The use of cover crops in soybean production systems has increased in recent years. There are many questions surrounding cover crops—specifically about benefits to crop production and most effective herbicides for spring termination. No studies evaluating cover crop termination have been conducted across a wide geographic area, to our knowledge. Therefore, field experiments were conducted in 2016 and 2017 in Arkansas, Indiana, Mississippi, Missouri, and Wisconsin for spring termination of regionally specific cover crops. Glyphosate-, glufosinate-, and paraquat-containing treatments were applied between April 15 and April 29 in 2016 and April 10 and April 20 in 2017. Visible control of cover crops was determined 28 days after treatment. Glyphosate-containing herbicide treatments were more effective than paraquat- and glufosinate-containing treatments, providing 71% to 97% control across all site years. Specifically, glyphosate at 1.12 kg ha−1 applied alone or with 2,4-D at 0.56 kg ha−1, saflufenacil at 0.025 kg ha−1, or clethodim at 0.56 kg ha−1 provided the most effective control on all grass cover crop species. Glyphosate-, paraquat-, or glufosinate-containing treatments were generally most effective on broadleaf cover crop species when applied with 2,4-D or dicamba. Results from this research indicate that proper herbicide selection is crucial to successfully terminate cover crops in the spring.

Cover crops have increased in popularity in midwestern U.S. corn and soybean systems in recent years. However, little research has been conducted to evaluate how cover crops and residual herbicides are effectively integrated together for weed control in a soybean production system. Field studies were conducted in 2016 and 2017 to evaluate summer annual weed control and to determine the effect of cover crop biomass on residual herbicide reaching the soil. The herbicide treatments consisted of preplant (PP) applications of glyphosate plus 2,4-D with or without sulfentrazone plus chlorimuron at two different timings, 21 and 7 d prior to soybean planting (DPP). Cover crops evaluated included winter vetch, cereal rye, Italian ryegrass, oat, Austrian winter pea, winter wheat, and a winter vetch plus cereal rye mixture. Herbicide treatments were applied to tilled and nontilled soil without cover crop for comparison. The tillage treatment resulted in low weed biomass at all collection intervals after both application timings, which corresponded to tilled soil having the highest sulfentrazone concentration (171 ng g−1) compared with all cover crop treatments. When applied PP, herbicide treatments applied 21 DPP with sulfentrazone had greater weed (93%) and waterhemp (89%) control than when applied 7 DPP (60% and 69%, respectively). When applied POST, herbicide treatments with a residual herbicide resulted in greater weed and waterhemp control at 7 DPP (83% and 77%, respectively) than at 21 DPP (74% and 61%, respectively). Herbicide programs that included a residual herbicide had the highest soybean yields (≥3,403 kg ha−1). Results from this study indicate that residual herbicides can be effectively integrated either PP or POST in conjunction with cover crop termination applications, but termination timing and biomass accumulation will affect the amount of sulfentrazone reaching the soil.

Two field experiments were conducted during 2018 at Paskeville and Arthurton, South Australia, to identify effective herbicide options for the control of thiocarbamate-resistant rigid ryegrass in wheat. Dose–response experiments confirmed resistance in both field populations (T1 and A18) of rigid ryegrass to triallate, prosulfocarb, trifluralin, and pyroxasulfone. T1 and A18 were 17.9- and 20-fold more resistant to triallate than susceptible SLR4. The level of resistance detected in T1 to prosulfocarb (5.9-fold) and pyroxasulfone (4-fold) was lower compared to A18, which displayed 12.1- and 7.8-fold resistance to both herbicides, respectively. Despite resistance, the mixture of two different preplant-incorporated (PPI) site-of-action herbicides improved rigid ryegrass control and wheat yield compared to a single PPI herbicide only. Prosulfocarb + triallate and prosulfocarb + S-metolachlor + triallate did not reduce rigid ryegrass seed set when compared to prosulfocarb applied alone at the higher rate (2,400 g ai ha–1). Pyroxasulfone + triallate PPI followed by glyphosate (1,880 g ai ha-1) as a weed seed set control treatment reduced rigid ryegrass seed production by 93% and 95% at both sites, respectively. These herbicides also significantly improved grain yield of wheat at Paskeville (22%) and Arthurton (38%) compared to the untreated.

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.

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.

Weeds can cause significant yield loss in watermelon production systems. Commercially acceptable weed control is difficult to achieve, even with heavy reliance on herbicides. A study was conducted to evaluate a spring-seeded cereal rye cover crop with different herbicide application timings for weed management between row middles in watermelon production systems. Common lambsquarters and pigweed species (namely, Palmer amaranth and smooth pigweed) densities and biomasses were often lower with cereal rye compared with no cereal rye, regardless of herbicide treatment. The presence of cereal rye did not negatively influence the number of marketable watermelon fruit, but average marketable fruit weight in cereal rye versus no cereal rye treatments varied by location. These results demonstrate that a spring-seeded cereal rye cover crop can help reduce weed density and weed biomass, and potentially enhance overall weed control. Cereal rye alone did not provide full-season weed control, so additional research is needed to determine the best methods to integrate spring cover cropping with other weed management tactics in watermelon for effective, full-season control.

Cover crops provide a number of agronomic benefits, including weed suppression, which is important as cases of herbicide resistance continue to rise. To effectively suppress weeds, high cover crop biomass is needed, which necessitates later termination timing. Cover crop termination is important to mitigate potential planting issues and prevent surviving cover crop competition with cash crops. Field studies were conducted in Virginia to determine the most effective herbicide options alone or combined with glyphosate or paraquat to terminate a range of cover crop species. Results revealed that grass cover crop species were controlled (94% to 98%) by glyphosate alone 4 wk after application (WAA). Overall, legume species varied in response to the single active-ingredient treatments, and control increased with the addition of glyphosate or paraquat. Mixes with glyphosate provided better control of crimson clover and hairy vetch by 7% to 8% compared with mixes containing paraquat 4 WAA. Mix partner did not influence control of Austrian winter pea. No treatment adequately controlled rapeseed in this study, with a maximum of 58% control observed with single active-ingredient treatments and 62% control with mixes. Height reduction for all cover crop species supports visible rating data. Rapeseed should be terminated when smaller, which could negate weed suppressive benefits from this cover crop species. Growers should consider herbicide selection and termination timing in their cover crop plan to ensure effective termination.

Living mulches can provide many sustainability benefits. However, living mulch–cash crop competition and unreliable weed control are major challenges in living mulch systems. In this study, we evaluated the potential of herbicides used at reduced rates in combination with living mulch to suppress weeds, while simultaneously reducing living mulch vigor. Herbicide treatments were a combination of two POST applications, each consisting of a single, different herbicide. Field trials were conducted in Freeville, NY, USA, using: fresh market field tomato as cash crop; sesbania and sunn hemp as living mulch; and the herbicides fomesafen, halosulfuron, metribuzin, and rimsulfuron. In 2015, when water was not limiting, tomato yield and living mulch biomass were positively correlated. This relationship was negative in 2016, likely because of drought during the growing season. Compared with the untreated living mulch check, using the herbicide treatments in combination with living mulch reduced tomato yield losses by up to 71% in 2015 and 51% in 2016. In these herbicide plus living mulch plots, weed biomass was reduced by up to 97%, compared with the weedy check. Living mulch in herbicide treatments generated up to 2500 kg ha−1 of dry matter during both 2015 and 2016, with an average ground cover of 63% in 2015 and 85% in 2016. A predominantly PRE herbicide with residual soil activity (metribuzin), followed by a herbicide with greater POST activity (halosulfuron/rimsulfuron) was the most effective herbicide application sequence. Results from our study indicate that well-designed herbicide applications may enhance the practicability of living mulch systems.

Horseweed is one of Kentucky’s most common and problematic weeds in no-till soybean production systems. Emergence in the fall and spring necessitates control at these times because horseweed is best managed when small. Control is typically achieved through herbicides or cover crops (CCs); integrating these practices can lead to more sustainable weed management. Two years of field experiments were conducted over 2016 to 2017 and 2017 to 2018 in Versailles, KY, to examine the use of fall herbicide (FH; namely, saflufenacil or none), spring herbicide (SH; namely, 2,4-D; dicamba; or none), and CC (namely, cereal rye or none) for horseweed management prior to soybean. Treatments were examined with a fully factorial design to assess potential interactions. The CC biomass in 2016 to 2017 was higher relative to 2017 to 2018 and both herbicide programs reduced winter weed biomass in that year. The CC reduced horseweed density while growing and after termination in 1 yr. The FH reduced horseweed density through mid-spring. The FH also killed winter weeds that may have suppressed horseweed emergence; higher horseweed density resulted by soybean planting unless the CC was present to suppress the additional spring emergence. If either FH or CC was used, SH typically did not result in additional horseweed control. The SH killed emerged plants but did not provide residual control of a late horseweed flush in 2017 to 2018. These results suggest CCs can help manage spring flushes of horseweed emergence when nonresidual herbicide products are used, though this effect was short-lived when less CC biomass was present.

Cover crops (CCs) play an important role in integrated weed management. Data necessary to evaluate the role of CCs in weed management at the watershed scale with topographic positions are lacking. We evaluated the effects of cereal rye and hairy vetch CCs on weed suppression at different topographic positions (shoulder, backslope, and footslope) at a watershed scale. Watersheds with a CC treatment followed a crop rotation of corn–cereal rye–soybean–hairy vetch, whereas watersheds without a CC (no-CC) had a crop rotation of corn–winter fallow–soybean–winter fallow. A negative relationship was present between CCs and weed biomass at the shoulder, backslope, and footslope topographic landscape positions, with R2 values of 0.40, 0.48, and 0.50, respectively. In 2016, a cereal rye CC reduced weed biomass 46% to 50% at footslope and shoulder positions compared to no CC. In 2018, a cereal rye CC reduced weed biomass between 52% and 85% at all topographic positions in CC treatment watersheds compared to no-CC watersheds. Hairy vetch in 2017 reduced weed biomass 62% to 72% at footslope and shoulder topographic positions in CC watersheds compared to no-CC. The C:N ratio of weed biomass in CC treatment watersheds was generally higher compared to watersheds without CCs. In this study, several significant interactions were found between the topographic positions and CC treatments. Cover crop–induced weed suppression at different topographic positions can lead to developing better site-specific weed control strategies. Therefore, CC interactions with topography, weed germination potential, and the role of soil moisture at the watershed scale should be further evaluated.

Field trials were conducted near Lubbock, TX, in 2013, 2014, and 2015 to evaluate non–2,4-D–resistant cotton response to low rates of glyphosate plus 2,4-D choline. Cotton was treated with five rates of glyphosate plus 2,4-D choline (0.0183, 0.183, 1.83, 18.3, and 183 g ae ha−1) at two application timings (nine leaf and first bloom). These rates correspond to contamination rates of 0.0008%, 0.008%, 0.08%, 0.8%, and 8%, respectively. Visual cotton injury, boll retention, lint yield, and fiber properties were recorded. When averaged over contamination rates, visual injury after applications made to nine-leaf cotton was greater than for first-bloom cotton in three of 3 yr and yield loss was greater when applications were made to nine-leaf cotton when compared with first-bloom cotton in two of 3 yr. Averaged over application timing, lint yield in 2013, 2014, and 2015 after glyphosate plus 2,4-D choline contamination rates of 0.0008% and 0.008% were not different than that of the nontreated control, whereas contamination rates of 0.08%, 0.8%, and 8% decreased yield by 3% to 20%, 45% to 58%, and 80% to 96%, respectively. Contamination rates of 0.0008%, 0.008%, 0.08%, and 0.8% rarely affected fiber quality; however, a contamination rate of 8% frequently decreased micronaire, fiber length, fiber length uniformity, and fiber strength. This decrease in fiber quality also resulted in a reduction in cotton loan value and potential financial return. Although decreases in fiber quality parameters were not observed with the 0.8% contamination rate, significant reductions in financial return occurred due to yield loss caused by injury from glyphosate plus 2,4-D choline.

The occurrence of herbicide tank contamination with dicamba or 2,4-D will likely increase with the recent commercialization of dicamba- and 2,4-D-resistant soybean. High-value sensitive crops, including dry bean, will be at higher risks for exposure. In 2017 and 2018, two separate field experiments were conducted in Michigan to understand how multiple factors may influence dry bean response to dicamba and 2,4-D herbicides, including 1) the interaction between herbicides applied POST to dry bean and dicamba or 2,4-D, and 2) the impact of low rates of glyphosate with dicamba or 2,4-D. Dry bean injury was 20% and 2% from POST applications of dicamba (5.6 h ae ha−1) and 2,4-D (11.2 g ae ha−1), respectively, 14 days after treatment (DAT). The addition of glyphosate (8.4 g ae ha−1) did not increase dry bean injury from dicamba or 2,4-D. Over 2 site-years the addition of dry bean herbicides to dicamba or dicamba + glyphosate (8.4 g ae ha−1) increased dry bean injury and reduced yield by 6% to 10% more than when dicamba or dicamba + glyphosate was applied alone. The interaction between 2,4-D (11.2 g ae ha−1) and dry bean herbicides was determined to be synergistic. However, 2,4-D (11.2 g ae ha−1) had little effect on dry bean with or without the addition of a dry bean herbicide program. These studies document that synergy also occurs between dicamba and dicamba + glyphosate and both common dry bean herbicide programs tested: 1) imazamox (35 g ha−1) + bentazon (560 g ha−1), and 2) fomesafen (280 g ha−1). The synergy between dry bean herbicide and dicamba and dicamba + glyphosate can increase plant injury, delay maturity, and reduce yield to a greater extent than dicamba or dicamba + glyphosate alone. This work emphasizes the need to properly clean out sprayers after applications of dicamba to reduce the risk of exposure to other crops.

Sensitive cotton varieties planted into soil treated with 2,4-D or dicamba utilized in burndowns can result in stunting and stand loss if use rate is too high and the plant-back interval is too short. The objective of this study was to evaluate cotton stunting and yield responses resulting from 2,4-D or dicamba residues in soil after preplant burndown applications at three locations in 2016 and 2017. Treatments with 2,4-D included 532 and 1,063 g ae ha−1 applied 3 wk before planting (WBP) and 53, 160, 266, 532, 1,063 g ae ha−1 applied at planting. Dicamba treatments included 560 and 1,120 g ae h−1 applied 3 WBP and 56, 168, 280, 560, 1,120 g ae ha−1 applied at planting. Dicamba or 2,4-D treatments applied 3 WBP resulted in no adverse effects on cotton stand, plant height, or yield. Dicamba 560 g ae h−1 applied at planting reduced cotton stand by 36% at 21 to 24 d after planting (DAP) over all locations in 2016. In 2017, stands were reduced by dicamba at 168, 280, 560, and 1,120 g ae ha−1 by 17% to 25% at 20 to 23 DAP. Moreover, cotton stands were not affected by 2,4-D in 2016, and only 266, 532, and 1,063 g ae ha−1 of 2,4-D caused stand reductions of 26% to 36% at 20 to 23 DAP over all locations in 2017. Dicamba at 560 g ae ha−1 at planting was the only treatment in this study that reduced plant height. Although stand losses were observed in both years, no yield loss occurred. The data suggest that stunting and stand reduction may occur if susceptible varieties are planted soon after burndown applications with 2,4-D or dicamba, but yield may not be affected after a full growing season. Dicamba showed greater potential to cause stunting and stand reduction than 2,4-D.

Herbicide carrier water hardness and pH can be variable depending on the source and geographic location. Herbicide efficacy can be affected by the pH and hardness of water used for spray solution. Field and greenhouse studies were conducted to evaluate the effect of carrier water pH and hardness on premixed dicamba and glyphosate efficacy. Treatments were combinations of water pH at 4, 6.5, or 9; and water hardness at 0 (deionized water), 400, or 800 mg L−1 of CaCO3 equivalent. In the field study, dicamba and glyphosate were applied at 0.55 and 1.11 kg ae ha−1, respectively, and half of these rates were applied in the greenhouse study. There was no interaction between carrier water pH and hardness on dicamba and glyphosate efficacy; however, the main effects of carrier water pH and hardness were significant. Herbicide efficacy was reduced with carrier water at pH 9 compared with pH 4. In the field study, common lambsquarters, common ragweed, horseweed, or Palmer amaranth control was improved 6% or more at carrier water at pH 4 compared with pH 9. Similar results were observed with water pH for giant ragweed, Palmer amaranth, or pitted morningglory control in the greenhouse study. Carrier water hardness at 400 or 800 mg L−1 reduced common ragweed, giant ragweed, or horseweed control compared with 0 mg L−1. Similarly, common lambsquarters, Palmer amaranth, or pitted morningglory control was reduced at least 10% with carrier water hardness at 800 mg L−1 compared with 0 mg L−1. These results indicate carrier water at acidic pH and of no hardness is critical for dicamba and glyphosate application, and spray solution needs to be amended appropriately for an optimum efficacy.

Herbicides are an important tool in managing weeds in turf and agricultural production. One of the earliest selective herbicides, 2,4-D, is a weak acid herbicide used to control broadleaf weeds. Water-quality parameters, such as pH and hardness, influence the efficacy of weak acid herbicides. Greenhouse experiments were conducted to evaluate how varying water hardness level, spray solution storage time, and adjuvant inclusion affected broadleaf weed control by 2,4-D dimethylamine. The first experiment evaluated a range of water-hardness levels (from 0 to 600 mg calcium carbonate [CaCO3] L−1) on efficacy of 2,4-D dimethylamine applied at 1.60 kg ae ha−1 for dandelion and horseweed control. A second experiment evaluated dandelion control from spray solutions prepared 0, 1, 4, 24, and 72 h before application. Dandelion and horseweed control by 2,4-D dimethylamine was reduced when the CaCO3 level in water was at least 422 or at least 390 mg L−1, respectively. Hard-water antagonism was overcome by the addition of 20 g L−1 ammonium sulfate (AMS) into the mixture. When AMS was included in spray mixtures, no differences were observed at 600 mg CaCO3 L−1, compared with distilled water. Spray solution storage time did not influence dandelion control, regardless of water-hardness level or adjuvant inclusion. To prevent antagonism, applicators should use a water-conditioning agent such as AMS when applying 2,4-D dimethylamine in hard water.

Dicamba and 2,4-D exposure to sensitive crops, such as dry bean, is of great concern with the recent registrations of dicamba- and 2,4-D–resistant soybean. In 2017 and 2018, field experiments were conducted at two Michigan locations to understand how multiple factors, including dry bean market class, herbicide rate, and application timing, influence dry bean response to dicamba and 2,4-D. Dicamba and 2,4-D at rates of 0.1%, 1%, and 10% of the field use rate for dicamba and 2,4-D choline were applied to V2 and V8 black and navy bean. Field-use rates for dicamba and 2,4-D choline were 560 and 1,120 g ae ha−1, respectively. There were few differences between market classes or application timings when dry bean was exposed to dicamba or 2,4-D. Estimated rates to cause 20% dry bean injury 14 d after treatment were 4.5 and 107.5 g ae ha−1 for dicamba and 2,4-D, respectively. When dicamba was applied at 56 g ae ha−1, light interception was reduced up to 51% and maturity was delayed up to 16 d. Although both herbicides caused high levels of injury to dry bean, yield reductions were not consistently observed. At four site-years, 2,4-D did not reduce dry bean yield or seed weight with any rate tested. However, when averaged over site-years, dicamba rates of 3.7, 9.8 and 17.9 g ae ha−1 were estimated to cause 5%, 10%, and 15% yield loss, respectively. Dicamba also reduced seed weight by 10% when 56 g ae ha−1 was applied. However, the germination of harvested seed was not affected by dicamba or 2,4-D. Long delays in dry bean maturity from dicamba injury can also indirectly increase losses in yield and quality due to harvestability issues. This work further stresses the need for caution when using dicamba or 2,4-D herbicides near sensitive crops.

Bermudagrass is a major forage species throughout Georgia and the Southeast. An essential part of achieving high-yielding, top-quality forages is proper weed control. Indaziflam is a residual herbicide that controls many broadleaf and grass species by inhibiting cellulose biosynthesis. Research conducted in Tift and Colquitt counties in Georgia determined optimal PRE rates for indaziflam for bermudagrass forage production. Treatments applied at spring greenup of established ‘Alicia’ bermudagrass included indaziflam at 47, 77, 155, or 234 g ai ha−1 PRE, pendimethalin at 4,480 g ha−1 PRE, a split application of indaziflam at 47 g ha−1 PRE followed by the same rate applied POST after the first cutting, and a nontreated control (seven treatments in all). Forages were machine harvested three times each year for each location beginning at least 47 d after treatment (DAT), with final cuttings up to 168 DAT. For all treatments, fresh- and dry-weight yields at each harvest and totals for the season did not differ from the nontreated control. Indaziflam at 155 and 234 g ha−1 did cause minor stunting at 44 DAT, but this was transient and not observed at the second harvest. Indaziflam applied PRE has the potential to provide residual control of troublesome weeds in bermudagrass forage and hay production, with ephemeral stunting at the recommended application rates.

We conducted a survey in the major row-crop production regions of Texas to determine the response of waterhemp to glyphosate (5-enolpyruvylshikimate-3-phosphate synthase [EPSPS] inhibitor), atrazine (photosystem II [PSII] inhibitor), pyrithiobac (acetolactate synthase [ALS] inhibitor), tembotrione (hydroxyphenylpyruvate dioxygenase [HPPD] inhibitor), fomesafen (protoporphyrinogen oxidase [PPO] inhibitor), and dicamba (synthetic auxin). We evaluated 127 accessions for these herbicides. Resistance was confirmed on the basis of plant survival within an accession, and the injury ratings of surviving plants were used to categorize each accession as resistant (<50% injury) or less sensitive (50% to 89% injury). For glyphosate, approximately 27% of all tested accessions were resistant and 20% were less sensitive. The Gulf Coast region had the most glyphosate-resistant accessions (46% of the accessions from this region), followed by the Blacklands region (9%). A dose-response assay of the most resistant waterhemp accession (TX-25) exhibited 17-fold resistance to glyphosate when compared with a susceptible standard. Waterhemp resistance to atrazine also was common in the Gulf Coast region. The accession with the greatest atrazine resistance (TX-31) exhibited 47- and 68-fold resistance to this herbicide when applied POST and PRE, respectively. Widespread resistance to pyrithiobac was observed in waterhemp accessions throughout the Blacklands and Gulf Coast regions. The most resistant accession identified in this study was 61-fold resistant compared with a susceptible standard. No high-level resistance was detected for tembotrione, dicamba, or fomesafen, but high variability in sensitivity to tembotrione and dicamba was observed. One waterhemp accession exhibited reduced sensitivity to fomesafen; the rest were sensitive. Overall, at least two accessions exhibited resistance or reduced sensitivity to herbicides with five different sites of action. The study illustrates the prevalence of multiple herbicide resistance in waterhemp accessions in Texas and emphasizes the need to implement diversified management tactics.

Resistance to protoporphyrinogen oxidase (PPO) inhibitors was first observed in waterhemp in 2001 and was conferred by the deletion of a glycine residue at the 210th position (ΔGly-210) of the PPO enzyme. PPO-inhibitor resistance in Palmer amaranth was first observed in 2011, 10 years later. The objectives of this study were to directly compare PPO inhibitor responses in plants of both species with or without the ΔGly-210 mutation. Using greenhouse experiments, early (EPOST) and late (LPOST) postemergence dose responses using lactofen and fomesafen, and preemergence (PRE) dose responses using fomesafen and flumioxazin, were obtained for a sensitive and resistant population each of waterhemp and Palmer amaranth. An additional spray study confirmed each sensitive population used in the dose responses was representative of its respective species, with regards to PPO-inhibitor sensitivity. When treated at either POST timing, Palmer amaranth was more tolerant than waterhemp, and the ΔGly-210 mutation provided greater resistance in Palmer amaranth (48-fold to >3,440-fold, depending on timing and herbicide) than in waterhemp (31-fold to 123-fold). The level of tolerance in Palmer amaranth was striking; the sensitive Palmer amaranth population treated LPOST survived as well or better than the resistant waterhemp population treated EPOST. With PRE applications, response differences both between species and between resistant and sensitive populations generally were less pronounced, relative to POST applications. Collectively, this research indicates Palmer amaranth tolerance to POST-applied PPO inhibitors could have initially slowed (relative to waterhemp) evolution of resistance to these herbicides, and resistant and sensitive populations of both species are more likely to be effectively controlled with PRE rather than POST applications.

Horseweed, also known as marestail, is a problematic weed for no-till soybean producers that can emerge from late summer through the following spring. Overwintering cover crops can reduce both the density and size of fall-emerged weeds such as horseweed and reduce further spring emergence, although typically cover crops do not provide complete control. Cover crops may be integrated with additional spring herbicide applications to control emerged horseweed, and selective herbicides such as 2,4-D may be used to target horseweed while maintaining small grain cover crop growth. However, cover crops may affect herbicide deposition, which could reduce their efficacy to control weeds. The objective of this study was to determine how the amount and variability of 2,4-D ester spray solution deposition, measured with water-sensitive paper, was affected by a cereal rye cover crop and fall-applied saflufenacil. We also examined deposition at the soil surface relative to the cereal rye row position. In a year with greater cereal rye biomass accumulation, there was 44% less coverage and average deposit size was 45% smaller immediately adjacent to cereal rye rows compared with between rows and areas without cereal rye. Greater variability in these measurements was also noted in this position. Percent spray solution coverage was also 22% greater in plots that received saflufenacil in the fall, and deposits were 28% larger. In a year with less cover crop and winter weed biomass, no differences in spray deposition were observed. This suggests that small horseweed plants and other weeds immediately adjacent to cereal rye cover crop rows may be more likely to survive early spring herbicide applications, though the suppressive effects of cover crops may mitigate this concern.