S-metolachlor Treatments Research Articles

Effect of S-metolachlor and flumioxazin herbicides on sweetpotato treated with and without activated charcoal applied through transplant water

Abstract Flumioxazin and S-metolachlor are widely used in conventional sweetpotato production in North Carolina and other states; however, some growers have recently expressed concerns about potential effects of these herbicides on sweetpotato yield and quality. Previous research indicates that activated charcoal has the potential to reduce herbicide injury. Field studies were conducted in 2021 and 2022 to determine whether flumioxazin applied preplant and S-metolachlor applied before and after transplanting negatively affect sweetpotato yield and quality when activated charcoal is applied with transplant water. The studies evaluated five herbicide treatments and two activated charcoal treatments. Herbicide treatments included two flumioxazin rates, one S-metolachlor rate applied immediately before and immediately after transplanting, and no herbicide. Charcoal treatments consisted of activated charcoal applied at 9 kg ha−1, and no charcoal. No visual injury from herbicides or charcoal was observed. Likewise, no effect of herbicide or charcoal treatment on no. 1, marketable (sum of no. 1 and jumbo grades), or total yield (sum of canner, no. 1, and jumbo grades) was observed. Additionally, shape analysis conducted on calculated length-to-width ratio (LWR) for no. 1 sweetpotato roots found no effect from flumioxazin at either rate on sweetpotato root shape. However, both S-metolachlor treatments resulted in lower LWR of no. 1 sweetpotato roots in 2021. Results are consistent with prior research and indicate that flumioxazin and S-metolachlor are safe for continued use on sweetpotato at registered rates.

Effect of fire on weed dynamics and behavior of pre-emergent herbicides in Eucalyptus spp. plantations

The forest fires that occurred during summer 2021-22 in Uruguay generated concern in several areas, including silvicultural systems. The fire could generate changes at the flora level by changes in the dormancy of soil bank seeds, and in the behavior of frequently used pre-emergent herbicides due to inactivation by ashes. These possible changes impose a challenge in connection to weed control. In this study, an attempt was made to provide an answer to the problem by evaluating the dynamics of weeds and the behavior of pre-emergent herbicides in post-fire Eucalyptus plantations. The evaluations were carried out in two sites previously burned —one of them previously harvested and the other one not, being able to obtain data in different conditions. The weed dynamics was evaluated through the observational study of the percentage of total weed cover and present species were identified. With the data obtained, the parameters frequency, diversity and richness were calculated to analyze the different properties separately. Pre-emergent herbicides were evaluated in a randomized block experimental design: S-metolachlor, flumioxazin + S-metolachlor , isoxaflutole + S-metolachlor, oxifluorfen + S-metolachlor . Proposed treatments were assessed for possible interactions with fire and removal of the first cm of soil and ashes. Weed density was evaluated by characterizing and quantifying how many weeds could emerge in a square 30cm×30cm. While one of the fields was evaluated 8, 23, 31, 41, 61, and 82 days post application (DPA), the other was evaluated 8, 20, 31, 41, and 61 DPA. On both experiments the fire showed an increase in the weeds. In one experiment the weeding percentage increment was 64.9% on the burned area, while on the other it was 54.3 %. In addition, in the non-harvested field there were no large amounts of weeds on both treatments due to the conditions that standing trees generated. The results of weed control did not show interaction between efficiency and ashes presence, and were related to the species present in the weed bank, not showing a clear trend among the herbicide treatments. The S-metolachlor treatment was always the highest in number of weeds. The number of weed|m2 was about 17-25 on the harvested field, and 3-4 weed|m2 on the non-harvested one. The fire generated changes in the expression at seed bank of soil. This study was an important development in the generation of knowledge for future production.

Exploratory Analysis on Herbicide Metabolism and Very-Long-Chain Fatty Acid Production in Metolachlor-Resistant Palmer Amaranth (Amaranthus palmeri S. Wats.).

A Palmer amaranth (Amaranthus palmeri S. Wats.) biotype resistant to S-metolachlor was confirmed from crop fields in Arkansas, USA. This study investigated the metabolic effects of malathion (cytochrome P450 inhibitor) and 4-chloro-7-nitrobenzofurazan [NBD-Cl; glutathione S-transferase inhibitor] on the S-metolachlor-resistant A. palmeri biotype. Root elongation of the resistant biotype was 20% more inhibited by treatment of NBD-Cl (50 nM) and S-metolachlor (2 μM) in mixture than by treatment of S-metolachlor alone. Metabolites of S-metolachlor were 1.4-12.1 times greater produced in the resistant biotype for 7 d than in the susceptible standard. Production of cerotic acid, one of the very-long-chain fatty acids containing 26 carbons, was more reduced in the susceptible standard (3.8-fold) than in the resistant biotype (1.8-fold) by S-metolachlor treatment. Conclusively, evolution of S-metolachlor resistance observed in this study was likely associated with improved activity of glutathione S-transferases. Further studies are needed to genetically evaluate plant endogenous enzymes involving cerotic acid production.

Lethal and sublethal effects of five common herbicides on the wolf spider, Pardosa milvina (Araneae: Lycosidae).

We tested lethal and sublethal effects of five commonly applied herbicides on the agrobiont wolf spider Pardosa milvina. Pardosa were collected from two agricultural fields; one kept under continuous crop rotation and sprayed for over twenty years, the other had no pesticide application for the last twelve years. Male and female Pardosa from each site were exposed to one of seven herbicide treatments (atrazine, glyphosate, mesotrione, S-metolachlor, rimsulfuron, a combination of all five herbicides, or a distilled water control; N = 1201) and maintained for 52 days on the treated soil substrate. We recorded mortality, prey capture behavior, weight change, courtship behavior, and egg sac production across treatments. Mesotrione and the five-herbicide combination showed significantly higher mortality than control substrates while atrazine, glyphosate and S-metolachlor showed significantly higher survival than the control. Both male spiders and spiders collected from the conventional field had reduced survival under some herbicide treatments. Prey capture behavior varied significantly by herbicide treatment, sex, and site. We observed significant weight change differences in males and differences in egg sac production in females, with, compared to the control, significant male weight loss in the rimsulfuron treatment collected from the no herbicide field, and a decrease in egg sac production in rimsulfuron and S-metolachlor treatments among females collected from the no herbicide field. Our results show some herbicides may have modest but significant fitness benefits (atrazine, glyphosate, and S-metolachlor) while others strongly increase the mortality of a generalist predator (mesotrione and the combination herbicide treatment).

Sesame (Sesamum indicum L.) Response to Combinations of Ethalfluralin Plus S-metolachlor Applied Prior to Planting

Aims: Determine sesame response to ethalfluralin at 0.63, 0.84, or 1.05 kg ha-1 in combination with S-metolachlor at 1.07 or 1.42 kg ha-1 applied and incorporated prior to planting.
 Study Design: Randomized complete-block with 3 replications. 
 Place and Duration of Study: Studies conducted during the 2019 growing season in the Southern High Plains region of Texas near New Deal (33.5818o N, -101.7794o W) and in south Texas near Yoakum (29.2756o N, -97.1226o W).
 Methodology: At New Deal, two passes (in opposite directions) using a rolling cultivator with mixing wheels was used to incorporate herbicides within one hour of application. Mixing wheels consists of four to six spider gangs approximately 10 to 13 cm long mounted on a gang tube and set to incorporate the herbicide no greater than 2.0 cm. At the Yoakum locations, listed bed tops were cut so that they were no greater than 7 cm tall. At Yoakum 1, after beds were knocked down, herbicides were applied and incorporated approximately 2.0 cm deep with a similar piece of equipment as used at New Deal. At Yoakum 2, only the mixing action of the Monosem® precision planter was used to incorporate herbicides.
 Results: At Yoakum, Urochloa texana (Buckl.) control with all ethalfluralin plus S-metolachlor treatments when evaluated 31 to 39 DAP was inconsistent and varied from 62 to 95% while Amaranthus palmeri S. Wats control was 98 to 100% and Trianthema portulacastrum L. control was 81 to 99%. At New Deal, only ethalfluralin at 0.84 kg ha-1 + S-metolachlor at 1.42 kg ha-1 did not reduce stand. All treatments caused sesame injury when evaluated 16 and 72 days after planting; only ethalfluralin at 1.05 kg ha-1 + S-metolachlor at 1.42 kg ha-1 reduced yield. At Yoakum, all herbicide treatments reduced yield regardless of incorporation method (mixing wheel vs planter action).

Efficacy of S-metolachlor and flumetsulam (triazolopyrimidine sulfonanilide)+S-metolachlor (chloro-acetanilide) as pre-emergence herbicides in controlling weeds in maize at Chisumbanje Estate

Field trials were conducted during the 2019-2020 cropping season to assess the efficacy of S-metolachlor and flumetsulam (triazolopyrimidine sulfonanilide)+S-metolachlor (chloro-acetanilide) pre-emergence herbicides in controlling weeds in maize at Chisumbanje estate. The experiment was laid out as a randomised complete block design (RCBD) with three treatments and replicated thrice. Treatments used include hand weeding (control), S-metolachlor and flumetsulam (triazolopyrimidine sulfonanilide)+S-metolachlor (chloro-acetanilide) herbicide. The data collected was based on weed density, plant height, maize grain yield indicating significant differences (p<0.05) amongst the treatments. Flumetsulam (triazolopyrimidine sulfonanilide)+S-metolachlor (chloro-acetanilide) scored the least weed density per m-2 as compared to S-metolachlor showing that it is an effective pre-emergence herbicide (33, 27 and 22) on Mexican marigold (Tagetes minuta), shamva (Rottboellia cochinchinensis) grass and wild jute (Corchorous tridens) respectively. The control (hand weeding) scored the highest weed density per m-2, indicating that the method was not effective as compared to S-metolachlor and flumetsulam (triazolopyrimidine sulfonanilide)+S-metolachlor (chloro-acetanilide) treatments. Flumetsulam (triazolopyrimidine sulfonanilide)+S-metolachlor (chloro-acetanilide) scored the highest yield of 10 tonnes/hectare whilst S-metolachlor and control scored 7.6 and 5.6t/ha respectively. Herbicides reduced the weed spectrum in maize resulting in realisation of higher yield in flumetsulam (triazolopyrimidine sulfonanilide) + S-metolachlor (chloro-acetanilide) experiment followed by S-metolachlor. Farmers are recommended to use flumetsulam (triazolopyrimidine sulfonanilide)+S-metolachlor (chloro-acetanilide) as a pre-emergence herbicide in controlling weeds in maize so as to realise higher yields and low weed density.

Selectivity of Herbicides Applied in Preemergence and Diuron in Postemergence for Sesame

The expansion of sesame growth has led to one of the biggest challenges in weed management mainly due to the limited options of selective herbicides for the control of eudicots species. This study aimed to evaluate the selectivity of diuron, s-metolachlor or imazethapyr herbicide treatments in isolation, or in a tank mixture, when applied in preemergence only, or association with diuron herbicide sprayed in postemergence during sesame cultivation. Two experiments were carried out under Brazilian Cerrado conditions. In the first experiment, different doses (g ha-1) of diuron (500 and 750), s-metolachlor (480 and 960) and imazethapyr (40 and 80) were applied in preemergence in isolation or in a tank mixture. In the second experiment, doses (g ha-1) of the herbicides diuron (500 and 750) and s-metolachlor (480 and 720) were applied in preemergence isolated or in tank mixture, associated or not with diuron (500 g ha-1) in postemergence. The results indicated that diuron and s-metolachlor applied in isolation or in combination during preemergence are selective to the sesame crop up to the doses of 750 g ha-1 and 720 g ha-1, respectively, and when applied in association with diuron at a dose of 500 g ha-1 in postemergence are also selective for sesame crop.

Evaluation of pre-emergence herbicides for weed control in maize

Weed management is an ongoing constraint in southern Africa for conventional farming systems and in emerging conservation agriculture systems, which are more heavily reliant on herbicides for primary weed control. The challenge of rising labour costs and decreasing availability creates a greater need to develop effective and efficient weed management methods in key crops such as maize. Field experiments were conducted at Sebele Agricultural Research Station, Botswana in the 2011/12 and 2012/13 cropping seasons to evaluate pre-emergence application of atrazine at 1,000 and 2,000 g a.i. ha-1 and S-metolachlor at 1,440 and 2,880 g a.i. ha-1, and a tank mixture of atrazine at 1,000 and S-metolachlor at 1,440 g a.i. ha-1. Atrazine at either rate alone, effectively controlled annual broadleaf weeds: Acanthospermum hispidum, Datura ferox and Sesamum alatum, but failed to control annual grass weeds (Tragus berteronianus and Urochloa spp.). Conversely, sole application of S-metolachlor at either rate provided complete control of annual grass weeds, but poorly controlled annual broadleaf weeds except small-seeded Amaranthus hybridus and Amaranthus thunbergii. A tank mixture of atrazine and S-metolachlor provided broad-spectrum weed control and successfully controlled both annual broadleaf and grass weeds. Atrazine alone and in tank mixture with S-metolachlor significantly reduced annual broadleaf weed density and biomass and increased maize grain yield by more than 80% when compared with the weedy treatment. High weed density and biomass of annual broadleaf weeds in S-metolachlor treatments significantly reduced maize grain yield to levels similar to the weedy treatment. A pre-mixture of atrazine and S-metolachlor is recommended for broad-spectrum weed control. Using a combination of herbicides with different modes of action may reduce selection pressure for herbicide resistance.

Investigation of alternate herbicides for effective weed management in glyphosate-tolerant cotton

ABSTRACTGlyphosate-resistant (GR) weeds are the biggest concern for all cotton stakeholders worldwide. Currently, 43 weeds species are resistant to glyphosate and the number is increasing at an alarming rate. Soil residual/pre-emergence (PRE) herbicides like Pendimethalin and S-metolachlor can be effectively used for the control of GR weeds; however, their use is very limited at farmer’s side due to the adoption of herbicide-tolerant technology with complete reliance on glyphosate. The present study was conducted to evaluate the performance of PRE and post-emergence (POST) herbicides in glyphosate-tolerant (GT) cotton. The herbicide treatments were pendimethalin and S-metolachlor as PRE-residual, and glyphosate was applied as POST at 20 days after sowing (DAS) either alone or in combination with other herbicides like S-metolachlor, pendimethalin, and haloxyfop. A second application of glyphosate was made at 35 DAS. Results revealed that pendimethalin and S-metolachlor treatments gave 100% suppression of all dominant weeds and increased lint yield by 310–350% as compared to weedy control. In contrast, glyphosate applied once and twice, gave weed biomass reduction of only 10–86%, and increased lint yield by 136–185% over weedy control. This research established that PRE application of pendimethalin and S-metolachlor can be included in the weed management program of GT cotton.

Enantioselective Oxidative Stress Induced by S- and Rac-metolachlor in Wheat (Triticum aestivum L.) Seedlings.

The unfounded use of chiral pesticides has caused widespread concern. In this study, the enantioselective effects of S- and racemic (Rac)-metolachlor on the oxidative stress of wheat seedlings was determined based on physiological and gene transcription differences. Growth inhibition increased with increasing concentrations of tested metolachlor, and S-metolachlor had a stronger inhibitory effect than did Rac-metolachlor. Root growth was also significantly inhibited, but no enantioselective effects from the tested concentrations of the metolachlor enantiomers were observed. At a concentration of 5mgL-1, the maximal fresh weight inhibition reached 63.7% and 53.8% for S-metolachlor and Rac-metolachlor, respectively. In response to the S-metolachlor treatment, the maximum level of superoxide anions and malondialdehyde (MDA) increased to 1.73 and 2.55 times that in response to the control treatment, both of which were greater than those in response to the Rac-metolachlor treatment. The activity of superoxide dismutase (SOD) also increased in response to the S-metolachlor treatment, but the activity of peroxidase (POD) decreased. Real-time polymerase chain reaction (PCR) revealed that, compared with the Rac-metolachlor treatment, the S-metolachlor treatment attenuated the expression of several antioxidant genes. Together, these results demonstrate that S-metolachlor has a greater effect than does Rac-metolachlor on wheat seedlings.

Impacts of <i>S</i>-Metolachlor Application Timing on Sweetpotato Growth and Development

S-metolachlor is used to control/suppress yellow nutsedge, annual grasses and several broadleaf weeds in sweetpotato. However, a decline in storage root quality is suspected when excessive rainfall occurs within 24-h after application. A greenhouse study was conducted to determine the effect of S-metolachlor application timing on sweetpotato growth and development. S-metolachlor treatments (0 and 1 kg·ha-1) were applied over-the-top at 0, 5 and ten days after transplanting (DAT) and a simulated rainfall treatment delivered 25 mm of rain, 51 mm·h-1 intensity, immediately after herbicide application. Plants were harvested at 5, 10, 15, 20 and 80 DAT. During the first four harvests, roots were scanned and analyzed with WinRHIZO-Pro image analysis system to estimate root number, length, volume, and surface area along with aboveground growth parameters. At the final harvest, plant growth and biomass components, and quality of storage roots were recorded. Plants treated with S-metolachlor on day 0 and 5 DAT were significantly less than those of 10 DAT and untreated control for all measured parameters for the initial 20 days of plant growth. Even though vine length, leaf number, stem biomass, and total storage roots were not different among the treatments at 80 DAT, all other plant components and total biomass production and leaf area development for plants treated at 0 and 5 DAT were significantly (P % and 16% for plants treated at 0 and 5 DAT, respectively, relative to the untreated check. These results indicate that delaying S-metolachlor application to 10 DAT will be less damaging to sweetpotato growth and development, particularly marketable storage roots and yield.

Impacts of Rac- and S-metolachlor on cyanobacterial cell integrity and release of microcystins at different nitrogen levels

Pesticide residues and nitrogen overload (which caused cyanobacteria blooms) have been two serious environmental concerns. In particular, chiral pesticides with different structures may have various impacts on cyanobacteria. Nitrogen may affect the behavior between pesticides and cyanobacteria (e.g., increase the adverse effects of pesticides on cyanobacteria). This study evaluated the impacts of Rac- and S-metolachlor on the cell integrity and toxin release of Microcystis aeruginosa cells at different nitrogen levels. The results showed that (both of the configurations: Rac-, S-) metolachlor could inhibit M. aeruginosa cell growth under most conditions, and the inhibition rates were increased with the growing concentrations of nitrogen and metolachlor. However, cyanobacterial growth was promoted in 48 h under environmental relevant condition (1 mg/L metolachlor and 0.15 mg/L nitrogen). Therefore, the water authorities should adjust the treatment parameters to remove possible larger numbers of cyaonbacteria under that condition. On the other hand, the inhibition degree of M. aeruginosa cell growth by S-metolachlor treatments was obviously larger than Rac-metolachlor treatments. S-metolachlor also had a stronger ability in compromising M. aeruginosa cells than Rac-metolachlor treatments. Compared to control samples, more extracellular toxins (12%–86% increases) were detected after 5 mg/L S-metolachlor treatment for 72 h at different nitrogen levels, but the variations of extracellular toxins caused by 5 mg/L Rac-metolachlor addition could be neglected. Consequently, higher concentrations of metolachlor in source waters are harmful to humans, but it may prevent cyanobacterial blooms. However, the potential risks (e.g. build-up of extracellular toxins) should be considered.

Enantioselective oxidative stress and oxidative damage caused by Rac- and S-metolachlor to Scenedesmus obliquus

The rational use and environmental security of chiral pesticides has gained the interest of many researchers. The enantioselective effects of Rac- and S-metolachlor on oxidative stress in Scenedesmus obliquus were determined in this study. Stronger green fluorescence was observed in response to S-metolachlor treatment than to Rac-metolachlor treatment, suggesting that more reactive oxygen species (ROS) were stimulated by S-metolachlor. ROS levels following S-metolachlor treatment were 1.92-, 8.31-, and 1.08-times higher than those observed following Rac-metolachlor treatment at 0.1, 0.2, and 0.3 mg/L, respectively. Superoxide dismutase (SOD) and catalase (CAT) were stimulated with increasing herbicide concentrations, with S-metolachlor exhibiting a greater effect. Oxidative damage in terms of chlorophyll (Chl) content, cellular membrane permeability, and cellular ultrastructures of S. obliquus were investigated. Chla and Chlb contents in algae treated with Rac-metolachlor were 2–6-fold higher than those in algae treated with S-metolachlor at 0.1, 0.2, and 0.3 mg/L. The cellular membrane permeability of algae exposed to 0.3 mg/L Rac- and S-metolachlor was 6.19- and 42.5-times that of the control. Correlation analysis implied that ROS are the major factor responsible for the oxidative damage caused by Rac- and S-metolachlor. Damage to the chloroplasts and cell membrane of S. obliquus, low production of starch granules, and an increased number of vacuoles were observed upon ultrastructural morphology analysis by transmission electron microscope. These results indicate that S-metolachlor has a greater effect on S. obliquus than Rac-metolachlor.

Degradation of S-metolachlor and its Effects on Soil Enzymes and Microbial Communities in Vegetable Field Soil

S-metolachlor is a promising alternative to metolachlor. However, the extensive use of S-metolachlor as herbicide in vegetable fields in China has caused concerns about its environmental fate. Here we investigated the effects of temperature, relative humidity (RH) and PH on the degradation rate of S-metolachlor in vegetable soil. The degradation rates of S-metolachlor increased with increasing temperatures and RH and either acidic or basic PH facilitates S-metolachlor degradation. The degradation of S-metolachlor under these conditions followed the first-order kinetics resulting in the half-lives (T-1/2) ranging from 12.18 d to 70.71 d at 5–35°C, and 27.28 d to 53.72 d at RH 30–90%; and 29.62–19.69 d at pH 6–8. Stronger response of soil enzymes including catalase, dehydrogenase, urease and cellulase to S-metolachlor was detected in soil with high organic matter. PLFA profiles showed that, totally, the microbe populations including actinomycetes, fungi and bacteria increased gradually in the first 14 days after the treatment and decreased from 14d to 28d after the treatment. All the S-metolachlor treatments caused the increase of aerobe and anaerobe. High S-metolachlor concentration, 13.9 mg/kg, could cause significant variation at the first 2 weeks, stimulating growth of the entire soil microbial community. These findings might have practical implications for the fate of S-metolachlor residue in vegetable fields. Environmental factors, especially temperature, relative humidity and pH should be considered in combination with the appropriate application dose of S-metolachlor for achieving satisfactory weed-control efficacy, reducing runoff, and minimizing effects on environmental quality.

S-metolachlor and rainfall effects on sweetpotato (Ipomoea batatas L. [Lam]) growth and development

The herbicide S-metolachlor is used to control or suppress annual grasses, nutsedges and several broadleaf weeds in sweetpotato. However, a decline in storage root quality is suspected when an excessive rainfall occurs within 24h after application. A sunlit, controlled environment experiment was conducted to investigate sweetpotato response to five levels of S-metolachlor (0.00, 0.86, 1.72, 2.58 and 3.44kgha−1), and two levels of simulated rainfall (0 and 38mm at 51mmh−1) immediately after application. Sweetpotato slips were transplanted into white polyvinyl chloride pots filled with sandy loam soil. S-metolachlor treatments were applied to slips and a simulated rainfall treatment delivered immediately after transplanting and herbicide treatment. All pots were transferred to sunlit growth chambers that were maintained at 30/22°C, day/night temperatures and ambient carbon dioxide concentration (400μLL−1) for 60 days. An evapotranspiration-based irrigation system was used to supply water and nutrients. Plant biomass components and quality of storage roots were recorded 60 days after transplanting. There was no difference between rainfall treatments across S-metolachlor rates for vine lengths, leaf numbers and leaf area. These parameters, however, declined linearly and significantly with increase in S-metolachlor concentration. Total storage root weight declined linearly with increased S-metolachlor concentration; the decline was steeper with simulated rainfall. Yield of marketable storage roots declined by 18 and 31% in the absence of rainfall and 55 and 79% in the presence of rainfall with S-metolachlor at minimum (0.86kgha−1) and maximum (1.43kgha−1) recommended label rates, respectively, used to control weeds. Yield reduction was directly proportional to the rate of S-metolachlor applied in the absence or presence of rainfall; 77 and 123g fresh weight kg−1ha−1S-metolachlor for no-rainfall and rainfall treatment, respectively. These results can be used to improve management decisions to optimize yield under field conditions as well as to mitigate risk of injury that could be associated with the use of S-metolachlor in sweetpotato production systems.

Tolerance of Hooker’s Evening Primrose Transplants to Preemergence Herbicides

No research has investigated the phytotoxic response of hooker’s evening primrose (Oenothera elata) plug transplants to preemergence herbicides. Varied phytotoxic responses of common evening primrose (Oenothera biennis) to preemergence herbicides suggest that options may exist for the safe control of weeds present within hooker’s evening primrose when grown as an agronomic field crop. Enhanced weed control during early establishment may reduce competition for water and nutrients as well as increase seed yield and oil content. Therefore, the objective of this research was to determine the phytotoxic effect of preemergence herbicides on hooker’s evening primrose plug transplants grown in the greenhouse. Research was conducted in 2010 and 2011 at the Plant and Soil Science greenhouse complex at Texas Tech University in Lubbock, TX. Herbicide treatments were applied on 13 July 2010 and 5 Apr. 2011 and consisted of oxadiazon at 3 lb/acre, isoxaben at 0.5 lb/acre, oryzalin at 2 lb/acre, prodiamine at 1.5 lb/acre, dithiopyr at 0.5 lb/acre, s-metolachlor at 1.8 lb/acre, pendimethalin at 0.6 lb/acre, and isoxaben + trifluralin at 2.5 lb/acre. One 4-month-old hooker’s evening primrose plug (2 inches wide) was transplanted into each pot (3 gal) 2 days after treatment (DAT). Dithiopyr and s-metolachlor treatments exhibited similar lack of phytotoxicity as the untreated control 7 DAT. Phytotoxicity ≥13% was observed for trifluralin + isoxaben, pendimethalin, prodiamine, oryzalin, isoxaben, and oxadiazon 7 DAT, with the highest level of phytotoxicity (24%) exhibited by trifluralin + isoxaben treatments. Hooker’s evening primrose phytotoxicity decreased (plants grew out of the damage) for all treatments except trifluralin + isoxaben, pendimethalin, and oryzalin 28 DAT. Oryzalin (16%) and trifluralin + isoxaben (60%) were the only two treatments that did not exhibit similar phytotoxicity to the untreated control 28 DAT. There were no significant differences in aboveground or belowground biomass nor plant growth index (PGI) of any of the treatments when compared with the untreated control 28 DAT. Based upon the results of this trial, pendimethalin, prodiamine, dithiopyr, s-metolachlor, oryzalin, isoxaben, and oxadiazon may be used for preemergence weed control in hooker’s evening primrose without causing excessive phytotoxicity (>20%), potential yield loss, or both. Trifluralin + isoxaben treatments exhibited 60% hooker’s evening primrose phytotoxicity 28 DAT, which resulted in too low of an initial plant stand to warrant use.

Comparative phytotoxicity of Rac-metolachlor and S-metolachlor on rice seedlings

The toxicity of Rac-metolachlor and S-metolachlor on rice (IIYou92, Oryza sativa L.) seedlings was determined and compared in a hydroponics experiment. The elongation of shoot, main root, and the number of lateral roots were inhibited in both Rac- and S-metolachlor treatments. The 96h- IC50 of Rac- and S-metolachlor were 12.32 and 9.44 μM for shoot, and 4.69 and 1.75 μM for root, respectively. The content of chlorophyll a, chlorophyll b and total chlorophyll of leaf treated by Rac-metolachlor was higher than that treated by S-metolachlor. The activities of superoxide dismutase, peroxidase, cytochrome P450 2E1, glutathione S-transferase, and the content of glutathione increased after herbicide exposure, and the activities or the content was higher in the S-metolachlor treatment than in the exposure to Rac-metolachlor. Ultrastructural studies revealed that Rac- and S-metolachlor had adverse effects on leaf cells, and S-metolachlor treatment caused higher damage.

Enantioselective phytotoxicity of metolachlor against maize and rice roots

Rac-metolachlor, a widely used chloracetanilide herbicide, is now being replaced by S-metolachlor in many countries. The enantioselective effects of rac- and S-metolachlor on root growth of maize and rice was studied in hydroponics. Visible morphological changes in root growth were observed after treatment with rac- or S-metolachlor. The main root and lateral roots were shorter in length, and the number of lateral roots was reduced. The half inhibition (IC50,5d) values for root length of rac- and S-metolachlor were 18.86 and 10.61μM, respectively, for maize, and 7.33 and 5.35μM, respectively, for rice. The root system activity after treatment with rac- or S-metolachlor was lower than that of the control, while the root membrane permeability was higher. The activities of superoxide dismutase, peroxidase, and catalase in the roots were lower after rac- or S-metolachlor treatment compared to those of the control, while the malondialdehyde content was higher. After rice was treated with 3.1μM rac- or S-metolachlor, the cell wall separated from the cell membrane, and some destruction of nuclei and organelles was observed. The entire cell was destroyed after treatment with 12.4μM rac- or S-metolachlor. The results showed that S-metolachlor has stronger effects than rac-metolachlor on crop roots.

Enantioselectivity of racemic metolachlor and S-metolachlor in maize seedlings

Chiral herbicides may have enantioselective effects on plants. In this study, we assessed and compared the enantioselectivity of the chiral herbicides rac-metolachlor and S-metolachlor to maize seedlings. The superoxide dismutase activity (SOD) activity of roots and stem leaves treated by rac-metolachlor was 1.38 and 1.99 times that of roots and stem leaves treated by S-metolachlor. The peroxidase activity (POD) activity of roots and stem leaves was 1.48 and 2.79 times that of roots and stem leaves treated by S-metolachlor, respectively, while the catalase activity (CAT) activity was 4.77 and 8.37 times greater, respectively. The Hill reaction activity of leaves treated by rac-metolachlor were 1.45, 1.33, and 1.14 times those treated by S-metolachlor with treatments of 18.6, 37.2, and 74.4 μM. The differences observed between treatments of rac- and S-metolachlor were significant. Significant differences in maize seedling morphology were also observed between rac- and S-metolachlor treatments. The degradation rate of S-metolachlor in roots was greater than that of rac-metolachlor. The half-lives of rac- and S-metolachlor were 80.6 and 60.3 h at 18.6 μM; 119.5 and 90 h at 37.2 μM; and 169 and 164.8 h at 74.4 μM, respectively. Using the liquid chromatography-mass spectrometry method, hydroxymetolachlor, deschlorometolachlor and deschlorometolachlor propanol were considered to be possible metabolites. We determined the enantioselective toxicity of rac- and S-metolachlor to maize and speculated on the proposed metabolic pathway of metolachlor in maize roots. These results will help to develop an understanding of the proper application of rac- and S-metolachlor in crops, and give some information for environmental safety evaluation of rac- and S-metolachlor.

A comparative study of rac- and S-metolachlor on some activities and metabolism of silkworm, Bombyx mori L.

The racemic product of metolachlor contains two R-enantiomers and two S-enantiomers. S-Metolachlor is a more effective herbicidal agent than rac-metolachlor; currently, it is widely used instead of rac-metolachlor. The comparative toxicity of rac- and S-metolachlor was assessed on the basis of their effects on some enzyme activities of the fifth instar of the silkworm, Bombyx mori L. These results suggest that rac-metolachlor and S-metolachlor have different effects on enzyme activities of fifth-instar silkworm larvae. Hemolymph lactate dehydrogenase and catalase activity levels in silkworms treated with rac-metolachlor were much lower than those in silkworms treated with S-metolachlor. Control hemolymph ACP activity was 44–73% higher than that of rac-metolachlor-treated hemolymph, but the difference was not large between S-metolachlor treatment and the control. Midgut alkaline phosphatase activity of silkworms treated with rac-metolachlor was 46% lower than control activity, and the effect of S-metolachlor on midgut alkaline phosphatase activity was minor. The difference between the two herbicides was apparent mainly during the first 5 days of the fifth instar, and then gradually disappeared. It is shown that compared with S-metolachlor, rac-metolachlor is more toxic to economically important silkworms because of its effects on some enzymes and on metabolism.