Sunflower crops were planted on 1 million hectares in the United States during 2008 (Anonymous 2009a). A survey of sunflower growers conducted in 1999 indicated that 95% of the hectares received a herbicide application (Anonymous 2009b); 80% of this hectarage received PRE applications of ethalfluralin, pendimethalin, or trifluralin to control annual grasses and a limited number of small-seeded broadleaf weeds (Anonymous 2009b).

Competition from small-seeded broadleaf weeds such as kochia [Kochia scoparia (L.) Schrad.] or from grasses such as large crabgrass can cause significant yield loss to sunflower. Durgan et al. (1990) found that kochia decreased sunflower yield by 47% when 6 plants m⁻¹ of row emerged within 1 wk after sunflower emergence, whereas Johnson (1971) found that a combination of large crabgrass and goosegrass [Eleusine indica (L.) Gaertn.], sicklepod (Cassia obtusifolia L.), tall morningglory [Ipomea purpturea (L.) Roth], ivyleaf morningglory [I. hederacea (L.) Jacq.], and redroot pigweed (Amaranthus retroflexus L.) decreased sunflower yield by 62% when the weeds competed with sunflower for the entire growing season.

Pyroxasulfone is a herbicide developed by Kumiai America. The mode of action of this herbicide is seedling growth inhibition. University faculty from many states have evaluated this herbicide in a variety of crops and found that it can provide excellent weed control. For example, King and Garcia (2008) reported that pyroxasulfone provided 88% or better control of kochia and velvetleaf (Abutilon theophrasti Medik.) 4 mo after planting, which was greater than metolachlor in furrow-irrigated corn (Zea mays L.). In glyphosate-resistant corn, pyroxasulfone applied with glyphosate controlled shattercane [Sorghum bicolor (L.) Moench ssp. arundinaceum (Desv.) de Wet & Harlan] 96%, but the combination of glyphosate with metolachlor, acetochlor, or pendimethalin only provided 88% or less control (King et al. 2007).

Other researchers have studied the application rate of pyroxasulfone. Knezevic et al. (2009) indicated that the proposed label rate of pyroxasulfone of 200 to 300 g ha⁻¹ provided excellent control of green foxtail, field sandbur (Cenchrus spinifex Cav.), and large crabgrass. Geier et al. (2006) reported that pyroxasulfone at 250 g ha⁻¹ provided 86% control of green foxtail, Palmer amaranth, and puncturevine (Tribulus terrestris L.). At 4 wk after treatment (WAT), pyroxasulfone applied at 208 g ha⁻¹ or more provided at least 90% control of Texas panicum (Panicum texanum), Palmer amaranth, and velvetleaf (Gregory et al. 2005).

Research also has been conducted on the efficacy of pyroxasulfone for controlling weeds in sunflower. Zollinger and Ries (2007) reported that pyroxasulfone provided acceptable to excellent control of kochia, redroot pigweed, common ragweed (Ambrosia artemisiifolia L.), and green and yellow foxtail [Setaria pumila (Poir.) Roemer & J.A. Schultes] at three locations in North Dakota. These results suggest that pyroxasulfone would be an excellent addition to the list of herbicides registered for use in sunflower.

Sulfentrazone is a valuable tool for controlling weeds in sunflower, but it can injure the crop. Thompson et al. (2000) reported an average of 8% more injury from PRE applications of sulfentrazone after planting sunflower compared with sulfentrazone applied 2 wk before planting and also noted that sunflower injury was enhanced when sulfentrazone was applied in combination with pendimethalin. Wait and Johnson (2002) observed a similar increase in sunflower injury from sulfentrazone applied with pendimethalin. In their study, sulfentrazone alone injured sunflower 6% or less, whereas the combination of pendimethalin and sulfentrazone increased sunflower injury by 14%.

Pyroxasulfone has shown promise as a new herbicide to provide excellent weed control. However, researchers have not collected data on the response of sunflower to pyroxasulfone over multiple environments. In addition, the combination of sulfentrazone and pyroxasulfone could increase crop injury; research should be conducted to evaluate sunflower response to tank mixtures of pyroxasulfone with sulfentrazone. The objective of this study was to evaluate the effect of multiple rates of pyroxasulfone applied with and without sulfentrazone under various field conditions.

Materials and Methods

A multisite study was initiated in spring 2007 to evaluate the effect of pyroxasulfone on sunflower. Experiments were conducted at the Northwest Research-Extension Center at Colby, KS; Agricultural Research Center at Hays, KS; Southwest Research-Extension Center–Tribune Unit at Tribune, KS; Ashland Bottoms Research Unit at Manhattan, KS; North Central Research Extension Center at Minot, ND; Central Crops and Soils Research Station at Highmore, SD; and a farmer's field near Valley City, ND. Sunflower hybrids, planting rates and dates, soil types, soil pH, organic matter, and application information are presented in Table 1. Fertilizer was applied at each site in accordance with soil test levels and yield goals, and insecticides were used at each site as needed to prevent yield loss from insects. The standardized protocol used in 2007 was pyroxasulfone applied PRE at 167, 208, and 333 g ha⁻¹ and an untreated control. All treatments were applied to plots ranging in size from 2 to 3 m wide and 6.7 to 7.6 m long. Sunflower injury was evaluated visually on a scale of 0 (no injury) to 100 (crop death). Grain weight and moisture content were recorded and yield was calculated at all sites, except Valley City, ND. Grain yield was adjusted to a standard moisture of 10%. All locations were set up as randomized complete block designs with three or four replications.

Table 1

Description of experiment locations and operations, 2007.^a

In 2008, experiments were conducted on fields adjacent to the locations used in 2007. Descriptions of experimental locations are listed in Table 2. General production practices were similar to those used in 2007. An expanded protocol, examining three rates of pyroxasulfone at 125, 167, and 250 g ha⁻¹ (coarse-textured soils) or 167, 208, and 333 g ha⁻¹ (medium-textured soils) applied alone and in combination with sulfentrazone at 105 and 140 g ha⁻¹, was used at each location. Treatments were applied at 120 to 187 L ha⁻¹. Plot layout, experimental design, and evaluation of sunflower injury were the similar to the procedures used in 2007. Weed control was assessed visually on a scale of 0 (no injury) to 100 (weed death). Grain weight, test weight, and moisture content were recorded at each location, except Tribune, KS, and Valley City, ND. Yield was calculated and adjusted to 10% moisture. Data were statistically analyzed with PROC GLM in SAS¹ and separated with Fisher's Protected LSD with P ≤ 0.05.

Table 2

Description of experiment locations and operations, 2008.^a

Results and Discussion

Sunflower Response

In 2007, sunflower was not injured from PRE applications of pyroxasulfone at any of the locations, except Highmore, SD (data not shown). Organic matter ranged from 1.6 to 5.3%, and soil pH ranged from 5.5 to 8.3 across locations (Table 1). At Highmore, the mid-rate of pyroxasulfone (208 g ha⁻¹) injured sunflower 5% and the high rate of pyroxasulfone (333 g ha⁻¹) injured sunflower 17%, 4 WAT. This location received 18 mm of precipitation within 1 wk of planting and pyroxasulfone application. This rainfall event might have caused an increased accumulation of pyroxasulfone around the emerging sunflower shoot, resulting in sunflower injury. Another possible cause for sunflower injury might have been the result of pyroxasulfone coming into direct contact with the emerging sunflower cotyledons. Regardless of rate or location, sunflower yield was not affected by pyroxasulfone.

In 2008, five sunflower hybrids were planted at the seven different experimental locations with soil pH ranging from 5.8 to 7.9 and organic matter ranging from 2 to 3.8% (Table 2). Sunflower injury was < 2% for all rates of pyroxasulfone across the seven locations, 4 WAT (Table 3). Sulfentrazone also did not cause significant sunflower injury, and the combination of sulfentrazone and pyroxasulfone did not increase injury.

Table 3

Sunflower injury and yield and grass weed control from pyroxasulfone and sulfentrazone alone and in various combinations in 2008, 4 WAT.^a

Sunflower yield was only lower in treatments that did not provide adequate weed control. These treatments included the lowest rate of pyroxasulfone and the untreated control (Table 3).

Broadleaf Weed Control

Prostrate pigweed control ranged between 70 and 80% with pyroxasulfone applied alone at Minot (Table 4). Prostrate pigweed control with sulfentrazone was 79 and 83% with 105 and 140 g ha⁻¹, respectively. The addition of pyroxasulfone to sulfentrazone increased prostrate pigweed control by at least 10% compared with either herbicide applied alone.

Table 4

Broadleaf weed control from pyroxasulfone and sulfentrazone alone and in various combinations in 2008, 4 WAT.^a

At Brookings, pyroxasulfone rates of 208 g ha⁻¹ or less provided 68% or less control of wild buckwheat (Table 4). However, when pyroxasulfone was applied at 208 g ha⁻¹ in combination with sulfentrazone at 105 or 140 g ha⁻¹ wild buckwheat control was 83 and 85%, respectively, a 15 to 20% increase in control over that observed from either herbicide applied alone. However, this improved control was not observed at Minot. In fact, control of wild buckwheat from sulfentrazone alone was > 90%.

Palmer amaranth is an aggressive weed that has spread throughout most sunflower production fields in the Great Plains. A herbicide that can provide a high level of control of this weed would have an advantage in the marketplace. The combination of pyroxasulfone and sulfentrazone at the lowest tank mixture rate provided complete control of Palmer amaranth at Tribune and Manhattan (Table 4). Neither herbicide applied alone at the lowest application rate provided complete control of Palmer amaranth at either location.

At Valley City, pyroxasulfone applied at either the 167 or 208 g ha⁻¹ rate with either the 105 or 140 g ha⁻¹ rate of sulfentrazone enhanced marshelder control compared with the herbicides applied alone (Table 4). No difference in control was observed between the combination of pyroxasulfone and sulfentrazone applied at 333 and 140 g ai ha⁻¹, respectively, compared with pyroxasulfone applied at the 333 g ai ha⁻¹ alone.

Pyroxasulfone and sulfentrazone applied together at the 167 and 105 g ai ha⁻¹ rate provided higher levels of broadleaf weed control than either herbicide applied alone with the exception of wild buckwheat control at Minot, ND (Table 4). However, applying these two herbicides together did not reduce control compared with applying them separately.

These results indicate that pyroxasulfone has the potential to be a valuable tool in the future for growers to control annual grass and broadleaf weeds in sunflower when the herbicide is incorporated by rainfall or irrigation. Tank mixing pyroxasulfone with sulfentrazone broadens the spectrum of weeds controlled and improves the control of certain species. Sunflower has demonstrated excellent tolerance to pyroxasulfone, and the slight occasional injury did not reduce seed yield. This study was conducted over a wide range of soil and environmental conditions, which provides further support for the idea that pyroxasulfone has good potential for use in PRE weed control in sunflower. However, additional trials are needed to determine whether mixtures of pyroxasulfone and sulfentrazone or other herbicides will consistently provide improved broad-spectrum weed control compared with available herbicide treatments.[3]