**Abstract**

Field studies were conducted for 2 years in the High Plains of Texas (34.1826o N, 101.9505o W) and in South Texas (29.1634o N, 97.0725o W) to evaluate weed control when using different adjuvants with commonly used peanut herbicides. In the High Plains, *Amaranthus palmeri* L. control with acifluorfen, imazapic, lactofen, and 2,4- DB at the 1X dose improved with the use of an adjuvant over no adjuvant. *A. palmeri* control with imazethapyr was similar to that seen with imazapic and lactofen with the exception of the 1/2X rate of imazethapyr, which showed improved control with Agridex over the use of no adjuvant or Induce in 1 year, while Induce was better than no adjuvant or Agridex in the other year. In 1 year in South Texas, *A*. *palmeri* control with imazapic at the 1X dose was ≥73% with/without an adjuvant. In another year, the 1X dose of imazapic controlled *A. palmeri* 64% without an adjuvant, while the addition of Cide Kick II resulted in 83% control. An adjuvant did not improve *A. palmeri* control with lactofen or *Cucumis melo* L. control with either imazapic or lactofen. *Urochloa texana* (Buckl.) control with clethodim at the 1X dose was not improved by the addition of an adjuvant in either year. *U. texana* control was not improved when using the 1X dose of fluazifop-P with any adjuvant.

**Keywords:** herbicides, Palmer amaranth, smell melon, Texas millet, weed control

#### **1. Introduction**

An adjuvant is described as any compound that lowers the surface tension of a liquid, thereby increasing the contact between the liquid and another substance [1]. The efficacy of postemergence (POST) herbicides is influenced by several factors including weed species [2], weed size [2, 3], environmental conditions at the time of application [4, 5], application rate [2], interactions with other agrichemicals [6, 7], and the interaction with adjuvants [3, 7–10].

Adjuvants enhance herbicide efficacy primarily through increasing herbicide absorption [9–12]. Some adjuvants alter the formulation of a herbicide so that the herbicide more completely and evenly covers the plant surfaces, thereby keeping the herbicide in contact with plant tissue rather than beading up and rolling off [13, 14]. This is accomplished by the adjuvant reducing the surface tension and contact angle of herbicide solution, thereby improving the coverage of the solution and improving the chance for the herbicide to penetrate the plant surface [15–17].

Foy and Smith [18] studied the effect of adjuvants on surface tension and herbicide efficacy and found that minimum surface tension and contact angle occurred at concentrations of 0.1–0.5% for all adjuvants tested. However, maximum herbicidal activity was observed at 1% concentration, which indicated that there were other factors increasing herbicide activity besides surface tension and contact angle. They concluded that specific interactions of herbicide-adjuvant-plant surface were a part of the total adjuvant action.

Other adjuvants increase the herbicides' penetration through the cuticular wax, cell walls, and/or stomatal openings [13, 14, 19, 20]. Crop oil concentrates and vegetable oils fall into the category of penetrants [20]. This type of adjuvant improves cuticular penetration by softening, plasticizing, or dissolving cuticular waxes and allowing herbicide movement to the more hydrophilic regions underneath [20]. Although volatile herbicides easily penetrate stomata, stomatal penetration by an aqueous solution is not possible unless the surface tension of the spray solution is reduced significantly [20]. Most adjuvants are incapable of reducing surface tension enough to allow stomatal penetration. Prior to the development of the organosilicone surfactants, stomatal infiltration of herbicides into the leaf was considered to be of minor importance [20]. In contrast to other wetting agents, the organosilicone surfactants can reduce surface tension to levels low enough to allow stomatal infiltration of aqueous spray solutions [21–23]. When stomatal penetrations occur, it is greatest in the morning when stomates are more likely to be open.

The objectives of this research were (1) to compare efficacy of several grass and broadleaf herbicides commonly used in peanut (*Arachis hypogaea* L.) when applied with different adjuvants and (2) compare the different spray adjuvants when labeled and sublethal herbicide doses are used with acifluorfen, clethodim, fluazifop-P-butyl, imazapic, imazethapyr, lactofen, and 2,4-DB on four major weeds found in Texas peanut.

## **2. Materials and methods**

#### **2.1 Field studies**

These studies were conducted during the 2011 and 2012 peanut growing seasons in the Texas High Plains near Halfway (34.1826<sup>o</sup> N, 101.9505o W) and during the 2012 and 2013 growing seasons in the south-central Texas peanut growing region near Yoakum (29.276o N, 97.123o W). Soil type at the High Plains location was a Pullman clay loam (fine, mixed, thermic Torrertic Paleustoll) with less than 1% organic matter and pH 7.7, while at the South Texas location, the soil type was a Denhawken sandy loam (fine-silty, carbonitic, hyperthermic Fluventic Ustochrepts) with less than 1.0% organic matter and pH 7.6. Studies were conducted in the same field but moved from year-to-year to different areas within those fields. Irrigation was applied as needed to maintain soil moisture and plant growth.

#### **2.2 Herbicides, doses, and application**

Postemergence herbicide treatments at the High Plains location included acifluorfen {5-[2-chloro-4-(trifluoromethyl) phenoxy]-2-nitrobenzoic acid} at 0.28 (1/2X) and 0.56 kg ai/ha (1X), imazapic {(+)-2-[4,5-dihydro-4-methyl-4- 4(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-methyl-3-pyridinecarboxylic acid} at 0.035 (1/2X) and 0.07 kg ai/ha (1X), imazethapyr {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid}

**83**

in each study.

Cide-Kick II

**Table 1.**

*Influence of Adjuvants on Efficacy of Postemergence Herbicides Commonly Used in Peanut…*

**Adjuvant Adjuvant composition Dose (%, v/v) Manufacturer**

1.0 Helena Chem. Co.

1.0 Brewer International

1.0 Aurora Cooperative

0.25 Helena Chem. Co.

1.0 Precision

Laboratories

at 0.035 (1/2X) and 0.07 kg ai/ha (1X), lactofen {2-ethoxyl-1-methyl-2-oxoethyl 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzene} at 0.11 (1/2X) and 0.22 (1X) kg ai/ha, and 2,4-DB [4-(2,4-dichlorophenoxy) butanoic acid] at 0.14 (1/2X)

Herbicides were applied in water using a CO2 pressurized backpack sprayer with TeeJet® 11002 DG (Spraying Systems Company, P.O. Box 7900, North Avenue, Wheaton, IL 60188) nozzles calibrated to deliver 190 L/ha at 180 kPa at the South Texas location and TurboTee® 110015 nozzles calibrated to deliver 94 L/ha at

207 kPa at the High Plains location. Herbicides were applied POST when *Amaranthus palmeri* L. was up to 43 cm tall, while *Cucumis melo* L. var. Dudaim Naud. vines were vining up to 38 cm in length. *Urochloa texana* (Buckl.) R. Webster was up to 46 cm

The experimental design was a randomized complete block with three replications at both locations. *A. palmeri* was evaluated at the High Plains location in a 5-(herbicide)-by-2-(dose)-by-3 (adjuvant) factorial arrangement of treatments. At the South Texas location, two separate studies were completed. In the first study, *A. palmeri* and *C. melo* were evaluated using imazapic and lactofen, while in another study, *U. texana* was evaluated using clethodim and fluazifop-P butyl for control. Both studies in South Texas were a 2-(herbicide)-by-2-(dose)-by-6-

Individual plots at the High Plains location were four rows 9.5 m long spaced 101 cm apart, and the middle two rows of each plot were sprayed, while at the South Texas location, plots were two rows 7.9 m long spaced 97 cm apart. Natural

in height at the time of herbicide application (**Table 2**).

**2.3 Experimental design, weeds, and densities**

(adjuvant) factorial arrangement of treatments.

and 0.28 (1X) kg ai/ha. An untreated check was included for comparison. In South Texas, herbicides in the broadleaf weed study included imazapic and lactofen at the previously mentioned rates, while in the annual grass study, the herbicides included clethodim {(E)-2-[1-[[(3-chloro-2-propenyl) oxy]imino]propyl]5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} at 0.05 (1/2X) and 0.1 (1X) kg ai/ha and fluazifop-P-butyl {(butyl)(R)-2-[4-[[5- (trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoate} at 0.1 (1/2X) and 0.2 (1X) kg ai/ha. Adjuvants in both studies include Agridex®, Cide-Kick II®, ETA®, Induce®, and 90–10® (**Table 1**). An untreated check was included for comparison

*DOI: http://dx.doi.org/10.5772/intechopen.82708*

Agridex Paraffin-based petroleum oil (83%) and

ETA Paraffinic petroleum oil (60%) and

Induce Alkylarylpolyoxylkane ether, free fatty acids

90–10 Alkyl, polyethoxy ethers, ethoxylated and

*Adjuvants, composition, dose, and manufacturer.*

surfactant blend (17%)

100% d'limonene and related isomers plus selected emulsifiers

ethoxylated nonionic surfactant (40%); unsulfonated oil residue (UR) value, 90% minimum

isopropyl (90%) and water and formulation aids (10%)

soybean derivatives, and antifome 90–10

*Influence of Adjuvants on Efficacy of Postemergence Herbicides Commonly Used in Peanut… DOI: http://dx.doi.org/10.5772/intechopen.82708*


#### **Table 1.**

*Legume Crops – Characterization and Breeding for Improved Food Security*

of the total adjuvant action.

weeds found in Texas peanut.

**2. Materials and methods**

region near Yoakum (29.276o

**2.2 Herbicides, doses, and application**

sons in the Texas High Plains near Halfway (34.1826<sup>o</sup>

**2.1 Field studies**

Foy and Smith [18] studied the effect of adjuvants on surface tension and herbicide efficacy and found that minimum surface tension and contact angle occurred at concentrations of 0.1–0.5% for all adjuvants tested. However, maximum herbicidal activity was observed at 1% concentration, which indicated that there were other factors increasing herbicide activity besides surface tension and contact angle. They concluded that specific interactions of herbicide-adjuvant-plant surface were a part

Other adjuvants increase the herbicides' penetration through the cuticular wax, cell walls, and/or stomatal openings [13, 14, 19, 20]. Crop oil concentrates and vegetable oils fall into the category of penetrants [20]. This type of adjuvant improves cuticular penetration by softening, plasticizing, or dissolving cuticular waxes and allowing herbicide movement to the more hydrophilic regions underneath [20]. Although volatile herbicides easily penetrate stomata, stomatal penetration by an aqueous solution is not possible unless the surface tension of the spray solution is reduced significantly [20]. Most adjuvants are incapable of reducing surface tension enough to allow stomatal penetration. Prior to the development of the organosilicone surfactants, stomatal infiltration of herbicides into the leaf was considered to be of minor importance [20]. In contrast to other wetting agents, the organosilicone surfactants can reduce surface tension to levels low enough to allow stomatal infiltration of aqueous spray solutions [21–23]. When stomatal penetrations occur,

it is greatest in the morning when stomates are more likely to be open.

The objectives of this research were (1) to compare efficacy of several grass and broadleaf herbicides commonly used in peanut (*Arachis hypogaea* L.) when applied with different adjuvants and (2) compare the different spray adjuvants when labeled and sublethal herbicide doses are used with acifluorfen, clethodim, fluazifop-P-butyl, imazapic, imazethapyr, lactofen, and 2,4-DB on four major

These studies were conducted during the 2011 and 2012 peanut growing sea-

the 2012 and 2013 growing seasons in the south-central Texas peanut growing

was a Pullman clay loam (fine, mixed, thermic Torrertic Paleustoll) with less than 1% organic matter and pH 7.7, while at the South Texas location, the soil type was a Denhawken sandy loam (fine-silty, carbonitic, hyperthermic Fluventic Ustochrepts) with less than 1.0% organic matter and pH 7.6. Studies were conducted in the same field but moved from year-to-year to different areas within those fields. Irrigation was applied as needed to maintain soil moisture and plant

Postemergence herbicide treatments at the High Plains location included acifluorfen {5-[2-chloro-4-(trifluoromethyl) phenoxy]-2-nitrobenzoic acid} at 0.28 (1/2X) and 0.56 kg ai/ha (1X), imazapic {(+)-2-[4,5-dihydro-4-methyl-4- 4(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-methyl-3-pyridinecarboxylic acid} at 0.035 (1/2X) and 0.07 kg ai/ha (1X), imazethapyr {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid}

N, 97.123o

N, 101.9505o

W). Soil type at the High Plains location

W) and during

**82**

growth.

*Adjuvants, composition, dose, and manufacturer.*

at 0.035 (1/2X) and 0.07 kg ai/ha (1X), lactofen {2-ethoxyl-1-methyl-2-oxoethyl 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzene} at 0.11 (1/2X) and 0.22 (1X) kg ai/ha, and 2,4-DB [4-(2,4-dichlorophenoxy) butanoic acid] at 0.14 (1/2X) and 0.28 (1X) kg ai/ha. An untreated check was included for comparison.

In South Texas, herbicides in the broadleaf weed study included imazapic and lactofen at the previously mentioned rates, while in the annual grass study, the herbicides included clethodim {(E)-2-[1-[[(3-chloro-2-propenyl) oxy]imino]propyl]5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} at 0.05 (1/2X) and 0.1 (1X) kg ai/ha and fluazifop-P-butyl {(butyl)(R)-2-[4-[[5- (trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoate} at 0.1 (1/2X) and 0.2 (1X) kg ai/ha. Adjuvants in both studies include Agridex®, Cide-Kick II®, ETA®, Induce®, and 90–10® (**Table 1**). An untreated check was included for comparison in each study.

Herbicides were applied in water using a CO2 pressurized backpack sprayer with TeeJet® 11002 DG (Spraying Systems Company, P.O. Box 7900, North Avenue, Wheaton, IL 60188) nozzles calibrated to deliver 190 L/ha at 180 kPa at the South Texas location and TurboTee® 110015 nozzles calibrated to deliver 94 L/ha at 207 kPa at the High Plains location. Herbicides were applied POST when *Amaranthus palmeri* L. was up to 43 cm tall, while *Cucumis melo* L. var. Dudaim Naud. vines were vining up to 38 cm in length. *Urochloa texana* (Buckl.) R. Webster was up to 46 cm in height at the time of herbicide application (**Table 2**).

#### **2.3 Experimental design, weeds, and densities**

The experimental design was a randomized complete block with three replications at both locations. *A. palmeri* was evaluated at the High Plains location in a 5-(herbicide)-by-2-(dose)-by-3 (adjuvant) factorial arrangement of treatments.

At the South Texas location, two separate studies were completed. In the first study, *A. palmeri* and *C. melo* were evaluated using imazapic and lactofen, while in another study, *U. texana* was evaluated using clethodim and fluazifop-P butyl for control. Both studies in South Texas were a 2-(herbicide)-by-2-(dose)-by-6- (adjuvant) factorial arrangement of treatments.

Individual plots at the High Plains location were four rows 9.5 m long spaced 101 cm apart, and the middle two rows of each plot were sprayed, while at the South Texas location, plots were two rows 7.9 m long spaced 97 cm apart. Natural


*a A, A. palmeri; AT, air temperature; C, C. melo; D, dry; E, excellent; G, good; RH, relative humidity; ST, soil temperature; SM, soil moisture; U, U. texana; and WS, weed size. b Only A. palmeri was present at Halfway.*

#### **Table 2.**

*Environmental conditions at time of herbicide application at each location.*

infestations of *A. palmeri* at the High Plains location were present at a population range of 6–8 plants/m2 . *A. palmeri*, *C. melo*, and *U. texana* were present in South Texas at a population density of 6–10 plants/m2 in both years.

#### **2.4 Peanuts and planting**

At the High Plains location, OLin [24] was planted in both years at the rate of 100 kg/ha. Planting date in 2011 was April 27, while in 2012, the planting date was May 1. Tamrun OL07 [25] and Georgia 09B [26] peanut were planted at the rate of 110 kg/ha in South Texas on June 14, 2012 and June 6, 2013, respectively. At neither location was peanut harvested for yield.

#### **2.5 Weed efficacy ratings and data analysis**

Weed control or peanut injury was estimated visually using a scale of 0 (no weed control or peanut injury) to 100 (complete weed control or plant death) relative to the untreated control [27]. Weed control ratings and peanut injury consisting of chlorosis and/or stunting (where applicable) were taken 2 and 4 weeks after herbicide application.

Data from the High Plains were analyzed using a five by a two-by-three factorial analysis (POST herbicide by dose by adjuvant), while the data from South Texas were analyzed using a two-by-two-by-six factorial analysis (POST herbicides by dose by adjuvant). Significant differences among treatments were determined using analysis of variance and means were separated by protected Fisher's LSD test at P < 0.05 [28]. Visual estimates of weed control and peanut injury were transformed to the arcsine square root prior to analysis of variance, but are expressed in their original form for clarity because the transformation did not alter interpretation. The untreated check was not included in the weed control or peanut injury analysis.

**85**

*Influence of Adjuvants on Efficacy of Postemergence Herbicides Commonly Used in Peanut…*

No attempt was made to consolidate data over years since there was a treatment

by year interaction and environmental conditions (relative humidity, soil temperature, and soil moisture) at time of herbicide application varied between years (**Table 2**). Also, extremely hot, dry weather conditions were observed in 2011 (data not shown). Although the test area was irrigated, the record high temperatures and low rainfall [29] made it difficult to maintain adequate soil moisture for

In 2011, only the high dose of acifluorfen and 2,4-DB showed no response to the addition of an adjuvant, while the addition of either Agridex or Induce to the low dose of acifluorfen and 2,4-DB improved *A. palmeri* control over those herbicides with no adjuvant (**Table 3**). The addition of Induce to either imazapic or imazethapyr at 0.035 kg/ha or lactofen at 0.11 kg/ha improved control over those herbicides without any adjuvant, while the addition of Agridex to the high dose of these herbicides improved control over Induce or the use of the herbicide with no adjuvant. Other research has reported that herbicide rates can be reduced up to 75% with the use of adjuvants, usually when applications are made during early growth stages [30–32]. However, successful control using reduced herbicide rates depends on weed growth stage sensitivity [33, 34] and current environmental

In 2012, the low dose of either imazapic or lactofen showed no response to *A. palmeri* control with the addition of an adjuvant, while acifluorfen, imazapic, imazethapyr, or lactofen at the high dose and 2,4-DB at both doses resulted in greater control with the addition of either Agridex or Induce over the use of no adjuvant (**Table 3**). *A. palmeri* control with acifluorfen, imazapic, or lactofen herbicides was similar with either adjuvant. Imazethapyr, at either dose, provided better control with the addition of Agridex than the addition of Induce. Since soil moisture was low in 2011 and weed size at time of herbicide application was greater in 2012 than 2011 (**Table 2**), the use of an adjuvant proved beneficial. Adjuvants have been reported to increase absorption of bentazon in *Abutilon theophrasti* Medic. [37] although plants were water-stressed [38]. Bellinder et al. [39] reported that there was no benefit in using a crop oil concentrate (COC) with either bentazon or fomesafen at the 0–2 or 2–4-leaf stage of *A. theophrasti*; however, control was

In 2012, only the addition of ETA to imazapic at the low dose improved *A. palmeri* control over the use of either imazapic or lactofen without an adjuvant (**Table 4**). In 2013, the addition of either Induce or Cide-Kick II to the low dose of imazapic or Cide-Kick II and 90–10 to the high dose of imazapic improved control over both doses of imazapic without an adjuvant. No other adjuvants improved *A*. *palmeri* control over either dose of imazapic or lactofen without

In both years, *A. palmeri* amaranth control when using lactofen with or without an adjuvant was at least 88% with the exception of the addition of ETA to the high dose of lactofen in 2012, which resulted in 78% control. Grichar and Dotray [40]

inconsistent at the 4–6-leaf stage even when a COC was used.

*DOI: http://dx.doi.org/10.5772/intechopen.82708*

**3. Results and discussion**

*3.1.1 High Plains of Texas*

plant growth.

conditions [35, 36].

*3.1.2 South Texas*

an adjuvant.

**3.1** *Amaranthus palmeri* **control**

*Influence of Adjuvants on Efficacy of Postemergence Herbicides Commonly Used in Peanut… DOI: http://dx.doi.org/10.5772/intechopen.82708*
