新規除草剤ピロキサスルホンの除草効果

Abstract

Pyroxasulfone is a new herbicide developed by Kumiai Chemical Industry Co., Ltd. It has been registered in several countries for use in corn, soybeans, cotton, and wheat. Growth inhibition tests, greenhouse tests, and a field trial were conducted to evaluate the herbicidal efficacy of pyroxasulfone. Pyroxasulfone exhibited an excellent herbicidal activity at lower application rates as compared to S-metolachlor and has shown sufficient residual activity. Pyroxasulfone is one effective tool for the weed management programs.

Introduction

Weed management is an integral part of crop production systems and herbicides are necessary for stable and profitable production. The acreage of genetically modified (GM) crops has grown rapidly because GM crops provide easy weed control programs, require less field tillage and increase profit potential as compared to conventional crop production.^1,2) However, dependence on herbicide chemistry with the same mode of action exerts a great deal of selection pressure on weed species, and, thus, the development of herbicide-resistant weeds is accelerated.^3,4) Diversity of weed management programs, tillage, and using herbicides with different modes of action are important in preventing or delaying the evolution of herbicide-resistant weeds. A preemergence type of herbicide with sufficient residual activity is an effective tool for protecting crops from early weed competition and for managing herbicide-resistant weeds. Pyroxasulfone (Code: KIH-485) is a new preemergence herbicide developed by Kumiai Chemical Industry Co., Ltd. and is registered in the USA, Canada, Australia, and South Africa for use in corn, soybeans, cotton, and wheat.⁵⁾ Pyroxasulfone’s mode of action is the inhibition of the elongation steps of very-long chain fatty acid (VLCFA) synthesis.⁶⁾ Among preemergence herbicides pyroxasulfone has a unique chemistry that allows lower use rates (60 to 250 g a.i./ha) than other VLCFA synthesis-inhibiting herbicides, such as S-metolachlor and acetochlor. Pyroxasulfone also has a lower water solubility and lower vapor pressure as compared to the acetoanilide herbicides. The purpose of this study was to clarify the characteristics of the herbicidal efficacy and crop selectivity of pyroxasulfone.

Materials and Methods

1. Chemicals

Pyroxasulfone(3-[5-(difluoromethoxy)-1-methyl-3-(trifluoromethyl)pyrazol-4-ylmethylsulfonyl]-4,5-dihydro-5,5-dimethyl-1,2-oxazole) was synthesized by Ihara Chemical Industry Co., Ltd. (Fig. 1). It is a solid with a vapor pressure of 2.4×10⁻⁶ Pa (25°C) and a water solubility of 3.49 mg/L (20°C) and is stable at pH 5.7 and 9 (25°C, 15 days).⁹⁾ Formulations of pyroxasulfone (42.7% SC and 85% WG) were prepared by the Kumiai Formulation Technology Institute. Pyroxasulfone 42.7% SC was used in Experiments 1, 3, 4, and 5, and 85% WG was used in Experiment 2. S-Metolachlor (Dual II Magnum, 91.6% EC) was obtained from Syngenta Crop Protection.

Fig. 1. Chemical structure of pyroxasulfone.

2. Seedling growth inhibition tests on agar medium (Experiment 1)

Growth inhibition tests against Echinochloa crus-galli (ECHCG) were conducted to compare the herbicidal activity of pyroxasulfone with that of S-metolachlor. Eight grams of agar powder and Murashige & Skoog medium were dissolved in 1 L of distilled hot water. Twenty milliliters of agar solution was added to glass tubes (2.5 cm in diameter and 10 cm in height) and kept at room temperature. The test chemicals were added to warm agar medium at 0.004, 0.008, 0.016, 0.063, and 0.125 ppm. After the agar solidified, ECHCG seeds, preincubated at 25°C for 2 days, were planted five seeds/tube at a depth of 1 cm. The test tubes 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 7 days after planting, using a scale of 0 to 100 with 0 representing no effect and 100 representing complete control. The experimental design was a randomized complete block with three replications repeated two times.

Nonlinear regression analysis was performed to determine the effect of the chemicals against ECHCG. Data for percent visible growth inhibition were analyzed using probit analysis. Statistical analysis of the dose response curves is described in the procedure by Seefeldt et al.¹⁰⁾ Data were fitted to a four-parameter log-logistic model:

     

where Y=herbicidal activity, C=lower limit, D=upper limit, b=slope, ED₅₀=dose giving 50% response, and X=herbicide dose. Test chemical rates required to exhibit 50% (ED₅₀) or 90% (ED₉₀) control were calculated from regression equations.

3. Efficacy of pyroxasulfone applied preemergence and postemergence to Echinochloa crus-galli (Experiment 2)

Glass greenhouse experiments were conducted at the Kumiai Chemical Life Science Research Institute in Shizuoka, Japan, to compare the herbicidal efficacy of pyroxasulfone and S-metolachlor against ECHCG. Containers (11×11×11 cm) were filled with a loam soil (2.7% organic matter and pH 5.9). Approximately 40 ECHCG seeds were planted 1 cm deep. The herbicides were applied preemergence 1day after planting and initial watering or postemergence at the 2-leaf stages of ECHCG with 1, 3.9, 15.6, 62.5, and 250 g a.i./ha of pyroxasulfone or 8.4, 33.3, 133.8, 535, and 2140 g a.i./ha of S-metolachlor. The treatments were made using a micro sprayer calibrated to deliver 500 L/ha of water. Water was supplied to the containers from the top (<0.5 cm) with a mist sprayer just after the preemergence application and 2 days after the postemergence application. Herbicidal efficacy was evaluated visually 30 days after application (DAA) using a scale of 0 to 100, and nonlinear regression analysis was performed to determine the effect of the herbicides against ECHCG, as described above. There were three replicate containers for each herbicide treatment and the untreated check.

4. Efficacy of pyroxasulfone applied preemergence to upland weeds (Experiment 3)

Experiments were conducted in a glass greenhouse at the Kumiai Life Science Research Institute to evaluate the herbicidal efficacy of pyroxasulfone on 10 grass species, Cyperus esculentus (CYPES), and 6 broadleaf weed species. Seeds and tubers of the test weed species were planted 1 cm deep in plastic containers (1200 cm² containers for grass weeds and CYPES, 380 cm² containers for broadleaf weeds) and filled with a loam soil (2.7% organic matter and pH 5.9). Pyroxasulfone was applied preemergence 1 day after initial watering at 8, 16, 32, 63, and 125 g a.i./ha, using a sprayer calibrated to deliver 500 L/ha of water. Water was supplied to the containers from the top (<0.5 cm) with a mist sprayer just after the preemergence application. There were two replicate containers for each herbicide treatment, and the experiment was repeated two times. Herbicidal efficacy was evaluated visually at 28 DAA, as described in Experiment 1.

5. Phytotoxicity of pyroxasulfone to corn and soybeans (Experiment 4)

The experiment was conducted in a glass greenhouse at the Kumiai Life Science Research Institute to evaluate phytotoxicity of pyroxasulfone to corn and soybeans. Seeds of corn (Zea mays cv. Pioneer 33G26) and soybean (Glycine max cv. Fukuyutaka) were planted 3 cm deep in plastic containers (120 cm²) filled with a loam soil (2.7% organic matter and pH 5.9) for corn and with a sandy loam soil (0.9% organic matter and pH 6.2) for soybeans. Pyroxasulfone was applied 1 day after initial watering at 125, 250, and 500 g a.i./ha on the soil surface, using a micro sprayer calibrated to deliver 500 L/ha of water. S-metolachlor was applied at 1070 and 2140 g a.i./ha as recommended in the commercial label and also at 4280 g a.i./ha. There were three replicate containers for each herbicide treatment and the untreated check. Water was supplied to the containers from the top (<0.5 cm) with a mist sprayer right after the preemergence application. Phytotoxicity was evaluated visually 11, 21, and 28 DAA, using a scale of 0 to 100 with 0 representing no phytotoxicity and 100 representing crop death.

6. Field trial (Experiment 5)

A field trial was conducted at the K-I Chemical U.S.A. Mississippi Research Station (Leland Mississippi, U.S.A, in 2003) to evaluate the field performance of pyroxasulfone. The soil texture was a silty clay (sand 6.7%, silt 48.1%, and clay 45.2%) with 2.2% organic matter and a pH of 6.8. Weed seeds of Echinochloa crus-galli (ECHCG), Setaria viridis (SETVI), and Abutilon theophrasti (ABUTH) were sown in the test area and immediately mixed with soil using a Triple K field cultivator. Corn (ZEAMX: Dyna-Gro 57K66) was planted at a seeding rate of 2.5 seeds per 30 cm at a depth of 3.5 cm on April 12. The trial site had a natural population of Amaranthus palmeri (AMAPA), Amaranthus retroflexus (AMARE), and Hemp sesbania (SEBEX). The plot was 2 m wide by 4 m long, arranged in a randomized complete block design, and replicated three times. Herbicides were applied immediately after corn planting with a CO₂ backpack sprayer calibrated to deliver 200 L/ha at 276 kPa. Weed control and phytotoxicity were assessed visually 10, 30, and 41 DAA, using a scale of 0 to 100 with 0 representing no corn injury or no efficacy and 100 representing corn death or complete weed control. Weed control of AMAPA and AMARE was visually assessed together as Amaranthus species (AMASS). Corn injury and weed control data were subjected to ANOVA, and treatment means were separated using Student–Newman–Keuls (P=0.05), providing a general means of comparing the herbicide treatments.

Results and Discussion

1. Seedling growth inhibition tests on agar medium

The evaluated ED₉₀ for ECHCG were 0.019 ppm and 0.056 ppm, and the ED₅₀ were 0.009 ppm and 0.018 ppm for pyroxasulfone and S-metolachlor, respectively. These indicated that the ED₉₀ of pyroxasulfone was one third and the ED₅₀ was one half those of S-metolachlor (Fig. 2). The results, under the test condition without various environmental factors, support that the differences between the herbicidal activities of each herbicide are due to each molecule in itself.

Fig. 2. Effects of chemicals on the growth of ECHCG seedlings on agar medium. The error bars represent the standard error of each data point.

2. Efficacy of pyroxasulfone applied preemergence and postemergence to Echinochloa crus-galli (Experiment 2)

The estimated ED₉₀ of pyroxasulfone and S-metolachlor in preemergence application in a loam soil were 26.7 g a.i./ha and 127.5 g a.i./ha, respectively. In postemergence application, the values were 174.5 g a.i./ha and 1144 g a.i./ha for pyroxasulfone and S-metolachlor, respectively (Fig. 3). The ED₉₀ of pyroxasulfone was one fifth of that of S-metolachlor at preemergence application in Experiment 2. Provably, differences between the herbicidal activities of Experiment 1 and 2 are affected by the soil adsorptions, water solubility, and distributions in the soil layer of each herbicide.

Fig. 3. Dose–response curves of pyroxasulfone and S-metolachlor applied preemergence and postemergence to ECHCG in a loam soil. The error bars represent the standard error of each data point.

3. Efficacy of pyroxasulfone applied preemergence to upland weeds (Experiment 3)

Pyroxasulfone provided more than 90% control of seven grass weed species at 16 g a.i./ha and of Urochloa platyphylla (BRAPP), Sorghum halepense (SORHA), Eriochloa villosa (ERBVI) and AMARE at 32 g a.i./ha. CYPES and other broadleaf weeds were controlled at 63 or 125 g a.i./ha (Table 1). Pyroxasulfone was more effective on grasses than on broadleaf weeds, and AMARE was more effective among the broadleaf weeds tested.

Table 1. Herbicidal efficacy of pyroxasulfone against upland weeds using a preemergence application

Weeds		Pyroxasulfone (g a.i./ha)
		8	16	32	63	125
Echinochloa crus-galli	ECHCG	87.5	96.5	99	100	100
Setaria viridis	SETVI	88.5	94	99	100	100
Setaria faberi	SETFA	85	90	98	100	100
Setaria pumila	SETLU	87.5	98	100	100	100
Urochloa platyphylla	BRAPP	80	85	98	99	100
Sorghum halepense	SORHA	80	85	98	100	100
Digitaria sanguinalis	DIGSA	90	97.5	100	100	100
Panicum dichotomiflorum	PANDI	90	98	100	100	100
Eleusine indica	ELEIN	90	95	100	100	100
Eriochloa villosa	ERBVI	65	84.3	97.6	96.6	100
Cyperus esculentus	CYPES	10	40	82.5	97.5	100
Abutilon theophrasti	ABUTH	10	35	77.5	87.5	98
Amaranthus retroflexus	AMARE	70	85	95	99	100
Chenopodium album	CHEAL	53	65	75	96.7	100
Solanum nigrum	SOLNI	67.5	77.5	85	90	92.5
Polygonum lapathifolium	POLLN	55	81.6	75	92.6	96.6
Ipomoea hederacea	IPOHE	30	45	53.3	63.3	91.6

Visual rating was conducted herbicide at 28 DAA.

4. Phytotoxicity of pyroxasulfone to corn and soybeans (Experiment 4)

The phytotoxicity of pyroxasulfone was compared to that of S-metolachlor, which has been used widely for corn and soybeans (Fig. 4). For corn, pyroxasulfone exhibited approximately 5 to 10% injury at 11 and 21 DAA at 250 g a.i./ha; however, corn recovered quickly from the injury after 21 DAA. Corn injury at 500 g a.i./ha of pyroxasulfone was still observed at 28 DAA; this is similar to S-metolachlor at 4280 g a.i./ha. Soybean injury due to pyroxasulfone at 11 DAA was higher than with S-metolachlor; however, soybeans recovered quickly at all use rates of pyroxasulfone. The injury symptoms of pyroxasulfone were growth inhibition and a slight twisting on corn and growth inhibition and leaf cupping on soybeans. Although temporary injury was observed, the level of injury on both crops was similar to that caused by S-metolachlor. These data indicated that pyroxasulfone has sufficient crop safety for corn and soybeans.

Fig. 4. Phytotoxicity of pyroxasulfone and S-metolachlor applied preemergence to corn and soybeans. The error bars represent the standard error for each data point.

5. Field trial (Experiment 5)

The field trial was conducted in a silty clay soil using the recommended rate of S-metolachlor at 2140 g a.i./ha. The use rate of pyroxasulfone was 250 g a.i./ha, as determined from the results of several dose-response field studies that were conducted in various locations in the U.S.A. (data not shown).⁷⁾ Pyroxasulfone exhibited excellent control of ECHCG and SETVI at the use range, and it provided better control of ABUTH and AMASS than did S-metolachlor. Neither herbicide provided adequate control of SEBEX (Table 2).

Table 2. Weed control and phytotoxicity of pyroxasulfone and S-metolachlor applied preemergence in corn (Field trial)

Chemicals	g a.i./ha	ZEAMX	ECHCG	SETVI	ABUTH	AMASS	SEBEX
pyroxasulfone	125	0 a	100 a	99.7 a	71.7 abc	91.7 a	20 bcd
	250	0 a	100 a	100 a	88.3 a	97.3 a	35 a-d
S-metolachlor	1070	0 a	100 a	99 a	41.7 c	86.7 ab	3.3 d
	2140	0 a	100 a	100 a	50 bc	98.7 a	13.3 cd

Visual assessment was conducted at 41 days after application. Means followed by the same letter are not significantly different at P=0.05.

Pyroxasulfone is a new selective herbicide used to control a wide range of weeds in corn and soybeans. Pyroxasulfone is also registered for use in wheat and cotton and has potential for use in sunflowers, peanuts, potatoes, and other crops.¹¹⁾ In conclusion, the lower application rates of pyroxasulfone, 125 to 250 g a.i./ha, provided better weed control, although it caused slight injury of corn and soybeans in container trials; its performance can be compared favorably with the current standard preemergence herbicides with the same mode of action.^12,13) In addition, pyroxasulfone provides excellent control of Amaranthus. Herbicide resistance of Amaranthus palmeri and Amaranthus rudis to glyphosate has been spreading.^4,14) Herbicide-resistant weeds are a serious impediment to crop production, and the problem continues to become more serious.³⁾ Pyroxasulfone applied preemergence has an excellent fit in herbicide-resistance management programs, while controlling a wide range of weed species, including Amarantus species. It offers residual control and is a valuable part of any weed-control program, with or without the presence of herbicide-resistance problems.

References

• 1) The National Agricultural Statistics Service, Acreage (Published June 28, 2013).
• 2) http://www.ers.usda.gov/data-products/adoption-of-genetically-engineered-crops-in-the-us.aspx (Accessed 15 Dec., 2013).
• 3) http://www.weedscience.org (Accessed 15 Dec., 2013).
• 4) S. A. Culpepper, T. L. Grey, W. K. Vencill, J. M. Kichler, T. M. Webster, S. M. Brown, A. C. York, J. W. Davis and W. W. Hanna: Weed Sci. 54, 620–626 (2006).
• 5) T. Yoshimura: Kagaku To Seibutsu 50, 552–557 (2012) (in Japanese).
• 6) Y. Tanetani, K. Kaku, K. Kawai, T. Fujioka and T. Shimizu: Pestic. Biochem. Physiol. 95, 47–55 (2009).
• 7) P. J. Porpiglia, M. Nakatani, R. Ueno and Y. Yamaji: Abstr. 46th Annu. Meeting Weed Sci. Soc. Am. 172 (2006).
• 8) M. E. Kurtz, Y. Yamaji and Y. T. Deloach: Abstr. 49th Annu. Meeting Weed Sci. Soc. Am. 457 (2009).
• 9) Y. Yamaji, H. Honda, M. Kobayashi and O. Watanabe: Abstr. 51st Annu. Meeting Weed Sci. Soc. Am. 311 (2010).
• 10) S. S. Seefeldt, J. E. Jensen and E. P. Fuerst: Weed Technol. 9, 218–227 (1995).
• 11) http://ir4.rutgers.edu/FoodUse/Food_Use1.cfm (Accessed 15 Apr., 2014).
• 12) S. Z. Knezevic, A. Datta, J. Scott and P. J. Porpiglia: Weed Technol. 23, 34–39 (2009).
• 13) P. W. Geier, P. W. Stahlman and J. C. Frihauf: Weed Technol. 20, 622–626 (2006).
• 14) T. R. Legleiter and K. W. Bradley: Weed Sci. 56, 582–587 (2008).