Introduction

In 2017, Georgia producers harvested 333,865 hectares of peanut (Arachis hypogaea L.). This ranks Georgia as the leading producer of peanuts in the US (USDA-NASS, 2018). Currently, runner-type peanut cultivars are the most commonly grown in Georgia (Monfort, 2017). Peanut is characterized by having prostrate growth and indeterminate flowering. This growth habit limits peanut's ability to compete with weeds on its own (Buchanan et al., 1982). There is large variation in peanut yield loss due to weed interference. There are many factors, such as herbicide use, irrigation, and tillage practices that can influence weed populations. Because of this, producers must make sound decisions about weed control during the entire growing season (Hill and Santelmann, 1969; Hauser et al., 1975).

Dinoseb (2-sec-butyl-4,6-dinitrophenol) was heavily used in POST applications until its suspension. The loss of dinoseb in 1986 caused producers in the Southeastern US to rely heavily on paraquat (1,1'-dimethyl-4,4'-bipyridinium) as their staple POST herbicide in peanut (Wilcut et al., 1989). Paraquat is used to control broadleaf and grass weed species due to its nonselective nature (Wehtje et al., 1986). Paraquat must be applied no later than 28 d after emergence (DAE) in order to avoid significant foliar damage and yield loss to peanut (Wilcut and Swann, 1990). Peanut treated with paraquat past the 28 DAE mark may be injured with less time for recovery (Johnson et al., 1993). When paraquat is applied at the correct growth stage, foliar damage does not correlate with peanut yield loss (Wehtje et al., 1992; Wilcut et al., 1989).

Additionally, paraquat is tank-mixed with herbicides of different mechanisms of action to broaden the weed control spectrum, offset injury caused by paraquat, and provide longer weed control with residual herbicides (Wilcut et al., 1995). Producers tank-mix bentazon (3-(1-methylethyl)-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide) with paraquat to reduce injury and increase the flexibility of the application window (Wehtje et al., 1992). However, little information is available on the effects of these paraquat tank-mixtures on current runner-type peanut cultivars, which have varying growth characteristics, yield potential, and disease susceptibility. The main objectives for this research were to determine and quantify the level of injury and effects on vegetation, yield, and grade from POST herbicide tank-mixtures that include paraquat on runner-type peanut cultivars.

Materials and Methods

Peanut experiments conducted during 2016 and 2017 growing seasons were at the University of Georgia (UGA) Ponder Research Farm in Ty Ty (31.510624, -83.646659) and the Southwest Research and Education Center in Plains (32.046365, -84.366465). Trials in Ty Ty were conducted on a Tifton loamy sand (fine-loamy, Kaolinitic, thermic Plinthic Kandiudult) and trials in Plains were conducted on a Greenville sandy loam (fine, Kaolinitic, thermic Rhodic Kandiudult) (USDA-NRCS, 2018). The soils at these locations represent the southeastern US peanut production area. Two separate experiments were conducted at each location, one managed with supplemental irrigation and one entirely rainfed. Irrigation applied on an as-needed basis, in compliance with the UGA Peanut Production Guide Checkbook method (Porter, 2017; Stansell and Pallas, 1985; Stansell et al., 1976).

All trial sites were prepared by disk harrowing, moldboard plowing (30 cm deep) followed by rotary tilling to form 1.8 m wide soil beds. In Plains, plot length for 2016 was 12.2 m while in 2017 plot length was 9.1 m. In Ty Ty, plot length for 2016 was 10.7 m, 2017 irrigated plot length was 10.7 m, and 2017 non-irrigated plot length was 7.6 m. Plot length was determined by field dimensions for the given year and location. Fertilizer applications were based on UGA Extension recommendations and a pre-plant soil sample (Harris, 2018). Protective fungicide programs based on the high-risk management program from the Peanut Rx were followed (Kemerait et al., 2017). Fungicides applied to peanut at the R1 growth stage (Boote, 1982), and continued on 14-d intervals. Peanuts were planted on 11 May 2016 and 30 May 2017 in Ty Ty and 16 May 2016 and 2 May 2017 in Plains using a two-row Monosem air planter to a 5 cm depth at 19 seeds/m of row (Monosem-Inc., Edwardsville, KS). Seeding rate and depth remained constant across all site-year locations.

Trials were a split-plot design arranged in a randomized complete block with four replications. The whole plots (main effect) were a nontreated control (no herbicides added), a preemergence (PRE) control (flumioxazin at 0.107 kg ai/ha, plus pendimethalin at 0.90 kg ai/ha), PRE followed by (fb) paraquat (0.21 kg ai/ha) plus nonionic surfactant (NIS at 0.25% v/v) POST, PRE fb paraquat (0.21 kg ai/ha) plus acifluorfen (0.28 kg ai/ha) plus bentazon (0.56 kg ai/ha) POST, PRE fb paraquat (0.21 kg ai/ha) plus acifluorfen (0.28 kg ai/ha) plus bentazon (0.56 kg ai/ha) plus S-metolachlor (1.47 kg ai/ha) POST, and PRE fb paraquat (0.21 kg ai/ha) plus acifluorfen (0.28 kg ai/ha) plus bentazon (0.56 kg ai/ha) plus acetochlor (1.26 kg ai/ha) POST. The PRE application was made at planting and activated with 1.3 cm of irrigation. POST herbicide treatments were applied 28 d after planting. Plots were maintained weed free by handweeding. The sub-plot effect consisted of four cultivars assigned randomly within each whole-plot. Georgia-06G (Branch, 2007), Georgia-14N (Branch and Brenneman, 2015), TUF-Runner™ '511' (Tillman and Gorbet, 2017), and FloRun™ '157' were evaluated.

Data collection included visual foliar injury ratings, visual stunting ratings, yield (kg/ha), and grade as percentage of total sound mature kernels (%TSMK). Visual foliar injury ratings (% chlorosis/necrosis) were evaluated at 3, 7, 11, and 14 d after treatment (DAT). Visual stunting (%) was measured at 3, 7, 11, and 14 DAT. The visual injury ratings (foliar injury and stunting) were ranked on a percentage scale of 0 to 100, with 100% being complete necrosis/death while 0% represented no injury.

The hull scrape method established peanut maturity (Williams and Drexler, 1981). Peanut digging and inversion occurred with a 2-row digger for Ty Ty, and a 6-row digger (Kelley Mfg. Co., Tifton, GA) in Plains. Pods were allowed to desiccate to approximately 10 to 15% moisture before harvest with a 2-row KMC harvester (Kelley Mfg. Co., Tifton, GA) in Ty Ty and a Columbo harvester (Columbo North America, Adel, GA) in Plains. Yields were then adjusted to 7% moisture for uniformity. Grade was determined by the USDA-AMS grading standards by the USDA Federal-State Inspection Service in Tifton, GA (USDA-AMS, 1997).

For data analyses, SAS 9.4 utilized the PROC MIXED function. Location and year were treated as random effects (there were no interactions), while herbicide and cultivar, as well as their interactions, were fixed effects. Similar trends allowed for data to be combined across site-year locations. Data were analyzed by analysis of variance (ANOVA) and differences between means were determined using Tukey's honestly significant difference test (𝜶=0.05) (Tukey, 1949).

Results and Discussion

Irrigated experiments

There were no significant interactions between herbicide and cultivar (Table 1). Herbicide treatment was significant for all variables except grade. For cultivars, there were differences for yield and grade.

Table 1.

Analysis of variance for the effect of paraquat tank-mixtures on irrigated and non-irrigated peanut in Georgia. Data were combined across location (Ty Ty and Plains) and year (2016 and 2017)^a.

Herbicide effect

Leaf burn (% chlorosis/necrosis) and stunting (% stunting) decreased over time across all herbicide treatments (Table 2). At 3 DAT, paraquat plus acifluorfen plus bentazon, and paraquat plus acifluorfen plus bentazon plus acetochlor, resulted in the least amount of injury when compared to the other herbicide treatments. Including acifluorfen plus bentazon in tank-mixture with paraquat significantly reduced leaf burn at 3 and 7 DAT but subsequently had no influence on leaf burn. By 14 DAT all herbicide treatments exhibited less than 3% injury. Overall, there were negligible differences in vegetative injury between acetochlor and S-metolachlor, similar to previous research (Chaudhari et al., 2018).

Table 2.

Influence of herbicide treatment on peanut foliar injury (% chlorosis/necrosis) on irrigated and non-irrigated peanut in Georgia. Data combined across location (Ty Ty and Plains) and year (2016 and 2017).

Paraquat alone caused the greatest amount of stunting injury (Table 3). The herbicide combinations that included acifluorfen plus bentazon all had less stunting than paraquat alone, for all ratings. Including bentazon in paraquat tank-mixtures is known to cause a reduction in stunting injury when compared to paraquat alone (Wehtje et al., 1992). S-metolachlor caused greater stunting at 3 and 7 DAT than acetochlor, but by 11 DAT both treatments were less than 7%. Stunting from acetochlor was similar to paraquat plus acifluorfen plus bentazon by 7 DAT.

Table 3.

Influence of herbicide treatment on peanut foliar injury (% stunting) on irrigated and non-irrigated peanut in Georgia. Data combined across location (Ty Ty and Plains) and year (2016 and 2017).

Although paraquat alone caused the greatest amount of peanut foliar leaf injury, there was no difference in yield from the nontreated control (Table 4). Paraquat plus acifluorfen plus bentazon plus acetochlor yielded greater than the nontreated control and paraquat alone. Herbicide treatment had no effect on grade (% TSMK) (Table 5).

Table 4.

Influence of herbicide treatment on yield for irrigated peanut in Georgia. Data combined across location (Ty Ty and Plains) and year (2016 and 2017).

Table 5.

Peanut yield and grade for multiple runner-type peanut cultivars under irrigated and non-irrigated conditions in Georgia. Data combined across location (Ty Ty and Plains) and year (2016 and 2017).

Cultivar response

There were no differences in leaf burn or stunting injury for any of the tested cultivars (Table 1). Georgia-06G and TUFRunner™ '511' yielded greater than Georgia-14N and FloRun™ '157' (Table 5). Similar results were noted by Branch (2017). FloRun™ '157' had the lowest grade. Similar trends in yield and grade were noted by the UGA Statewide Variety Testing irrigated trials (Gasset et al., 2017) for the 2016 growing season.

Summary and Conclusions

Leaf burn and stunting declined over time for both experiments across all herbicide treatments indicating peanuts plants ability to produce new tissue after paraquat injury. Including acifluorfen plus bentazon in the POST herbicide tank-mixture containing paraquat reduced visible injury to the peanut crop similar to previous research (Grey et al., 1995; Wehtje et al., 1992). Grade variation was due to cultivar differences for both the irrigated and non-irrigated studies. Similar variation in grade was noted previously by Wright et al. (1986) using different runner-type peanut cultivars. For the non-irrigated study, while herbicide injury did occur, herbicide treatment had no overall effect on peanut yield. Paraquat injury has been shown to have negligible effects on peanut yield (Carley et al., 2009; Johnson et al., 1993) previously as well.

With the given supporting data, the use of the herbicide tank-mixtures from these experiments can be recommended with the given runner-type peanut cultivars without fear of negative yield impact for irrigated and non-irrigated peanut. Future research is warranted to determine the effects of these herbicide tank-mixtures on different market-type peanut cultivars with other growth characteristics.

This research was supported by the National Peanut Board and the Georgia Peanut Commission, and seed donations were received from the Georgia Seed Development Commission. The authors would like to thank Kristen Pegues, Hunter Bowen, Hunter Hayes, Sarah Chance, Chris Cromer, Sidney Cromer, and the staff members of the UGA Ponder farm and UGA Southwest Research and Education Center for technical assistance.

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