Herbicide-resistant Italian ryegrass [Lolium perenne L. spp. multiflorum (Lam.) Husnot] is a significant problem in multiple cropping systems because of its rapid growth, open pollination, prolific seed production, and multiple cases of resistance worldwide, except to protoporphyrinogen oxidase (PPO) inhibitors. This research evaluated tiafenacil, a new PPO inhibitor, in mixtures with glutamine synthetase or acetyl-CoA carboxylase (ACCase)-inhibiting herbicides to manage resistant L. multiflorum populations. Tiafenacil efficacy against L. multiflorum was growth stage-dependent, with increased efficacy at earlier stages in greenhouse studies. The LD90 was 41.06 g ai ha-1 at BBCH23, and increased to 9.0-fold at BBCH33. Field studies indicated that changes in carrier volume did not affect tiafenacil’s efficacy; the highest tested rate of tiafenacil (75 g ai ha-1) reduced L. multiflorum inflorescence weight by 50-90%. Mixtures of tiafenacil and glufosinate (1,150 g ai ha-1) improved L. multiflorum control (+24-43%) and reduced inflorescence weight (+15-34%), particularly at the highest tested rates (50 and 75 g ai ha-1), suggesting synergistic effects based on Colby’s test. Tiafenacil with ACCase inhibitors improved L. multiflorum control (+19-49%) and inflorescence weight reduction (+8-13%). These mixtures exhibited an additive effect when combined with fluazifop and a synergistic effect with clethodim. Herbicide mixtures and application strategies are critical to effective L. multiflorum management. Tiafenacil, especially when used with glufosinate or ACCase inhibitors, offers an effective solution to L. multiflorum management and is a strategic tool against herbicide resistance, as resistance to PPO inhibitors has not evolved. Further research should assess practices to ensure the long-term viability of these mixtures for resistance management.

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.

Herbicides are the foundation for row-middle weed control in Florida plasticulture production. Paraquat is commonly used as a burndown herbicide, and resistance issues have subsequently developed. Halosulfuron is mixed with PRE and POST herbicides to provide additional control of nutsedge. The objective of the study was to determine glufosinate efficacy on weeds emerging in the row-middle and suitability in mixture with halosulfuron for nutsedge control. For total weed control, the high dose of glufosinate (983 g ai ha–1) gave the highest overall control (98% and 64% at 4 wk after treatment for experiments 1 and 2, respectively), and the low rate of glufosinate (656 g ha–1) (67% and 39%) gave results comparable to paraquat (57% and 44%). The high glufosinate dose and paraquat gave comparable control of Brazil pusley (74% to 77% control). Glufosinate + halosulfuron mixture had lower efficacy on Brazil pusley than halosulfuron + paraquat mixture. Glufosinate application reduced grass densities, whereas paraquat did not. Increasing the glufosinate dose did not further decrease grass densities. Similar trends in grass control were also demonstrated in their respective mixtures. Mixing halosulfuron with glufosinate or paraquat did not provide consistent reductions in nutsedge densities, nor did adding paraquat or glufosinate further reduce densities compared with halosulfuron alone for the 4-wk study period. Both paraquat and glufosinate antagonized halosulfuron and reduced efficacy on nutsedge. Compared to controls, there was a reduction between expected and actual nutsedge control for paraquat and glufosinate (25% and 36%), respectively. For total weed control, glufosinate is a suitable alternative to paraquat for row-middle weed management in vegetable production.

Greenhouse and field experiments were conducted at the Lacombe Research Station to evaluate mixtures of sethoxydim and fluazifop on green foxtail, wild oat, wheat, and barley in canola. In both environments the two herbicides interacted on the grass species in a synergistic manner. Many of the observed responses to mixtures of sethoxydim and fluazifop were 100% greater than those expected assuming an additive interaction between the herbicides. Mixtures with at least 80 g ha-1 of sethoxydim and 80 g ha-1 of fluazifop controlled more than 90% of green foxtail, wild oat, wheat, and barley under field conditions. These experiments indicate that the sethoxydim/fluazifop mixture is both complementary and synergistic. The mixture may allow reduced herbicide application rates and therefore reduced herbicide costs and less potential for negative environmental impact.

Postemergence applications of MSMA (monosodium methanearsonate) for weed control in spring wheat (Triticum aestivum L.) were evaluated in the field, greenhouse, and controlled environmental chamber. MSMA controlled weeds better when applied to wheat at the four- to five-leaf stage than when applied at the two- to three-leaf stage. MSMA, tank mixed with barban (4-chloro-2-butynyl-m-chlorocarbanilate) or diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid}, controlled wild oat (Avena fatua L.), green foxtail, [Setaria viridis (L.) Beauv.] and broadleaf weeds better than did MSMA applied alone. Wild oat control with tank-mix applications of MSMA and difenzoquat (1,2-dimethyl-3,5-diphenyl-1H-pyrazolium) was variable. Weed control with MSMA was enhanced by 30 C air temperatures, 90% relative humidity, and adequate soil moisture. A simulated rainfall of 0.5 mm within 0.5 h or 4 mm with 4 hr after application reduced wild oat control with MSMA.

Field experiments were conducted during 1984 and 1985 to determine the influence of 2,4-D and MCPA amine and ester formulations on control of foxtail millet with diclofop. Foxtail millet control increased as the diclofop rate increased from 0.6 to 1.7 kg ai/ha whether diclofop was applied alone or with 2,4-D or MCPA amine and ester formulations; however, foxtail millet control was lower when diclofop was applied with 2,4-D or MCPA amine and ester formulations compared to diclofop applied alone. MCPA ester, MCPA amine, 2,4-D ester, or 2,4-D amine at 0.14 kg ai/ha added to a diclofop spray mixture reduced foxtail millet control 8, 15, 20, and 30% compared to diclofop applied alone, averaged over diclofop rates of 0.6, 0.8, 1.1, 1.4, and 1.7 kg/ha. Adding a petroleum oil to diclofop increased foxtail millet control with diclofop at 0.6 kg/ha but not at 0.8 kg/ha. The petroleum oil additive did not overcome the antagonism of foxtail millet control when diclofop at 0.6 kg/ha was applied with 2,4-D or MCPA amine and ester formulations. The amount of 14C-diclofop absorbed by oats was greater when diclofop was applied with the ester formulations of 2,4-D or MCPA compared to the amine formulations. Either once-refined sunflower oil or sunflower oil methyl ester applied with 14C-diclofop increased the amount of 14C absorbed and translocated in oat plants compared to 14C-diclofop applied alone.

The influence of chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} on control of wild oat (Avena fatua L. ♯3 AVEFA) when mixed in the spray tank with three herbicides was tested in greenhouse and in field trials at Lacombe and Scott, Canada, during 1981 and 1982. In field experiments, chlorsulfuron mixed with diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy]propanoic acid} caused a 4 to 35% reduction in control of wild oat. Mixtures of chlorsulfuron with difenzoquat (1,2-dimethyl-3,5-diphenyl-1H-pyrazolium) caused a 0 to 19% reduction and mixtures with flamprop {N-benzoyl-N-(3-chloro-4-fluorophenyl)-DL-alanine} a 2 to 30% reduction in control of wild oat. Addition of chlorsulfuron to the wild oat herbicides under greenhouse conditions also resulted in losses in control of wild oat. Increasing the rate of the wild oat herbicide in the mixture tended to overcome these losses.

Response of weeds and sugarbeets (Beta vulgaris L. ‘Mono Hy D2′) to preplanting treatments of mixtures of cycloate (S-ethyl N-ethylthiocyclohexanecarbamate) and ethofumesate [(±)-2-ethoxy-2,3-dihydro-3,3-dimethyl-5-benzofuranyl methanesulfonate] was evaluated in five field studies. A mixture containing 1.7 kg/ha of each herbicide reduced the stand of common lambsquarters (Chenopodium album L.) 90%, green foxtail [Setaria viridis (L.) Beauv.] and yellow foxtail [Setaria lutescens (Weigel) Hubb.] 97 to 99%, kochia [Kochia scoparia (L.) Schrad.] 64 to 77%, and redroot pigweed (Amaranthus retroflexus L.) 82 to 99%. The response of weeds to preplanting applications of mixtures of cycloate and ethofumesate was independent of soil texture, whereas response of sugarbeets was dependent on soil texture. The herbicide mixtures significantly reduced the pre-thinning stand of sugarbeets and root and sucrose yields on two sandy loams, but not on a clay loam.

The absorption, translocation, metabolism, and retention of an isopropylamine formulation of glyphosate [N-(phosphonomethyl)glycine] in the presence and absence of the commercial dimethylamine salt and isooctyl ester formulations of 2,4-D [(2,4-dichlorophenoxy)acetic acid], or the commercial isooctyl ester formulation of bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) was investigated in barley (Hordeum vulgare L. 'Summit′). In all treatments, absorption of 14C-glyphosate continued for up to 24 h, but no further absorption occurred with a further 48 h of treatment. A considerable reduction in absorption and translocation of 14C-glyphosate occurred when it was applied to the leaf surface in a mixture with the commercial formulation of either 2,4-D amine, 2,4-D ester, or bromoxynil. When a 14C-glyphosate solution was spotted on the leaf adjacent to drops of the commercial formulation of either 2,4-D amine, 2,4-D ester, or bromoxynil, no reduction in absorption or translocation of 14C-glyphosate occurred. The herbicide components of the 2,4-D amine and bromoxynil formulations were solely responsible for reduced absorption and translocation of 14C-glyphosate. No 14C-glyphosate metabolites were detected in barley following treatment with 14C-glyphosate either alone or mixed with the commercial formulations of 2,4-D amine or bromoxynil. No 14C-glyphosate complexes were detected in mixtures of the commercial formulations of 2,4-D amine, 2,4-D ester, or bromoxynil with 14C-glyphosate. Barley leaves retained less glyphosate when it was spray-applied together with the commercial formulation or the solvent system of 2,4-D amine.

Field experiments were conducted over a 3-yr period at three locations to evaluate lactofen at rates of 110 to 220 g ai ha−1 applied POST in combination with chlorimuron at 9 g ae ha−1 for weed control in soybean. At St. Joseph in 1989 and Baton Rouge in 1990, lactofen at 110, 150, and 170 g ha−1 in combination with chlorimuron controlled prickly sida (3 to 5 cm) and entireleaf and pitted morningglory (5 to 13 cm) comparable with the full rate of lactofen at 220 g ha−1 plus chlorimuron and the standard treatment of acifluorfen at 280 g ae ha−1 plus bentazon at 560 g ae ha−1. Entireleaf morningglory and sicklepod control at Alexandria with lactofen at 110 to 170 g ha−1 plus chlorimuron was comparable with that from the full rate of lactofen plus chlorimuron and better than with acifluorfen plus bentazon. Weed control was reduced when soil moisture was deficient at treatment time or when prickly sida height was more than 5 cm and entireleaf morningglory, pitted morningglory, and sicklepod more than 15 cm. In related studies at St. Joseph, a reduced rate of lactofen (170 g ha−1) in combination with chlorimuron controlled prickly sida 98%, pitted morningglory 93%, and entireleaf morningglory 90% in 1988, which was comparable with the control from the full rate of lactofen plus chlorimuron and with acifluorfen plus bentazon.

Field experiments were conducted from 1986 to 1988 at Lacombe and Lethbridge, Alberta and Scott, Saskatchewan to determine growth and yield response of canola to mixtures of ethametsulfuron with specific grass herbicides. Ethametsulfuron did not usually cause canola injury when mixed with sethoxydim. However, ethametsulfuron mixtures with the following grass herbicides listed in decreasing order of injury potential, often caused canola injury and yield loss: haloxyfop > fluazifop > fluazifop-P > quizalofop > quizalofop-P. Canola yield losses were severe in some experiments, ranging from 59% with quizalofop mixtures to 97% with haloxyfop mixtures; in other experiments, the same mixtures did not cause significant yield losses. ‘Tobin,’ a Brassica rapa cultivar, tended to be more susceptible to injury than the B. napus cultivars ‘Pivot’ and ‘Westar.’ Canola injury symptoms were consistent with those expected from sulfonylurea herbicides. Therefore, we suggest that specific grass herbicides differentially impair the ability of canola to metabolize ethametsulfuron to inactive forms.

Interaction of glyphosate and 2,4-DB was evaluated in greenhouse and laboratory studies with respect to control of four species of annual morningglory. The greenhouse study included smallflower, pitted, palmleaf, and ivyleaf morningglory at three growth stages: seedling, juvenile, and vining. A series of glyphosate rates (0.28 to 1.26 kg ae/ha) and 2,4-DB (0.022 and 0.045 kg ae/ha) were applied alone, and in all possible two-way tank mixtures. Morningglory response to both glyphosate and 2,4-DB applied alone was species- and growth stage-dependent. All combinations were either additive or synergistic for control of pitted, palmleaf, and ivyleaf morningglory. Response of smallflower morningglory ranged from antagonistic to synergistic. Absorption and translocation of 14C-2,4-DB was independent of the addition of glyphosate in all four species. By contrast, absorption of foliar-applied 14C-glyphosate and translocation of 14C from treated leaves to other plant parts were enhanced by the addition of 2,4-DB in all species except palmleaf morningglory. In palmleaf morningglory, 14C-glyphosate absorption and translocation were independent of the addition of 2,4-DB. Absorption and translocation data were in agreement with greenhouse studies.

Amine formulations of 2,4-D [(2,4-dichlorophenoxy) acetic acid] and MCPA {[(4-chloro-o-tolyl)oxy] acetic acid} caused a reduction in paraquat (1,1-dimethyl-4,4′-bipyridinium ion) phytotoxicity to barley (Hordeum vulgare L. 'Summit′), wheat (Triticum aestivum L. ‘Neepawa′), and wild oats (Avena fatua L.). Dicamba (3,6-dichloro-o-anisic acid) reduced paraquat activity in barley only. Paraquat phytotoxicity was antagonized more by 2,4-D amine at high than at low rates. Higher rates of paraquat relative to a fixed rate of 2,4-D amine overcame the antagonism. Paraquat phytotoxicity was not antagonized by 2,4-D or MCPA amine applied as a separate treatment. The technical components of the 2,4-D- or MCPA-amine formulations were solely responsible for the antagonism. Ester formulations did not affect paraquat phytotoxicity.

The isopropylamine salt of glyphosate at 420, 560, and 840 g ae/ha applied alone or mixed with the sodium salt of bentazon at 840 g ai/ha, chlorimuron at 9 g ai/ha, the sodium salt of fomesafen at 350 g ai/ha, or the ammonium salt of imazethapyr at 70 g ae/ha was evaluated for control of large crabgrass and broadleaf signalgrass. Neither grass was controlled by bentazon, fomesafen, or chlorimuron. Imazethapyr controlled large crabgrass and broadleaf signalgrass 30 and 72%, respectively, 3 weeks after treatment (WAT). Glyphosate at all rates controlled both grasses 100%. Control 3 WAT was unaffected by mixing bentazon, chlorimuron, fomesafen, or imazethapyr with glyphosate. Broadleaf signalgrass control 1 WAT was reduced 4 to 15% by mixing bentazon with glyphosate.

Field experiments were conducted for 2 yr to determine the influence of picloram (4-amino-3,5,6-trichloropicolinic acid) and a commercial mixture of picloram plus 2,4-D [(2,4-dichlorophenoxy)acetic acid] (1:16, w/w) on control of wild oats (Avena fatua L. # AVEFA) with four postemergence herbicides. The phytotoxicity to wild oats of barban (4-chloro-2-butynyl m-chlorocarbanilate) or difenzoquat (1,2-dimethyl-3,5-diphenyl-1H-pyrazolium) in 1981 and diclofop {2-[4-(2,4-dichlorophenoxy)-phenoxy] propanoic acid} or flamprop [N-benzoyl-N-(3-chloro-4-fluorophenyl)-DL-alanine] in 1981 and 1982 was reduced when these herbicides were applied in a mixture with picloram plus 2,4-D. Consequently, the use of these mixtures for broad-spectrum weed control in one spray operation is not recommended. Picloram applied at a rate equivalent to the amount present in the picloram plus 2,4-D mixture did not influence the control of wild oats obtained with any herbicide, indicating that the antagonism was due to the 2,4-D component of the picloram plus 2,4-D mixture.

Experiments were conducted in growth chambers by growing oats (Avena sativa L. ‘Ansi’) in water culture and soil with admixed TCA (trichloroacetic acid), pyrazon [5-amino-4-chloro-2-phenyl-3-(2H)-pyridazinone], and two mixtures (pyrazon: less TCA; 1:25 and 1:50). The sigmoid log-dose relationship for dry-matter yield was fitted with a four-parameter logistic equation. Generally, the mixtures appeared to be less active than would be expected under an additive dose model. Around GR50 the departure of joint action of the mixtures from the reference model was almost similar in the two growth media. At higher dose levels the mixtures approached the reference model, and the bioactivity of one mixture (1:25) in the water culture experiment even showed a higher activity than would be expected from the reference model.

Experiments were conducted at the Lacombe and Lethbridge Research Stations to determine tralkoxydim activity at two leaf stages of green foxtail and wild oat and to determine tralkoxydim activity in tank mixtures with bromoxynil and thifensulfuron. Green foxtail and wild oat control was usually better when tralkoxydim was applied at the 2- to 3-leaf stage. However, tralkoxydim applied at the 4- to 5-leaf stage of the weeds often led to less weed topgrowth at harvest because later flushes of green foxtail and wild oat often occurred after the early applications. Nevertheless, wheat and barley yields usually indicated the importance of early weed removal. In greenhouse and field tank mixture studies, thifensulfuron antagonized tralkoxydim activity on green foxtail more than wild oat. Bromoxynil was less antagonistic than thifensulfuron, but significant reductions in green foxtail and wild oat control were detected in tralkoxydim tank mixtures with bromoxynil. In field studies, control of false cleavers with thifensulfuron or bromoxynil was not reduced in tank mixtures with tralkoxydim.

Residual herbicides are routinely recommended to aid in control of glyphosate-resistant (GR) Palmer amaranth in cotton. Acetochlor, a chloroacetamide herbicide, applied PRE, controls Palmer amaranth. A microencapsulated (ME) formulation of acetochlor is now registered for PRE application in cotton. Field research was conducted in North Carolina to evaluate cotton tolerance and Palmer amaranth control by acetochlor ME alone and in various combinations. Treatments, applied PRE, consisted of acetochlor ME, pendimethalin, or no herbicide arranged factorially with diuron, fluometuron, fomesafen, diuron plus fomesafen, and no herbicide. The PRE herbicides were followed by glufosinate applied twice POST and diuron plus MSMA directed at layby. Acetochlor ME was less injurious to cotton than pendimethalin. Acetochlor ME alone or in combination with other herbicides reduced early season cotton growth 5 to 8%, whereas pendimethalin alone or in combinations injured cotton 11 to 13%. Early season injury was transitory, and by 65 to 84 d after PRE treatment, injury was no longer noticeable. Before the first POST application of glufosinate, acetochlor ME and pendimethalin controlled Palmer amaranth 84 and 64%, respectively. Control by acetochlor ME was similar to control by diuron plus fomesafen and greater than control by diuron, fluometuron, or fomesafen alone. Greater than 90% control was obtained with acetochlor ME mixed with diuron or fomesafen. Palmer amaranth control was similar with acetochlor ME plus a full or reduced rate of fomesafen. Acetochlor ME controlled large crabgrass and goosegrass at 91 and 100% compared with control at 83 and 91%, respectively, by pendimethalin. Following glufosinate, applied twice POST, and diuron plus MSMA, at layby, 96 to 99% control was obtained late in the season by all treatments, and no differences among herbicide treatments were noted for cotton yield. This research demonstrated that acetochlor ME can be safely and effectively used in cotton weed management programs.

Field studies were conducted in South Carolina and Georgia to evaluate weed control and soybean tolerance and yield after nonionic surfactant addition to combinations of chlorimuron plus an adjuvant-containing glyphosate formulation. Treatments included glyphosate alone, at 420 or 840 g ae/ha, or in combination with 6 or 9 g ai/ha chlorimuron and all possible combinations with or without 0.25% (v/v) nonionic surfactant. Other treatments included a weed-free and nontreated check. Chlorimuron plus glyphosate improved entireleaf, smallflower, and tall morningglory control over glyphosate alone, but nonionic surfactant addition did not further improve the control of any species, except tall morningglory. Up to 31% early-season injury was observed with the three-way mixture. Soybean injury was greater, and yields were reduced in one of three trials when nonionic surfactant was added to chlorimuron plus glyphosate combinations. This research indicates that there would be no benefit from the nonionic surfactant addition to this adjuvant-containing glyphosate formulation when combined with chlorimuron.

Imazethapyr-based weed control treatments were evaluated in water-seeded imidazolinone-resistant (IR) rice in Louisiana in 2000 and 2001. Treatments included imazethapyr applied to soil before seedling flood (surface [SURF] applied) or no soil application (no SURF), followed by imazethapyr applied postemergence (POST). Averaged over SURF application treatments (SURF, no SURF), all imazethapyr mixtures increased control of barnyardgrass at 14 and 42 d after POST treatment (DAT) compared with imazethapyr alone. Treatments containing imazethapyr-applied SURF followed by POST treatments controlled red rice 90 to 96% at 21 DAT. Alligatorweed and hemp sesbania control increased with POST mixtures compared with imazethapyr-only treatments at 35 DAT. Rice yield increased with imazethapyr applied SURF compared with no SURF application, averaged over POST applications. Other herbicides will be required to control troublesome weeds such as hemp sesbania and alligatorweed in an IR rice production system.