Introduction

Paraquat is one of the most frequently used postemergence (POST) herbicides in Southeastern US peanut (Arachis hypogaea L.) production systems but is seldom used in the Southwestern US production region, because crop injury may occur, reducing yield and grade characteristics (Knauft et al., 1990; Wilcut and Swann, 1990). The addition of bentazon to paraquat is a common practice to reduce peanut injury, although it may be either antagonistic or synergistic in its effect on weed control depending on the weed species and herbicide rate (Wehtje et al., 1992) and often does not improve peanut yield despite the reduction in crop injury (Wehtje et al., 1986, 1992; Wilcut et al., 1989). Due to the low price of paraquat, growers in the southwest continue to express an interest in its use (authors' personal observations). However, concerns have been expressed about peanut cultivar response to paraquat applications. Evaluations of cultivar response to herbicide treatments containing paraquat have been studied (Knauft et al., 1990; Wehtje et al., 1991; Wilcut and Swann, 1990). However, most of the cultivars that have been evaluated are no longer grown in the U.S.

Metolachlor is commonly used in peanut for control of small-seeded broadleaf weeds, some annual grasses, and yellow nutsedge (Grichar et al., 1996). S-metolachlor is labeled for either PPI, POST-plant incorporated, PRE, POST, or lay-by in peanut (Anonymous, 2004). Peanut cultivars commonly used in the Southwest production area have not been evaluated with respect to response to S-metolachlor, paraquat, or combinations of the two. Therefore, the objective of this research was to evaluate the effect of S-metolachlor, paraquat, or a combination of the two herbicides applied at several timings POST on peanut stunting, yield, and grade of four runner and two Virginia market-type peanut cultivars.

Materials and Methods

Peanut tolerance studies under weed-free conditions were conducted at Yoakum and near Lamesa with runner and Virginia market-types during the 2007 and 2008 growing seasons. The soils at Yoakum were a Denhawken sandy clay loam (fine, smectitic, hyperthermic, Vertic Haplustepts, 1.6% organic matter, pH 7.6) and at Lamesa the soils were a Amarillo fine sandy loam (fine-loamy, mixed, superactive, thermic Aridic Paleustalf, 0.4% organic matter, pH 7.8). Planting date, application dates, and other variables for each study are given in Table 1.

Table 1.

Planting and application dates, peanut cultivars, and time of application for various studies in south Texas and the High Plains of Texas.^a

At Yoakum, treatments consisted of a factorial arrangement of three runner market-type peanut varieties, ‘Tamrun OL02’ (Simpson et al. 2006) in both years, ‘York’ (Gorbet and Tillman, 2011) in 2007 and ‘Florida 07’ (Gorbet and Tillman, 2009) in 2008, three herbicide treatments (S-metolachlor at 1.6 kg ai/ha, paraquat at 0.14 kg ai/ha, or a combination of the two herbicides at the represented rate), and four application timings [7, 14, 21, or 28 days after peanut cracking (DAC)]. The peanut cultivar York was not used in 2008 due to lack of availability. At Lamesa, the treatments consisted of a factorial arrangement of three herbicide treatments by four application timings. The runner market-type used in each year was Flavorrunner (FL) 458 (Beasley and Baldwin, 2009) while NC-7 (Wynne et al., 1979) and Gregory (Isleib et al., 1999) were the Virginia market-types used in 2007 and 2008, respectively. An untreated check was included in each study at both locations. Each study was replicated three times. Paraquat only treatments include Induce (Helena Chemical Company, 225 Schilling Boulevard, Suite 300, Collierville, TN 38017) at 0.25% v/v. Individual plot size was 1.9 by 7.6 m at Yoakum and 2.1 by 9.5 m at Lamesa. Seasonal rainfall at Yoakum (June through Oct) was 61.8 cm in 2007 and 20.2 cm in 2008 while at Lamesa seasonal rainfall (May through Oct) was 37.3 cm in 2007 and 30.7 cm in 2008. Supplemental irrigation was supplied as needed at both locations. Traditional production practices were used to maximize peanut growth, development, and yield. All plots received a dinitroaniline herbicide applied preplant incorporated and were cultivated and hand-weeded throughout the growing season to maintain weed-free conditions. Clethodim at 0.18 kg ai/ha was applied POST to control annual grass escapes at Yoakum. No insecticides were needed at any location in any year.

Herbicides were applied using water as a carrier with a CO₂-pressurized backpack sprayer using Teejet 11002 DG flat fan nozzles ( Teejet Spraying Systems Co., P.O. Box 7900, Wheaton, IL 60188) that delivered 190 L/ha at 180 kPa at Yoakum or Teejet 80015 flat fan nozzles that delivered 140 L/ha at 207 kPa at Lamesa. Peanut stunting was evaluated approximately 60 d after peanut planting with the runner peanuts cultivars at Yoakum and Lamesa, while the Virginia cultivars were rated prior to peanut digging. Peanut stunting were based on a scale of 0 (no peanut stunting) to 100% (peanut death). Peanut yield was determined by digging the pods based on maturity of untreated control plots determined by mesocarp pod color (Williams and Drexler, 1981), air-drying in the field for 6 to 10 d, and harvesting individual plots with a combine. Yield samples were cleaned and adjusted to 10% moisture. For grades, a 200-g pod sample from each plot was obtained and grades determined following procedures described by the Federal-State Inspection Service (USDA, 1993).

Data for percentage of peanut injury and stunting were transformed to the arcsine square root prior to analysis; however, nontransformed means are presented because arscine transformation did not affect interpretation of the data. Data were subjected to ANOVA and analyzed using SAS PROC MIXED (SAS, 2002). Treatment means were separated using Fisher's Protected LSD at P ≤ 0.05. The untreated check was used for peanut yield and grade calculation comparison and a visual comparison for peanut injury and was only included in yield and grade analysis.

Results and Discussion

Yoakum

Early season injury from paraquat alone or in combination consisted of leaf bronzing as is normally seen with paraquat (Wilcut et al. 1995) (data not shown). No attempt was made to combine data over years since neither York or Florida 07 were not used in both years. Analysis of variance indicated no herbicide treatment by peanut cultivar interaction for peanut stunting; therefore, that data were combined over cultivars. There was a herbicide, application timing, and cultivar interaction for peanut yield and grade; therefore, that data were presented as an interaction of the three parameters at P≤ 0.05.

Peanut stunting

In 2007, peanut stunting was no greater than 8% with any herbicide treatment or application timing (Table 2). S-metolachlor plus paraquat caused the greatest stunting (8%) when applied 14 DAC while S-metolachlor alone caused peanut stunting (7%) when applied 7 DAC. Many growers have reported peanut stunting when soil applications of metolachlor have been followed by rain (Grichar et al., 1996). Combinations of factors, such as herbicide rate, moisture conditions at planting, soil organic matter, and pH may affect peanut injury by chloroacetamide herbicides such as S-metolachlor (Cardina and Swann, 1988; Wehtje et al., 1988; Osborne et al., 1995; Mueller et al., 1999). Cardina and Swann (1988) reported that metolachlor often delayed peanut emergence and reduced peanut growth when irrigation followed planting. However, yield loss was observed only when metolachlor was applied at a 3X rate. Metolachlor applied POST followed by irrigation within 24 h was effective for yellow nutsedge control and reduced the chance of peanut injury from preplant incorporated or preemergence applications of metolachlor (Grichar et al., 1996).

Table 2.

Peanut stunting in south Texas to S-metolachlor and paraquat alone and in combination when applied 7, 14, 21, and 28 days after ground cracking.^a,b

In 2008, S-metolachlor alone did not cause any peanut stunting at any application timing while paraquat alone caused at least 10% stunting when applied 21 and 28 DAC (Table 2). S-metolachlor plus paraquat caused at least 10% stunting with all application timings with the exception of 14 DAC timing application.

Peanut tolerance to paraquat was first noted in the late 1970's (Wilcut et al., 1995). Wehtje et al. (1986) reported that paraquat controlled Texas panicum (Panicum texanum Buckl.) when applied sequentially, and Johnson et al. (1993) noted differences in cultivar tolerance. Paraquat is often tank-mixed with bentazon and can be applied up to 28 d after peanut emergence (Senseman, 2007; Wilcut et al., 1995). Bentazon increases control of paraquat-tolerant species such as bristly starbur (Acanthospermum hispidum DC.), coffee senna (Cassia occidentalis L.), prickly sida (Sida spinosa L.), and smallflower morningglory [Jacquemontia tamnifolia (L.) Griseb.] (Wehtje et al., 1992) and reduces paraquat-induced injury to peanut (Wehtje et al., 1992). Although paraquat is a standard herbicide in southeastern U. S. peanut weed management (Wehtje et al., 1992; Grey et al., 2003), it lacks residual activity and has a narrow window of application (Wilcut et al., 1995).

Peanut yield

In 2007 with the cultivar York, only paraquat alone applied 21 DAC reduced yield when compared with the untreated control (Table 3). However, paraquat alone applied 7 DAC did produce a greater yield than the untreated check or S-metolachlor plus paraquat applied 14 or 21 DAC. None of the herbicide treatments affected Tamrun OL02 yields when compared with the untreated control. However, paraquat alone applied 28 DAC produced the greatest yield, higher than all herbicide treatments with the exception of the untreated check, S-metolachlor alone at 14 DAC, and S-metolachlor plus paraquat applied 7 and 28 DAC.

Table 3.

Peanut yield in south Texas as influenced by herbicide, application timing, and cultivar.^a,b

In 2008 with the cultivar Florida 07, no response to herbicide treatment was noted when compared with the untreated check (Table 3). S-metolachlor alone applied 7 DAC produced the greatest yield and this was greater than S-metolachlor alone applied 14 DAC, paraquat alone applied 7 and 14 DAC, or S-metolachlor plus paraquat applied 7, 21, and 28 DAC. With Tamrun OL02, no yield differences were noted between the untreated control and any herbicide treatments (Table 3). Paraquat alone applied 14 DAC produced the highest yield and this was greater than paraquat alone applied 28 DAC or S-metolachlor plus paraquat applied 28 DAC.

Other studies have not reported any yield reductions with the use of paraquat (Carley et al., 2009; Johnson et al., 1993; Wehtje et al., 1991). Brecke (1989) reported that the peanut cultivars Early Bunch and Southern Runner exhibited yield suppression from a treatment that included two applications of paraquat at 0.3 kg/ha per application. However, this is twice the normal use rate of paraquat (Wehtje et al., 1991). Wehtje et al. (1991) noted that paraquat can be used on different peanut cultivars without a detrimental cultivar response. However, they concluded that adverse environmental conditions, such as drought stress, or other growth limiting factors could result in a cultivar-by-paraquat interaction.

Peanut grade

With the cultivar York, grades were low (<70%) for all treatments (Table 4). No reduction in grade from the untreated control was noted with any herbicide treatments and this may be due to its extremely low grade for the untreated control (63.2%). However, several herbicide treatments including S-metolachlor alone applied 21 and 28 DAC, paraquat alone applied 7 and 28 DAC, and S-metolachlor plus paraquat applied 28 DAC improved peanut grade over the untreated control. Tamrun OL02 grades with any herbicide treatments were not different from the untreated control. However, S-metolachlor alone applied 28 DAC, paraquat alone applied 14 DAC, and S-metolachlor plus paraquat applied 7 DAC produced greater grades than S-metolachlor alone applied 14 DAC.

Table 4

Peanut grade in south Texas as influenced by herbicide, application timing, and cultivar.^a,b,c

With the cultivar Florida 07 in 2008, no differences from the untreated control were noted with any herbicide treatments although S-metolachlor alone applied 7, 14, and 28 DAC and S-metolachlor plus paraquat applied 28 DAC produced higher grades than S-metolachlor plus paraquat applied 21 DAC (Table 4). Previous research suggested that paraquat may affect peanut grade (especially Virginia types) by increasing the proportion of other kernels (Knauft et al., 1990), which may indicate that the herbicide affected grade by delaying maturity (Carley et al., 2009).

Lamesa

There was a herbicide treatment by timing by year interaction for peanut stunting; therefore, that data are presented as a herbicide treatment by timing interaction for each year. There was a herbicide effect for peanut yield in each year; therefore, that data are presented for herbicide for each year, while there was no effect of herbicide or timing on peanut grade (data not presented).

Peanut stunting

In 2007, S-metolachlor alone caused stunting when applied 7 DAC but no stunting was observed at any other application timing with this herbicide (Table 5). Stunting from paraquat alone was greatest when applied 14 DAC or later. The combination of S-metolachlor plus paraquat caused peanut stunting at all application timings. In 2008, no stunting was noted with any S-metolachlor alone application timing. Similar trends as seen in 2007 were noted with paraquat alone and combination treatments of S-metolachlor plus paraquat (Table 5).

Table 5.

Stunting of runner peanut in Texas High Plains to S-metolachlor and paraquat alone and in combination when applied 7, 14, 21, and 28 days after ground cracking.^a,b

Peanut yield

In 2007, the untreated control produced the lowest yield when compared with S-metolachlor while paraquat and S-metolachlor plus paraquat were intermediate in yield (Table 6). In 2008, S-metolachlor plus paraquat reduced peanut yield when compared with the untreated control and S-metolachlor and paraquat alone. No response with respect to yield was noted with application timing in either year (Table 6). The indeterminate growth habit of peanut often allows compensation from early season stress (such as herbicide injury), and if given sufficient recovery time and good growing conditions, peanut yields and grade may not be compromised (Carley et al., 2009; Mozingo et al., 1991).

Table 6.

Yield of runner peanut (Flavorrunner 458) in Texas High Plains as influenced by S-metolachlor and paraquat alone and in combination and timing of application.^a,b

Peanut grade

No peanut response to either herbicide or application timing were noted in either year (data not shown). Wehtje et al. (1991) reported no effect on peanut yield when using paraquat at 0.14 or 0.28 kg/ha; however, these treatments were applied only once, 4 d after crop emergence.

Virginia market type response

No attempt was made to combine data over years due to different cultivars being used in each year of study. For peanut stunting, there was a herbicide treatment by application timing interaction; therefore, that data are presented separately by herbicide treatment and application timing for each year. Analysis of variance indicated no significant interaction for peanut yield; therefore, that data are presented as main effects by year.

Peanut stunting

In 2007, S-metolachlor alone applied 7 DAC caused peanut stunting; however, no stunting was noted with any other application of S-metolachlor alone when rated prior to peanut digging (Table 7). Paraquat alone or the combination of S-metolachlor plus paraquat applied at 7, 14, 21, or 28 DAC resulted in stunting when compared with S-metolachlor alone applied 21 and 28 DAC. In 2008, S-metolachlor alone applied 7 or 21 DAC, paraquat alone at all application timings, and S-metolachlor plus paraquat applied 21 and 28 DAC resulted in peanut stunting when compared with S-metolachlor plus paraquat applied 7 DAC (Table 7). Carley et al. (2009) reported when using the cultivar Gregory, that in one year, paraquat alone at 0.14 kg/ha was more injurious than paraquat plus bentazon at 0.28 kg/ha applied with diclosulam at 0.027 kg/ha, dimethenamid at 0.84 kg/ha, or S-metolachlor at 1.1 kg/ha when rated 3 weeks after treatment.

Table 7.

Stunting of Virginia peanut in Texas High Plains to S-metolachlor and paraquat alone and in combination when applied 7, 14, 21, and 28 days after ground cracking.^a,b

Peanut yield

No yield differences were noted with herbicide treatment or application timing in either year (Table 8). Although Carley et al. (2009) noted early season injury to Gregory with paraquat alone or in combination with bentazon, diclosulam, dimethenamid, or S-metolachlor no yield difference were noted.

Table 8.

Virginia peanut yield as influenced by S-metolachlor and paraquat alone and in combination and timing of application.^a,b,c

Peanut grade

No grade differences were noted with herbicide treatment or application timing in either year (data not shown). Carley et al. (2009) reported that paraquat applied to Virginia peanut reduced extra-large kernels (ELK) from 45% (non-treated peanut) to 41% in one year but did not affect percentage ELK in another.

The results from these studies demonstrated although runner peanut cultivars were stunted by paraquat or S-metolachlor plus paraquat combination treatments, peanut yield and grade response was variable and no consistent trend could be determined. In laboratory studies, Wehtje et al. (1991) reported the absorption and translocation of paraquat did not vary significantly between peanut cultivars. They concluded that paraquat could be used on different peanut cultivars without a detrimental cultivar response. They also felt that adverse environmental conditions, such as drought stress, or other growth limiting factors may result in a cultivar-by paraquat interaction. With respect to Virginia market type, although peanut stunting was noted, no negative yield or grade response were noted. Under growing conditions at Lamesa where the growing season can be shortened by cold weather, the use of paraquat did not delay peanut maturity as has been noted in other peanut growing regions (Carley et al., 2008; Mozingo et al., 1991).

The Texas Peanut Producers Board provided financial support for this research. Lyndell Gilbert, Kevin Brewer, Bill Klesel, and Dwayne Drozd provided technical assistance.

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