Off-target movement of rice herbicides is a concern in California, where sensitive crops are often grown nearby. Florpyrauxifen-benzyl and triclopyr are auxin-mimics commonly used in rice systems. To steward florpyrauxifen-benzyl around the time of its initial registration in the state, research was conducted to compare the onset of foliar symptoms from simulated florpyrauxifen-benzyl and triclopyr drift onto grapevine, peach, and plum. The rates were 1/200X, 1/100X, 1/33X, and 1/10X of the 29.4 g ai ha–1 florpyrauxifen-benzyl and 1/200X, 1/100X, and 1/33X of the 420.3 g ae ha–1 triclopyr rice use rates. Herbicides were applied on one side of one- to two-year-old peach and plum trees and one side of established grapevines in 2020 and 2021. The general symptoms for florpyrauxifen-benzyl and triclopyr were similar and included chlorosis, leaf curling, leaf distortion, leaf malformation, leaf crinkling, necrosis, and twisting on leaves. The symptoms from herbicides were observed on both sides of the grapevine canopy, whereas florpyrauxifen-benzyl symptoms on peach and plum were mostly observed on the treated side of the tree. Florpyrauxifen-benzyl and triclopyr symptoms were observed three days-after-treatment (DAT) for grapevines and seven DAT for peach and plum. In all crops, most symptoms persisted through 42 DAT. Some grape clusters showed deformation and dropping of berries. All treated crops gradually recovered during the season regardless of application rates. Because symptoms were relatively minor in peach and plum, this research suggested that application precautions that reduce off-site drift are likely to minimize the occurrence of significant injury. However, grapevines were more sensitive and showed injury symptoms up to 71% at 14 DAT with a 1/10X florpyrauxifen-benzyl simulated drift rate. Therefore, extra precautions such as using drift-management agents and proper wind speed conditions at the time of florpyrauxifen-benzyl applications may be necessary if there are nearby vineyards.

Rice herbicide drift poses a significant challenge in California, where rice fields are near almond, pistachio, and walnut orchards. This research was conducted as part of a stewardship program for a newly registered rice herbicide and specifically aimed to compare the onset of foliar symptoms resulting from simulated florpyrauxifen-benzyl drift with residues in almond, pistachio, and walnut leaves at several time points after exposure. Treatments were applied to one side of the canopy of 1- and 2-yr-old trees at 1/100X and 1/33X of the florpyrauxifen-benzyl rice field use rate of 29.4 g ai ha–1 in 2020 and 2021. Symptoms were observed 3 d after treatment (DAT) for pistachio and 7 DAT for almond and walnut, with peak severity at approximately 14 DAT. While almond and walnut symptoms gradually dissipated throughout the growing season, pistachio still had symptoms at leaf out in the following spring. Leaf samples were randomly collected from each tree for residue analysis at 7, 14, and 28 DAT. At 7 DAT with the 1/33X rate, almond, pistachio, and walnut leaves had florpyrauxifen-benzyl at 6.06, 5.95, and 13.12 ng g–1 fresh weight (FW) leaf, respectively. By 28 DAT, all samples from all crops treated with the 1/33X drift rate had florpyrauxifen-benzyl at less than 0.25 ng g–1 FW leaf. At the 1/100X rate, pistachio, almond, and walnut residues were 1.78, 2.31, and 3.58 ng g–1 FW leaf at 7 DAT, respectively. At 28 DAT with the 1/100X rate, pistachio and almond samples had florpyrauxifen-benzyl at 0.1 and 0.04 ng g–1 FW leaf, respectively, but walnut leaves did not have detectable residues. Together, these data suggest that residue analysis from leaf samples collected after severe symptoms may substantially underestimate actual exposure due to the relatively rapid dissipation of florpyrauxifen-benzyl in nut tree foliage.

Field studies were conducted to determine hazelnut tolerance to quinclorac and clopyralid and control efficacy of Canada thistle and field bindweed at three commercial orchards in western Oregon. Hazelnut cultivars evaluated included ‘Jefferson’, ‘Wepster’, and ‘McDonald’. Clopyralid at 278, 547, and 1,090 g ae ha−1, and quinclorac at 420, 840, and 1,680 g ai ha−1 were applied once a year as basal-directed applications to trees that were 1, 2, and 5 yr old. Treatments were imposed in the early spring of 2019 and reapplied in 2020. In both years, treatments covered hazelnut suckers. Hazelnut injury from clopyralid and quinclorac was consistently between 0% and 13% and not different from nontreated control plants (P > 0.05) between 14 d and 455 d after initial treatment. Similarly, there was no treatment effect on plant canopy index, leaf chlorophyll content, trunk cross-sectional area, internode length, or yield among treatments, even at the highest rates of clopyralid and quinclorac. In separate efficacy studies, clopyralid (278 g ae ha−1) resulted in 68% Canada thistle control and did not differ when clopyralid was mixed with carfentrazone (278 + 35 g ai ha−1) or glufosinate (278 + 1,148 g ai ha−1). Clopyralid-containing herbicide treatments suppressed field bindweed growth but did not kill plants even when mixed with carfentrazone or glufosinate. Quinclorac (420 g ha−1) alone provided 80% control of field bindweed and 93% and 98% control when combined with rimsulfuron (35 g ai ha−1) or carfentrazone (35 g ai ha−1), respectively. Still, all herbicide treatments resulted in similar field bindweed biomass. Results indicate that clopyralid and quinclorac are effective tools to help manage Canada thistle and field bindweed and that hazelnut can tolerate clopyralid and quinclorac at rates equivalent to 4-fold commercial-use rates not affecting plant growth and yield.

Renewed interest in studying auxin herbicides (WSSA Group 4) is increasing as a result of the release of genetically engineered crop varieties that are tolerant to preemergence and postemergence applications of specific formulations of dicamba. Auxin-resistant crops were developed in response to the development of weed species resistant to glyphosate and other herbicides. Research was conducted at multiple field locations in Georgia in 2018 and 2019 to examine weed control when postemergence herbicides were applied to dicamba- and glyphosate-resistant cotton at eight different points in time over a 24-h period. Applications were made at 1 h prior to sunrise all the way up to midnight during the same day to examine the effect of herbicide application timing on broadleaf weed control. Glyphosate, dicamba, and glyphosate plus dicamba were applied at each timing. Visual ratings of weed control were scored at 7, 14, 21, and 28 d after treatment (DAT). Weed control was affected by herbicide application timing. Midnight applications resulted in the lowest levels of control. Sicklepod, pitted morningglory, and prickly sida control was 49%, 38%, and 41%, respectively. Greatest control of all three species (up to 99%) occurred from the noon to 1 h prior to sunset application timings. Orthogonal contrasts of timing of application indicated that weed control was improved with day > night and pre-dawn > midnight.

Waterhyacinth [Eichhornia crassipes (Mart.) Solms] has been the focus of national legislation efforts and has been listed as noxious, invasive, potentially invasive, or prohibited by at least seven U.S. states. Auxinic herbicides are one of the most effective control methods labeled for use in aquatic sites. In the United States, florpyrauxifen-benzyl, a synthetic auxin, was recently (2018) registered for use in aquatic sites, but limited information has been published on efficacy, especially differences between the two formulations. Therefore, the purpose of this work was to evaluate two formulations of florpyrauxifen-benzyl—suspension concentrate (SC) and emulsifiable concentrate (EC)—at three rates each (14.8, 29.5, and 58.9 g ai ha−1) for control of E. crassipes under outdoor and greenhouse conditions. All rates of each florpyrauxifen-benzyl formulation reduced E. crassipes biomass by 90% to 100% when compared with nontreated plants at 5 wk after treatment. Based on plant recovery in the outdoor trial, there was some evidence that the lowest rate (14.8 g ai ha−1) of florpyrauxifen-benzyl SC and EC may not be as efficacious at reducing E. crassipes biomass as the SC and EC formulations when applied at 29.5 and 58.9 g ai ha−1. Future work should evaluate the florpyrauxifen-benzyl rates tested in this research against E. crassipes in field trials and/or an operational setting to confirm findings.

Management of volunteer glyphosate-resistant (GR) corn may be problematic in soybean resistant to glyphosate and 2,4-D or dicamba, as auxinic herbicides often antagonize graminicide efficacy. Field and greenhouse trials were conducted using mixtures of 2,4-D or dicamba in combination with glyphosate and clethodim-A (formulated without an adjuvant) or clethodim-SM (adjuvant-inclusive formulation) to determine the effect on volunteer GR corn control. Neither auxinic herbicide reduced clethodim efficacy, regardless of clethodim rate or formulation in field trials. However, the addition of glyphosate to these mixtures at the 35 g ai ha−1 clethodim dose reduced control from clethodim-A and clethodim-SM by 62% to 75% and 27% to 47%, respectively. Increasing the clethodim dose to 105 g ha−1 or greater in combination with glyphosate and either auxinic herbicide generally restored clethodim efficacy (74% to 98% control); in one site-year, the addition of glyphosate plus dicamba to clethodim-A at 140 g ha−1 still reduced control by 34%. In greenhouse experiments, clethodim-A efficacy was reduced by 17% and 28% when applied with glyphosate plus 420 and 1,680 g ae ha−1 2,4-D, respectively, in the absence of crop oil concentrate (COC). Increasing the dose of dicamba in a similar mixture had a negligible effect. Irrespective of auxinic herbicide dose, the inclusion of COC to clethodim-A mixtures with glyphosate plus 2,4-D or dicamba resulted in ≥ 90% control. These results specify an enhanced risk of reduced clethodim efficacy on volunteer GR corn when glyphosate is added to mixtures containing 2,4-D or dicamba. To optimize control from these mixtures, clethodim should be applied at ≥ 105 g ha−1 and should include an activator adjuvant in the form of COC and/or an adjuvant-inclusive clethodim formulation. This recommendation contrasts with several labels of clethodim that do not require COC when applied with adjuvant-loaded glyphosate products.

Application timing and environmental factors reportedly influence the efficacy of auxinic herbicides. In resistance-prone weed species such as Palmer amaranth (Amaranthus palmeri S. Watson), efficacy of auxinic herbicides recently adopted for use in resistant crops is of utmost importance to reduce selection pressure for herbicide-resistance traits. Growth chamber experiments were conducted comparing the interaction of different environmental effects with application time to determine the influence of these factors on visible phytotoxicity and hydrogen peroxide (H2O2) formation in A. palmeri. Temperature displayed a high degree of influence on 2,4-D and dicamba efficacy in general, with applications at the low-temperature treatment (31/20 C day/night) resulting in an increase in phytotoxicity compared with high-temperature treatments (41/30 C day/night). Application time across temperature treatments significantly affected 2,4-D–induced phytotoxicity, resulting in a ≥30% increase across rates with treatments at 4:00 PM compared with 8:00 AM. Temperature differential had a significant influence on dicamba efficacy based on visible phytotoxicity data, with a ≥46% increase with a high (37/20 C day/night) compared with a low differential (41/30 C day/night). Concentration of H2O2 in herbicide-treated plants was 34% higher under a high temperature differential compared with the low differential. Humidity treatments and application time interactions displayed undetected or inconsistent effects on visible phytotoxicity and H2O2 production. Overall, temperature-related influences seem to have the largest environmental effect on auxinic herbicides within conditions evaluated in this study. Leaf concentration of H2O2 appears to be generally correlated with phytotoxicity, providing a potentially useful tool in determining efficacy of auxinic herbicides in field settings.

The noxious weed yellow starthistle is commonly controlled by the auxinic herbicide picloram. Induction of ethylene synthesis, epinasty, and reduction in shoot growth are typical symptoms of picloram treatment. Picloram did not induce ethylene evolution in the resistant accession RDW-1, though it caused a 250% increase in ethylene evolution in the susceptible wildtype SCI-1. The ethylene synthesis inhibitor aminoethoxyvinylglycine reduced the amount of ethylene induced by picloram in SCI-1 to control levels, but only reduced epinasty by 20% after 6 d. Aminoethoxyvinylglycine did not affect the reduction in shoot weight caused by picloram. The ethylene-releasing compound ethephon induced only a small amount of epinasty and had little effect on shoot weight in either accession. These results suggest that ethylene induced by picloram in wildrype plants plays only a minor role in the herbicidal effects of picloram. Furthermore, the resistance of the RDW-1 accession is not due to the lack of ethylene biosynthesis following picloram application to this accession.

The relative competitive abilities of yellow starthistle accessions that are resistant (R) and susceptible (S) to picloram were compared using a replacement series experiment. With no herbicide treatment, total shoot dry weights at vegetative and early reproductive stages of plant growth were similar for the two accessions, although S plants accumulated more total shoot dry weight by the late reproductive stage, mainly as a result of a greater contribution of vegetative growth. Without herbicide, relative yield of total biomass or reproductive structures did not differ from theoretical competitive equivalence at any accession ratio, thereby indicating that interaccession interference was similar. For picloram-treated plants, R plants accumulated more total, vegetative, and reproductive dry weight than did S plants at the early and late reproductive stages, and there was no difference between S and R plants at the vegetative growth stage. Seed production by R plants was 10-fold greater than that observed in S plants, but seed size remained unchanged, regardless of accession ratio. With herbicide present, the relative yield of S plants differed from theoretical competitive equivalence as S:R accession ratios decreased, but relative yield of R plants did not. Therefore, only in the presence of picloram will R plants have a competitive advantage over S plants. Some of the progeny from mixed populations of S and R plants that were cross-pollinated, even at low R frequency (25%), expressed resistance to picloram.

Studies were initiated to evaluate absorption, translocation, and distribution of 14C-quinclorac in propanil- and quinclorac-resistant (R-BYG) and -susceptible (S-BYG) barnyardgrass. No differential absorption of 14C-quinclorac was observed between R-BYG and S-BYG, but more 14C remained in the treated leaf of S-BYG (57% of total absorbed) compared with the R-BYG leaf (34% of total absorbed) at 72 h after treatment (HAT). After 12 HAT, 20 and 15% of the amount absorbed had been translocated basipetally by R-BYG and S-BYG, respectively. At 72 HAT, 27 and 17% of the total absorbed 14C had been translocated acropetally by R-BYG and S-BYG, respectively. The levels of 14C above the treated leaf continued to increase throughout the duration of the experiment in R-BYG while levels of 14C above the treated leaf in S-BYG remained relatively constant. Seven percent more of the total absorbed 14C was exuded from roots of R-BYG than S-BYG at 72 HAT. Although differential translocation was observed between R-BYG and S-BYG, it is unclear whether this difference is a cause of quinclorac resistance or an effect of some other physiological process. Further research is needed to determine if differential translocation is due to metabolism or other physiological factors.

Cotton genetically engineered to be resistant to topical applications of 2,4-D could provide growers with an additional tool for managing difficult-to-control broadleaf species. However, the successful adoption of this technology will be dependent on the ability of growers to manage off-target herbicide movement. Field experiments were conducted in Moultrie, GA, to evaluate cotton injury resulting from the volatilization of 2,4-D when formulated as an ester, an amine, or a choline salt. Each formulation of 2,4-D (2.24 kg ha−1) was applied in mixture with glyphosate (2.24 kg ha−1) directly to the soil surface (10 to 20% crop residue) in individual square blocks (750 m2). Following herbicide applications, replicate sets of four potted cotton plants (five- to seven-leaf stage) were placed at distances ranging from 1.5 to 48 m from the edge of each treatment. Plants were allowed to remain in-field for up to 48 h before being removed. Cotton exposed to 2,4-D ester for 48 h exhibited maximum injury ratings of 63, 57, 48, 29, 13, and 2% at distances of 1.5, 3, 6, 12, 24, and 48 m, respectively. Less than 5% injury was noted for the amine and choline formulations at any distance. Plant height was also affected by formulation and distance; plants that were located closest to the ester-treated block were smaller than their more distantly-positioned counterparts. Exposure to the amine and choline formulations did not affect plant heights. Additionally, two plastic tunnels were placed inside of each treated block to concentrate volatiles and maximize the potential for crop injury. Injury ratings of 76, 13, and 5% were noted for cotton exposed to the ester, amine, and choline formulations, respectively when under tunnels for 48 h. Results indicate that the choline formulation of 2,4-D was less volatile and injurious to cotton than the ester under the field conditions in this study.

The role of ethylene in kochia injury following the application of dicamba and other auxinic herbicides is not known. Experiments were conducted to evaluate kochia response to ethylene generated within plant tissue and to quantify ethylene emission from kochia following dicamba application to determine if ethylene production could be used to differentiate between dicamba-susceptible (S) and dicamba-resistant (R) kochia accessions. Dicamba at 0.28 kg ha−1 and fluroxypyr at 0.28 kg ha−1 caused severe stem and petiole twisting within 6 h of application, with S kochia exhibiting a greater level of epinasty than R kochia and fluroxypyr causing more epinasty than dicamba. Fluroxypyr provided similar control of S and R kochia, but dicamba provided less control of R kochia than S kochia. Ethylene evolution in S kochia from ethephon at 1.1 kg ha−1 and dicamba at 0.28 kg ha−1 was similar, approximately 70 ng (g fresh weight)−1 h−1. Elevated ethylene concentration from as much as 2 kg ha−1 ethephon did not produce epinastic symptoms or other plant response in S or R kochia. Ethylene evolution following dicamba application increased with time up to 48 h after application and with increasing dicamba rate up to 0.56 kg ha−1, which were the end points of the research. Ethylene evolution from the S accession was 2.5 times that of the R accession averaged across dicamba rates and harvest times. The greatest difference in ethylene evolution between S and R accessions occurred 24 h after treatment at 0.14 kg ha−1 dicamba, with S kochia producing ethylene at 470%, compared with R kochia.

Quinclorac drift has been speculated as the cause of injury to tomato crops throughout northeast Arkansas. In this study, we set out to determine whether tomato plant injury and yield reduction were correlated with simulated quinclorac drift. Experiments were carried out at Fayetteville, AR, in 1999 and 2000. Maximum plant injury (visual ratings) was about 20% when plants were treated with one, two, or three quinclorac applications (weekly intervals beginning at first flower) at 0.42 g ai ha−1 (0.001 times the normal use rate to simulate drift). Maximum plant injury ranged from 48 to 68% with quinclorac simulated drift treatment of 42 g ha−1. Overall, increasing quinclorac rate and number of applications increased tomato injury. In both years, tomato plant fresh-weight accumulation was not influenced by one, two, or three applications of quinclorac at 0.42 g ha−1 compared with the untreated control. In 1999, increasing the rate of quinclorac from 0.42 to 4.2 g ha−1 reduced plant fresh-weight accumulation. In 2000, there was no significant difference in plant fresh weight when plants were treated with quinclorac at 2.1 to 4.2 g ha−1. Evaluation of the herbicide rate effect indicated that quinclorac at 0.42 g ha−1 did not reduce tomato fruit yield (total weight of edible fruit) compared with the untreated control, but yield decreased as rate increased above 0.42 g ha−1. Increasing the number of applications generally decreased tomato yield, and overall as maximum visual plant injury increased, tomato yield reduction ALSo increased linearly. We conclude that quinclorac at simulated drift rates can adversely affect tomato plant growth and yield.

The herbicide resistance to dicamba (HRd) biotype of kochia is resistant to several auxinic herbicides and is impaired in shoot gravitropism and other auxin-mediated responses. To better characterize the biotype and investigate its mechanism of resistance, we used messenger RNA (mRNA) differential display to compare patterns of dicamba-induced gene expression in HRd and susceptible (S1) plants. More than 60,000 complementary DNA fragments were generated and examined, 106 of which were isolated and used as probes on Northern blots to confirm gene expression levels. Steady-state levels of > 90% of mRNAs did not change after dicamba application. However, several mRNAs were detected whose levels were decreased, increased, or differentially regulated between the biotypes within minutes of dicamba treatment. The abundance of three mRNAs decreased after treatment, two of which had significant sequence similarity to choline monooxygenase and 5,10-methylenetetrahydrofolate reductase, respectively. Conversely, increased expression levels were observed for a putative chloride channel protein, 1-aminocyclopropane-1-carboxylate synthase, and an unknown gene. Genes differentially expressed between HRd and S1 plants included those similar to a putative translation initiation factor, xyloglucan endotransglycosylase, and a hypothetical protein cloned from several organisms. The results demonstrate that mRNA differential display is a useful technique for discovering genes that are rapidly regulated as part of a physiological response, and that this approach may provide insight into the mechanism of auxinic herbicide resistance in kochia.