Relationship of soil properties to pyroxasulfone bioactivity in a range of prairie soils

ABSTRACT

The relationship between pyroxasulfone bioactivity and soil properties has not been investigated in a wide range of soils typical of western Canada. In this study, 47 soils from Saskatchewan, Manitoba and Alberta, with varying organic matter content (1.5%–22.1%), pH (5.0–7.9), and clay content (6.8%–59.4%) were used to evaluate the effect of soil properties on pyroxasulfone bioactivity and its relevance to field application rates. Bioactivity was assessed by measuring the reduction of sugar beet shoot length after 7 days in response to 0, 92, 184, and 368 µg ai kg⁻¹ pyroxasulfone concentration in soil. Multiple regression analysis showed that pyroxasulfone bioactivity was related to soil organic matter content, pH and clay content. Grouping the soils according to these properties allowed for a summarization of pyroxasulfone field application rates required to achieve bioactivity based on the magnitude of sugar beet shoot length inhibition (%). The estimated field application rates ranged from less than 120–480 g ai ha⁻¹.

KEYWORDS:

• Pyroxasulfone
• sugar beet
• bioassay
• soil properties
• bioactivity
• field application rate

Introduction

Pyroxasulfone herbicide has attracted considerable interest in Western Canada because it is regarded to be a suitable alternative to herbicides to which target weeds have developed resistance. The pyroxasulfone mode of action is via seedling shoot growth inhibition, and the primary target enzyme is the very-long-chain fatty acid elongase (Tanetani et al. 2009, 2013). Pyroxasulfone is classified as a Group 15 herbicide by the Weed Science Society of America and as Group K3 herbicide by the Herbicide Resistance Action Committee. Pyroxasulfone is typically applied pre-emergence to the soil for control of annual grasses and small-seeded broadleaf weeds, and can be used alone or in mixtures with other herbicides.

Walsh et al. (2011) reported that pyroxasulfone at 100 g ai ha⁻¹ provided effective weed control of both resistant and susceptible rigid ryegrass (Lolium rigidum Gaud.) populations in Australia with little or no effect on the growth and survival of wheat (Triticum aestivum L.). Studies conducted in western Canada on pyroxasulfone control of Bromus spp. in winter wheat indicated that pre-emergence applications at rates of 112–150 g ai ha⁻¹ provided excellent control of downy and Japanese brome with acceptable crop tolerance (Johnson et al. 2013). Knezevic et al. (2009) reported that pyroxasulfone at 200–300 g ai ha⁻¹ provided control of most grasses and certain broadleaf weeds on soils containing up to 3% organic matter. Because pyroxasulfone may not provide complete weed control, it is used in sequential herbicide applications or in mixtures with other herbicides. Pyroxasulfone applied alone (up to 333 g ai ha⁻¹) or in combination with sulfentrazone in sunflower (Helianthus annuus L.) improved control of many weeds and did not cause sunflower injury (Olson et al. 2011). Tidemann et al. (2014a) also reported improved control of herbicide-resistant wild oat (Avena fatua L.) and false cleaver (Galium spurium L.) in field pea (Pisum sativum L.) with a combination of soil-applied pyroxasulfone and sulfentrazone in Canadian prairie soils under field and greenhouse conditions. In glyphosate-resistant corn (Zea mays L.), pre-emergence application of pyroxasulfone alone (up to 250 g ai ha⁻¹) followed by post-emergence application of a pyroxasulfone/glyphosate mixture resulted in excellent control of herbicide-resistant sorghum (Sorghum bicolor L.) (King et al. 2007).

Pyroxasulfone field application rates depend on soil type, and are generally higher in soils with high organic matter and clay contents (Knezevic et al. 2009; Walsh et al. 2011). In Ontario, Canada where pyroxasulfone is registered for weed control in corn, the recommended field application rates are 123 g a.i. ha^-1 on coarse (sandy) soil, 166 and 208 g a.i. ha⁻¹ on medium to medium-fine soil with organic matter content ≤ 3% and > 3%, respectively, and 247 g a.i. ha⁻¹ on fine-textured (high clay content) soil (Health Canada 2012). The application rates recommended by FMC Corporation Canada (2016) for a premix containing pyroxasulfone (447 g L⁻¹) and carfentrazone ethyl (53 g L⁻¹) are 125 and 150 g ai ha⁻¹ for coarse to medium soils with 1%–4% organic matter content and for medium-fine to fine soils with 4%–7% organic matter content, respectively.

Limited information is available on the relationship of different soil properties and pyroxasulfone bioactivity in soils of the northern Great Plains and on how pyroxasulfone application rates may be adjusted in accordance with soil characteristics. Tidemann et al. (2014b) reported that in western Canadian soils, increased pyroxasulfone bioavailability was observed in locations with low organic matter content, especially under moist conditions. Laboratory studies of prairie soils showed that pyroxasulfone bioactivity was reduced in soils with high organic carbon content and low pH; however, only five soils with a relatively narrow range of soil properties were used in that study (Szmigielski et al. 2014). The objectives of this research were to establish the relationship of pyroxasulfone bioactivity to the combined effect of soil organic matter content, pH, and clay content using a large number (47) of soils, and to relate bioactivity to field application rates in Canadian prairie soils.

Materials and methods

Soils

Soil samples were collected from 47 sites in western Canada (Figure 1).

Figure 1. Forty-seven locations throughout the Canadian prairies where soil was collected.

The soils used in this study were selected to be representative of the Canadian Prairies, which encompasses the northernmost portion of the Great Plains of North America. Generally, the northern Great Plains are considered to stretch from north of 58° latitude in northern Alberta, Canada to south of 42° in central Nebraska, USA and from the Rocky Mountain foothills in the west to the Red River Valley in the east. A complete description of soil-climatic conditions in the northern Great Plains region may be found in Bullock et al (2010). The agricultural soils in the Canadian prairies were developed on glacially derived parent material, under both grassland and boreal forest vegetation prior to cultivation and onset of annual cropping about 120 years ago. The area is characterized by moisture deficit and a short growing season, which generally limits productivity.

After sampling, soils were air-dried, and ground to pass through a 2-mm sieve. Soils were analyzed for organic carbon content, pH and texture. Soil organic carbon content was measured using a LECO C632 Carbon Analyzer (LECO Corporation, St. Joseph, MI, USA; Wang & Anderson 1998), following a 6% H₂SO₃ pre-treatment to remove the inorganic C (Skjemstad & Baldock 2008). Organic matter content was calculated by multiplying organic carbon content by a factor of 1.7. Soil pH (1:2 soil:water on a weight basis; Hendershot et al. 2008) was analyzed using a Beckman 50 pH Meter (Beckman Coulter, Fullerton, CA, USA). Particle size distribution was determined using the modified pipette procedure (Indorante et al. 1990). Clays in these soils are dominantly 2:1 clay minerals, including smectites and illite. The soils used in this study covered a wide range of basic soil properties (Table 1).

Table 1. Number of soil samples and range of properties of 47 western Canadian soils used in this study.

Number of samples	Organic matter (%)	pH	Clay (%)	Sand (%)
13	1.5–3	5.5–7.9	6.8–55.3	0.0–60.9
16	3–5	5.0–7.7	14.4–59.4	4.3–52.4
9	5–7	6.0–7.8	16.1–50.1	15.7–62.7
9	7–22	6.4–7.6	15.4–48.6	21.0–68.7

Addition of pyroxasulfone to soil and bioassay

A commercial formulation KIH-485 (85% pyroxasulfone) was obtained from FMC Corporation (Saskatoon, SK, Canada). The stock solution containing pyroxasulfone at a concentration of 92 mg ai L⁻¹ was prepared by dissolving 0.108 g KIH-485 in 1 L of water:acetone mixture (4:1 v/v). This solution was further diluted with water to a concentration of 9.2 mg ai L⁻¹. Addition of 0.5, 1, and 2 mL of this solution to 50 g of soil yielded pyroxasulfone concentrations in soil of 92, 184, and 368 µg ai kg⁻¹. These concentrations are equivalent to approximately 120, 240, and 480 g ai ha⁻¹ field application rates if the herbicide applied is confined to the 0–10-cm soil depth and the soil bulk density is approximately 1.3 g cm⁻³ (Eliason et al. 2004). Rates of pyroxasulfone for application in Saskatchewan to wheat, corn and soybean range from 125 g ai ha⁻¹ in coarse to medium textured soils to 153 g ai ha⁻¹ in medium – fine to fine textured soils, with pyroxasulfone not recommended for use on peat or muck soils and soils with 7% or more organic matter.

Pyroxasulfone bioactivity in soil was measured using sugar beet (Beta vulgaris L. ‘Beta 1385’) as a bioindicator plant (Szmigielski et al. 2014). Soil with added pyroxasulfone solution and water (herbicide-treated soil) or with water only (non-treated soil) was hand-mixed in a small plastic tray, transferred to a 2-oz WhirlPack® bag (VWR International, Mississauga, ON, Canada), gently packed to form a layer approximately 8 cm high, 6 cm long and 1 cm wide, and six sugar beet seeds were planted. Plants were grown for 7 days in the laboratory under fluorescent light with photon flux density of 16 µmol m⁻²s⁻¹, and were watered every day. After a 7-day growth period, intact plants were removed from soil after opening the bag and adding a small amount of water to soften the soil. In soils with organic matter content less than 7%, shoot length of sugar beet was measured in response to 0, 92, 184 µg ai kg⁻¹, while in soils with organic matter content greater than 7%, it was measured in response to 0, 184 and 368 µg ai kg⁻¹. These rates were selected based on our previous study of pyroxasulfone bioactivity in Canadian prairie soils (Szmigielski et al. 2014). The shoot length inhibition (%) was calculated using the formula:where L_t and L₀ is the shoot length measured in pyroxasulfone-treated and non-treated soil, respectively. Higher values of shoot length inhibition (%) indicate greater bioactivity of a herbicide in soil. Each measurement was replicated three times.

Statistical analyses

Multiple regression analysis was performed for the shoot length inhibition (%) measured at pyroxasulfone concentration of 184 g ai kg⁻¹ as a dependent variable, and organic matter content, pH, clay and sand contents as independent variables by SigmaPlot version 11.0 (2008, Systat Software, Inc., Chicago, IL, USA).

Results and discussion

Pyroxasulfone bioactivity

Stepwise multiple regression analysis showed that pyroxasulfone bioactivity in soil was related to the organic matter content, pH and clay content, and that sand content was insignificant in predicting pyroxasulfone bioactivity (Table 2). Pyroxasulfone was less bioavailable in soils having high organic matter content, because of the enhanced pyroxasulfone adsorption to the high molecular weight organic compounds formed in soil through decomposition of organic material. Pyroxasulfone bioactivity increased with increasing soil pH and was most likely associated with the shift in pH-dependent charge on organic matter and clay surfaces from positive to negative, and not with dissociation of pyroxasulfone as pyroxasulfone does not have an ionizable hydrogen. High clay content lowered pyroxasulfone bioactivity through binding of pyroxasulfone molecules to the surface of clay particles that characteristically have a large surface area in relation to their weight. Somewhat different results were reported by Westra (2012) who examined pyroxasulfone adsorption in 25 U.S. soils and concluded that sorption coefficients were significantly related to organic matter content only. Typically both chemical properties and texture influence herbicide binding to soil (Grey et al. 1997; Walsh et al. 2011; Szmigielski et al. 2014); however the nature of the herbicide-soil interactions may be difficult to reveal and may vary with the origin and range of soils used.

Table 2. Stepwise multiple regression analysis for sugar beet shoot length inhibition (%) in response to 184 g ai kg⁻¹ pyroxasulfone in soil and selected soil characteristics.

Step	Regression coefficients					R^2	Standard error
	Constant of regression	Organic matter (%)	pH	Clay (%)	Sand (%)
1	33.77	−2.14***	–	–	–	0.398	11.95
2	−6.77	−2.30***	6.05**	–	–	0.492	11.10
3	−11.98	−2.26***	8.12***	−0.40**	–	0.577	10.25
4	−14.58	−2.22***	8.20***	−0.36	0.02	0.577	10.37

**, ***significance at 1% and 0.1% level of probability, respectively.

Relation to field application rates required for bioactivity

To examine and simplify the depiction of patterns in pyroxasulfone bioactivity as related to the selected soil properties, soils were grouped based on organic matter content using the following criteria: less than 3%, 3%–5%, 5%–7%, and larger than 7%. Then within each organic matter group, soils were sorted into two groups with pH greater than 7 and pH less than 7. Next, the soils were arranged into further subgroups: coarse (sand content greater than 50%) and medium to fine (sand content less than 50%). For each soil subgroup, the average shoot length inhibition (%) of sugar beet at 92, 184, and 368 g ai kg⁻¹ pyroxasulfone was calculated (Table 3).

Table 3. Average shoot length inhibition (mean ± SE) of sugar beet for each soil subgrouping in response to varied pyroxasulfone concentration as evaluated by a 7-day laboratory bioassay in 47 western Canadian soils.

Organic matter (%)	pH	Texture	Number of samples	Shoot length inhibition (%)
				92 g ai kg^−1 (120 g ai ha^−1)	184 g ai kg^−1 (240 g ai ha^−1)	368 g ai kg^−1 (480 g ai ha^−1)
1.5–3	Above 7	Coarse sand	4	33 ± 2	47 ± 3	–
		Medium - fine	3	18 ± 4	33 ± 3	–
	Below 7	Coarse sand	5	15 ± 5	29 ± 4	–
		Medium - fine	1	7 ± n/a^a	18 ± n/a	–
3–5	Above 7	Coarse sand	2	13 ± 4	32 ± 7	–
		Medium - fine	8	17 ± 4	32 ± 5
	Below 7	Coarse sand	4	8 ± 2	17 ± 2	–
		Medium - fine	2	6 ± 3	19 ± 7	–
5–7	All	Medium - fine	9	2 ± 1	15 ± 2	–
7–22	All	Medium - fine	9	–	2 ± 2	13 ± 3

^anot available

The magnitude of sugar beet shoot length inhibition (%) after 7 days of growth could be used for estimation of pyroxasulfone field application rates required for bioactivity in different soil types. Pyroxasulfone field application rate of approximately 120 g ai ha⁻¹ has been reported to be effective in weed control in soils with less than 3% organic matter content, neutral to acidic pH, and sandy to medium texture (Walsh et al. 2011; Johnson et al. 2013). In this study, for similar soil types (soil subgrouping with below 3% organic matter content, pH below 7, and coarse texture), pyroxasulfone concentration of 92 g ai kg⁻¹ resulted in 15% ± 5% shoot length inhibition (%) of sugar beet (Table 3). It could therefore be approximated that pyroxasulfone concentration causing ca. 15% ± 5% shoot length inhibition (%) of sugar beet in a 7-day laboratory bioassay would be effective in weed control. Using this approximation, respective pyroxasulfone field application rates were estimated for each soil subgrouping (Table 4). The proposed field application rates ranged from less than 120 g ai ha⁻¹ for soils with 1.5%–3% organic matter content, pH above 7 and coarse texture, to 480 g ai ha⁻¹ for soils with 7%–22% organic matter content, all pHs and medium to fine texture. Additional field experiments may be required to confirm that the estimated pyroxasulfone field application rates are suitable for weed control and crop tolerance in soils of varying characteristics.

Table 4. Estimated pyroxasulfone field application rates.

Organic matter (%)	pH	Texture	Field application rate (g a.i. ha^−1)
		Coarse sand	<120
1.5–3	Above 7	Medium - fine	120
		Coarse sand	120
	Below 7	Medium - fine	240
		Coarse sand	120
3–5	Above 7	Medium - fine	120
		Coarse sand	240
	Below 7	Medium - fine	240
5–7	All	Medium - fine	240
7–22	All	Medium - fine	480

In conclusion, pyroxasulfone bioactivity in soil and consequently its field application rates were shown to be strongly related to organic matter content, pH, and clay content. Therefore rates of application should be adjusted for these properties in western Canadian soils as they can vary greatly from region to region, field to field, and even within individual fields. Knowledge of basic soil properties i.e. organic matter content, pH and clay content is important, as pyroxasulfone will be less effective in weed control in soils of high organic matter and clay content and low soil pH, such as may be found in depressions and potholes in prairie soil landscapes. Further field trials conducted in western Canada will aid in validating the proposed pyroxasulfone field application rates for both the efficacy in weed control and crop tolerance.

Acknowledgements

Assistance from FMC Corporation Canada is gratefully acknowledged.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes on contributors

Anna M. Szmigielski is a research scientist in the Soil Science Department at the University of Saskatchewan. Her research interests include development of plant bioassays for detection of herbicides in soil, and evaluation of herbicide bioactivity and persistence as related to soil properties.

Ryan D. Hangs is a post doctoral fellow in the Department of Soil Science, University of Saskatchewan. His research interests are bioenergy crop production, nutrient cycling and availability, and soil management.

Jeff Schoenau is a professor of soil science in the Soil Science Department, University of Saskatchewan and holds the Saskatchewan Ministry of Agriculture chair in Soil Nutrient Management. Dr. Schoenau has nearly 30 years’ experience conducting research and teaching in soil fertility, agronomy, nutrient cycling, and soil management in prairie farming systems.