Weed Technology 2009 23:544–549

Interaction of and Pelargonic Acid in Ready-to-Use Weed Control Products

Glenn Wehtje, James E. Altland, and Charles H. Gilliam*

Glyphosate-based, ready-to-use weed control products often contain pelargonic acid (PA) at a concentration equivalent to that of the glyphosate. It remains unclear what benefit, if any, this combination provides. Greenhouse experiments using large crabgrass, yellow nutsedge, longstalked phyllanthus, and prostrate spurge were conducted to determine whether the addition of PA improved weed control efficacy compared to glyphosate alone. Glyphosate was applied at a series of rates, ranging from 0.11 to 1.12 kg ae/ha, either alone or with an equal rate of PA. Addition of PA to glyphosate was synergistic only in longstalked phyllanthus and yellow nutsedge, and this synergism was manifested only as an increase in the amount of early (i.e., 5 to 7 d after treatment) visual injury. Conversely, longer-term control and control of regrowth was either not affected or reduced by the addition of PA. We conclude that the addition of PA to glyphosate in ready-to-use weed control products is neither warranted nor justified. However, we also note that the increase in early injury that was observed in only two of the four species evaluated could be an important attribute for the consumers for which these products are targeted. Nomenclature: Glyphosate; pelargonic acid; large crabgrass, Digitaria sanguinalis (L.) Scoop DIGSA; longstalked phyllanthus, Phyllanthus tenellus Roxb.; prostrate spurge, Chamaesyce masculata (L.) Small EPHPT; yellow nutsedge, Cyperus esculentus L. CYPES. Key words: efficacy, linear regression, nonlinear regression, synergism and antagonism.

A wide variety of ready-to-use weed control products With respect to combining PA with other , the marketed to homeowners have glyphosate as their main Herbicide Handbook states that ‘‘synergistic responses with component. Glyphosate is ideal for ready-to-use products for certain POST herbicides show enhanced speed of top kill and several reasons. Glyphosate is relatively nonselective, and improved control of numerous weed species’’ (Senseman therefore is effective against a wide diversity of weed species. 2007b). However, a search of the literature by the authors was Because of the systemic nature of glyphosate, it can kill the unable to recover any published data that directly supports roots and other tissues not directly contacted by the spray. synergism. Pline et al. (1999) reported that the translocation, Glyphosate has low mammalian toxicity and is correctly but not absorption, of glufosinate in redroot pigweed perceived as relatively safe (Senseman 2007a). An informal (Chenopodium album L.) was increased with the addition of survey of these ready-to-use weed control products revealed PA. However, nearly all published data conclude that PA that glyphosate is rarely marketed as a single-component either has no effect or is antagonistic toward other herbicides. product. Pelargonic acid (PA) is a common additive and, Chachalis and Reddy (2004) evaluated trumpetcreeper when used, is usually included in an amount equal to [Campis radican (L.) Seem. ex Bureau] control with glyphosate. Fatty acids, including PA, constitute an extensive glyphosate alone and in combination with PA at rates up to and diverse group of aliphatic carboxylic acids. Fatty acids that 3% (2.86 kg/ha). (Rates of PA in this and other referenced typically occur in nature are monocarboxylic acids, containing studies are often expressed as volume of the total spray a linear hydrocarbon chain with an even number of carbons, solution, comparable to the method in which surfactant rates generally in the range of 4 to 22 (DeMan 1990). Currier and are typically expressed. The weight-based rates as presented Peoples (1954) first reported on the phytotoxicity of selected here in parentheses are conversions made by the authors for fatty acids in 1954. Coleman and Penner (2006) evaluated the clarity based upon the product identified in the Sources of desiccant activity of nine fatty acids in which the carbon chain Materials section.) In general, control was not improved by ranged from 2 to 10 carbons in length. (eight addition of PA. Furthermore, neither glyphosate absorption carbons) and PA (nine carbons) were the most effective. PA is nor translocation was improved. In a compilation of reports a contact, nonselective, nontranslocating, broad-spectrum on herbicide tank-mixture interactions prepared by Hatzios herbicide (Senseman 2007b). It is only effective against and Penner (1985), 15 out of 16 glyphosate-containing actively growing emerged vegetation. Herbicidal activity is mixtures were deemed antagonistic. Bradley and Hagood attributed to rapid cell death resulting from a sudden decrease (2002) evaluated various herbicides and their rates for in intracellular pH which in turn causes loss of membrane mugwort (Artemisia vulgaris L.) control. It was noted that integrity and, ultimately, cell death (Currier and Peoples the addition of PA to either glyphosate, glufosinate, or the 1954; Senseman 2007b). dimethylamine salt of 2,4-D did not enhance weed control. These authors also noted that when the rates of these DOI: 10.1614/WT-08-044.1 herbicides were held constant but the rate of the added PA was * First and third authors: Professor of Agronomy and Professor of Horticulture, varied from 0.625 to 20.0% (1.06 to 33.60 kg/ha), control Auburn University, Auburn AL 36849; second author: Research Horticulturist, USDA-ARS. Applications Technology Research Unit, 208 Agricultural Engi- was reduced with the higher rates of PA, indicating neering Building, 1680 Madison Ave., Wooster, OH 44691. Corresponding antagonism. Pline et al. (2000) evaluated glufosinate and author’s E-mail: [email protected] glyphosate alone and in combination with 3% (i.e., 3.75 kg/

544 N Weed Technology 23, October–December 2009 ha) PA on various weeds and transgenic crops that are resistant previous season and stored at 3 C. Tubers (three per pot) to these two herbicides. PA was deemed either to have no obtained from a commercial source4 were used to establish effect or was antagonistic with respect to weed control. yellow nutsedge. All species were approximately 4 to 6 wk old However, the addition of PA improved yellow nutsedge at time of treatment. At this time large crabgrass, longstalked control with glufosinate at 6 d after treatment; but this benefit phyllanthus, and prostrate spurge had extensive foliar was transitory. These authors also noted that ‘‘the desiccating development; flowering and seed production was evident. action of PA in most cases did not seem to inhibit the long- Yellow nutsedge was approximately 20 cm tall but not term control of glufosinate or glyphosate as would be expected flowering. Treatments consisted of a factorial arrangement for contact, nontranslocated herbicides. This desiccation in of six glyphosate rates (1.12, 0.63, 0.40, 0.25, 0.16, and combination with a slower acting systemic herbicide, could be 0.11 kg ae/ha), which were applied with and without PA. appealing to growers’’ (Pline et al. 2000). Glyphosate rates between 1.12 and 0.34 kg ae/ha are generally Glyphosate inhibits aromatic amino acid production considered to be adequate for the control of annuals (Jaworski 1972; Steinru¨cken and Amrhein 1980). One aspect (Senseman 2007a). When included, PA was included at a of this mode of action is that treated plants generally do not rate equivalent to that of the glyphosate. Additional exhibit injury until several days after application (Senseman treatments included PA alone at 1.12 kg/ha (i.e., the highest 2007a). This delay is actually beneficial in that it allows for pelargonic rate used in any of the mixed treatments) and a symplastic translocation, and ultimately more complete nontreated control. Treatments were applied with an control. However, this delay in symptom occurrence does enclosed-cabinet sprayer calibrated to deliver 280 L/ha at not give the consumer the immediate visual satisfaction that 193 kPa. the product is working. With respect to consumer satisfaction Data collected included an early control rating taken at 5 to with ready-to-use products, the rapid onset of injury is 7 d after treatment, termed ‘‘early injury’’ followed by a desirable and may justify any long-term loss of glyphosate- control rating at 2 wk after treatment (WAT), termed based activity. Although this phenomena was reported by ‘‘control.’’ These two visual ratings used a scale where 0% Pline et al. (2000), they had included PA at approximately indicated no visual injury, and 100% indicated death of all 6.3 kg/ha, which is much higher than typical glyphosate use foliage. After the 2 WAT rating, plants were clipped at rates. Concentrations of glyphosate and PA are generally equal approximately 15 cm above the soil surface and allowed to in the ready-to-use products; a 1.0 kg ae/ha application rate of regrow for an additional 2 wk. At this time, regrowth (if any) glyphosate results in the equivalent rate of PA. According to was removed and its fresh weight determined. Regrowth was the product label,1 the use rate of PA when applied alone expressed as a percentage relative to that of the nontreated and ranges from 10.6 to 94.1 kg/ha. However, the ready-to-use then subtracted from 100. The resultant value was termed products claim ‘‘visible results in 6 hours.’’ The intent of this ‘‘regrowth control.’’ All experiments were repeated in time. 5 research was to determine to what extent (if any) glyphosate Data were subjected to ANOVA by PROC GLM in SASH. activity is altered when PA is included as a tank mixture at For all experiments, data were pooled over the two repetitions rates equivalent to that of glyphosate. because no treatment-by–experimental repetition interactions were detected in the initial ANOVA. There is no consensus among researchers as to what Materials and Methods constitutes proof of antagonism or synergism when two or more active components are combined. An excellent review of Studies were conducted between May and September of this topic, including merits and weakness of various models 2007. Four weed species were selected that are problematic in used to identify antagonistic or synergistic interactions, has nurseries and landscapes across the southeastern United States, been published by Streibig and Jensen (2000). These and and thus a common target of ready-to-use weed control other authors (Morse 1978; Seefeldt et al. 1995) support the products. These species were large crabgrass, yellow nutsedge, position that comparison of response over a series of doses/ longstalked phyllanthus, and prostrate spurge. Test plants rates through linear or nonlinear regression is the most valid were grown and maintained after treatment in a double-layer means of identifying antagonistic or synergistic interactions. polyethylene greenhouse located at Auburn University, Consequently, nonlinear regression using the log-logistic Auburn, AL. The greenhouse was equipped with evaporative model, followed by a lack-of-fit test, was used to compare cooling, which operated whenever the internal temperature between the two treatment series (i.e., glyphosate alone and exceeded 23 C. Only natural lighting was received and day glyphosate + PA) using SAS and the procedure described by length (latitude 32uN) ranged from 12.3 to 14.0 h. Seefeldt et al. (1995). The log-logistic model is as follows: Species were grown individually in 10-cm square plastic pots. Pots were filled with a pine-bark and sand (7 : 1 v/v) D{C y~Cz substrate, which had been amended with a controlled-release b 1zðÞx= I50 granular fertilizer,2 dolomitic limestone, and a micronutrient fertilizer3 at 10.9, 3.0, and 1.0 kg/m3, respectively. Substrate- where y 5 the measured response (i.e., control); C and D are filled containers received 0.6 cm irrigation daily from the lower and upper limits of the response, respectively; I50 5 overhead irrigation. For large crabgrass, longstalked phyl- rate resulting in 50% of the observed response; b is the slope lanthus, and prostrate spurge, pots were seeded with near the I50 value, and x is the herbicide rate. Briefly, approximately 20 seeds of the respective species. Seed of assuming that the response for both treatment series can be these three species had been collected by the authors the described by the log-logistic model, this procedure first

Wehtje et al.: Glyphosate plus pelargonic acid N 545 determines whether the response is equivalent or different between the two treatment series. If a significant difference is detected, this procedure can subsequently determine which of the four parameters (C, D, I50,orb) of the log-logistic model are equivalent or different between the two treatment series. SigmaPlotH6 was used to summarize and graph data for presentation. In a few cases where data did not fit either a nonlinear or a linear response, treatment means and standard deviations are graphically presented.

Results and Discussion PA applied alone at 1.12 kg/ha had no herbicidal effect and was equivalent to the nontreated control for all four weed species. This was expected because 1.12 kg/ha is approxi- mately one-tenth of the lowest labeled rate for pelargonic applied alone. Large Crabgrass. Both early injury and control could be described by the log-logistic model (r2 $ 0.95; Table 1; Figure 1). Neither early injury nor control was influenced by the addition of PA (P $ 0.60). Regrowth control also could be described by the log-logistic model (r2 $ 0.83; Table 1; Figure 1); however, the response differed between glyphosate alone and glyphosate + PA (P , 0.001). The lower and upper limits were equivalent (Table 1), indicating that the lowest and highest glyphosate rates were equivalent whether applied alone or with PA. However, both the I50 and slope were significantly different. I50 was 0.39 kg ae/ha for glyphosate alone vs. 0.59 kg ae/ha for glyphosate + PA, and the slope was 6.3 and 3.4 for the two treatment series, respectively. Graphically, the I50 variable indicates the glyphosate rate at which the sigmoid curves changes (inflects) from concave to convex, which is also the rate that provides a response that is midway between the lower and upper limits. The lower slope for glyphosate + PA indicates that the increase in regrowth control in response to increasing glyphosate rate is less when PA is included. A consequence of the lower I50 for glyphosate alone is that adequate control can be achieved with a lower application rate. For large crabgrass, the addition of PA to glyphosate had no effect on either early injury or control, but was antagonistic with respect to regrowth control. Regression analysis revealed that the glyphosate rate would have to be increased by approximately 46% to compensate for the PA- based antagonism.

Yellow Nutsedge. Early injury was minimal and only Figure 1. Log-logistic response in large crabgrass for early injury as a function of occurred with the higher glyphosate rates; thus neither a glyphosate rate when applied alone or in combination with an equal rate of linear nor nonlinear response could be detected (Table 1; pelargonic acid. (Top) 5 d after treatment; (middle) control, 2 wk after Figure 2). Inspection of individual treatment means revealed treatment; and (bottom) regrowth control, 1 mo after treatment. See Table 1 for summary of parameter estimates. that glyphosate + PA produced more injury than glyphosate alone. Pline et al. (2000) reported that the addition of PA improved glufosinate-based control of yellow nutsedge. was different between glyphosate alone and glyphosate + PA However, this improvement was only evident for a brief (P , 0.001; Table 1). Although the lower limit was period soon after treatment (i.e., 6 d). Control could be equivalent, the upper limit, the I50, and the slope were described by the log-logistic model, and the lack-of-fit test different. The I50 was 0.27 kg ae/ha with glyphosate alone, revealed that the response was equivalent between glyphosate but 0.33 kg ae/ha with glyphosate + PA. In yellow nutsedge, alone and glyphosate + PA (P 5 0.99; Table 1; Figure 2). the addition of PA to glyphosate enhanced early injury, had Regrowth control also could be described by the log-logistic no effect on control, and was antagonistic toward regrowth model (r2 $ 0.85; Table 1). However, the regrowth response control. Glyphosate rate would have to be increased

546 N Weed Technology 23, October–December 2009 Table 1. Summary of regression analysis for four weed species when treated with a series of glyphosate rates applied alone and tank mixed with an equal rate of pelargonic acid. Parameters for log-logistic equation

a 2 b Treatment series Response variable Upper limit Lower limit Inflection (I50) Slope r Lack of fit ------% ------kg/ha Probability Large crabgrass (see Figure 1) Pooled Early injury 90 0 0.97 3.5 0.97 0.66 Pooled Control 78 1.6 0.38 3.4 0.95 0.60 Glyc alone Regrowth control 97 1.3 0.39 6.3 0.96 , 0.001 Gly + PA Regrowth control 97 1.3 0.59 3.4 0.83 Yellow nutsedge (see Figure 2) Gly alone Early injury Regression NA; see Figure 2, top. Gly alone Control 53 21 0.49 4.1 0.84 0.99 Gly alone Regrowth control 100 3.7 0.27 3.7 0.88 , 0.001 Gly + PA Regrowth control 87 3.7 0.33 8.9 0.85 Longstalked phyllanthus (see Figure 3) Gly. alone Early injury 69 0 0.40 3.1 0.93 ,0.001 Gly + PA Early injury 64 0 0.40 9.1 0.98 Gly alone Control 85 21 0.23 9.7 0.92 , 0.001 Gly + PA Control 80 21 0.23 3.2 0.90 Pooled Regrowth control 90 7 0.34 4.8 0.82 0.91 Prostrate spurge (see Figure 4) Gly alone and Early injury Regression NA; See Figure 4, top. + PA Gly alone Control 86 22 0.44 3.9 0.94 , 0.001 Gly + PA Control 71 22 0.49 3.9 0.86 Gly alone Regrowth control 96 21 0.36 3.6 0.87 NA Gly + PA Regrowth control linear regression: control 5 9.8 + 42(log gly. rate), r2 5 0.43.

a Inflection refers to the point (i.e., kg/ha) at which the model predicts that the response is midway between the upper and lower limits. b Lack-of-fit comparison is between glyphosate alone and glyphosate + PA, assuming that the responses from treatment series could be adequately described by the log- logistic model. Comparison followed procedure described by Seefeldt et al. (1995). c Abbreviations: gly, glyphosate; PA, pelargonic acid; NA, not applicable. approximately 22% to compensate for the PA-based antag- described by the log-logistic model (r2 $ 0.86; Table 1); onism, as determined from regression analysis. however, the response differed between the two treatment series (P # 0.001). Nonlinear regression analysis revealed that Longstalked Phyllanthus. Early injury from both glyphosate the upper limit was greater with glyphosate alone (86% vs. alone and glyphosate + PA could be described by the log- 2 71%); and that the I was 0.44 kg ae/ha with glyphosate logistic model (r $ 0.93; Table 1; Figure 3). However, the 50 alone, compared to 0.49 kg ae/ha with glyphosate + PA response differed between the two treatment series (Table 1). Graphical presentation revealed that glyphosate (P , 0.001). Nonlinear regression analysis revealed that the alone was consistently more effective than glyphosate + PA. upper limit was greater with glyphosate alone (69% vs. 64%); (Figure 4). Regrowth control with glyphosate alone also could and the slope was 3.1 with glyphosate alone vs. 9.1 with 2 be described by the log-logistic model, r 5 0.87. Regrowth glyphosate + PA (Table 1). Control from both treatment control was much less with glyphosate + PA than with series could be described by the log-logistic model (r2 $ 0.90; glyphosate alone, and the response between regrowth control Table 1); however, the response differed between the two and glyphosate rate was sufficiently suppressed that only a treatment series (P # 0.001). Nonlinear regression analysis 2 nominal linear relationship (r 5 0.43) could be detected. In revealed that the upper limit was greater with glyphosate alone prostrate spurge, the addition of PA to glyphosate had no (85% vs. 80%); and the slope was 9.7 with glyphosate alone effect on either early injury or control, but regrowth control vs. 3.2 with glyphosate + PA (Table 1). Regrowth control was markedly reduced. from both treatment series also could be described by the log- 2 The addition of PA to glyphosate was synergistic in two logistic model (r 5 0.82); and the response was equivalent (P (longstalked phyllanthus and yellow nutsedge) of the four 5 0.91; Table 1). In longstalked phyllanthus, the addition of species evaluated, but this synergism was manifested only in PA to glyphosate was synergistic toward early injury, the degree of early visual injury. In none of the four species antagonistic to control, and had no effect on regrowth control. was the addition of PA beneficial to longer-term response, i.e., Prostrate Spurge. Early injury was minimal and evident only control and regrowth control. With respect to regrowth with the higher glyphosate rates. Consequently, neither a control, the addition of PA was antagonistic to glyphosate in linear nor nonlinear response could be detected (Table 1). all species except longstalked phyllanthus. Our results are Inspection of treatment means revealed that early injury was consistent with previous studies that also concluded that not altered by the addition of PA (Figure 4). Control could be glyphosate efficacy is reduced by tank mixing with other

Wehtje et al.: Glyphosate plus pelargonic acid N 547 Figure 3. Log-logistic response in longstalked phyllanthus for (top) early injury Figure 2. Treatment means in yellow nutsedge for (top) early injury (5 d after (5 d after treatment), (middle) control (2 wk after treatment) and (bottom) treatment), and log-logistic response for (middle) control (2 wk after treatment) regrowth control (1 mo after treatment) as a function of glyphosate rate when and (bottom) regrowth control (1 mo after treatment) as a function of glyphosate applied alone or in combination with an equal rate of pelargonic acid. See Table 1 rate when applied alone or in combination with an equal rate of pelargonic acid. for summary of parameter estimates. See Table 1 for summary of parameter estimates. herbicides including PA (Bradley and Hagood 2002; justified. However, it must be noted that the ready-to-use Chachalis and Reddy 2004; Hatzios and Penner 1985; Pline products are targeted to consumers that generally do not et al. 2000). The PA-based antagonism of glyphosate observed understand the importance of herbicide translocation nor here was obtained with much lower PA rates than those used appreciate the need for control of belowground tissues. These in the aforementioned studies. Our data support the customers are likely more influenced by the satisfaction conclusion that the addition of PA to glyphosate in ready- obtained in seeing fairly rapid onset of weed dessication that to-use weed control products is neither warranted nor occurs with products that contain contact-type herbicides such

548 N Weed Technology 23, October–December 2009 unaware of the negative aspects of these products. This research makes no assumptions about the validity of marketing strategies for products targeted for casual herbicide users, but instead provides data on how PA affects glyphosate across several weed species.

Sources of Materials 1 Pelargonic acid, ScytheH,57% or 0.50 kg/L of pelargonic acid, Mycogen Co., 5501 Oberlin Dr., San Diego, CA 92121. 2 PolyonH 17N-6P-12K, available from Pursell Technologies INC., 203 W. 4th St., Sylacauga, AL 35105. 3 MicromaxH, O. M. Scott Corp., 14111 Scotts Lawn Road, Marysville, OH 43401. 4 Azlin Seed Service, P.O. Box 914, Leland, MS 38756. 5 SASH software, release 8.3, SAS Institute, Inc., P.O. Box 8000, SAS Circle, Cary, NC 27513. 6 SigmaPlotH 2000 for WindowsH, version 6.00, Systat Software Inc., 501 Canal Blvd., Suite E, Point Richmond, CA 94804-2058.

Literature Cited Bradley, K. W. and E. S. Hagood Jr.. 2002. Evaluations of selected herbicides and rates for long term mugwort (Artemsia vulgaris) control. Weed Technol. 16:164–170. Chachalis, D. and K. N. Reddy. 2004. Pelargonic acid and rainfall effects on glyphosate activity in trumpertcreeper (Campis radicans). Weed Technol. 18:66–72. Coleman, R. and D. Penner. 2006. Desiccant activity of short chain fatty acids. Weed Technol. 20:410–415. Currier, H. B. and S. A. Peoples. 1954. Phytotoxicity of hydrocarbons. Hilgardia 23:155–173. DeMan, J. M. 1990. Principles of Food Chemistry. 2nd ed. New York, NY: Chapman and Hall. Pp. 36–88. Hatzios, K. K. and D. Penner. 1985. Interaction of herbicides with other agrochemicals in higher plants. Rev. Weed Sci. 1:1–63. Jaworski, E. G. 1972. The mode of action of N-(phosphonomethyl)glycine: inhibition of aromatic amino acid biosynthesis. J. Agric. Food Chem. 20:1195–1198. Morse, P. A. 1978. Some comments on the assessment of joint action on herbicide mixtures. Weed Sci. 26:58–71. Pline, W. A., K. K. Hatzios, and E. S. Hagood. 2000. Weed and herbicide- resistant response to glufosinate and glyphosate plus sulfate and pelargonic acid. Weed Technol. 14:667–674. Pline, W. A., J. Wu, and K. K. Hatzios. 1999. Absorption, translocation and metabolism of glufosinate in five weed species as influenced by ammonium sulfate and pelargonic acid. Weed Sci. 47:636–643. Seefeldt, S. S., J. E. Jensen, and E. P. Fuerst. 1995. Log-logistic analysisof herbicide dose–response relationships. Weed Technol. 9:218–227. Senseman, S. A., ed. 2007a. Herbicide Handbook. 9th ed. Lawrence, KS: Weed Figure 4. (Top) Treatment means of early injury (5 d after treatment), (middle) Science Society of America. Pp. 186–187. log-logistic response for control (2 wk after treatment), and (bottom) regression Senseman, S. A., ed. 2007b. Herbicide Handbook. 9th ed. Lawrence, KS: Weed analysis for regrowth control (1 mo after treatment) for prostrate spurge as a Science Society of America. Pp. 379–381. function of glyphosate rate when applied alone or in combination with an equal Steinru¨cken, H. C. and P. Amrhein. 1980. The herbicide glyphosate is a potent rate of pelargonic acid. See Table 1 for summary of parameter estimates. inhibitor of 5-enolpyruvylshikimic acid-3-phosphate synthase. Biochem. Biophys. Res. Comm. 94:1207–1212. as PA or diquat in addition to glyphosate. Providing faster Streibig, J. C. and J. E. Jensen. 2000. Actions of herbicides in mixtures., Pp. 153–180 in A. H. Cobb and R. C. Kirkwood, eds. Herbicides and their dessication on some weed species despite the deleterious Mechanisms of Action. Boca Raton, FL: CRC. effects of longer-term control could be the intent of these products. Conversely, the manufacturers may simply be Received March 12, 2008, and approved June 26, 2009.

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