Introduction

Peanut (Arachis hypogaea L.) production has challenges with weed management. First, most peanut cultivars grown in the U.S. require a fairly long growing season of 140 to 160 d depending on cultivar and geographical region (Grichar et al., 2022; Branch et al., 2021; Lamb et al., 2017). In turn, soil-applied at-plant herbicides do not provide season-long control which may result in mid- to late-season weed problems. Secondly, peanut has a prostrate growth habit with a relatively shallow canopy, and is slow to shade row middles allowing weeds to be more competitive (Wilcut et al., 1995; Walker et al., 1989). Thirdly, peanut fruit develops underground on pegs that originate from stems and grow along the soil surface, and the prostrate growth habit and pattern of fruit development limits cultivation to an early season control option (Wilcut et al., 1995; Brecke and Colvin, 1991).

Pyroxasulfone is a Group 15 herbicide labeled for use in peanut in the U.S. since 2017 (Anonymous, 2017) and is a very long-chain fatty-acid biosynthesis inhibitor, similar to chloroacetamide, oxyacetamide, and tetrazolinone herbicides (Curran and Lingenfelter, 2016; Tanetani et al., 2009). Pyroxasulfone has a low water solubility of 3.49 mg/L at 20 C, and there is a strong correlation between soil binding, reduced herbicide dissipation, and increased soil organic matter content (Westra et al., 2014). Although pyroxasulfone has a similar weed control spectrum as S-metolachlor and dimethenamid-P, it has a higher specific activity allowing for use rates approximately eight times lower than dimethenamid-P (Curran and Lingenfelter, 2016).

Application timing is crop specific and pyroxasulfone can be applied from preplant through postemergence in corn (Zea mays L.), cotton (Gossypium hirsutum L.), peanut, soybean (Glycine max L.), and wheat (Triticum aestivum L.) (Cahoon et al., 2015; Hardwick, 2013; King and Garcia, 2008; Knezevic et al., 2009; Mangin et al., 2017; Tanetani et al., 2009; Tanetani et al., 2011). In peanut, pyroxasulfone may be applied from ground cracking (CRACK) through beginning of pod development stage (Anonymous, 2017). It provides good to excellent control of many weeds in peanut including Amaranthus spp., Lolium spp, Urochloa spp., goosegrass (Eleusine indica L.), crowfootgrass (Dactyloctenium aegyptium L.), and Digitaria spp. (Cahoon et al., 2012; Eure, 2013; Koger et al., 2008; Nurse et al., 2011; Odero and Wright, 2013).

Research has not reported any injury from pyroxasulfone in corn (Mueller and Steckel, 2011). Also, Sikkema et al. (2008) reported that pyroxasulfone was safe on several sweet corn hybrids. In other crops, pyroxasulfone was reported to injure pinto and small red Mexican beans (Phaseolus vulgaris L.) when applied preplant incorporated (PPI) (Soltani et al., 2008). Peanut has a good crop tolerance to pyroxasulfone; however, pyroxasulfone applied preemergence (PRE) has been documented to cause early-season stunting without any yield loss (Eure et al., 2015).

Studies in other crops have reported some yield reductions when using pyroxasulfone and results can vary by crop (Boydston et al., 2012; Hulting et al., 2012; Soltani et al., 2012; Mahoney et al., 2014; McNaughton et al., 2014; Tidemann et al., 2014). Winter wheat showed minimal injury or yield reductions at doses up to 0.15 kg/ha (Hulting et al., 2012). Potato (Solanum tuberosum L.) also exhibited tolerance to pyroxasulfone at rates up to 0.15 kg/ha with minor yield reduction and quality losses (Boydston et al., 2012). Pyroxasulfone at 0.125 kg/ha caused unacceptable yield losses in barley (Hordeum vulgare L.) as well as durum wheat (Triticum durum L.) and oats (Avena sativa L.) (Soltani et al., 2012). Sunflower (Helianthus annus L.) has shown acceptable tolerance to pyroxasulfone up to 0.33 kg/ha although injury (but not yield loss) did occur at locations with heavy precipitation events shortly after application (Olsen et al., 2011).

To further document peanut variety tolerance to pyroxasulfone, field studies were conducted at two rates (0.09 and 0.12 kg/ha) and three application timings, peanut ground cracking (CRACK), early postemergence (EPOST), and mid-postemergence (MPOST), in the southwestern Oklahoma and south and southern High Plains of Texas peanut growing regions.

Materials and Methods

Peanut tolerance studies were conducted during the 2019 and 2020 growing seasons at the Oklahoma State University Caddo Research Station near Ft. Cobb, a commercial field near Seminole, TX and the Texas A&M AgriLife Research site near Yoakum. Soils at Ft. Cobb were a Cobb fine sandy loam (fine-loamy, mixed, active, thermic Typic Haplustalfs) with less than 1% organic matter and a pH 7.3. Soils at the Seminole location were a Patricia loamy fine sand (fine-loamy, mixed, superactive, thermic Aridic Paleustalfs) with 1.4% organic matter and a pH 7.9. Soils at Yoakum were a Denhawken sandy clay loam (fine, smectitic, hyperthermic, Vertic Haplustepts) with less than 1.0 % organic matter and pH 7.6.

Treatments consisted of a factorial arrangement of two pyroxasulfone rates (0.09 or 0.012 kg ai/ha) and three application timings, CRACK, EPOST, or MPOST. The CRACK applications were applied 8 to 15 days after planting (DAP), EPOST applications were applied 17 to 30 DAP, and MPOST applications were applied 34 to 58 DAP, depending on location. This is within the application timing suggested on the pyroxasulfone label for it use on peanut (Anonymous, 2017). Herbicides were applied using water as a carrier with a CO₂-pressurized backpack sprayer. An untreated weed-free check was included in each study and each treatment was replicated three to four times depending on location. Other specifics of each study can be seen in Table 1.

Table 1

Variables associated with the pyroxasulfone study in Texas and Oklahoma.^a

Peanut cultivars evaluated were those grown in each production area. In south Texas and the Texas High Plains, the runner type peanut, ‘Georgia-09B’ was planted (Branch, 2010) while in Oklahoma the Virginia type peanut, ‘Wynne’ (Roberson, 2013) was grown. Georgia-09B has been grown extensively in Texas for a number of years and is a high-oleic and low-linoleic fatty acid composition with partial resistance to the Tomato Spotted Wilt (TSW) caused by the Tomato spotted wilt virus (Anonymous, 2024). Wynne yields on the average 68% jumbo and 21% fancy and also has a high oleic fatty acid composition with partial resistance to early leaf spot caused by Passalora arachidicola (syn. Cercospora arachidicola Hori.), Cylindrocladium black rot (CBR) caused by Cylindocladium crotalaria (Loos) Bell & Sobers, Sclerotinia blight caused by Sclerotinia minor Jagger, and TSWV (Anonymous, 2016).

Each plot consisted of two rows spaced 91 cm apart and 7.6 m long at Ft. Cobb, four rows spaced 102 cm apart and 9.1 m long at the Seminole location, and 97 cm apart and 7.6 m long at Yoakum. At Seminole only the center two rows were treated. Traditional production practices as recommended by the Texas A&M AgriLife Extension Service (Baughman et al., 2012) and the Oklahoma Cooperative Extension Service (Godsey et al., 2011) were used to maximize peanut growth, development, and yield at all locations. Plots were maintained weed-free with the use of POST herbicides such as clethodim or 2,4-DB and/or hand-weeding.

At Yoakum, lateral hand moved irrigation lines were used while at the Lubbock and Ft. Cobb locations, a center pivot irrigation system was used. Irrigation was applied throughout the growing season as needed.

Peanut stunting was based on visual subjective estimates using a scale of 0 to 100 (0 = no peanut stunting, 100 = peanut death) (Frans, et al., 1986). Peanut yield was determined by digging the pods based on maturity of the untreated check plots, air-drying in the field for 6 to 10 d, and harvesting with a 2-row combine. Yield samples were cleaned and adjusted to 10% moisture. Pod, shell, and peanut kernel weight were determined from each sample. Grades [percent sound mature kernels (SMK) plus sound splits (SS)] were determined for a 200-g pod sample from each plot following procedures described by the Federal-State Inspection Service (Anonymous, 2019). Grade data were collected both years at Yoakum and in 2019 at Ft. Cobb.

Since arscine transformation did not affect interpretation of the data, original means are presented. Data were subjected to ANOVA and analyzed using the SAS PROC MIXED procedure 23, version 9.2 (SAS 9, 2019). Treatment means were separated using Fisher’s Protected LSD at P < 0.05.

Results and Discussion

Peanut injury (stunting) was estimated visually throughout the growing season at all locations; however, only the 28 to 32 days after planting (DAP) evaluations are presented since injury symptoms were most apparent during this time-frame (Table 2).

Table 2

Early-season stunting 28 to 32 days after pyroxasulfone application in Oklahoma.^a

Stunting

Stunting was not observed in either year at the Yoakum or Seminole locations. The south Texas (Yoakum) results are similar to that seen in a previous study with no stunting due to pyroxasulfone (Grichar et al., 2019). In the High Plains area, pyroxasulfone has resulted in some peanut stunting but was not consistent over years (Dotray et al., 2018).

At Ft. Cobb, there was a pyroxasulfone rate by application timing interaction. When evaluated 28 to 32 days after pyroxasulfone application, stunting was seen with pyroxasulfone at 0.09 kg ai/ha in both years (Table 2). In 2019, pyroxasulfone at 0.09 kg/ha applied CRACK resulted in 3% stunting; however, no stunting was noted with any other application timing or rate. In 2020, stunting was noted with pyroxasulfone at the 0.09 kg/ha rate regardless of application timing. Pyroxasulfone at 0.12 kg/ha applied CRACK or EPOST resulted in stunting; however, the MPOST application resulted in no stunting. When evaluated late-season prior to harvest, stunting was < 1% with pyroxasulfone at either rate or application timing (data not shown). In previous work with pyroxasulfone in Oklahoma, peanut stunting was observed with PPI and PRE treatments with injury ranging from 4 to 13% (Baughman et al., 2018).

In Georgia, Eure et al. (2015) reported peanut stunting during the two test years with pyroxasulfone ranged from 3 to 11% in one year and 38 to 55% in another, depending on peanut cultivar. They reported several factors played a role in the differences observed between the two years. More rainfall occurred through the EPOST application in the year with greater injury than in the year with lesser injury (50.8 mm vs. 25.4 mm). At Ft. Cobb in 2019, high rainfall amounts were noted in May (Table 3) which may have been a factor with pyroxasulfone stunting at 0.09 kg/ha. However, in 2020 rainfall totals were not extremely high (Table 3) and therefore would not appear to be a factor in the peanut stunting with pyroxasulfone. In contrast, the Yoakum location had high rainfall amounts in May of both 2019 and 2020 (Table 3) and this did not affect peanut growth after the application of pyroxasulfone (data not shown).

Table 3

Monthly rainfall at Ft. Cobb, Seminole, and Yoakum during the 2019 and 2020 growing seasons.

Enhanced peanut stunting has been observed following the application of PRE herbicides under cool, wet conditions (Grichar et al., 2004). In previous research, Prostko (2013) documented transient peanut stunting at one of two locations following pyroxasulfone applied PRE.

Research in other crops has shown greater crop injury from pyroxasulfone applied PRE on coarse-textured soils than on fine-textured or organic soils (Cahoon et al., 2012; Eure, 2013; Koger et al., 2008; Nurse et al., 2011; Odero and Wright, 2013). Sweet corn injury has been documented to be greater than 10% following pyroxasulfone at 0.25 kg/ha on soils with 82% sand (Nurse et al., 2011) while no injury has been observed on soils high in organic matter (Odero and Wright, 2013). In cotton, Koger et al. (2008) reported only transient injury on a silt loam soil following pyroxasulfone applied PRE while Cahoon et. al. (2015) observed injury and stand reductions on loamy sand and sandy loam soils.

Yield

The effect of pyroxasulfone rate on peanut yield was only seen at the Seminole location in 2019 (Table 4). In that year, the untreated check resulted in an 11% yield increase over pyroxasulfone at 0.09 kg/ha while there was not a difference in yield between the untreated check and pyroxasulfone at 0.12 kg/ha. The rainfall pattern in 2019 at the Seminole location indicates that May rainfall was actually lower (21.8 mm) while June (63.2 mm) and July (57.2 mm) rainfall was slightly above the 23-year average. Averages for May are 45.7 mm while June and July averages are 45.7 and 43.2 mm, respectively (Anonymous, 2025). Therefore, the different observed levels of moisture did not appear to be a factor in the yield differences.

Table 4

Peanut response to pyroxasulfone rate in Texas and Oklahoma.^a,b,c

The effect of pyroxasulfone application timing on yield was only seen at the Yoakum location in 2019 (Table 5). Pyroxasulfone applied at CRACK produced a 21% higher yield than pyroxasulfone applied MPOST. No other application timing differences were noted. No explanation for the higher yield with the CRACK application or the lower yield with the MPOST application can be provided.

Table 5

Peanut response to pyroxasulfone application timing in Texas and Oklahoma.^a,b,c,d

In a previous two-year herbicide efficacy study in Oklahoma, in one year there was no difference in yield between the untreated check and any pyroxasulfone treatment and the authors attributed this to the lack of adequate U. texana control with any herbicide treatment (Baughman et al., 2018). High populations of U. texana in peanut can reduce yield due to competition for nutrients, moisture, and sunlight (Wilcut et al., 1995; Brecke and Colvin, 1991). Also, U. texana can reduce peanut yield through reduced harvest efficiency. The tight fibrous root system of this weed becomes intertwined with the peanut plant, causing peanut pods to be stripped from the vine during digging (Buchanan et al., 1982; Henning et al., 1982; Wilcut et al., 1995). Peanuts that become detached from the plant remain unharvested in or on the soil (Buchanan et al., 1982). In another year, with lower U. texana populations, all herbicide systems yielded more than the untreated check. The herbicide system that included pyroxasulfone at 0.06 kg/ha applied PRE and late postemergence (LPOST) provided the greatest yield.

Conclusion

The results of these studies indicate that pyroxasulfone is safe to use on peanut in Texas and Oklahoma. Peanut injury was less than 5%, and yield was only reduced in one out of 6 site years and was not consistent over the two pyroxasulfone rates. Additional research is needed on peanut cultivars that have not been evaluated to make sure they are compatible with pyroxasulfone at the different rates and application timings.

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