Introduction

The introduction and mass adoption of glyphosate resistant crops in the late 1990's led to the reliance of glyphosate alone as a weed control method in many instances (Vencill et al., 2012; Sosnoskie and Culpepper, 2014). This reliance on glyphosate and the reduction in use of herbicides with different modes of action (MOA) have led to the evolution of herbicide resistant (HR) weed species (Vencill et al., 2012; Cahoon et al., 2015). Glyphosate resistance has now been confirmed in 17 species in the United States and one weed specie in Georgia (Heap, 2017). Herbicide resistance in Georgia has been documented for 4 other herbicide MOA's (Heap, 2017). The increasing occurrence of HR-weeds due to selection pressure, has led agri-chemical companies to develop auxin-resistant corn (Zea mays L.), cotton (Gossypium hirsutum L), and soybean [Glycine max (L.)] (USDA-APHIS, 2015). The addition of auxin herbicides into a grower's weed management program may extend the lifespan of glyphosate, glufosinate, and other critical herbicides (Behrens et al., 2007).

One concern with applying auxin herbicides in current production systems is the sensitivity that many other plant species have to these herbicides (Egan et al., 2014). For some broadleaf species, sensitivity is so great that significant damage can be seen at sub-lethal or drift rates (Egan et al., 2014; Leon et al., 2014; Mohsen-Moghadan and Doohan, 2015). In order to mitigate the potential of off-target movement of these herbicides current labels contain certain application requirements. Auxin herbicide labels denote specific environmental conditions for application, buffer zones between tolerant and susceptible crops, applicator speeds and boom height, and nozzle type. For example, the current label for 2,4-D choline + glyphosate permits 23 different nozzle types designed to produce coarser (larger) droplets, thus minimizing potential off-target movement (Anonymous, 2017). By increasing droplet size, applicators can successfully reduce the number of fine droplets that are considered driftable and minimize off-site movement (Mueller and Womac, 1997; Taylor et al., 2004).

Herbicide efficacy can be directly related to droplet size but also can differ greatly depending on herbicide and weed species being controlled (Mckinlay et al., 1974; Ramsdale and Messersmith, 2001). Generally, finer droplet nozzles are needed for use with contact herbicides, where increased coverage is required for control (Etheridge et al., 2001). Weed control programs in Georgia for agronomic crops such as corn, cotton, peanut and soybean contain both systemic and contact herbicides (Horton, 2017).

Auxin-resistant technologies will likely be widely adopted by growers. In 2016, 43% of the cotton ha in the Southeastern United States were planted to dicamba-resistant cultivars (USDA-AMS, 2016). It is anticipated that the addition of these auxin-resistant crops to production practices, and the subsequent application of auxin herbicides would mean a change to coarse-droplet producing nozzles for every herbicide application.

Peanut has long been an important rotational crop with cotton for Georgia growers due to its many benefits, such as reduced disease and insect pressure, decreased/easier to manage weed pressure, and its ability to provide nitrogen to the soil (Elkan, 1995; and Vencill et al., 2012). In 2016, 308,000 ha of peanut were planted in Georgia, comprising approximately 49% of the United States total production (USDA-NASS, 2016). The importance of having a cotton/peanut crop rotation and the knowledge that some Georgia growers do not routinely change nozzles depending upon pesticide application could lead to a reduction in weed control in peanut due to compromised spray coverage.

Weed control research has been conducted on the effect of nozzle type and droplet size on individual herbicides and herbicide tank-mixture efficacy (Etheridge et al., 2001; Creech et al., 2015; Meyer et al., 2016). However, there is limited data regarding the use of an entire (i.e. season-long) herbicide program with the nozzle types required for use when applying auxin herbicides to auxin-resistant crops. Growers do not rely solely on a single herbicide or a single herbicide application to successfully manage weeds. Therefore, the objective of this research was to evaluate the performance of complete peanut weed control programs applied using nozzles that produce coarse droplets.

Materials and Methods

In-crop study

An in-crop trial was also conducted at the Ponder Research Farm and the Attapulgus Research and Education Center (30.763629⁰N, -84.479938⁰) on a Faceville loamy sand with 84% sand, 10% clay, 6% silt, 1.6% organic matter, and pH of 6.0 during 2015 and 2016 (4 site-years). Conventional tillage practices were used and 'Georgia-06G' (Branch, 2007) peanut was planted at both locations. A vacuum planter (Monosem Precision Planters, 1001 Blake St., Edwardsville, KS 66111) was calibrated to deliver 18 peanut seed/m at a depth of 5 cm. Peanut was planted in 2 twin rows (90 cm by 22 cm spacing) at Ponder and 2 single rows (90 cm spacing) in Attapulgus. Plot size was 7.6 m by 0.9 m.

The trial was arranged in a randomized complete block design with a 4 by 3 factorial design (4 herbicide programs and 3 nozzle types) with 4 replications. The herbicide programs presented in Table 1 were applied at their specified timings. Each herbicide program was applied with each nozzle throughout the entire season (DG11002, AIXR11002, TTI11002). Treatments were applied using a CO₂-pressurized backpack sprayer calibrated to deliver 141 L/ ha at 262 kPa and 4.83 km/h_. Visual estimates of peanut crop injury were obtained 7 days after the EPOST treatment. Visual injury ratings of peanut crop injury consisted of a combination of leaf burn and stunting (0%= no crop injury; 100%= no crop present). Later crop injury ratings were not collected due to dense weed populations in the non-treated checks which prevented reliable visual evaluations. Visual estimates of weed control were collected 7, 14, and 21 days after the POST treatment. Annual grass control ratings were not taken at 21 days after treatment due to dense Palmer amaranth populations in the NTC preventing accurate visual estimates. Palmer amaranth and a non-uniform mixture of annual grasses including, Texas millet, crowfootgrass, goosegrass, and crabgrass were present in the NTC plots as described in the non-crop test. Peanuts were inverted, allowed to air dry, and harvested 4 days later using commercial equipment. Peanut yields were adjusted to 10% moisture.

Table 1.

Herbicide program, active ingredient, rate, and timings for in-crop/peanut nozzle studies, 2015-2016.

University of Georgia Extension peanut production recommendations were used and supplemental irrigation was applied to maximize peanut growth and development (Anonymous, 2017). Soil types, planting, application peanut stages of growth, weed heights, and harvest dates are presented in Table 2.

Table 2.

Soil type, planting dates, application dates, peanut stages of growth, weed heights, and harvest dates for in-crop/peanut nozzle studies in Georgia, 2015-2016^a.

Data for all parameters in both the non-crop and in-crop studies were analyzed as factorial plot designs and subjected to ANOVA using the PROC MIXED procedure in SAS (SAS Institute 107 Inc., Cary, NC 27511). Nozzle type and herbicide treatment/program were considered fixed effects and locations and replications (nested within year) were considered random effects. No treatment by location and herbicide/program by nozzle interactions were significant (P ≥ 77), therefore data were combined over locations and only treatment main effects are presented. Least square means of significant main effects were separated using pairwise t-tests (alpha=0.10).

Results and Discussion

Non-crop

When data were pooled over the four herbicide treatments, nozzle type had no effect on the control of Palmer amaranth and a non-uniform mixture of annual grasses (Table 3). Nozzle type has previously been reported to impact control of grass species, with coarse-droplet producing nozzles reducing control (Etheridge et al. 2001). However, other research has revealed that nozzle type did not play a factor in weed control (Ramsdale and Messersmith, 2001; Berger et al., 2014). Our tests differ from previous reports, in that multiple herbicide active ingredients are included in a single treatment in order to more accurately represent a typical grower program. A similar test using multiple active ingredients also indicated that nozzle type did not influence Palmer amaranth control (Meyer et al., 2016).

Table 3.

Influence of nozzle type on weed control (non-crop study) in Georgia, 2015-2016^a

At 7 and 14 DAT, Palmer amaranth control with imazapic + 2,4-DB + S-metolachlor was 35% less when compared to the other herbicide treatments (Table 4). This reduction in control can be attributed to the fact that this population of Palmer amaranth is resistant to the ALS-inhibiting herbicides. At 7 and 14 DAT, treatments with imazapic + 2,4-DB + S-metolachlor provided better control of annual grasses than paraquat, lactofen, and acifluorfen treatments. Although primarily used for nutsedge (Cyperus spp.) and broadleaf weed control in peanut, imazapic provides some level of annual grass control (Monks et al., 1996; Wilcut et al., 1999; Jordan et al., 2009). Although commercially unacceptable, lactofen based treatments provided better control of annual grasses than acifluorfen treatments.

Table 4.

Influence of herbicide treatment on weed control (non-crop study) in Georgia, 2015-2016^abc.

In-crop

Crop injury

Nozzle type had no effect on peanut injury (Table 5). In cotton, crop injury was reduced when single fan nozzles that delivered larger droplets were used (Reeves et al., 2016). When herbicide programs were pooled over nozzle type, the programs that included paraquat were more injurious than similar programs without paraquat (Table 6). Although paraquat causes stunting and necrosis, peanut tolerance has been thoroughly studied (Wilcut et al., 1991; Tubbs et al., 2010; Eure et al., 2015). Paraquat continues to be an important component of many peanut weed control programs.

Table 5.

Influence of nozzle type on peanut weed control programs and yield in Georgia, 2015-2016^a.

Table 6.

Herbicide program effects on peanut weed control and yield in Georgia, 2015-2016^ab.

Palmer amaranth

Palmer amaranth control, when averaged over all four herbicide programs, was not significantly influenced by nozzle type at any rating date (Table 5). This is similar to what was reported for the non-crop test as well as previous research suggesting that nozzle type does not influence broadleaf weed control (Etheridge et al., 2001; Berger et al., 2014). Broadleaf weed control is often less affected by nozzle type than grass control. The compact more upright structure of grasses makes them more difficult to control with a less uniform spray pattern produced by drift reducing nozzle types compared to broadleaf weeds (McKinlay et al., 1974; Etheridge et al., 2001). When Palmer amaranth control was pooled over all nozzle types, herbicide programs provided similar control at all rating dates (Table 6). The fact that a significance difference in Palmer amaranth control was observed in the non-crop test and not in the in-crop test can be attributed to the multiple herbicide modes of action that were incorporated into the in-crop programs. By using a complete herbicide program, issues with resistant species and reduced herbicide efficacy can be minimized. The sequential herbicide applications in a complete program also increase control by killing survivors of previous applications.

Annual Grass Control

Herbicides applied with the TTI11002 nozzle which produced the coarsest droplet of the nozzles evaluated, were 5 to 6% less effective at controlling annual grasses at both rating dates than the AIXR and DG nozzles (Table 5). Reduced grass control has previously been reported when using coarse droplet producing nozzles when compared to nozzles that produce smaller droplet sizes (Mckinlay et al., 1974; Etheridge et al., 2001; Meyer et al., 2016). The narrower leaf structure of grass species allows for a more difficult area for herbicide to contact, thus the more thorough coverage provided by smaller droplet sizes should provide better control. As observed in the non-crop study, herbicide programs that included imazapic resulted in greater control of annual grass than those that contained lactofen (Table 6). Although typically a broadleaf and sedge herbicide, imazapic can exhibit a range of control on small grass species. Previous research has shown upwards of 90% control on Texas panicum and crabgrass (Monks et al., 1996; Ducar et al., 2004). Lactofen is a broadleaf herbicide and does not adequately control grasses (Grichar, 1991; Minton et al., 1989).

Peanut Yield

Peanut yield was not affected by nozzle type or herbicide program (Tables 5 and 6). The reduction in grass control observed with the TTI11002 did not result in a reduction in yield. Research has shown that peanut yield loss from grasses varies with the species, density, and duration of interference (Everman et al., 2008).

In summary, growers who use coarse-droplet producing nozzles for weed control in auxin tolerant crops should not have to change nozzles for weed control in peanut when Palmer amaranth is present. In some instances, annual grass control may be slightly reduced when TTI nozzles are used. It is also important to note that these trials were conducted under irrigated conditions and results could differ in non-irrigated or dryland production systems. Additional nozzle performance data is needed for other troublesome weeds in peanut including sicklepod (Senna obtusifolia, L. Irwin & Barneby), yellow/purple nutsedge (Cyperus spp.), Florida beggarweed (Desmodium tortuosum, Sw.), smallflower morningglory (Jacquemontia tamnifolia, L. Griseb.), and annual morningglories (Ipomoea spp.). Future nozzle studies should evaluate herbicide treatments applied in lower carrier volumes, since reduced carrier volumes can negatively influence control (Etheridge et al., 2001; Sikkema et al., 2008; Berger et al., 2014 )

The technical support of Charlie Hilton, Dwayne Dales, Timothy Richards, and the Attapulgus Research and Education Center is greatly appreciated.

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