameturon in tank mixes. Similarly, control of common lambsquarters and wild mustard with chlorsulfuron or thiameturon was not affected by adding either sethoxydim or fluazifop-P in tank mixes. Plant height, date of flowering, seed yield, and quality data indicated that safflower tolerated these herbicides applied alone or in combination. Sethoxydim or fluazifop-P tank mixed with chlorsulfuron or thiameturon provide a postemergent alternative for selective control of grass and broadleaf weeds in safflower.

The basis for chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] caronyl] benzenesulfonamide} antagonism of diclofop {(±)-2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid} was investigated in Italian ryegrass [Lolium multiflorum (Lam.) # LOLMU] and wheat (Triticum aestivum L. ‘McNair 1813′). Chlorsulfuron did not affect the chemical stability or volatility of diclofop in spray mixtures over the time and concentration ranges evaluated. Addition of chlorsulfuron (0.11 μg/μl) to a diclofop (2.0 μg/μl) foliar-applied spotting solution did not affect the absorption, translocation, or metabolism of diclofop in 17-day-old Italian ryegrass or wheat. Italian ryegrass and wheat responded differentially to diclofop alone. At 72 h after treatment, 69 and 40% of labeled diclofop penetrated into Italian ryegrass and wheat leaves, respectively. Although little movement of labeled diclofop occurred in either species, more translocated from the treated zone of Italian ryegrass. At 72 h after treatment, the percentages of 14C recovered as parent methyl ester, diclofop (free acid), and conjugates in treated leaf extracts were 9, 62, and 25%, respectively, for Italian ryegrass, and 50, 20, and 29%, respectively, for wheat. Differential penetration and metabolism of diclofop are the probable bases for selectivity between wheat and Italian ryegrass. The antagonism of diclofop caused by chlorsulfuron does not appear to be due to an alteration of the molecular fate of diclofop in Italian ryegrass.

2,4-D has been reported to antagonize glyphosate activity on grass weed species. In addition, sodium bicarbonate antagonized 2,4-D activity on broadleaf species. Experiments were conducted using Nuclear Magnetic Resonance (NMR) technology to evaluate interactions of 2,4-D dimethylamine and 2,4-D butoxyethylester formulations with glyphosate in solution. The 2,4-D dimethylamine formulation associated with glyphosate to form the dimethylamine salt of glyphosate. NMR spectra also showed that the molecular orientation of the glyphosate molecule was affected by the 2,4-D butoxyethylester formulation. The antagonist was not the active 2,4-D butoxyethylester molecule itself, but appeared to be due to an association of glyphosate with organic components in the 2,4-D butoxyethylester formulation. The molecular influence of sodium bicarbonate on technical grade 2,4-D free acid, and dimethylamine and butoxyethylester formulations was also evaluated. Sodium bicarbonate was found to react with the 2,4-D free acid and 2,4-D dimethylamine to form the sodium salt of 2,4-D. The 2,4-D butoxyethylester formulation did not react with sodium bicarbonate.

Experiments were conducted in 1991 and 1992 to evaluate annual grass and broadleaf weed control with bromoxynil applied alone and with graminicides. Bromoxynil controlled velvetleaf better than entireleaf morningglory and controlled both species better than sicklepod. Mixing bromoxynil with fluazifop-P, quizalofop-P, and sethoxydim reduced control of barnyardgrass, green foxtail, large crabgrass, and shattercane. Control of broadleaf signalgrass with quizalofop-P was reduced by bromoxynil. Annual grass control with clethodim was not affected by bromoxynil.

Studies were conducted to determine the usefulness of HOE-39866 (HOE-00661) in chemical fallow systems on the Canadian prairies. HOE-39866 at 0.5 to 1.0 kg ai/ha controlled Russian thistle, kochia, green foxtail, wild oats, and wheat comparable to paraquat, glyphosate, and glyphosate plus the isopropylamine salt of 2,4-D. However, control of barley with HOE-39866 was unacceptable. HOE-39866 was compatible in tank mixtures with ammonium sulfate, paraquat, chlorsulfuron, and metsulfuron. Ammonium sulfate improved weed control when HOE-39866 was applied at 0.25 kg/ha but not at 0.75 kg/ha. Adding paraquat at 0.07 to 0.21 kg ai/ha to HOE-39866 improved control of grass species over HOE-39866 alone. Adding chlorsulfuron and metsulfuron to HOE-39866 provided greater initial control of certain species as well as residual control of many weeds. HOE-39866 alone or in conjunction with other herbicides is an alternative to the herbicides used in chemical fallow systems.

Field studies were conducted to study the interaction of sethoxydim or fluazifop-P with clopyralid and/or ethametsulfuron applied as tank mixtures to canola. Control of the indicator species barley, broadbean, and wild mustard with the tank mixtures was comparable to, or sometimes better than, that attained with each herbicide alone. Canola tolerated all herbicides applied individually and all tank mixtures of these herbicides except fluazifop-P and ethametsulfuron. Tank mixtures of fluazifop-P and ethametsulfuron, with or without clopyralid included in the mixture, suppressed early growth of canola in two of four tests and reduced seed yield in one. Sethoxydim, clopyralid, and ethametsulfuron combined in tank mixtures provide an effective POST alternative for selective control of grass and broadleaf weeds in canola.

A synthetic data set was created by incorporating results from previously published papers on antagonistic and synergistic herbicide interactions between two herbicides applied as a tank mixture or sequentially, and then analyzed on the basis of various properties of the herbicides and target plants. Generally, interactions between herbicides were antagonistic more frequently than synergistic. This trend held no matter whether the interacting herbicides were absorbed by the same or different parts of the plant, had the same or different translocating abilities, had the same or different modes of action, and regardless of whether the target plants were annual or perennial plants, or crops or weeds. Antagonistic interactions occurred much more frequently when the target plants were monocot than dicot, and in the Compositae, Gramineae, or Leguminosae than in the Chenopodiaceae or Convolvulaceae families.

A single application of 0.56 or 0.84 kg ai/ha tridiphane [2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl) oxirane] plus 2.3 kg ai/ha of atrazine [6-chloro-N-ethyl-N′-(1-niethylethyl)-1,3,5-triazine-2,4-diamine] at the one-to three-leaf stage controlled large crabgrass [Digitaria sanguinalis (L.) Scop. # DIGSA] and fall panicum (Panicum dichotomiflorum Michx. #PANDI) 96% or more. A sequential application of 1.1 kg/ha of atrazine, in addition to the above single application, was required for control of the two grass species at the four-to six-leaf stage. Application of tridiphane plus atrazine to large crabgrass and fall panicum beyond the six-leaf stage was ineffective. Tridiphane at 0.75 kg/ha, applied alone, reduced the shoot dry weight of large crabgrass by 71%. The response of tridiphane and atrazine combinations at the four- to five-leaf stage of large crabgrass was additive.

Field studies were conducted in 1995 and 1996 at three locations in Illinois to determine soybean response to combinations of thifensulfuron and bentazon. Thifensulfuron was applied at 2.2 to 8.8 g ai/ha alone or in combination with 280 to 560 g/ha of bentazon. Soybean injury 30 d after treatment ranged from 0 to 22% when thifensulfuron was applied alone at 2.2 g/ha. Increasing thifensulfuron rate to 8.8 g/ha increased soybean injury to a range of 12 to 44%. Soybean grain yield was significantly reduced compared to the yield of untreated soybean when thifensulfuron was applied at 4.4 and 8.8 g/ha in two of five and four of five experiments, respectively. The addition of bentazon to thifensulfuron consistently reduced soybean injury and stunting. In many cases, increasing the bentazon rate to 420 g/ha decreased soybean injury from thifensulfuron to a greater extent than 280 g/ha. In cases where thifensulfuron decreased soybean yield, the addition of 420 or 560 g/ha of bentazon restored yields to levels that were not lower than untreated soybeans. These studies demonstrate that thifensulfuron at 2.2 to 8.8 g/ha in combination with bentazon at 420 g/ha may be safely applied to soybean for broadleaf weed control.

The effect of methylated seed oil (MSO), the organosilicone adjuvant DC-X2-5394, and ammonium sulfate on the efficacy, absorption, and spray retention of primisulfuron applied alone or with atrazine, dicamba, and bentazon to shattercane and giant foxtail was evaluated. Primisulfuron efficacy on both species was reduced by the three tank-mix combinations. Atrazine antagonism was not explained by decreases in foliar absorption and/or spray retention. Reductions in primisulfuron absorption and/or foliar spray retention appeared to cause bentazon antagonism on both weeds and dicamba antagonism on shattercane. MSO, DC-X2-5394, and ammonium sulfate completely reversed dicamba and bentazon antagonism on shattercane and partially reversed bentazon antagonism on giant foxtail by increasing foliar absorption and/or spray retention. Compared with non-ionic surfactant, MSO and DC-X2-5394 consistently increased giant foxtail control with primisulfuron by increasing foliar absorption and/or spray retention.