In New Mexico chile pepper production, pendimethalin is traditionally applied shortly after crop thinning, which is 9 to 10 wk after crop seeding. Pendimethalin applications before crop thinning may be a method for controlling early-season weeds in chile pepper; however, chile pepper tolerance to early-season applications of pendimethalin is poorly understood. We conducted a greenhouse study to evaluate young chile pepper responses to pendimethalin. We also conducted a field study to determine weed and chile pepper responses to early-season, postemergence-directed pendimethalin in combination with herbicides registered for preemergence applications. The greenhouse study included three treatments administered when chile pepper was at the four-leaf stage: (i) pendimethalin applied to foliage and soil, (ii) pendimethalin applied to soil only, and (iii) a nontreated control. The field study included four treatments: (i) preemergence applications of napropamide followed by postemergence-directed pendimethalin at 5 wk after crop seeding, (ii) preemergence applications of clomazone followed by postemergence-directed pendimethalin at 5 wk after crop seeding, (iii) postemergence-directed pendimethalin without preemergence herbicides, and (iv) nontreated, weed-free control. We conducted the field study at two sites that differed in soil texture. Pendimethalin application rates were maximum labeled rates for the specific soil. Results from the greenhouse study indicated that pendimethalin applied to foliage and soil stunted two of five cultivars, whereas pendimethalin applied to soil did not affect chile pepper height, fresh weight, dry weight, or root area. Results from the field study indicated that postemergence-directed pendimethalin did not affect chile pepper height or fruit yield, or cause visible symptoms of herbicide injury. Postemergence-directed pendimethalin reduced the densities of weeds, including junglerice. The results of this study indicate that postemergence-directed applications of pendimethalin at 5 wk after crop seeding do not cause crop injury or yield loss in chile pepper, while providing some weed control benefits.

Widespread occurrence of herbicide-resistant weeds and more variable weather conditions across the United States has made weed control in many crops more challenging. Preemergence (PRE) herbicides with soil residual activity have resurged as the foundation for early season weed control in many crops. Field experiments were conducted in Janesville and Lancaster, Wisconsin, in 2021 and 2022 (4 site-years) to evaluate the weed control efficacy of solo (single site of action [SOA]) and premix (two or more SOAs) PRE herbicides in conventional tillage corn. Treatments consisted of 18 PRE herbicides plus a nontreated check. At the Janesville-2021 site, S-metolachlor + bicyclopyrone + mesotrione, atrazine + S-metolachlor + bicyclopyrone + mesotrione, and clopyralid + acetochlor + mesotrione provided >72% giant ragweed control. At the Janesville-2022 site, none of the PRE herbicides evaluated provided >70% giant ragweed control due to the high giant ragweed density and the lack of timely rainfall. At the Lancaster-2021 site, atrazine, dicamba, and flumetsulam + clopyralid provided <45% waterhemp control, but the remaining treatments provided >90% control. At the Lancaster-2022 site, the efficacy of some PRE herbicides was reduced due to the high waterhemp density; however, most herbicides provided >75% control. At the Lancaster-2021 and Lancaster-2022 sites, only dicamba and S-metolachlor did not provide >75% common lambsquarters control. Group 15 PRE herbicides provided >75% control of giant foxtail. Across weed species, PRE herbicides with two (78%) and three (81%) SOAs provided greater weed control than PRE herbicides with a single SOA (68%), indicating that at least two SOA herbicides applied PRE result in better early season weed control. The efficacy of the PRE herbicide treatments evaluated herein varied according to the soil seedbank weed community composition and environmental conditions (i.e., rainfall following application), but the premixes were a more reliable option to improve early season weed control in conventional tillage corn.

Northern highbush blueberry is an important fresh market product in New Jersey where the plant was first domesticated in the early 20th century. Because of the short period for safely and timely applying postemergence (POST) herbicides, reliance on residual herbicides that provide season-long control of weeds is essential for blueberry growers to minimize the detrimental effect of weed competition on berry yield and quality and bush growth. Field studies were conducted from 2018 to 2020 in Chatsworth, New Jersey, on ‘Bluecrop’, ‘Duke’, and ‘Elliott’ blueberry cultivars growing on sandy acidic soil to evaluate weed control and crop tolerance in response to repeated annual applications of indaziflam at 73 or 146 g ai ha−1 applied in fall or spring. The efficacy of indaziflam treatments were compared to those of fall-applied dichlobenil at 3,300 g ai ha−1 or a spring-applied mix of diuron at 1,800 g ai ha−1, oryzalin at 3,360 g ai ha−1, and mesotrione at 210 g ai ha−1. Indaziflam at the currently labeled rate of 73 g ai ha−1 provided ≥85% and season-long control of horseweed, Canadian toadflax, and large crabgrass with fall applications on dormant blueberry, whereas spring applications were less effective. Whereas minor (≤8%) and transient leaf crinkling was noted in response to spring-applied indaziflam at 146 g ai ha−1, a fall application never caused leaf crinkling greater than that observed in the nontreated weedy and weed-free controls, regardless of rate. No negative effects on plant growth or fruit production were observed from indaziflam applied at 73 or 146 g ai ha−1 in fall or spring. Findings of this study suggest that indaziflam applied at 73 (1× commercial use rate) and 146 g ai ha−1 is safe to use on blueberry grown on New Jersey sandy acidic soils despite restrictions for using this herbicide on such soils.

Wild radish is the most problematic broadleaf weed in Australian grain production. The propensity of wild radish to evolve resistance to herbicides has led to high frequencies of multiple herbicide–resistant populations present in these grain production regions. The objective of this study was to evaluate the potential of mesotrione to selectively control wild radish in wheat. The initial dose response pot trials determined that at the highest mesotrione rate of 50 g ha−1 applied preemergence (PRE) was 30% more effective than when applied postemergence (POST) on wild radish. This same rate of mesotrione applied POST resulted in a 30% reduction in wheat biomass compared to 0% for the PRE application. Subsequent mesotrione PRE dose response trials identified a wheat selective rate range of >100 and <300 g ai ha−1 that provided greater than 85% wild radish control with less than 15% reduction in wheat growth. Field evaluations confirmed the efficacy of mesotrione at 100 to 150 g ai ha−1 in reducing wild radish populations by greater than 85% following PRE application and incorporation by wheat planting. Additionally, these field trials demonstrated the opportunity for season-long control of wild radish when mesotrione applied PRE was followed by bromoxynil applied POST. The sequential PRE application of mesotrione, a herbicide that inhibits p-hydroxyphenylpyruvate dioxygenase, followed by POST application of bromoxynil, a herbicide that inhibits photosystem II, has the potential to provide 100% wild radish control with no effect on wheat growth.

A chloroacetamide herbicide by application timing factorial experiment was conducted in 2017 and 2018 in Mississippi to investigate chloroacetamide use in a dicamba-based Palmer amaranth management program in cotton production. Herbicides used were S-metolachlor or acetochlor, and application timings were preemergence, preemergence followed by (fb) early postemergence, preemergence fb late postemergence, early postemergence alone, late postemergence alone, and early postemergence fb late postemergence. Dicamba was included in all preemergence applications, and dicamba plus glyphosate was included with all postemergence applications. Differences in cotton and weed response due to chloroacetamide type were minimal, and cotton injury at 14 d after late postemergence application was less than 10% for all application timings. Late-season weed control was reduced up to 30% and 53% if chloroacetamide application occurred preemergence or late postemergence only, respectively. Late-season weed densities were minimized if multiple applications were used instead of a single application. Cotton height was reduced by up to 23% if a single application was made late postemergence relative to other application timings. Chloroacetamide application at any timing except preemergence alone minimized late-season weed biomass. Yield was maximized by any treatment involving multiple applications or early postemergence alone, whereas applications preemergence or late postemergence alone resulted in up to 56% and 27% yield losses, respectively. While no yield loss was reported by delaying the first of sequential applications until early postemergence, forgoing a preemergence application is not advisable given the multiple factors that may delay timely postemergence applications such as inclement weather.

Residual herbicides applied PRE provide early season weed control, potentially avoid the need for multiple POST herbicides, and can provide additional control of herbicide-resistant weeds. Thus, field studies were conducted in 2017 and 2018 at Concord, NE, to evaluate the influence of PRE herbicides on critical time for postemergence weed removal (CTWR) in corn. The studies were arranged in a split-plot design that consisted of three herbicide regimes as main plot treatments and seven weed removal timings as subplot treatments in four replications. The herbicide regimes included no PRE herbicide, atrazine, and a premix of saflufenacil/dimethenamid-P mixed with pyroxasulfone. The weed removal timings were at V3, V6, V9, V12, and V15 corn growth stages and then plots were kept weed-free until harvest. A weed-free and nontreated control were included for comparison. The relationship between corn growth or yield, and weed removal timings in growing degree days (GDD) was described by a four-parameter log-logistic model. This model was used to estimate the critical time for weed removal based on 5% crop yield loss threshold. A delay in weed removal until the V2 to V3 corn growth stage (91 to 126 GDD) reduced corn biomass by 5% without PRE herbicide application. The CTWR started at V3 without PRE herbicide in both years. Atrazine delayed the CTWR up to V5 in both years, whereas saflufenacil/dimethenamid-P plus pyroxasulfone further delayed the CTWR up to the V10 and V8 corn growth stages in 2017 and 2018, respectively. Herbicide applied PRE particularly with multiple sites of action can delay the CTWR in corn up to a maximum growth stage of V10, and delay or reduce the need for POST weed management.

In glyphosate-resistant (GR) cropping systems, paraquat applied in mixtures with residual herbicides prior to crop emergence offers an alternative herbicide mode of action (MOA) to aid in GR weed management. Rice is sensitive to off-target herbicide movement; however, severity of injury can vary with herbicide, rate, and formulation. Therefore, research was conducted from 2015 to 2017 in Stoneville, MS, to characterize rice response to a sublethal concentration of paraquat applied at 84 g ai ha–1 in combination with common residual herbicides. Paraquat plus metribuzin injured rice 68% to 69% 14 and 28 d after treatment (DAT), which was 10% to 13% greater than injury following paraquat alone or paraquat plus fomesafen. Pooled across metribuzin and fomesafen treatments, paraquat reduced rough rice yields 23%. Paraquat plus 10 different residual herbicides injured rice ≥51% 28 DAT and reduced rough rice yields ≥21%. These studies indicate a severe negative impact on rice growth and development following exposure to a sublethal concentration of paraquat alone or in mixture with common residual herbicides. Therefore, applications of paraquat plus residual herbicides to fields in proximity to rice should be avoided if conditions are conducive for off-target movement.

Understanding the critical time of weed removal (CTWR) is necessary for designing effective weed management programs in popcorn production that do not result in yield reduction. The objective of this study was to determine the CTWR in popcorn with and without a premix of atrazine and S-metolachlor applied PRE. Field experiments were conducted at the University of Nebraska–Lincoln, South Central Agricultural Laboratory near Clay Center, NE in 2017 and 2018. The experiment was laid out in a split-plot design with PRE herbicide as the main plot and weed removal timing as the subplot. Main plots included no herbicide or atrazine/S-metolachlor applied PRE. Subplot treatments included a weed-free control, a non-treated control, and weed removal timing at V3, V6, V9, V15, and R1 popcorn growth stages and then kept weed free throughout the season. A four-parameter log-logistic function was fitted to percentage popcorn yield loss and growing degree days separately to each main plot. The number of growing degree days, when 5% yield loss was achieved, was extracted from the model and compared between main plots. The CTWR was from the V4 to V5 popcorn growth stage in absence of PRE herbicide. With atrazine/S-metolachlor applied PRE, the CTWR was delayed until V10 to V15. It is concluded that, to avoid yield loss, weeds must be controlled before the V4 popcorn growth stage when no PRE herbicide is applied, and PRE herbicide, such as atrazine/S-metolachlor in this study, can delay the CTWR until the V10 growth stage.

Timely herbicide applications for no-till soybean can be challenging given the diverse communities of both winter and summer annual weeds that are often present. Research was conducted to compare various approaches for nonselective and preplant weed control for no-till soybean. Nonselective herbicide application timings of fall (with and without a residual herbicide) followed by early-spring (4 wk before planting), late-spring (1 to 2 wk before planting), or sequential-spring applications (4 wk before planting and at planting) were compared. Spring applications also included a residual herbicide. For consistent control of winter annual weeds, two herbicide applications were needed, either a fall application followed by a spring application or sequential-spring applications. When a fall herbicide application did not include a residual herbicide, greater winter annual weed control resulted from early- or sequential-spring treatments. However, application timings that effectively controlled winter annual weeds did not effectively control summer annual weeds that have a prolonged emergence period. Palmer amaranth and large crabgrass control at 4 wk after planting was better when the spring residual treatment (chlorimuron plus metribuzin) was applied 1 to 2 wk before planting or at planting, compared with 4 wk before planting. Results indicate that in order to optimize control, herbicide application programs in soybean should coincide with seasonal growth cycles of winter and summer annual weeds.

Double-crop soybean after winter wheat is a component of many cropping systems across eastern and central Kansas. Until recently, control of Palmer amaranth and common waterhemp has been both easy and economical with the use of sequential applications of glyphosate in glyphosate-resistant soybean. Many populations of Palmer amaranth and common waterhemp have become resistant to glyphosate. During 2015 and 2016, a total of five field experiments were conducted near Manhattan, Hutchinson, and Ottawa, KS, to assess various non-glyphosate herbicide programs at three different application timings for the control of Palmer amaranth and waterhemp in double-crop soybean after winter wheat. Spring-POST treatments of pyroxasulfone (119 g ai ha–1) and pendimethalin (1065 g ai ha–1) were applied to winter wheat to evaluate residual control of Palmer amaranth and waterhemp. Less than 40% control of Palmer amaranth and waterhemp was observed in both treatments 2 wk after planting (WAP) double-crop soybean. Preharvest treatments of 2,4-D (561 g ae ha–1) and flumioxazin (107 g ai ha–1) were also applied to the winter wheat to assess control of emerged Palmer amaranth and waterhemp. 2,4-D resulted in highly variable Palmer amaranth and waterhemp control, whereas flumioxazin resulted in control similar to PRE treatments that contained paraquat (841 g ai ha–1) plus residual herbicide(s). Excellent control of both species was observed 2 WAP with a PRE paraquat application; however, reduced control of Palmer amaranth and waterhemp was noted 8 WAP due to subsequent emergence. Results indicate that Palmer amaranth and waterhemp control was 85% or greater 8 WAP for PRE treatments that included a combination of paraquat plus residual herbicide(s). PRE treatments that did not include both paraquat and residual herbicide(s) did not provide acceptable control.

Field experiments were conducted in 2014 and 2015 in Fayetteville, Arkansas, to evaluate the residual activity of acetyl-CoA carboxylase (ACCase)–inhibiting herbicides for monocot crop injury and weed control. Conventional rice, quizalofop-resistant rice, grain sorghum, and corn crops were evaluated for tolerance to soil applications of six herbicides (quizalofop at 80 and 160 g ai ha–1, clethodim at 68 and 136 g ai ha–1, fenoxaprop at 122 g ai ha–1, cyhalofop at 313 g ai ha–1, fluazifop at 210 and 420 g ai ha–1, and sethoxydim at 140 and 280 g ai ha–1). Overhead sprinkler irrigation of 1.3 cm was applied immediately after treatment to half of the plots, and the crops planted into the treated plots at 0, 7, and 14 d after herbicide treatment. In 2014, injury from herbicide treatments increased with activation for all crops evaluated, except for quizalofop-resistant rice. At 14 d after treatment (DAT) in 2014, corn and grain sorghum were injured 19% and 20%, respectively, from the higher rate of sethoxydim with irrigation activation averaged over plant-back dates. Conventional rice was injured 13% by the higher rate of fluazifop in 2014. Quizalofop-resistant rice was injured no more than 4% by any of the graminicides evaluated in either year. In 2015, a rainfall event occurred within 24 h of initiating the experiment; thus, there were no differences between activation via irrigation or by rainfall. However, as in 2014, grain sorghum and corn were injured 16% and 13%, respectively, by the higher rate of sethoxydim, averaged over plant-back dates. All herbicides provided little residual control of grass weeds, mainly broadleaf signalgrass and barnyardgrass. These findings indicate the need to continue allowing a plant-back interval to rice following a graminicide application, unless quizalofop-resistant rice is to be planted. The plant-back interval will vary by graminicide and the amount of moisture received following the application.

Field trials were conducted in 2000, 2001, and 2002 at Tifton, GA, and Plains, GA, to evaluate the effects of simulated imazapic residues on cotton growth and yield. Preemergence applications of imazapic at 1, 2, 5, 9, 18, and 36 g ai/ha were made to four different cotton varieties (two at each location) and included a nontreated control. There were no differences in cotton variety response to imazapic. Each cotton variety responded to imazapic in a similar manner. Analysis of cotton yield as a percentage relative to the nontreated control indicated no difference in variety for location, so data for varieties were combined. At Tifton, cotton injury was exponentially related to imazapic rate with a maximum injury of 44% for 35 g/ha. Seed cotton yields at this location were reduced 0, 6, 6, 14, 16, 34, and 61% at 1, 2, 5, 9, 18, and 36 g/ha, respectively. For Plains, cotton exhibited extreme sensitivity with injury exceeding 70% for imazapic at 5 g/ha and greater than 95% for 18 g/ha. Seed cotton yields at this location were reduced 60% or more from imazapic rates of 5 g/ha and greater. These results indicated that soil type is a key factor in the response of cotton to imazapic.

Field studies were conducted from 2007 to 2009 in East Lansing, MI to evaluate three residual herbicide programs, three POST herbicide application timings, and two POST herbicides in glyphosate- and glufosinate-resistant corn. Herbicide programs included a residual PRE-applied herbicide followed by (fb) POST application (residual fb POST), a residual herbicide tank-mixed with a POST herbicide (residual + POST), and a nonresidual POST. Three POST herbicide application timings included early POST (EP), mid-POST (MP), and late POST (LP) at an average corn growth stage of V3/V4, V4/V5, and V5/V6, respectively. The two POST herbicides evaluated were glyphosate and glufosinate. Control of common lambsquarters and giant foxtail was evaluated 28 d after the LP application. Glyphosate often provided greater weed control than glufosinate. The LP application resulted in greater giant foxtail control compared with the EP application timing, which may be attributed to control of late-emerging weeds. The EP application timing improved common lambsquarters control compared with the LP application timing. The residual + POST program resulted in greater weed control compared with the residual fb POST program in all years. The effect of residual herbicide program, POST herbicide, and POST application timing on corn grain yield varied by year. In 2007, the use of glyphosate resulted in higher grain yield compared with glufosinate. In 2008, corn grain yield was the highest in the PRE fb POST program and with POST applications at EP and MP. To provide the most consistent weed control and minimize the likelihood of grain yield reductions, a PRE fb POST program applied at EP or MP is recommended.

Field studies were conducted in Alabama in 1998 and 1999 to evaluate fomesafen preemergence (PRE) in glyphosate-resistant cotton. Fomesafen (0.3 and 0.4 kg ai/ha), fluometuron (1.4 kg ai/ha), and pyrithiobac (0.05 kg ai/ha) were applied alone or in tank mixtures. Glyphosate (0.4 kg ae/ha) was applied postemergence over-the-top (POT) and postemergence directed (PD). Regardless of PRE treatment, POT followed by (fb) PD applications of glyphosate were necessary for greater than 82% sicklepod control at midseason. In the absence of glyphosate, fomesafen, and fomesafen-containing tank mixtures controlled common cocklebur and Ipomoea species 77 and 72%, respectively, 14 d after PD application. However, two applications of glyphosate were needed for >94% season-long control of common cocklebur and Ipomoea species because of continued germination throughout the growing season. Postemergence applications of glyphosate added a 1,000 kg/ha seed cotton yield increase to all PRE treatments at both locations. Common cocklebur, Ipomoea species, and sicklepod control was not significantly increased by the addition of fomesafen PRE tank mixtures fb glyphosate postemergence as compared with glyphosate postemergence only.

The earliest possible seeding of wheat crops in the southern Australian dryland cropping zone is prevented by the lack of a weed control practice that adequately controls initial weed seedling emergence at the start of the growing season. The objective of this study was to determine the potential for using residual herbicides applied up to 1 mo before the start of the growing season to control rigid ryegrass seedlings that emerge after the season-opening rains. In a series of glasshouse studies, S-metolachlor and propyzamide were found to effectively persist on the soil surface through prolonged exposure to hot, dry, and intense sunlight conditions, preventing the establishment of rigid ryegrass seedlings. In addition, these herbicides caused little or no effect on subsequently seeded wheat. It also was determined that S-metolachlor had the potential to retain efficacy on rigid ryegrass seedlings after 12 wk of exposure on the soil surface to these conditions. These studies have identified two herbicides with the potential for use at the novel application timing, i.e., before the commencement of the growing season, in Mediterranean climates of southern Australia.