Original Article
Soil and environmental factors affecting the efficacy of pyroxasulfone for weed control
2016 Volume 41 Issue 1 Pages 1-5
Details
2016 Volume 41 Issue 1 Pages 1-5
Pyroxasulfone inhibits very-long-chain fatty acid elongase in susceptible weeds and facilitates preemergence weed control. Pyroxasulfone showed 98% control of Echinochloa crus-galli (ECHCG) for up to 63 days at a use rate lower than that of chloroacetamide herbicides. ED90 of pyroxasulfone against ECHCG in soil with high organic matter (OM) content did not differ significantly from that in lower OM soils. The low water solubility and the low vapor pressure of pyroxasulfone resulted in limited horizontal diffusion on the soil surface and low risk of volatilization, respectively. The herbicidal efficacy of pyroxasulfone was affected by clod size and improved with smooth soil surface preparation.
Pyroxasulfone is a new herbicide for preemergence control of grass and broadleaf weeds in corn, soybeans, and wheat.1) The target site of pyroxasulfone is very-long-chain fatty acid elongase, and it is classified as a K3 herbicide by the Herbicide Resistance Action Committee.2–5) The dosage rate of pyroxasulfone for use in corn and soybeans is 90 to 240 g a.i./ha, which is lower than those of chloroacetamide herbicides widely used in corn and soybeans.6,7) The duration and efficacy of preemergence herbicides for weed control are affected by various environmental factors such as soil properties, rainfall, and field preparation as well as the physicochemical properties of the herbicide that are related to its behavior in the field.8,9) Knowledge of the factors influencing weed control will facilitate the effective use of herbicides and decision making for weed management programs. The influence of environmental factors on weed control by chloroacetamide herbicides has been published, thereby contributing knowledge about herbicide behavior and fate and facilitating proper herbicide use.10–12) In the present study, the effects of physicochemical properties as well as environmental factors on the weed control efficacy of pyroxasulfone were evaluated by bioassay to clarify optimal conditions under which the herbicidal efficacy of pyroxasulfone is maximized.
The pyroxasulfone was synthesized by the Ihara Chemical Industry Co., Ltd. Under application conditions, pyroxasulfone is in a solid state with a vapor pressure of 2.4×10−6 Pa (25°C), a Koc of 57 to 114 (4 soils), and water solubility of 3.49 mg/L (20°C); it is stable at a pH between 5.7–9 (25°C, 15 days).6) Formulations of pyroxasulfone (60% water-dispersible granules (WG) and 42.7% suspension concentrate (SC)) were prepared by the Kumiai Formulation Technology Institute, and 10% wettable powder (WP) was prepared by the K-I Research Institute. Pyroxasulfone 60% WG was used in Experiment 1, 42.7% SC was used in Experiments 2 and 3, and 10% WP was used in Experiments 4 and 5. Commercial herbicides metolachlor, S-metolachlor, dimethenamid-P, and alachlor were purchased and used.
2. Field plant back test (Experiment 1)A field plant back test was conducted in a field in Kumiai Kakegawa (clay loam, 0.9% OM, pH 6.3) to compare the residual weed control of pyroxasulfone with that of metolachlor, alachlor, and dimethenamid-P. Herbicides were applied 1 day after initial Echinochloa crus-galli (ECHCG) planting with a CO2 backpack sprayer calibrated to deliver 1000 L/ha. ECHCG seeds were planted using a mechanical planter 1 day before application (DBA) and 28, 63, 93, and 127 days after application (DAA). Planting depth was 1 to 3 cm. Weed control was visually assessed 28 days after planting (DAP) using a scale of 0 to 100, with 0 representing no efficacy and 100 representing complete weed control.
3. Seedling growth inhibition tests in different soils (Experiment 2)Tests assessing growth inhibition of ECHCG seedlings were conducted at the Kumiai Life Science Research Institute in Shizuoka Japan (KLSRI) to compare the herbicidal activity of pyroxasulfone with that of S-metolachlor in different soils (A: sandy loam, 0.7% OM, pH 6.6; B: sandy loam, 4.7% OM, pH 5.4; C: loam, 12.5% OM, pH 4.7; D: loam, 1.3% OM, pH 6.8; E: loam, 4.7% OM, pH 6.5; F: clay loam, 10.0% OM, pH 5.6). A plastic cup (60 mL) was filled with 10 g of air-dried soil filtered through a 2-mm sieve. Herbicides were applied by micropipette in 2 mL chemical solutions to the soil surface at concentrations ranging from 0.016 to 2 ppm. Approximately 40 ECHCG seeds were mixed with soil, and the total volume of water in each soil was adjusted to the field water capacity. Test cups were kept in a growth chamber maintained at 25°C for 7 days under fluorescent lights (12 hr light/dark). Growth inhibition was evaluated visually with three replications at 7 DAP using a scale of 0 to 100, with 0 representing no effect and 100 representing complete control. Data for visible growth inhibition were analyzed by probit analysis.
4. Horizontal movement of pyroxasulfone on the soil surface (Experiment 3)Glass greenhouse experiments were conducted at KLSRI to clarify the horizontal movement of pyroxasulfone on the soil surface. Containers (11×11×11 cm) were filled with clay soil (9.6% OM, pH 5.4). Cynodon dactylon seeds were mixed with soil filtered through a 1 mm sieve (10 g seeds/1 L soil), and the mixed soil (25 mL) was uniformly spread on the container’s surface. Herbicides were applied to the center of the containers within a diameter of 1.5 cm using a micropipette. The total amount of applied herbicide active ingredient (a.i.) was the amount required for application to the container’s surface area at the required dosage. Water was supplied to the containers from above (0.5 cm) with a mist sprayer just after application, and 2, 4, 6, and 8 DAA. The area in which the herbicide completely controlled seedling growth was described as a circle, and the diameter was measured by scale at 14 DAA. There were two replicate containers for each herbicide treatment and the untreated check.
5. Seedling growth inhibition test on the location of the herbicide layer (Experiment 4)A greenhouse trial was conducted at KLSRI to determine the relationship between the spatial positions of the pyroxasulfone layer and weed control. Trays (14×24×3 cm, 1 L) were filled with clay soil (9.6% OM, pH 5.4). Pyroxasulfone was applied on the soil surface with a micro sprayer calibrated to deliver 500 L/ha, and the soil was well mixed in a plastic bag. The treated soils were placed in layers at depths from 0 to 1 cm, 1 to 2 cm, and 2 to 3 cm in a plastic container (11×11×11 cm), and Setaria viridis (SETVI) seeds were placed at a depth of 1.5 cm. Water was supplied to the containers from the bottom of the containers at 1 DBA and when the container surface was dry. Growth inhibition was visually evaluated at 21 DAA using a scale of 0 to 100. There were three replicate containers for each herbicide treatment and the untreated check.
6. Influence of soil surface conditions on weed control by pyroxasulfone (Experiment 5)Glass greenhouse experiments were conducted at KLSRI to clarify the influence of soil surface conditions on weed control by pyroxasulfone. A container (11×11×11 cm) was filled with clay (9.6% OM, pH 5.4) to 3 cm from the top, and initial watering was conducted. Sieved soils of three particle sizes (< 5 mm, 5–15 mm, and >15 mm) were mixed with SETVI seeds (200 mg seeds/100 mL soil), and placed into the pre-prepared containers up to their maximum capacity 1 day before planting (DBP). Pyroxasulfone was applied on the soil surface with a micro sprayer calibrated to deliver 500 L/ha. Water was supplied from the bottom of the tested containers. Weed control was individually evaluated at 28 DAA using a scale of 0 to 100. Nonlinear regression analysis was performed to determine the effect of the chemicals on SETVI growth inhibition. Data for visible weed control were analyzed using R software (R version 3.1.2 has been released by The R Foundation, http://www.R-project.org). Data were fitted to a four-parameter log-logistic model:
 | (1) |
Pyroxasulfone is used at a rate of 90 to 240 g a.i./ha in corn and soybeans and provides good weed control with residual activites.1,6,7) The efficacy and weed control duration of pyroxasulfone was compared with those of herbicides with the same mode of action metolachlor, alachlor, and dimethenamid-P at the labeled use rate by a plant back test in the field. Pyroxasulfone applied at a rate of 200 g a.i./ha showed an efficiency of 98% weed control up to 63 DAA, whereas the residual weed control period, the length of ECHCG control greater than 90%, was 0 to 28 DAA for alachlor at 1736 g a.i./ha, 28 to 63 DAA for dimethenamid-P at 1138 g a.i./ha, and 63 to 93 DAA for metolachlor at 1800 g a.i./ha. These results showed that pyroxasulfone was effective for a longer period at dosages 1/9 to 1/6 that of other herbicides examined (Fig. 1).
), and 28 (
), 63 (
), 93 (
), and 127 days after application ().Residual weed control of preemergence herbicide is affected by herbicide physicochemical properties and dissipation due to decomposition in soil, volatilization, runoff, and leaching.13–15) Several model tests were conducted to determine the efficacy and residual activity of pyroxasulfone.
The labeled use dosage of pyroxasulfone is 8- to 10-fold lower than that of S-metolachlor, however, the ED90 for ECHCG control without soil shows only a 2-fold difference between pyroxasulfone and S-metolachlor.7) The growth inhibition tests were conducted to determine the ED90s of pyroxasulfone and S-metolachlor in different soils, and ED90 data were calculated and analyzed by probit. Test results demonstrated that the efficacy of pyroxasulfone in the presence of soil is 4- to 11-fold higher than that of S-metolachlor. These results support the assertion that the rate differences between pyroxasulfone and S-metolachlor are due to differences in the chemical properties and the soil–herbicide interactions (Table 1).
| Soil | A | B | C | D | E | F |
|---|---|---|---|---|---|---|
| Pyroxasulfone (pED90) ppm | 1.7 | 3.5 | 3.9 | 3.9 | 5.9 | 6.6 |
| S-Metolachlor (mED90) ppm | 7.4 | 23.4 | 43.8 | 20.9 | 31.3 | 62.5 |
| mED90/pED90 | 4.4 | 6.7 | 11.2 | 5.4 | 5.3 | 9.5 |
| Organic matter % | 0.7 | 4.7 | 12.5 | 1.3 | 4.7 | 10 |
| Texture | SL | SL | L | L | L | CL |
| pH | 6.6 | 5.4 | 4.7 | 6.8 | 6.7 | 5.6 |
SL: sandy loam, L: loam, CL: clay loam.
The correlation between pyroxasulfone efficacy and OM was calculated from the results of Experiment 2, which showed no significant correlation between OM and ED90 (the correlation coefficient was 0.51); however, the correlation between efficacy and OM without soil C (12.5% OM) was 0.83. On the other hand, the correlation coefficient of S-metolachlor was 0.86 for all soil types and 0.96 without soil C. In general, herbicide efficacy shows a correlation with the OM content, as demonstrated for several herbicides, including pyroxasulfone.16–18) The results showed that pyroxasulfone at 200 to 300 g a.i./ha was required to achieve effective weed control in soils with up to 3% OM, and higher doses of pyroxasulfone are required in soils with higher OM contents. Odero et al. demonstrated the efficacy of pyroxasulfone use in soils with high organic matter (80% OM soil).19) They showed that a high dosage of pyroxasulfone was required to control weeds in a high OM soils; however, the labeled use rate of pyroxasulfone at 214 g a.i./ha achieved effective weed control of grass and broadleaf, even in high OM soils. In general, organic carbon affects soil adsorption, and soil adsorption is related to the efficacy of preemergence herbicides. As soil adsorption affects the efficacy of preemergence herbicides, the ED90 of pyroxasulfone for each soil type exhibited a significant correlation with soil adsorption; S-metolachlor shows a similar tendency (data not shown). OM is an important factor in soil-herbicide interactions; however, soil adsorption of an organic compound may have different interactions caused by the quality of organic carbons, clay in the soils, hydrogen bondings, and other factors. This result suggested that high OM does not necessarily influence the efficacy of pyroxasulfone, and, under certain conditions, pyroxasulfone may be effective even when applied to high OM soils.
To estimate residual weed control, loss of the herbicide through runoff from the soil surface and volatilization to the air was considered.6,20) A greenhouse test to estimate the potential of runoff was conducted (Experiment 3). The horizontal movement of pyroxasulfone on the soil surface appeared to show a spatial distribution resembling a circle, and the diameter of diffusion of pyroxasulfone and S-metolachlor at a rate of 250 g a.i./ha and 2140 g a.i./ha on the soil surface, respectively, was 4.3 cm and 9.5 cm (Table 2). These results indicate that the movement on the soil surface of pyroxasulfone is half that of S-metolachlor; thus, diffusion of pyroxasulfone on a soil’s surface is limited. S-Metolachlor (means Koc 189, water solubility 480 mg/L) shows a Koc value approximately 2 times higher and a water solubility approximately 100 times larger than those of pyroxasulfone; therefore, water solubility rather than Koc value appears to affect the horizontal movement of those herbicides.21)
| Herbicide | Dosage (g a.i./ha) | Diameter* (cm) |
|---|---|---|
| Pyroxasulfone | 250 | 4.3±0.4** |
| 125 | 4.3±0.4 | |
| S-Metolachlor | 2140 | 9.5±0.7 |
| 1070 | 7.8±1.1 |
*Diameter represents the diameter of a circle of Cynodon dactylon growth inhibition area controlled by herbicide on soil surface.**Each data represents diameter±standard deviation.
Gish et al. conducted an 8-year study to quantify field-scale herbicide volatilization and runoff.19) It has been reported that the losses of metolachlor by runoff never exceed 2.5%; therefore, it is likely that pyroxasulfone has a low potential for herbicide loss from the soil’s surface. On the other hand, the cumulative volatilization of metolachlor (vapor pressure 4.2×10−3 Pa, 25°C) was 5 to 63% of applied metolachlor within 5 DAA, and the isomer, S-metolachlor (vapor pressure 3.7×10−3 Pa, 25°C), is expected to show similar behavior.20,21) There are no data showing the loss of pyroxasulfone by volatilization under the same conditions; however, vapor pressure of pyroxasulfone (2.4×10−6 Pa, 25°C) is approximately 1/1000 that of metolachlor and S-metolachlor.21) This suggests that pyroxasulfone has a lower potential for loss through volatilization.22)
Residual weed control is affected by the soil persistence of a herbicide. Previous studies have shown that field dissipation (the time for 50% disappearance) occurs within 6 to 49 days (12 sites) for S-metolachlor and 16 to 26 days (4 sites) for pyroxasulfone.6,21) Dissipation studies of S-metolachlor and pyroxasulfone under the same field conditions were conducted by Mueller et al.23) and Westra et al.,24) and pyroxasulfone showed a tendency for longer persistence in the soil than did S-metolachlor. It is considered that the longer residual weed control at lower dosages demonstrated by pyroxasulfone is due to higher herbicidal activities with a lower risk of loss of the herbicide, resulting in longer soil persistence. Pyroxasulfone has a relatively low Koc; it may move into the soil under excess water conditions and cause a certain degree of crop injury.25) To assess the concern regarding the balance between weed control and the location of the herbicide layer, a greenhouse test was conducted at KLSRI. Pyroxasulfone and S-metolachlor were sprayed on the soil for uniform mixing. SETVI seeds were planted above, within, and under the treated layer (1 cm thick). Pyroxasulfone showed good efficacy when seeds were located under or within the treated layer, whereas S-metolachlor showed strong efficacy when seeds were located under the treated layer (Fig. 2). These results indicate that pyroxasulfone is highly effective when it is in contact with the plumule or mesocotyl, and it is also effective when taken up by the roots of weeds.
) and 63 () g a.i./ha for pyroxasulfone and 1070 () and 535 () g a.i./ha for S-metolachlor. The herbicide-treated soils were mixed in plastic bags and placed in layers at depths from 0 to 1 cm, 1 to 2 cm, and 2 to 3 cm in a plastic container; Setaria viridis seeds were placed at a depth of 1.5 cm. Growth inhibition was visually evaluated 21 days after application.Soil moisture conditions have an influence on herbicide efficacy.26) Pyroxasulfone showed >85% inhibition of growth of SETVI during a greenhouse test at up to 1/8 of the recommend dosage rate for use in sandy loam soils and 1/4 of the recommend dosage rate for use in clay soils with no irrigation over a period of 10 days (initial water was approximately 60% of field capacity (data not shown)). The amount of rainfall or irrigation required for herbicide activation is difficult to determine and is dependent on a field’s soil texture, initial soil moisture, and dew point. When a grower uses this herbicide, knowledge of the required the water amount required for herbicide activation is important for effective use of the herbicide and irrigation efficiency. Data on the efficacy of pyroxasulfone from more than 100 field trials (125 g a.i./ha, all soil types, control of Setaria faberi) were analyzed by the total amount of rainfall after application (data not shown). Based on the analysis of the field trial data, pyroxasulfone exhibits >88% control in the event of >12.5 mm rainfall within 7 DAA. However, only 62% control was observed for <6.25 mm rainfall within 7 DAA.
The results of this study have shown that the diffusion of pyroxasulfone on the soil’s surface is limited, due to its low water solubility. Under conditions of a rough field surface and large clods, pyroxasulfone may not have a chance to make contact with weeds, and decreased in weed growth inhibition may occur. A greenhouse trial was conducted to determine the influence of the soil’s surface condition (clod size: <5 mm, 5 to 15 mm, and >15 mm) for SETVI control at preemergence by pyroxasulfone. The results showed that the ED50s of pyroxasulfone were 6.2 g a.i./ha, 10.2 g a.i./ha, and 14.3 g a.i./ha, respectively, for each of the clod sizes mentioned above (Fig. 3). A smooth soil surface will increase the chances for pyroxasulfone’s contact with weeds, and preparing of a smooth seedbed is important for maximizing the weed-control potential of pyroxasulfone.
Preemergence herbicide is applied for proactive weed control, however, the results of herbicide application are affected by environmental conditions and may not be sufficient under certain conditions. Environmental factors influencing the efficacy of pyroxasulfone were evaluated and discussed. The use rate of pyroxasulfone mostly depends on the soil OM content; however, it is not necessary to use a high rate of pyroxasulfone in soil with high OM content. Pyroxasulfone has the potential to provide weed control for an extended duration with low risk of runoff or volatilization. Rainfall or irrigation of approximately 12.5 mm within 7 DAA is required to optimize the activity of pyroxasulfone for effective weed control, and smooth soil preparation additionally maximizes the efficacy of weed control. Usage of pyroxasulfone under optimal conditions not only affords effective weed control but contributes to reducing the environmental load by chemicals.
The authors thank Dr. Hiroshi Matsumoto, Tukuba University, for instruction and advices of this research, and also the suggestions of the anonymous reviewers that improved this manuscript.
