The herbicides that inhibit very-long-chain fatty acid (VLCFA) elongases are primarily used for residual weed control in corn, barley, oat, sorghum, soybean, sugarcane, certain vegetable crops, and wheat production fields in the United States. They act primarily by inhibiting shoot development of susceptible species, preventing weed emergence and growth. The objectives of this review were to summarize 1) the chemical family of VLCFA-inhibiting herbicides and their use in the United States, 2) the VLCFA biosynthesis in plants and their site of action, 3) VLCFA-inhibitor resistant weeds and their mechanism of resistance, and 4) the future of VLCFA-inhibiting herbicides. After their reclassification as Group 15 herbicides to include shoot growth-inhibiting herbicides (Group 8), the VLCFA-inhibiting herbicides are currently represented by eight chemical families (benzofurans, thiocarbamates, α-chloroacetamides, α-oxyacetamides, azolyl-carboxamides, isoxazolines, α-thioacetamides, and oxiranes). On average, VLCFA-inhibiting herbicides are applied once a year to both corn and soybean crops in the United States with acetochlor and S-metolachlor being the most used VLCFA-inhibiting herbicides in corn and soybean production, respectively. The site of action of Group 15 herbicides results from inhibition of the VLCFA synthase, which is encoded by several fatty acid elongase (FAE1)-like genes in VLCFA elongase complex in an endoplasmic reticulum. The VLCFA synthase is a condensing enzyme, and relies on a conserved, reactive cysteinyl sulfur in its active site that performs a nucleophilic attack on either the natural substrate (fatty acyl-CoA) or the herbicide. As of August 2023, 13 weed species have been documented to be resistant to VLCFA inhibitors, including 11 monocot weeds and two dicot weeds (Palmer amaranth and waterhemp). The isoxazolines (pyroxasulfone and fenoxasulfone) are the most recently (2014) discovered VLCFA-inhibiting herbicides. Although the intensity of VLCFA-inhibitor-directed discovery efforts has decreased over the past decade, this biochemical pathway remains a viable mechanistic target for the discovery of herbicide premixes and a valuable component of them.

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.

Residual herbicides applied to summer cash crops have the potential to injure subsequent winter annual cover crops, yet little information is available to guide growers’ choices. Field studies were conducted in 2016 and 2017 in Blacksburg and Suffolk, Virginia, to determine carryover of 30 herbicides commonly used in corn, soybean, or cotton on wheat, barley, cereal rye, oats, annual ryegrass, forage radish, Austrian winter pea, crimson clover, hairy vetch, and rapeseed cover crops. Herbicides were applied to bare ground either 14 wk before cover crop planting for a PRE timing or 10 wk for a POST timing. Visible injury was recorded 3 and 6 wk after planting (WAP), and cover crop biomass was collected 6 WAP. There were no differences observed in cover crop biomass among herbicide treatments, despite visible injury that suggested some residual herbicides have the potential to effect cover crop establishment. Visible injury on grass cover crop species did not exceed 20% from any herbicide. Fomesafen resulted in the greatest injury recorded on forage radish, with greater than 50% injury in 1 site-year. Trifloxysulfuron and atrazine resulted in greater than 20% visible injury on forage radish. Trifloxysulfuron resulted in the greatest injury (30%) observed on crimson clover in 1 site-year. Prosulfuron and isoxaflutole significantly injured rapeseed (17% to 21%). Results indicate that commonly used residual herbicides applied in the previous cash crop growing season result in little injury on grass cover crop species, and only a few residual herbicides could potentially affect the establishment of a forage radish, crimson clover, or rapeseed cover crop.

Research was conducted to evaluate the sensitivity of cover crops to a low rate of soil-applied herbicides and investigate the likelihood of herbicide carryover to fall-seeded cover crops following an irrigated corn crop. In the sensitivity study, herbicides were applied at a 1/16×rate (to simulate four half-lives) 1 d after cover crop planting, whereas for the carryover study residual herbicides were applied at a 2×rate at the maximum label corn height or growth stage and cover crops sown immediately after corn harvest. In the sensitivity experiment, atrazine, diuron, fluridone, fomesafen, metribuzin, pyrithiobac, and sulfentrazone reduced emergence of the leguminous cover crops Austrian winterpea, crimson clover, and hairy vetch. However, reduced biomass production of leguminous cover crops in the spring was only observed for atrazine, fluridone, and pyrithiobac. For rapeseed, atrazine, flumioxazin, fluridone, pyrithiobac, pyroxasulfone, sulfentrazone, and tembotrione reduced emergence, but biomass production was reduced only by atrazine and fluridone. Conversely, wheat, cereal rye, barley, oats, and triticale were not affected by soil-applied herbicides. Barley was the only cereal cover crop that showed biomass reduction due to the application of flumioxazin, fluridone, mesotrione, S-metalochlor, and sulfentrazone. In the carryover study, with the exception of crimson clover, Austrian winterpea, cereal rye, hairy vetch, rapeseed, and wheat showed no negative affect on biomass production following a 2×rate of postemergence-applied residual herbicide in corn.

Imazapyr and imazamox are frequently applied postemergence to control grass and broadleaf weeds in imidazolinone-resistant sunflower in Argentina. Herbicide carryover to rotational crops represents a disadvantage of these herbicides, particularly in regions with low rainfall during the months prior to rotational crop sowing. Between 2009 and 2012, field and greenhouse studies were conducted on four important sunflower-cropped areas of Argentina. The objective was to quantify the effects of imazapyr alone and imazamox plus imazapyr applied in sunflower crops on the subsequent establishment, growth, and yield of barley, oat, and wheat. In all field experiments, imazapyr alone and imazamox plus imazapyr were applied at recommended rates (80 gha–1 and 66 plus 30 gha–1, respectively), and also, in some experiments, at double the recommended rates. Soil bioassays were also conducted in the greenhouse to study the effect of these herbicides on barley, oat, and wheat seedlings. The mixture of imazamox plus imazapyr was safer for rotational crops than imazapyr applied alone, because of the reduced rate of imazapyr in the mixture treatments. Barley was more sensitive to imidazolinones, particularly imazapyr, than the other winter cereals. Imazapyr at double rate (160 gha–1) reduced barley yield by 45% when seeds were sown 165 d after herbicide application and with 240 mm rainfall after herbicide application.

The effectiveness of preharvest applications of glyphosate on yellow toadflax was evaluated at five sites in Alberta from 1992 to 1994. At each site, glyphosate at 0.9 to 1.8 kg ae/ha was applied with or without nonionic surfactant and/or ammonium sulfate. Glyphosate at 2.7 kg/ha and glufosinate at 0.6 kg ai/ha were applied without additional adjuvants. The treatments were applied 7 to 10 d before crop harvest, when the majority of the yellow toadflax was in a flowering stage. Eleven to 12 mo after glyphosate application, yellow toadflax density was reduced by more than 80%. In most instances, there was no advantage in increasing the glyphosate rate above 0.9 kg/ha. The addition of nonionic surfactant and/or ammonium sulfate did not enhance glyphosate activity. Glufosinate did not control yellow toadflax in the year following treatment. Barley, canola, and flax yields in the year following applications were significantly higher in all preharvest glyphosate-treated plots than in untreated plots.

One year after application under irrigated conditions, atrazine at 1.2 kg/ha injured oat but there was no effect after two years. On dryland, wheat and barley were injured for up to two years after atrazine was applied at 1.5 kg/ha. Atrazine concentrations in the soil were related to accumulated rainfall in an exponential equation. The equation predicted that, under the most severe drought conditions ever recorded for southern Alberta, injury to cereal crops could occur for at least three years after atrazine application at 1.2 to 1.5 kg/ha. Cyanazine did not injure subsequent crops under either irrigated or dryland conditions.

Chlorsulfuron was applied to wheat at rates of 0, 10, 20, and 40 g ai ha-1 in three locations at Lethbridge and Brooks between 1980 and 1984. The soil pH and organic matter contents at Lethbridge and Brooks were 8.0 and 2.1% and 7.4 and 3.0%, respectively. Rotational crops were seeded for up to 7 yr after chlorsulfuron application. The criteria used to indicate when susceptible crops will grow safely were no significant yield loss at the 40 g ha-1 rate and the magnitude of the upper confidence limit for this difference. The following times after application before crop yields were not affected were required at Lethbridge: barley, 2 yr; canola, 3 yr; peas, beans, 4 yr; flax and potatoes, 5 yr; alfalfa and sugarbeets, 6 yr; and lentils at least 7 yr. The effect of chlorsulfuron residues in the soil on rotational crops appeared to be similar at Brooks and at Lethbridge. Estimated chlorsulfuron concentrations were related to the tolerance of rotational crops.

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.

Jointed goatgrass and downy brome continue to plague winter wheat producers in the western United States. Because there are no effective herbicides for in-crop control of these weeds, producers are seeking cultural practices that stimulate seed germination and deplete the soil seed bank. We determined the effect of limited tillage and crop canopy on seedling emergence of these grasses. One tillage operation with a sweep plow increased jointed goatgrass seedling emergence 74% in the first year but did not affect emergence in later years. Downy brome emergence was not affected by tillage. Jointed goatgrass seedlings emerged over 5 yr, whereas downy brome did not emerge after 3 yr. Seedling emergence of both species was two times greater in corn and barley than in proso millet. Producers will accrue more benefit for seedbank management with cultural strategies such as alternative rotations and competitive wheat canopies than with limited tillage using a sweep plow.

A practical expert system is needed to handle POST weed control in herbicide-intensive crops such as wheat, barley, oat, rye, sugarbeet, corn, and sorghum. SELOMA is an expert system having a step-by-step problem-solving procedure closely resembling what a weed management expert would follow. It is based on field surveys of weed density, and crop and weed growth stage and height. SELOMA evaluates weed competitiveness and provides weed management advice. It suggests whether or not to intervene, chemical and mechanical weed control treatments, and selects the best herbicides, including commercial formulations, costs, and optimal dosages.

The objective of this paper was to evaluate the effect of preplant applications of trifluralin on barley stand and yield, and control of grass weeds in field experiments during 1992 and 1993. Factors examined were: (1) crop planting patterns (conventional drill with rows 15 cm apart and deep-seeder drill with rows 25 cm apart), (2) herbicide application times (22 d before sowing and immediately before sowing), and (3) herbicide application. During 1993, hand-weeded plots also were established. Trifluralin applied preplant at 528 g ai/ha reduced weed density and biomass. Weed control was higher under conventional planting than under the deep planting pattern, and there was no effect of the time of application on herbicide efficacy. There was no herbicide injury to the crop, and grain yield was higher in treated than in untreated plots due to successful weed control.

The effectiveness of single and repeated annual preharvest glyphosate applications (1 to 2 wk before crop harvest) to reduce Canada thistle populations was evaluated in field experiments near Bezanson, AB, Scott, SK, and St. Albert, AB. Single applications of glyphosate at 0.45 to 1.8 kg/ha at Bezanson and St. Albert reduced Canada thistle shoot density by more than 75% but in the three experiments at Scott 1.8 kg/ha was required to consistently obtain a similar reduction. Four consecutive, annual applications of glyphosate at 0.45 kg/ha or more did not eliminate Canada thistle but reduced shoot densities by more than 98% at Bezanson. At Scott, similar reductions occurred in two of three sites following two consecutive, annual preharvest applications of glyphosate at the same rates. In the third Scott site the 1.8 kg/ha rate of glyphosate provided a similar reduction but lower rates did not. At Bezanson, barley yields following the first, second, and third applications of glyphosate at 0.45 kg/ha or more were 14% greater than check plots where MCPA was applied in the spring. Yields of wheat seeded 1 yr after a single application of glyphosate at St. Albert were 26% greater than from unsprayed checks. Barley yields seeded 1 yr after each glyphosate application at Scott were not affected because Canada thistle populations were sparse.

Field experiments were conducted at the Lacombe Research Station from 1989 to 1991 to determine the influence of various adjuvants on sethoxydim activity. In all experiments sethoxydim was applied at 100 g ai ha-1 to green foxtail, wheat, wild oat, and barley seeded in a canola crop. Of the four grass species, green foxtail was the most susceptible and barley was the least susceptible to sethoxydim. CC 16255 was the most effective adjuvant followed by either of two sources of ammonium sulphate (liquid or granular) and then Merge. Liquid and granular forms of ammonium sulphate were equally effective in enhancing sethoxydim activity. Several other adjuvants, including Enhance, Savol, and XE 1167, were moderately effective in the enhancement of sethoxydim activity. Adding Canplus 411 to Merge was not usually beneficial, but additions of Canplus 411 to Enhance often increased sethoxydim activity compared with sethoxydim and Enhance alone. Agral 90 and LI-700 were of little or no value as adjuvants with sethoxydim.

Experimental controlled release starch granules (CRSG) containing 5.3% a.i. (w/w) norflurazon or 6% a.i. (w/w) simazine retarded the leaching of both herbicides in loamy sand soil columns when compared to commercial formulations of norflurazon [80% (w/w) dry flowable] or simazine [90% (w/w) water dispersible granule]. Barley bioassays indicated norflurazon and simazine remained in the surface 0 to 2.5 cm of soil when applied as CRSG formulations and moved to a depth of 15 cm when applied as commercial dry formulations and leached with 6 cm of water. CRSG placed on pre-wetted soil columns began to release norflurazon by 7 d at 25 C or 14 d at 15 C, and subsequent leaching moved norflurazon beyond the top 2.5 cm of the soil column.

AC 222,293, a postemergence herbicide for wild oat control in wheat, barley, and sunflower, was applied at five rates at three locations in Montana in the spring of 1985. Twelve and 11 crops were seeded into treated soil in 1985 and 1986, respectively. Sugarbeet, lentil, and rape were injured by residual levels of AC 222,293 14 months after application. Injury was significantly greater at one location, indicating herbicide bioavailability varied by location. Soil pH was correlated negatively with crop injury, indicating the herbicide persists longer on lower pH soils.

An accurate method is described for evaluating the relative effectiveness of isolates of Fusarium avenaceum as potential biological control agents for weeds such as spotted knapweed and wild oat. The method uses seeds that have been disinfected, placed in tubes containing water agar, and challenged with various isolates of F. avenaceum. Evaluatory data on the pathogenicity of the fungi are obtained after 14 d, when plants and fungi both are grown on water agar in the lab. Commonly, potential biocontrol agents are tested on seeds placed in the soil, but up to 60 d may be required for results. However, comparative studies using the soil test vs. the water agar test indicated that the water agar tests are giving comparable results to those from greenhouse tests but in a shorter time (14 vs. 60 d). Also, both water agar and soil tests were done with a number of crop plants including wheat, barley, alfalfa, and wild oat with comparable results. Thus, this method has the potential to screen many pathogens that may have potential biocontrol activities in a short time.

A field study was conducted from 1987 to 1992 to determine the effectiveness of several treatments consisting of combinations of glyphosate and/or quizalofop with tillage to control quackgrass in legume plowdown and continuous annual crop rotations. The treatments were imposed on the quackgrass in 1987 and/or 1988, evaluated in 1989 (cycle 1), reimposed on the same plots in 1990 and/or 1991 and re-evaluated in 1992 (cycle 2). Where continuous barley was grown, glyphosate applied at 0.45 kg/ha before seeding in 1987 (cycle 1) and 1990 (cycle 2), and combined with fall tillage in the first two years of each cycle, reduced quackgrass shoot density and rhizome dry weight by more than 96%. In a legume plowdown rotation, consisting of barley underseeded to red clover followed by plowdown and barley in subsequent years, quackgrass was reduced by a similar amount by glyphosate at 0.45 kg/ha applied in 1987 and 1990 before seeding or in 1988 and 1991 at 5 d before or 6 wk after red clover plowdown. Quackgrass reduction from all of these treatments was as effective as the labelled rate of glyphosate (0.90 kg/ha) applied at the same times, and also as effective as glyphosate applied at 0.45 kg/ha in combination with tillage in a year of fallow. Without fall tillage the efficacy of glyphosate applied before seeding was reduced. Quizalofop at 0.20 kg/ha applied to canola in a rotation of canola followed by two years of barley, reduced quackgrass, but was less effective than glyphosate treatments with fall tillage. Reducing quackgrass populations resulted in significant increases in crop yields.

Greenhouse and field experiments were conducted from 1987 to 1990 at the Lacombe Research Station to determine the influence of ammonium sulfate (AS) on various grass control herbicides. In field studies, AS had slight or no effects on the phytotoxicity of aryloxyphenoxypropanoate (APP) herbicides (fenoxaprop, fluazifop, haloxyfop, and quizalofop). The largest AS-mediated increase in APP herbicide phytotoxicity was 19% (based on fresh weight reduction) for wild oat with haloxyfop at 50 g/ha. AS consistently mediated increases in cyclohexanedione (CHD) herbicide phytotoxicity. With added AS, barley fresh weight was reduced 75% (1988) with BAS 517 at 50 g/ha, and 100% (1990) with clethodim at 25 g/ha. Greenhouse studies confirmed field studies, but differences were less substantial and consistent. It is suggested that APP herbicides are either less susceptible to UV degradation than CHD herbicides, and/or that APP herbicides may penetrate plant cuticles quickly enough to nullify any protection from UV degradation that AS might provide via rapid absorption.

The response of Persian darnel, green foxtail, wild oat, and volunteer cereals to clethodim applied at two growth stages, and the effect of these treatments on flax yield and quality were determined in a series of field experiments in which each weed species was evaluated separately. Clethodim applied at 45 g/ha provided excellent control of Persian darnel, green foxtail, wild oat, and volunteer wheat and barley. Sethoxydim applied at 150 g/ha provided excellent control of Persian darnel, green foxtail, wild oat, and volunteer barley. Rax was tolerant at five times the recommended rate of application and was unaffected by plant growth stage at time of application. When applied at an early (2- to 3-leaf) stage, control of Persian darnel, green foxtail, wild oat, and volunteer wheat and barley was excellent at 20 g/ha. Except for Persian darnel, weed control as determined by reduction in plant dry weight, was negatively affected by growth stage at time of application. Although visible control and reduction in plant dry weight were somewhat less when clethodim was applied at the 4-to 5-leaf stage, control of each weed species was acceptable. When weight of water in plant tissue (fresh weight – dry weight) was used as a measure of living plant tissue, wild oat and volunteer barley control were unaffected by growth stage at time of application.