This research examined dicamba measurements following an application to soil inside a humidome. The dicamba formulations examined were the diglycolamine (DGA) and diglycolamine plus VaporGrip® (DGA+VG), both applied with glyphosate. Post-application dicamba measurements were related to ambient temperature, with more dicamba detected as the temperature increased. There also appeared to be a minimum temperature of ~15 C at which dicamba decreased to low levels. The addition of glyphosate to dicamba formulations decreased the spray mixture pH and increased the observed dicamba air concentrations. Adding glyphosate to DGA+VG increased detectable dicamba air concentrations by 2.9 to 9.3 times across the temperature ranges examined. Particle drift would not be expected to be a factor in the research, as applications were made remotely before treated soil was transported into the greenhouse. The most probable reason for the increased detection of dicamba at higher temperatures and with mixtures of glyphosate is via volatility.

The pH of spray mixtures is an important attribute that affects dicamba volatility under field conditions. This report examined the effect of different components added to water sources that ranged in initial pH from 4.6 to 8.4. Commercial products were used, which include formulations of dicamba, glyphosate, the drift retardant Intact, ammonium sulfate (AMS), and several pH modifiers. Adding BAPMA salt of dicamba always increased the mixture pH, whereas diglycolamine + VaporGrip® (DGA+VG) had a mixed response. The addition of AMS decreased pH slightly (usually <0.5 pH unit), whereas the addition of potassium salt of glyphosate (GLY-K) always decreased the measured pH (from 1.0 to 2.1 pH units). A substantial pH change could have profound effects on dicamba volatility. Moreover, the 1.0 to 2.1 pH units would not be consistent with the registrant’s report stating that GLY-K decreased mixtures with DGA+VG pH by only 0.2 to 0.3 units. The drift retardant Intact had no effect on pH. There was no difference in resultant pH when comparing K salt and isopropylamine (IPA) salts of glyphosate. Spray carrier volume, ranging from 94 to 187 L ha–1, had only a minor effect on measured pH after the addition of various spray components. The addition of selected pH modifiers raised the pH above 5.0, which is a critical value according to the latest dicamba application labels. The order of mixing of various pH modifiers, including AMS, had only limited effect on measured spray solution pH.

Field experiments were conducted in 2012 and 2013 across four locations for a total of 6 site-years in the midsouthern United States to determine the effect of growth stage at exposure on soybean sensitivity to sublethal rates of dicamba (8.8 g ae ha−1) and 2,4-D (140 g ae ha−1). Regression analysis revealed that soybean was most susceptible to injury from 2,4-D when exposed between 413 and 1,391 accumulated growing degree days (GDD) from planting, approximately between V1 and R2 growth stages. In terms of terminal plant height, soybean was most susceptible to 2,4-D between 448 and 1,719 GDD, or from V1 to R4. However, maximum susceptibility to 2,4-D was only between 624 and 1,001 GDD or from V3 to V5 for yield loss. As expected, soybean was sensitive to dicamba for longer spans of time, ranging from 0 to 1,162 GDD for visible injury or from emergence to R2. Likewise, soybean height was most affected when dicamba exposure occurred between 847 and 1,276 GDD or from V4 to R2. Regarding grain yield, soybean was most susceptible to dicamba between 820 and 1,339 GDD or from V4 to R2. Consequently, these data indicate that soybean response to 2,4-D and dicamba can be variable within vegetative or reproductive growth stages; therefore, specific growth stage at the time of exposure should be considered when evaluating injury from off-target movement. In addition, application of dicamba near susceptible soybean within the V4 to R2 growth stages should be avoided because this is the time of maximum susceptibility. Research regarding soybean sensitivity to 2,4-D and dicamba should focus on multiple exposure times and also avoid generalizing growth stages to vegetative or reproductive.

Carrier water pH is an important factor for enhancing herbicide efficacy. Coapplying agrochemical products with the herbicide might save time and resources; however, the negative effect of foliar fertilizers on herbicide efficacy should be thoroughly evaluated. In greenhouse studies, the effect of carrier water pH (4, 6.5, and 9), foliar fertilizer (zinc [Zn], manganese [Mn], or without fertilizer), and ammonium sulfate (AMS) at 0% or 2.5% vol/vol was evaluated on 2,4-D and premixed 2,4-D plus glyphosate efficacy for giant ragweed, horseweed, and Palmer amaranth control. In addition, a field study was conducted to evaluate the effect of carrier water pH (4, 6.5, and 9); and Zn or Mn foliar fertilizer on premixed 2,4-D plus glyphosate efficacy for horseweed and Palmer amaranth control. In the greenhouse study, 2,4-D and premixed 2,4-D plus glyphosate provided 5% greater weed control at acidic compared with alkaline carrier water pH. Coapplied Mn foliar fertilizer reduced 2,4-D and premixed 2,4-D plus glyphosate efficacy at least 5% for weed control. Addition of AMS enhanced 2,4-D and premixed 2,4-D plus glyphosate efficacy at least 6% for giant ragweed, horseweed, and Palmer amaranth control. In the field study, few significant differences occurred between coapplied Zn or Mn foliar fertilizer for any treatment variables. Therefore, carrier water pH, coapplied foliar fertilizer, and water-conditioning adjuvants have potential to influence herbicide performance. However, weed species could play a role in the differential response of these factors on herbicide efficacy.

Weed size can nfluence herbicide performance and herbicide interactions in mixtures. To control a broad range of species in soybean or cotton, POST herbicide mixtures will likely be commonplace in Roundup Ready® XtendFlex® and Enlist™ technologies. The impact of weed size on herbicide interactions that could occur in Roundup Ready XtendFlex or Enlist crops was assessed in two field experiments conducted in 2015 and 2016 at the Northeast Research and Extension Center in Keiser, AR. Combinations of glufosinate, glyphosate, dicamba, and 2,4-D were applied to either 10-cm or 30-cm weeds and evaluated for percent weed control, height reduction, and density reduction, collected 5 wk after treatment. Colby’s method was used to analyze treatments for herbicide interactions for control of barnyardgrass, Palmer amaranth, and pitted morningglory. Antagonism was identified with at least one treatment on all species. Almost all treatments were antagonistic for percent weed control, height reduction, and density reduction on barnyardgrass. When glyphosate in mixture with 2,4-D or dicamba was applied to 30-cm barnyardgrass, control declined 9% for both mixtures relative to glyphosate alone. Glufosinate plus glyphosate was antagonistic when applied to both 30-cm pitted morningglory and barnyardgrass. Glufosinate plus dicamba provided less control and density reduction of Palmer amaranth than what was expected from Colby’s equation. Overall, antagonism was more likely to be identified when applications were made to 30-cm weeds compared with 10-cm weeds. The utility of a given herbicide mixture will depend on the species present in the field and the size of those species at the time of application.

Grape hyacinth is a perennial bulbous species in the Liliaceae. It is commonly grown as an ornamental plant, but it can spread into agricultural fields and become weedy, potentially interfering with harvest and fall-planted crops. There has been limited research on controlling grape hyacinth in cropping systems. Fall and spring applied field-research studies were conducted to determine grape hyacinth control with herbicides labeled for use in wheat or winter fallow before planting soybean. Among fall-applied herbicides, paraquat resulted in the greatest initial grape hyacinth control (90% to 100%). Grape hyacinth control, 16 months after application (MAA), was variable, but the top-performing treatments were glyphosate and metsulfuron plus paraquat, resulting in 65% and 50% control, respectively. After spring applications, grape hyacinth control in November (7 MAA) was variable, but top-performing treatments were glyphosate and metsulfuron, which resulted in at least 26% control. Spring-applied paraquat, carfentrazone, metsulfuron, and sulfosulfuron resulted in 73%, 68%, 69%, and 60% reductions in grape hyacinth bulb counts, compared with the nontreated control 7 MAA, and were the top-performing treatments. Despite product-label prohibitions on rotation to soybeans, no soybean yield reductions were observed from any treatment in either study. Single applications of certain herbicides in the fall or spring can result in good control (>80%) of grape hyacinth initially, but long-term control is poor, and additional research is required.

Field dodder is an obligatory stem and leaf plant parasite that causes significant damage in field and vegetable crops in all agricultural regions of the globe. Selective and effective measures to control the parasite are extremely limited. In recent studies, we have shown that granular formulations of dinitroaniline cell division–inhibiting herbicides applied after crop establishment and before dodder germination fit our dodder control strategy and kill the parasite effectively and selectively. The aim of our study conducted from 2014 to 2018 was to evaluate the efficacy and selectivity of granular pendimethalin for dodder control in chickpea under laboratory, greenhouse, and field conditions. Petri dish experiments revealed that the herbicide reduces dodder seed germination while its main effect is a restriction of shoot elongation. Greenhouse experiments demonstrated that the inhibition and distortion of dodder shoot growth impede shoot twining and prevent attachment to the host plant. In dose–response experiments conducted in the greenhouse, we observed that half the recommended rate of granular pendimethalin provides efficient dodder control with no damage to chickpea seedlings. In 3 yr of chickpea field trials, GPM applied across the seeding bed at the recommended rate resulted in high crop yields that were not significantly different from those observed for the untreated no-dodder control, while half of the recommended dose efficiently controlled dodder and other weeds with no damage to the crop, resulting in significantly increased chickpea yields and profitability. These studies indicate that GPM can provide efficient and selective dodder control in chickpea.

Research was conducted from 2013 to 2015 across three sites in Mississippi to evaluate corn response to sublethal paraquat or fomesafen (105 and 35 g ai ha−1, respectively) applied PRE, or to corn at the V1, V3, V5, V7, or V9 growth stages. Fomesafen injury to corn at three d after treatment (DAT) ranged from 0% to 38%, and declined over time. Compared with the nontreated control (NTC), corn height 14 DAT was reduced approximately 15% due to fomesafen exposure at V5 or V7. Exposure at V1 or V7 resulted in 1,220 and 1,110 kg ha−1 yield losses, respectively, compared with the NTC, but yield losses were not observed at any other growth stage. Fomesafen exposure at any growth stage did not affect corn ear length or number of kernel rows relative to the NTC. Paraquat injury to corn ranged from 26% to 65%, depending on growth stage and evaluation interval. Corn exposure to paraquat at V3 or V5 consistently caused greater injury across evaluation intervals, compared with other growth stages. POST timings of paraquat exposure resulted in corn height reductions of 13% to 50%, except at V7, which was most likely due to rapid internode elongation at that stage. Likewise, yield loss occurred after all exposure times of paraquat except PRE, compared with the NTC. Corn yield was reduced 1,740 to 5,120 kg ha−1 compared with the NTC, generally worsening as exposure time was delayed. Paraquat exposure did not reduce corn ear length, compared with the NTC, at any growth stage. However, paraquat exposure at V3 or V5 was associated with reduction of kernel rows by 1.1 and 1.7, respectively, relative to the NTC. Paraquat and fomesafen applications near corn should be avoided if conditions are conducive for off-target movement, because significant injury and yield loss can result.

Crop plants have been used as mimic weeds to substitute for real weeds in competition studies. These mimic weeds have the advantages of availability of seed, uniform germination and growth, and the potential to confer better experimental controllability and repeatability. However, the underlying assumption that the competitive effects of mimic weeds are similar to real weeds has not been tested. We compared a range of morphological traits (plant height, node and leaf number, leaf area, leaf size, and dry weight) between the mimic weeds and real weeds: Japanese millet vs. junglerice, mungbean vs. bladder ketmia, and common sunflower vs. fierce thornapple. The impact of these mimic and real weeds on cotton was also assessed. There were similarities and differences between the mimic and real weeds, but impact on cotton lint yield was most closely associated with weed height and dry weight at mid-season. Mimic weeds may be satisfactorily substituted for real weeds in competition experiments where seasonal and environmental conditions are not limiting, such as with fully irrigated cotton, provided the plants have similar dry weight and height at mid-season. Alternatively, one can account for the differences in dry weight and height. We define here a generalized relationship estimating the yield loss of high-yielding, irrigated cotton from weed competition over a range of weed dry weights and heights, allowing extrapolation from the results with mimic weeds to the competitive effects of a range of weeds.

Palmer amaranth accessions resistant to protoporphyrinogen oxidase (PPO), 5-enolpyruvyl-shikimate-3-phosphate synthase, and acetolactate synthase (ALS)-inhibitor herbicides are widespread in the Midsouth, making control difficult. Field experiments were conducted in Marion and Crawfordsville, AR, in 2016 and 2017 to assess PRE and POST herbicides labeled for use in corn, cotton, or soybean for control of multiresistant Palmer amaranth. Accessions at both locations were resistant to glyphosate and ALS inhibitors and segregating for both the R128 and ΔG210 PPO resistance mechanisms. Of the 15 herbicide treatments tested, only atrazine (1,120 g ai ha−1), pyroxasulfone (149 g ha−1), and flumioxazin (144 g ha−1) provided 85% or greater Palmer amaranth control 14 days after treatment (DAT). Visible control ratings at 35 DAT declined sharply, with no treatment providing more than 84% control, suggesting POST applications should be made no later than 28 DAT. Glufosinate (594 and 818 g ha−1), dicamba (560 g ae ha−1), 2,4-D plus glyphosate (784 g ae ha−1 plus 834 g ae ha−1), and paraquat (700 g ha−1) applied POST to 7- to 10-cm plants reduced Palmer amaranth density 83% or more 14 DAT. Both glyphosate (1,266 g ha−1) and pyrithiobac sodium (73 g ha−1) provided less than 7% Palmer amaranth control. Although flumioxazin alone at a labeled rate controlled Palmer amaranth 82% in the PRE experiment, PPO inhibitors by themselves applied POST provided no more than 37% control at 14 DAT. Effective foliar herbicides applied POST, including residual herbicides, should be made when Palmer amaranth are less than 10-cm tall for optimal control of these multiresistant Palmer amaranth accessions.

Two species, torpedograss and Southern watergrass, are very difficult to selectively control when they invade desirable turfgrass stands. The purpose of this study was to evaluate selective control of torpedograss and Southern watergrass in ‘Tifway’ bermudagrass turf. Greater than 86% control of torpedograss was observed 4 wk after sequential treatment (WAST) with quinclorac, trifloxysulfuron-sodium, quinclorac and trifloxysulfuron-sodium, sulfentrazone + imazethapyr and quinclorac and trifloxysulfuron-sodium, and quinclorac and trifloxysulfuron-sodium followed by (fb) glyphosate. However, by 8 WAST, control was reduced to <36% for all treatments. Greatest Southern watergrass control was achieved 4 WAST with trifloxysulfuron-sodium (83%), and thiencarbazone-methyl + foramsulfuron + halosulfuron-methyl (75%). Limited control (<30%) was observed with other treatments. By 8 WAST, Southern watergrass control was <12% for all treatments. This study suggests that short-term control/suppression of these two species is possible; however, long-term control is limited with single-year programs. These weeds will probably require multiple applications in successive years to reduce infestations. Future research should continue to screen other herbicides, combinations, and timings for control of these and other perennial grass weeds.

Auxin herbicides are used in combinations to control glyphosate-resistant horseweed preplant burndown. Herbicide labels for 2,4-D–containing products require a 30-d rotation interval for planting cotton cultivars not resistant to 2,4-D. Dicamba labels require an accumulation of 2.5 cm of rain plus 21 d per 280 g ae ha–1 rotation interval for planting cotton cultivars not resistant to dicamba. Previous research has shown that cotton injury caused by dicamba applied 14 d before planting was transient with little effect on cotton yield, whereas 2,4-D has little effect on cotton when applied 7 d prior to planting. Injury caused by dicamba and 2,4-D is inversely related to rainfall received between herbicide application and cotton planting. Experiments were conducted to evaluate cotton tolerance to halauxifen-methyl, a new Group 4 herbicide, applied at intervals shorter than labeled requirements. Experiments were established near Painter and Suffolk, VA, and Belvidere, Clayton, Eure, Lewiston, and Rocky Mount, NC, during the 2017 and 2018 growing seasons. Herbicide treatments included halauxifen, dicamba, and 2,4-D applied 4, 3, 2, 1, and 0 wk before planting (WBP). Visible estimates of cotton growth reduction and total injury were collected 1, 2, and 4 wk after cotton emergence (WAE). Cotton stand and percentage of plants with distorted leaves were recorded 2 and 4 WAE. Cotton plant heights were recorded 4 and 8 WAE. Halauxifen was less injurious (9%) than dicamba (26%) or 2,4-D (21%) 2 WAE when herbicides were applied 0 WBP. Cotton stand reduction 2 WAE by halauxifen was less than 2,4-D and dicamba when applied 0 WBP. Injury observed from herbicides applied 1, 2, 3, and 4 WBP was minor, and no significant differences in cotton stand were observed. Early-season cotton injury was transient, and seed cotton yield was unaffected by any treatment.

Herbicide resistance is a major problem in United States and global agriculture, driving farmers to consider other methods of weed control. One of these methods is harvest weed seed control (HWSC), which has been demonstrated to be effective in Australia. HWSC studies were conducted across Virginia in 2017 and 2018, targeting Italian ryegrass in continuous winter wheat as well as common ragweed and Palmer amaranth in continuous soybean. These studies assessed the impact of HWSC (via weed seed removal) on weed populations in the next year’s crop compared with conventional harvest (weed seeds returned). HWSC reduced Italian ryegrass tillers compared with the conventional harvest at two locations in April (29% and 69%), but no difference was observed at a third location. At wheat harvest, HWSC at one location reduced Italian ryegrass seed heads (41 seed heads m−2) compared with conventional harvest (125 seed heads m−2). In soybean, before preplant herbicide applications and POST herbicide applications, HWSC reduced common ragweed densities by 22% and 26%, respectively, compared with the conventional harvest plots. By soybean harvest, no differences in common ragweed density, seed retention, or crop yield were observed, because of effectiveness of POST herbicides. No treatment differences were observed at any evaluation timing for Palmer amaranth, which is attributed to farmer weed management (i.e., effective herbicides) and low weed densities making any potential treatment differences difficult to detect. Across wheat and soybean, there were no differences observed in crop yield between treatments. Overall, HWSC was demonstrated to be a viable method to reduce Italian ryegrass and common ragweed populations.

The widespread use of herbicides in cropping systems has led to the evolution of resistance in major weeds. The resultant loss of herbicide efficacy is compounded by a lack of new herbicide sites of action, driving demand for alternative weed control technologies. While there are many alternative methods for control, identifying the most appropriate method to pursue for commercial development has been hampered by the inability to compare techniques in a fair and equitable manner. Given that all currently available and alternative weed control methods share an intrinsic energy consumption, the aim of this review was to compare methods based on energy consumption. Energy consumption was compared for chemical, mechanical, and thermal weed control technologies when applied as broadcast (whole-field) and site-specific treatments. Tillage systems, such as flex-tine harrow (4.2 to 5.5 MJ ha−1), sweep cultivator (13 to 14 MJ ha−1), and rotary hoe (12 to 17 MJ ha−1) consumed the least energy of broadcast weed control treatments. Thermal-based approaches, including flaming (1,008 to 4,334 MJ ha−1) and infrared (2,000 to 3,887 MJ ha−1), are more appropriate for use in conservation cropping systems; however, their energy requirements are 100- to 1,000-fold greater than those of tillage treatments. The site-specific application of weed control treatments to control 2-leaf-stage broadleaf weeds at a density of 5 plants m−2 reduced energy consumption of herbicidal, thermal, and mechanical treatments by 97%, 99%, and 97%, respectively. Significantly, this site-specific approach resulted in similar energy requirements for current and alternative technologies (e.g., electrocution [15 to 19 MJ ha−1], laser pyrolysis [15 to 249 MJ ha−1], hoeing [17 MJ ha−1], and herbicides [15 MJ ha−1]). Using similar energy sources, a standardized energy comparison provides an opportunity for estimation of weed control costs, suggesting site-specific weed management is critical in the economically realistic implementation of alternative technologies.