New agronomic practices are emerging in the green cane system to utilize sugarcane (Saccharum officinarum L.) straw for energy cogeneration, which necessitates its removal from the soil surface. This study has three main objectives: (1) evaluate the population dynamics and composition of Jamaican crabgrass (Digitaria horizontalis Willd.) and large crabgrass [Digitaria sanguinalis (L.) Scop.] under different sugarcane straw amounts, with and without herbicide treatment; (2) assess the development of sugarcane under different straw amounts; and (3) determine the amount of sugarcane straw that should be kept on the soil surface after harvest to ensure that it does not compromise the chemical control for Digitaria spp. in ratoon cane in a green cane system. We conducted this research at two experimental sites, one at the beginning and the other during the middle of the harvest season, over a span of 2 yr. Our primary treatments consisted of different amounts of sugarcane straw after harvest on the soil surface (0, 5, 10, and 15 Mg ha−1), while secondary treatments included the herbicide application (sulfentrazone + tebuthiuron for the beginning of harvest season and isoxaflutole + tebuthiuron for the middle of harvest season). The Digitaria spp. exhibited higher density (four times more) and dry matter (two times more) in scenarios with a lower sugarcane straw amount (5 Mg ha−1) on the soil surface and no herbicide application. However, a higher straw amount (15 Mg ha−1) contributed to reduced Digitaria spp. infestation and to improved sugarcane yield. According to this research, it is essential to maintain at least 10 Mg ha−1 of sugarcane straw on the soil surface and remove only 5 Mg ha−1 for energy cogeneration.

Multiple herbicide–resistant (MHR) Palmer amaranth is among the most problematic summer annual broadleaf weeds in Nebraska and several other states. A new MHR corn cultivar (resistant to 2,4-D/glufosinate/glyphosate, also known as Enlist corn) has been commercially available in the United States since 2018. Growers are searching for herbicide programs for control and reduce seed production of MHR Palmer amaranth among Enlist corn crops. The objectives of this study were to evaluate herbicides applied preemergence, early postemergence, or preemergence followed by (fb) late postemergence for the management of MHR Palmer amaranth in Enlist corn fields and to assess their effect on Palmer amaranth biomass, density, seed production, and corn yield. Field experiments were conducted near Carleton, NE, in 2020 and 2021, in a grower’s field of Enlist corn infested with acetolactate synthase–inhibitor/atrazine/glyphosate–resistant Palmer amaranth. Herbicides applied preemergence, such as flufenacet/isoxaflutole/thiencarbazone-methyl, acetochlor/clopyralid/flumetsulam, or acetochlor/clopyralid/mesotrione, provided 75% to 99% control of Palmer amaranth 30 d after preemergence. Preemergence fb late postemergence herbicides resulted in 94% Palmer amaranth control 90 d after late postemergence, reduced weed density to 0 to 8 plants m−2 30 d after late postemergence, and reduced biomass to 2 to 14 g m−2 15 d after late postemergence compared to preemergence-only (59% control, 0 to 15 plants m−2, and 4 to 123 g m−2) and early postemergence–only herbicides (78% control, 6 to 30 plants m−2, and 8 to 25 g m−2). Based on contrast analysis, Palmer amaranth seed production was reduced to 14,050 seeds m–2 in preemergence fb late postemergence herbicide programs compared with 325,490 seed m–2 in preemergence-only and 376,750 seed m–2 in early postemergence–only programs. Based on orthogonal contrast, higher corn yield of 12,340 and 11,730 kg ha−1 was obtained with preemergence fb late postemergence herbicide programs compared with preemergence-only (10,840 and 11,510 kg ha−1) and early postemergence–only programs (10,850 and 10,030 kg ha−1) in 2020 and 2021, respectively.

Sorption of the herbicide isoxaflutole and its main degradate, diketonitrile (DKN), to natural clays, SAz-1, SWy-2 and SHCa-1, and the organoclay derivatives (octadecylammonium (ODA) and hexadecyltrimethylammonium (HDTMA)) of these clays was investigated. Isoxaflutole hydrolysis to DKN was too rapid in aqueous solutions with organoclays to characterize sorption. No measurable DKN sorption was observed for the natural clays. Sorption of DKN was greater on organoclays with an interlayer paraffin-like complex that were prepared from the high-charge SAz-1 clay than on organoclays with a bilayer or monolayer interlayer complex prepared using lower-charge SWy-2 or SHCa-1 clays. Desorption isotherms indicated that sorption was irreversible. For SAz-1 with HDTMA at ∼100% of the clay CEC, the d001 values suggest that DKN enters the interlamellar space of the organoclay and dissociates into the anion. The DKN anion forms a very stable chelate complex with the residual cations and/or partially-coordinated structural cations. This strong interaction supports the irreversibility of the sorptive process.

Soil-applied herbicides are important for controlling weeds in many crops but risk damage to susceptible rotational crops if they persist. Field studies were conducted in Powell, WY, from 2015 through 2017 to evaluate the effect of reduced water availability on soil-applied herbicide dissipation. Eight soil-applied herbicides, applied to dry bean or corn, were exposed to three season-long irrigation treatments (100%, 85%, and 70% of estimated crop evapotranspiration [ETc]) by overhead sprinkler. Soil samples were collected to a depth of 10 cm from 0 to 140 d after application, and soil herbicide concentrations were quantified using gas or liquid chromatography and mass spectrometry. Herbicide concentrations were regressed over time to produce a soil half-life estimate for each herbicide and irrigation treatment. Reduced irrigation decreased dry bean yield by up to 77% and corn yield by up to 50%. After adjusting for precipitation, the lowest irrigation treatment received 78% and 76% as much water as the full irrigation treatment in 2015 and 2016, respectively. This significantly increased the soil half-life of imazethapyr but did not increase the soil half-life of atrazine, pyroxasulfone, saflufenacil, ethalfluralin, trifluralin, or pendimethalin. Reduced irrigation did not increase carryover injury to rotational crops from these herbicides 1 yr after application. Instead, carryover response was determined by the inherent persistence of individual herbicides. Imazethapyr (0.1 kg ai ha−1) injured rotational sugar beet, and isoxaflutole (0.1 kg ai ha−1) injured rotational dry bean. Pyroxasulfone (0.2 kg ai ha−1), atrazine (2.0 kg ai ha−1), saflufenacil (0.1 kg ai ha−1) + dimethenamid-P (0.6 kg ai ha−1), ethalfluralin (0.8 kg ai ha−1), trifluralin (0.6 kg ai ha−1), and pendimethalin (1.1 kg ai ha−1) did not injure rotational crops regardless of irrigation treatment. Drought stress sufficient to cause up to 77% crop yield loss did not increase soil-applied herbicide carryover.

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.

The southern United States produces 90% of the nation’s cotton, and the Texas High Plains is the largest contiguous cotton producing region. Since 2011, glyphosate-resistant Palmer amaranth has complicated cotton production, and alternatives to glyphosate are needed. Integrating soil residual herbicides into a weed management program is a crucial step to control glyphosate resistant weeds before emergence. The recent development of p-hydroxyphenylpyruvate dioxygenase (HPPD)-resistant cotton by BASF Corporation may allow growers to use isoxaflutole in future weed management programs. In 2019 and 2020, field experiments were conducted in New Deal, Lubbock, and Halfway, Texas, to evaluate HPPD-resistant cotton response to isoxaflutole applied preemergence (PRE) or early postemergence (EPOST) and to determine the efficacy of isoxaflutole when used as part of a season-long weed management program. At the New Deal location, cotton response was observed following the EPOST application, but it never exceeded 10%. Cotton response was greatest following the PRE application in Lubbock in 2019 but did not exceed 14%. In 2020 in Lubbock, cotton was replanted due to severe weather. There was <1% cotton response following the PRE application, and maximum cotton response observed was 9% following EPOST and mid-postemergence (MPOST) applications. Cotton lint yields were not different from those of the nontreated, weed-free control at either location. In non-crop weed control studies in Halfway, all treatments controlled Palmer amaranth ≥94% 21 d after the EPOST application. Twenty-one days after the MPOST treatment, systems with isoxaflutole applied EPOST controlled Palmer amaranth by 88% to 93%, while systems with isoxaflutole PRE controlled Palmer amaranth by 94% to 98%. End-of-season Palmer amaranth control was lowest in the system without isoxaflutole (88%) and when isoxaflutole was used EPOST (88% to 91%). These studies suggest that the use of isoxaflutole in cotton weed management systems may improve season-long control of several troublesome weeds with no adverse effects on cotton yield and quality.

The use of different herbicide-resistant soybean technologies in proximity could lead to injury and yield loss due to an application error. Research was initiated to evaluate the effect of 6.25%, 12.5%, and 100% doses of isoxaflutole and mesotrione field use doses applied postemergence to 4-hyroxyphenylpyruvate (HPPD) inhibitor-susceptible soybean, representing physical drift or misapplication doses. Visible injury manifested as chlorosis with slight necrosis, progressing to necrosis and height reduction, with visible height reduction as the only symptom. Isoxaflutole at 6.25% and 12.5% injured V2 soybean more than V1 or V4 soybean at all evaluation dates. This is supported by soybean height data at 14, 28, and 42 d after treatment (DAT). Following the 100% dose, maximum injury occurred at 28 DAT with V1, V2, and V4 soybean injured 90%, 85%, and 72%, but injury declined over time. All isoxaflutole treatments reduced yield at least 310 kg ha−1, with no differences among application timings or between the two lowest doses for a revenue loss of US$147 ha−1 to US$623 ha−1. Visible injury following mesotrione manifested as chlorosis and necrosis advancing to visible height reduction. Following mesotrione at 6.25% and 12.5%, injury ranged from 20% to 36% among application timings and did not differ at 3 to 21 DAT. Soybean heights at 28 and 42 DAT do not support injury observations; therefore, the greater injury was due to chlorosis and necrosis. Following mesotrione at 100%, sensitivity decreased at 3 to 14 DAT when applied to later growth stages, with no differences at 42 DAT. Yields did not differ among application timings, but yield losses were at least 240 kg ha−1, with revenue losses of US$60 ha−1 and US$373 ha−1. Producers are cautioned to prevent off-target movement or errant application of isoxaflutole or mesotrione to HPPD inhibitor-susceptible soybean.

Palmer amaranth has developed resistance to at least seven herbicide sites of action in the Cotton Belt of the United States, leaving producers with fewer options to manage this weed. Previous research with corn and newly commercially released soybean systems have found the use of 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides such as isoxaflutole (IFT) to be effective at managing Palmer amaranth. Consequently, a new transgenic cultivar of cotton is being developed with tolerance to IFT, allowing for in-crop applications of the herbicide. Two separate studies were conducted near Marianna, AR, in 2019 and replicated in 2020, to investigate the crop safety and utility of IFT when added to cotton herbicide programs. Herbicide programs featured IFT as a preemergence or early-postemergence option, residual herbicides in subsequent postemergence applications, and the presence or absence of a layby application. The use of IFT did not significantly impact cotton injury or yield, whereas the use of layered residual herbicides, including IFT, increased Palmer amaranth control compared to those without. Regardless of earlier use of IFT, layby applications were needed for season-long control of Palmer amaranth, entireleaf morningglory, broadleaf signalgrass, and johnsongrass, as evidenced by greater than a 20 percentage point improvement in control of all weeds when a layby application was made. Overall, findings from these studies indicate IFT to be a suitable tool for managing Palmer amaranth and will provide an additional site of action for cotton herbicide programs. Sequential herbicide applications and overlaying residuals were found to be paramount for managing Palmer amaranth throughout the season.

Studies were conducted in 2019 and 2020 in Lewiston, NC, to determine the crop response of 4-hydroxyphenylpyrivate dioxygenase (HPPD)-resistant cotton to isoxaflutole (IFT) and other cotton herbicides as part of a cotton weed management program that included herbicides applied preemergence, early postemergence (EPOST), and mid-postemergence (MPOST). IFT was applied PRE at 105 g ha−1 alone and in various combinations with acetochlor, diuron, fluometuron, fluridone, fomesafen, pendimethalin, and pyrithiobac. EPOST treatments included IFT at 53 or 105 g ha−1 alone or in combination with glyphosate or glufosinate, or dimethenamid-P + glufosinate. Glyphosate + glufosinate was applied MPOST to all treatments except the nontreated control. Cotton injury from IFT applied PRE was minimal (0% to 3%). Injury following EPOST application of dimethenamid-P + glufosinate ranged from 3% to 5% and 6% to 9% in 2019 and 2020, respectively. In both years, injury from IFT applied PRE followed by IFT applied EPOST never exceeded injury from IFT applied PRE followed by dimethenamid-P + glufosinate. Isoxaflutole applied PRE followed by IFT applied EPOST at 105 g ha−1 resulted in 0% to 2% cotton injury, indicating that IFT can be applied either PRE or EPOST with minimal risk to cotton. Late-season cotton height and cotton lint yield were not affected by any herbicide treatment. The experimental HPPD-resistant cotton cultivar was minimally injured by IFT applied PRE and EPOST, it tolerated standard cotton herbicides, and yield loss was not observed. Given these results, HPPD-resistant cotton and IFT may be integrated into cotton weed management systems with minimal risk for cotton injury and provide an additional effective mechanism of action for managing troublesome weeds in cotton.

Owing to the lack of effective POST herbicide options, producers typically rely on nicosulfuron as the main POST grass herbicide in sweet corn production systems. In 2019, a Wisconsin sweet corn producer reported fall panicum control escapes after spraying nicosulfuron. Seeds from mature plants were collected to (1) measure fall panicum response to acetolactate synthase (ALS)-inhibiting herbicides, (2) elucidate the resistance mechanism, and (3) evaluate its response to alternative POST herbicides. Greenhouse and laboratory investigations were conducted to assess fall panicum response to ALS-inhibiting herbicides and elucidate the resistance mechanism. Dose–response results showed that fall panicum was highly resistant to nicosulfuron with a resistance ratio of >12.9-fold (survived rates >254 g ai ha−1, or 8× the field label rate). Molecular and genetic studies indicated that there are multiple ALS gene copies in fall panicum and that resistance was due to a mutation in one copy, resulting in an Asp-376-Glu amino acid substitution. Additional greenhouse experiments indicate that clethodim (105 g ai ha−1), quizalofop-p-ethyl (70 g ae ha−1), glyphosate (864 g ae ha−1), and glufosinate (650 g ai ha−1) are effective POST options to manage the ALS-resistant fall panicum (>90.0% control and 96.8% biomass reduction) in rotational years. The 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides isoxaflutole (105 g ai ha−1), mesotrione (105 g ai ha−1), tembotrione (92 g ai ha−1), and tolpyralate (39 g ai ha−1) did not provide effective POST fall panicum control. Because these herbicides are commonly used for POST weed control in sweet corn, more investigations are required to evaluate combinations of HPPD-inhibiting herbicides with herbicides from other sites of action for POST fall panicum control. Herein we confirm the first case of herbicide resistance in fall panicum in the United States.

BASF Corp. has developed p-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor–resistant cotton and soybean that will allow growers to use isoxaflutole in future weed management programs. In 2019 and 2020, a multi-state non-crop research project was conducted to examine weed control following isoxaflutole applied preemergence alone and with several tank-mix partners at high and low labeled rates. At 28 d after treatment (DAT), Palmer amaranth was controlled ≥95% at six of seven locations with isoxaflutole plus the high rate of diuron or fluridone. These same combinations provided the greatest control 42 DAT at four of seven locations. Where large crabgrass was present, isoxaflutole plus the high rate of diuron, fluridone, pendimethalin, or S-metolachlor or isoxaflutole plus the low rate of fluometuron controlled large crabgrass ≥95% in two of three locations 28 DAT. In two of three locations, isoxaflutole plus the high rate of pendimethalin or S-metolachlor improved large crabgrass control 42 DAT when compared to isoxaflutole alone. At 21 DAT, morningglory was controlled ≥95% at all locations with isoxaflutole plus the high rate of diuron and at three of four locations with isoxaflutole plus the high rate of fluometuron. At 42 DAT at all locations, isoxaflutole plus diuron or fluridone and isoxaflutole plus the high rate of fluometuron improved morningglory control compared to isoxaflutole alone. These results suggest that isoxaflutole applied preemergence alone or in tank mixture is efficacious on a number of cross-spectrum annual weeds in cotton, and extended weed control may be achieved when isoxaflutole is tank-mixed with several soil-residual herbicides.

Palmer amaranth is the most problematic and troublesome weed in agronomic cropping systems in the United States. Acetolactate synthase (ALS) inhibitor and glyphosate-resistant (GR) Palmer amaranth has been confirmed in Nebraska and it is widespread in several counties. Soybean resistant to isoxaflutole/glufosinate/glyphosate has been developed that provides additional herbicide site of action for control of herbicide-resistant weeds. The objectives of this study were to evaluate herbicide programs for control of ALS inhibitor/GR Palmer amaranth and their effect on Palmer amaranth density and biomass, as well as soybean injury and yield in isoxaflutole/glufosinate/glyphosate–resistant soybean. Field experiments were conducted in a grower’s field infested with ALS inhibitor and GR Palmer amaranth near Carleton, Nebraska, in 2018 and 2019. Isoxaflutole applied alone or mixed with sulfentrazone/pyroxasulfone, flumioxazin/pyroxasulfone, or imazethapyr/saflufenacil/pyroxasulfone provided similar control (86%–99%) of Palmer amaranth 21 d after PRE (DAPRE). At 14 d after early-POST (DAEPOST), isoxaflutole applied PRE and PRE followed by (fb) POST controlled Palmer amaranth by 10% to 63% compared to 75% to 96% control with glufosinate applied EPOST in both years. A PRE herbicide fb glufosinate controlled Palmer amaranth 80% to 99% 21 d after late-POST (DALPOST) in 2018, and reduced density 89% to 100% in 2018 and 58% to 100% in 2019 at 14 DAEPOST. No soybean injury was observed from any of the herbicide programs tested in this study. Soybean yield in 2019 was relatively higher due to higher precipitation compared with 2018 with generally no differences between herbicide programs. This research indicates that herbicide programs are available for effective control of ALS inhibitor/GR Palmer amaranth in isoxaflutole/glufosinate/glyphosate-resistant soybean.

Control of waterhemp is becoming more difficult in Ontario because biotypes have evolved resistance to four herbicide sites of action (SOA), including groups 2, 5, 9, and 14. The objective of this study was to compare PRE, POST, and PRE followed by (fb) POST herbicide programs for their effect on control, density, and biomass of multiple-herbicide–resistant (MHR) waterhemp as well as corn injury and grain yield. Two separate field studies, each consisting of five field trials, were conducted over a 2-yr period (2018 and 2019) in fields where corn was grown in Ontario, Canada. The first experiment evaluated MHR waterhemp control with an inhibitor of 4-hydroxyphenyl-pyruvate dioxygenase (HPPD) applied PRE, PRE fb glufosinate applied POST, and glufosinate applied POST. The second experiment evaluated MHR waterhemp control with a non-HPPD inhibitor applied PRE, then PRE fb a POST application of atrazine + mesotrione, and then atrazine + mesotrione applied POST. Atrazine + isoxaflutole caused 3% to 5% corn injury at environment 1 (E1); no corn injury was observed with PRE and POST herbicide programs at environments E2, E3, E4, and E5. In general, atrazine/bicyclopyrone/mesotrione/S-metolachlor and dimethenamid-P/saflufenacil applied PRE controlled MHR waterhemp ≥95% 12 wk after POST application (WAA). A POST application of glufosinate following atrazine + tolpyralate PRE, and a POST application of atrazine + mesotrione following atrazine/dicamba or atrazine/S-metolachlor PRE, improved control at 4, 8, and 12 WAA in most environments. In general, PRE fb POST applications resulted in better control of MHR waterhemp throughout the growing season than single PRE and POST applications (P < 0.05). We conclude that herbicide programs based on multiple effective SOAs may offer effective control of MHR waterhemp where field corn is grown. It is advisable that when choosing an herbicide application program that excellent control of MHR waterhemp should be the goal given its high fecundity and competitive ability.

Weed competition severely constrains cassava root yield in sub-Saharan Africa; thus, good weed control measures, including the use of herbicides, are increasingly important. Herbicide trials were conducted at five locations across eastern, western, and north-central Nigeria over two cropping seasons (2014 and 2015). Nineteen premixed PRE herbicides applied at different rates were evaluated for efficacy on weeds and selectivity on cassava. Manual hoe-weeding at 4, 8, and 12 wk after planting (WAP) and two S-metolachlor + atrazine treatments commonly used by cassava growers were included for comparison. Six of the 19 PRE herbicide treatments (indaziflam + isoxaflutole, indaziflam + metribuzin, flumioxazin + pyroxasulfone, isoxaflutole, acetochlor + atrazine + terbuthylazine, and terbuthylazine + S-metolachlor) consistently provided 80% to 98% broadleaf and grass weed control up to 8 wk after treatment. Overall, PRE herbicide treatments and cassava yield were significantly positively correlated. Herbicide treatments terbuthylazine + S-metolachlor, flumioxazin + pyroxasulfone, diflufenican + flufenacet + flurtamone (respectively, 60 + 60 + 60, 120 + 120 + 120, 90 + 360 + 120, and 135 + 360 + 180 g ha−1), acetochlor + atrazine + terbuthylazine (875 + 875 + 875 g ha−1), S-metolachlor + atrazine (870 + 1,110 g ha−1), oxyfluorfen (240 g ha−1), indaziflam + isoxaflutole (75 + 225 g ha−1), indaziflam + metribuzin (75 + 960 g ha−1), and aclonifen + isoxaflutole (500 + 75 g ha−1) contributed to yields exceeding twice the Nigerian national average of 8.76 tonnes ha−1. These treatments had root yields of 1.4 to 2 times higher than plots that had been hoe-weeded three times. There were some adverse herbicide treatment effects such as delayed cassava sprouting and temporary leaf bleaching observed in indaziflam and diflufenican + flufenacet + flurtamone treatments, whereas sulfentrazone caused prolonged leaf crinkling. The PRE applications alone at rates safe for cassava did not provide adequate season-long weed control; supplemental POST weed control is needed about 10 WAP for satisfactory season-long control.

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.

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.

Transgenic crops are being developed with herbicide resistance traits to expand innovative weed management solutions for crop producers. Soybean with traits that confer resistance to the hydroxyphenylpyruvate dioxygenase herbicide isoxaflutole is under development and will provide a novel herbicide mode of action for weed management in soybean. Ten field experiments were conducted over 2 years (2017 and 2018) on five soil textures with isoxaflutole-resistant soybean to evaluate annual weed control using one- and two-pass herbicide programs. The one-pass weed control programs included isoxaflutole plus metribuzin, applied PRE, at a low rate (52.5 + 210 g ai ha−1), medium rate (79 + 316 g ai ha−1), and high rate (105 + 420 g ai ha−1); and glyphosate applied early postemergence (EPOST) or late postemergence (LPOST). The two-pass weed control programs included isoxaflutole plus metribuzin, applied PRE, followed by glyphosate applied LPOST, and glyphosate applied EPOST followed by LPOST. At 4 weeks after the LPOST application, control of common lambsquarters, pigweed species, common ragweed, and velvetleaf was variable at 25% to 69%, 49% to 86%, and 71% to 95% at the low, medium, and high rates of isoxaflutole plus metribuzin, respectively. Isoxaflutole plus metribuzin at the low, medium, and high rates controlled grass species evaluated (i.e., barnyardgrass, foxtail, crabgrass, and witchgrass) 85% to 97%, 75% to 99%, and 86% to 100%, respectively. All two-pass weed management programs provided 98% to 100% control of all species. Weed control improved as the rate of isoxaflutole plus metribuzin increased. Two-pass programs provided excellent, full-season annual grass and broadleaf weed control in isoxaflutole-resistant soybean.

In the mid-Atlantic region, there is increasing interest in the use of intercropping strategies to establish cover crops in corn cropping systems. However, intercropping may be limited by potential injury to cover crops from residual herbicide programs. Field experiments were conducted from 2013 to 2015 at Pennsylvania, Maryland, and New York locations (n=8) to evaluate the effect of common residual corn herbicides on interseeded red clover and annual ryegrass. Cover crop establishment and response to herbicide treatments varied across sites and years. S-metolachlor, pyroxasulfone, pendimethalin, and dimethenamid-P reduced annual ryegrass biomass relative to the nontreated check, whereas annual ryegrass biomass in acetochlor treatments was no different compared with the nontreated check. The rank order of observed annual ryegrass biomass reduction among chloroacetamide herbicides was S-metolachlor>pyroxasulfone>dimethenamid-P>acetochlor. Annual ryegrass biomass was not reduced by any of the broadleaf control herbicides. Mesotrione reduced red clover biomass 80% compared to the nontreated check. No differences in red clover biomass were observed between saflufenacil, rimsulfuron and atrazine treatments compared to the nontreated check. Red clover was not reduced by any of the grass control herbicides. This research suggests that annual ryegrass and red clover can be successfully interseeded in silt loam soils of Pennsylvania following use of several shorter-lived residual corn herbicides, but further research is needed in areas with soil types other than silt loam or outside of the mid-Atlantic cropping region.

Greenhouse and laboratory studies were conducted to examine certain characteristics of RPA 201772 and how they may affect its utility. 14C-RPA 201772 was used to determine the stability of RPA 201772 in various pH spray solutions over time. After 24 h, degradation of RPA 201772 was 20, 36, and 93% in spray solutions adjusted to pH 4.0, 7.0, and 10.0, respectively. The major metabolite was diketonitrile (DKN), which is herbicidally active. In addition, 9 and 15% of the RPA 201772 had degraded into an inactive benzoic acid derivative at pH 7.0 and 10.0, respectively. The differences in herbicidal activity of RPA 201772 and its metabolite DKN were also evaluated. Corn seeds and velvetleaf seeds readily imbibed RPA 201772, but only low levels of DKN were imbibed. Preemergence (applications of RPA 201772 and DKN were equally active on corn and velvetleaf. Further studies determined that the principal sites of uptake of RPA 201772 by corn was roots and seeds for four different corn hybrids. Another study determined that light was required for RPA 201772 activity. Corn shoots from seeds germinated under 14 h of light exhibited rate-dependent injury, while corn germinated and grown in the darkness was not injured.

The antidotes dichlormid, MON-4660, CGA-154281, R-29148, and MON-13900 were tested in the greenhouse to protect Zea mays L. (corn) against RPA 201772 injury. High rates of RPA 201772 injured four Z. mays hybrids > 30%. R-29148 and MON-13900 were the most effective of the five antidotes evaluated. R-29148 applied at rates ⩾ 45 g ha−1 provided excellent protection against RPA 201772 injury and also prevented injury to Z. mays from diketonitrile, the active metabolite of RPA 201772. In laboratory studies, R-29148 did not alter absorption of 14C-RPA 201772 from soil; however, R-29148 significantly enhanced the rate of RPA 201772 metabolism and inactivation in Z. mays. The mixed function oxidase inhibitor piperonyl butoxide (PBO) increased RPA 201772 injury on all hybrids. These results demonstrate that Z. mays tolerance to RPA 201772 can be enhanced with the use of antidotes such as R-29148 and MON-13900, that R-29148 protects Z. mays from RPA 201772 and diketonitrile by the enhancement of metabolism, and that oxidative reactions may be involved in the metabolism of RPA 201772 in Z. mays.