Effects of some agricultural tank-mix adjuvants on the deposition efficiency of aqueous sprays on foliage

Crop Protection 19 (2000) 27}37

E!ects of some agricultural tank-mix adjuvants on the deposition e$ciency of aqueous sprays on foliage P.J. Holloway!,*, M.C. Butler Ellis", D.A. Webb!, N.M. Western!, C.R. Tuck", A.L. Hayes!, P.C.H. Miller" !IACR-Long Ashton Research Station, Department of Agricultural Sciences, University of Bristol, Long Ashton, Bristol BS41 9AF, UK "Silsoe Research Institute, Wrest Park, Silsoe, Bedford MK45 4HS, UK Received 6 August 1999; accepted 24 September 1999

Abstract The e!ects of 10 commercially available tank-mix adjuvants on the retention and coverage of aqueous sprays on foliage were examined quantitatively under track sprayer conditions, following application at their maximum recommended rates. Substantial enhancement of #uorescein retention was observed only on water-repellent barley and peas, but the di!erences in performance between the additives were considerable. Addition of the water-soluble tallow amine and nonylphenol surfactants gave the largest increases in retention, whereas there was little improvement in e$ciency compared with water alone after inclusion of either the latexor pinolene-based products or ammonium sulphate. Retention enhancement was also achieved using the mineral oil, vegetable oil, methylated vegetable oil and phospholipid ECs and the organosilicone surfactant, but this was often much less than that obtained for the water-soluble surfactants; the best EC was the methylated vegetable oil which also had the highest emulsi"er content. Although spray quality was altered signi"cantly in the presence of many of the adjuvants, modi"cations to this parameter alone could not account for changes observed in deposition e$ciency, because retention enhancement was recorded in sprays with volume median diameters both smaller and larger than water. There was a better correlation between retention e$ciency and the dynamic surface tension of the corresponding spray liquids, with the exception of the organosilicone, which, as expected from its high surface activity, gave essentially complete spray coverage on leaves. Nevertheless, good coverage could still be achieved by adding the two water-soluble surfactants, as well as the methylated vegetable oil and phospholipid ECs. Coverage performance of the other adjuvants tested was poor in comparison, re#ecting, in part, their inferior retention enhancing properties. ( 2000 Elsevier Science Ltd. All rights reserved. Keywords: Agricultural adjuvants; Retention; Coverage; Liquid properties; Droplet size; Image analysis

1. Introduction Tank-mix adjuvants are used worldwide in order to improve the e$cacy of foliage applied pesticide formulations, especially if these are to be used at reduced dose rates. It is generally agreed that there are two main ways in which adjuvants can enhance ultimate biological performance. Firstly, by increasing the amount of active ingredient retained by the target and, secondly, by promoting its uptake. Although a wide variety of additives have been shown to increase spray deposition on foliage,

* Corresponding author. Tel.: #44-(0)1275-549274; fax: #44(0)1275-394007. E-mail address: [email protected] (P.J. Holloway).

notably surfactants (de Ruiter et al., 1990; Holloway, 1994; Nalewaja et al., 1996; Hoyle et al., 1998), emulsi"able oils (Hall et al., 1997a,b, 1998) and polymers (Wirth et al., 1991; Richards et al., 1998), there is little quantitative information regarding the relative e!ectiveness of di!erent classes of commercially available adjuvants as retention enhancers. These products are often described in the technical literature as wetting and/or spreading agents. In the present work, we have compared under standardised track sprayer conditions, the retention and coverage performance of 10 adjuvants currently authorised for use with pesticides in the UK. These additives comprised three types of emulsi"able oils, three classes of surfactant, two "lm-formers, a phospholipid-based product and an inorganic salt; all were applied to foliage in aqueous

0261-2194/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 2 1 9 4 ( 9 9 ) 0 0 0 7 9 - 4

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P.J. Holloway et al. / Crop Protection 19 (2000) 27}37

sprays and their e$ciencies quanti"ed using appropriate #uorescent tracers. Deposition characteristics are discussed in relation to liquid properties and additional measurements of adjuvant e!ects on spray quality.

2. Materials and methods 2.1. Additives Details of the composition of the products selected for evaluation are summarised in Table 1; all were tested at the maximum concentration recommended. Water and acetone}water (1 : 1 v/v) were used as low- and high-end benchmarks, respectively, for retention and coverage e$ciency in all experiments. 2.2. Plant material Field bean (Vicia faba L. cv. Maris Bead), pea (Pisum sativum L. cv. Early Onward) and barley (Hordeum vulgare L. cv. Triumph) were grown from seed in 90 mm diameter pots of peat-based compost; seedlings were culled to three, three and 10 per pot, respectively, after

establishment. Plants were raised in a heated glasshouse during March}April 1998. Two retention experiments were carried out. The "rst used beans 19 d after sowing (19DAS) (height 160} 240 mm, 3}6 leaf pairs), peas 19DAS (height 160} 220 mm, 6}10 leaf pairs) and barley 11DAS (height 220}270 mm, decimal growth stage (GS) 11/12 (Tottman and Broad, 1987), the second, peas 24DAS (height 190} 250 mm, 12}18 leaf pairs) and barley 15DAS (height 250}350 mm, GS 12/13). Detached leaves taken from peas 26DAS and barley 21DAS were used for coverage assessments.

2.3. Application Additives dissolved or dispersed in glass-distilled water (250 ml) were applied to foliage using a gear and toothdriven laboratory track sprayer "tted with an even spray nozzle (80-015 ex Spraying Systems, Wheaton, USA; BCPC code FE80/0.6/3.0). The nozzle was positioned 400mm above the top of all targets and travelled at 0.45 m s~1, giving a nominal application rate of ca 200l ha~1 (#ow rate 0.44l min~1 at 190 kPa).

Table 1 Authorised! agricultural adjuvants tested Type

Formulation

Amount (g l~1) in spray liquid

Emulsi"able oil

"EC: 968 g kg~1 mineral oil 32 g kg~1 emulsi"ers #EC: 950 g kg~1 rapeseed oil 50 g kg~1 emulsi"ers $EC: 750 g kg~1 methylated rapeseed oil 250 g kg~1 emulsi"ers %SL: 870 g l~1 polyethoxylated tallow amine &SL: 948 g l~1 polyethoxylated nonylphenol 'SL: 800 g kg~1 polyethoxylated heptamethyl trisiloxane )EC: 450 g l~1 synthetic latex 100 g l~1 polyethoxylated alkylphenol *EC: 960 g kg~1 poly-1-p-menthene (pinolene) 40 g kg~1 emulsi"ers +EC: 350 g l~1 modi"ed soya lecithin 350 g l~1 propionic acid 100 g l~1 polyethoxylated alkylphenol ,SL: 486 g l~1 ammonium sulphate

10

Emulsi"able oil Emulsi"able oil Surfactant Surfactant Surfactant Film-former Film-former Phospholipid

Inorganic

!The Pesticides Register Revised Adjuvants List, April 1998 Supplement, Pesticides Safety Directorate, York, UK. "Actipron (ADJ 0013). #Codacide (ADJ 0011). $Phase (ADJ 0279). %Ethokem (ADJ 0146). &Agral (ADJ 01546). 'Silwet L-77 (ADJ 0193). )Bond (ADJ 0184). *Barclay Clinger (ADJ 0198). +Li-700 (ADJ 0176). ,Team 2000 (ADJ 0229).

10 10 5 1 1.5 1.4 2.3 5

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P.J. Holloway et al. / Crop Protection 19 (2000) 27}37

2.4. Retention All spray liquids contained 0.05 g l~1 sodium #uorescein (BDH, Poole, UK). For each additive, blocks of "ve replicates of each target species and a block of four replicate polypropylene discs (Sonoco, Slough, UK; area 1633 mm2) were sprayed consecutively in a single run. The latter were mounted horizontally and used to record spray deposition per unit ground area. Fluorescein recoveries from individual discs and replicates of excised foliage were determined spectro#uorimetrically (van Toor et al., 1994) and then converted to deposit per unit emission (DUE) values, viz., ng tracer g~1 foliage dry weight g~1 tracer applied ha~1 (Courshee, 1960). Theoretical recovery e$ciencies for the tracer were '95%.

29

by the PMS are known to underestimate the numbers of small droplets and their velocities (Tuck et al., 1997). Therefore, although the percentage of the liquid volume contained in droplets with diameters less than 100 lm is more commonly used as an indicator of drift potential, the corresponding values for droplets smaller than 200 lm in diameter is a more reliable estimate of SDCs using the PMS (Tuck et al., 1997). These droplet sizes were used for comparative measurements of SDCs with the PDPA. No signi"cant di!erences were observed between the VMDs of distilled water sprays at SRI and LARS or between those of the corresponding tap and distilled waters at both sites. Similarly, sodium #uorescein or Uvitex 2B at the concentrations used had no e!ect on the spray quality of aqueous solutions containing them.

2.5. Coverage 2.7. Dynamic surface tension All spray liquids contained 0.5g l~1 Uvitex 2B (Tinopal CBS-X, Ciba-Geigy, Manchester, UK). Freshly excised barley (10 replicates) and pea (eight replicates) leaves were "xed onto glass plates using double-sided adhesive tape and sprayed as described above under Application, with their adaxial surfaces horizontal to the incident spray direction. When dry, spray deposits on individual replicates with typical areas of 200 mm2 for barley and 500 mm2 for pea, were viewed under ultraviolet light (ca 375 nm) and images captured at ca ]9 magni"cation using a video camera (JVC KY55-B, Synoptics, Cambridge, UK) and commercial software (AcQuis, Synoptics, Cambridge, UK). These were subsequently quanti"ed using image analysis software (AnalySIS, Synoptics, Cambridge, UK) in order to determine mean values for percentage leaf area cover, numbers of individual deposits and their dimensions. The system was calibrated using a graticule (Graticules Ltd, London, UK) and at ]9 magni"cation gave a resolution of ca 20 lm.

Values for the various spray liquids (Table 1) were recorded at 21}233C using the maximum bubble pressure method (BP2 Mark 2 tensiometer, KruK ss GmbH, Hamburg, Germany) over the surface age range of ca 5}5000 ms. The dynamic surface tensions of aqueous solutions containing sodium #uorescein or Uvitex 2B at tracer concentrations were identical to water. 2.8. Statistical analysis All data obtained were subjected to analysis of variance using logarithmic transformations where necessary. Signi"cance was assessed at the 95% level throughout. Regression analysis was also performed on various data set combinations using transformed or original values as appropriate.

3. Results and discussion

2.6. Spray quality

3.1. Retention

Independent measurements of droplet sizes and velocities from the even spray nozzle were made with a laser imaging probe (Particle Measuring Systems (PMS), Boulder, USA) at SRI and with a phase Doppler particle analyser (PDPA) (Aerometrics Inc., Sunnyvale, USA) at LARS. Spray clouds for each additive were sampled 400 mm below the nozzle over the central 160 mm; this is the portion of the spray that impacts with target foliage in the corresponding retention and coverage experiments. Three scans at 40 mm s~1 were made parallel to the short axis at !80, 0 and #80 mm from the centre; each measurement was repeated three or four times. The data obtained were then analysed to determine volume median diameter (VMD) and the corresponding small droplet component (SDC). Size distributions measured

Plant species were chosen in order to provide di!erent growth habits and leaf surface characteristics. At the growth stages used, the foliage of both "eld beans and peas was an essentially horizontal target for incident sprays but the leaves of the former are wettable in contrast to the waxy, water-repellent surfaces of the latter. On the other hand, young barley foliage was orientated mainly vertically and, again, the leaves are di$cult to wet because of their dense covering of microcrystalline epicuticular wax. None of the additives examined increased #uorescein retention by "eld beans by more than 20% (mean of all spray liquids, DUE 1796), when compared with water alone (DUE 1657) (Table 2). Such results are in accordance with previous data recorded on easy-to-wet foliage

30

P.J. Holloway et al. / Crop Protection 19 (2000) 27}37

Table 2 Adjuvant e!ects on #uorescein retention by "eld bean, pea and barley foliage after application with an even spray nozzle (Experiment 1)

Table 3 Adjuvant e!ects on #uorescein retention by pea and barley foliage after application with an even spray nozzle (Experiment 2)

Additive!

Additive!

None Mineral oil Vegetable oil Methylated vegetable oil Tallow amine Nonylphenol Organosilicone Latex Pinolene Phospholipid Ammonium sulphate Acetone}water LSD (p"0.05)

Mean DUE values" Field bean

Pea

Barley

7.41 (1657) 7.58 (1961) 7.57 (1931) 7.49 (1792)

4.80 (121) 5.34 (208) 6.24 (513) 6.91 (1005)

4.88 (131) 5.01 (149) 5.56 (259) 5.81 (333)

7.40 (1636) 7.61 (2018) 7.17 (1293) 7.59 (1986) 7.60 (1992) 7.46 (1734) 7.46 (1730)

6.94 (1030) 6.80 (893) 6.66 (783) 4.94 (140) 5.83 (340) 5.77 (319) 4.76 (117)

7.47 (1749) 0.13

Mean DUE values" Pea

Barley

6.16 (475) 5.78 (324) 5.61 (274) 4.90 (134) 4.80 (121) 4.99 (147) 4.69 (109)

None Mineral oil Vegetable oil Methylated vegetable oil Tallow amine Nonylphenol Organosilicone Latex Pinolene Phospholipid Ammonium sulphate Acetone}water

4.95 (141) 5.74 (312) 6.29 (537) 6.37 (584) 6.92 (1016) 6.50 (662) 6.40 (601) 5.18 (177) 5.81 (334) 6.30 (543) 4.93 (138) 7.13 (1253)

4.85 (127) 5.25 (191) 5.94 (380) 6.34 (558) 6.58 (721) 5.89 (362) 5.81 (334) 5.03 (153) 5.09 (162) 5.35 (210) 4.78 (119) 6.85 (945)

7.27 (1432)

6.56 (707)

LSD (p"0.05)

0.24

0.25

0.24

0.23

!Application rates and composition given in Table 1. "ln values for statistical analysis; back-transformed values in parenthesis.

!Application rates and composition given in Table 1. "ln values for statistical analysis; back-transformed values in parenthesis.

with sprays containing a wide variety of adjuvant types (de Ruiter et al., 1990; Holloway, 1994; van Toor et al., 1994; Hall et al., 1997a; Hoyle et al., 1998; Richards et al., 1998; Leaper and Holloway, 2000). The signi"cantly lower retention of the organosilicone (DUE 1293) on "eld beans, compared with water alone, is probably caused by excessive spreading, resulting in spray run-o! or splash (Holloway, 1994). On pea and barley targets, DUE values for #uorescein di!ered considerably according to additive composition but consistent e!ects were observed between the two species (r2"0.84 for Experiment 1 (Table 2), y"1.379x!1.421; r2"0.85 for Experiment 2 (Table 3), y"0.970x#0.564, where y"ln DUE pea and x"ln DUE barley), as well as between the two growth stages examined for each species (r2"0.91 for pea at 19 and 24DAS, y"1.189x!1.164; r2"0.96 for barley at 11 and 15DAS, y"0.852x#0.583, where y"ln DUE for Experiment 1 and x"ln DUE for Experiment 2 (Tables 2 and 3)). Nevertheless, none of the adjuvants tested were superior in performance to the aqueous acetone benchmark on either species. As expected, ammonium sulphate had no signi"cant e!ect on the deposition of aqueous sprays. Addition of the latex- and pinolene-based adjuvants also did little to improve #uorescein retention compared with water alone, except on peas 24DAS, where the latter increased DUE values from 141 for water to 334 (Table 3). The surfactant adjuvants produced the largest increases in spray retention, with the tallow amine being superior to both the nonylphenol and organosilicone in performance; however, these di!erences can probably be ascribed to the di!erent concentrations

used, viz., 5, 1 and 1.5 g l~1, respectively. For example, on peas 19DAS, DUE values for the tallow amine, nonylphenol and organosilicone were 1030, 893 and 783, respectively (cf. water 121); the corresponding values on barley 11DAS were 475, 324 and 274, respectively (cf. water 127) (Table 3). The three oil ECs also increased #uorescein retention when applied at 10 g l~1, with the methylated vegetable oil generally giving better enhancement than either the mineral or vegetable oil, di!erences between them probably being related to emulsi"er content or type (Table 1) in the spray rather than oil composition. On peas 19DAS, DUE values for the methylated vegetable, vegetable and mineral oil adjuvants were 1005, 513 and 208, respectively (cf. water 121, Table 2), comparable to those on barley 15DAS of 558, 380 and 191, respectively (cf. water 127, Table 3). Retention enhancement by the phospholipid adjuvant was better on peas than on barley. On young foliage, DUE values on pea were 319 (cf. water 121), compared with barley 147 (cf. water 131, Table 2); di!erences between the two species were even greater on older foliage (Table 3). There are few reports in the open literature which describe direct comparisons between the spray retention enhancing properties of di!erent types of spray additives. Such information is apposite where authorised products are described as `wetting agentsa; this occurs frequently. The present work has clearly demonstrated marked differences in the intrinsic ability of some commonly used agricultural adjuvants to increase the spray deposition of a water-soluble tracer on water-repellent foliage. As expected, the best performance was achieved using the

P.J. Holloway et al. / Crop Protection 19 (2000) 27}37

water-soluble surfactant products (Holloway, 1994). Enhancement obtained with the mineral oil EC was poor in comparison with the corresponding vegetable and methylated vegetable oils, with the latter often being better than the organosilicone surfactant. Hall et al. (1997a,b) have previously reported that adjuvant oil ECs are generally less e!ective as retention enhancers than the best surfactants. The latex-, pinolene- and phospholipid-based adjuvants tested proved to be ine$cient as spray deposition agents when applied in water; yet, all three are designated as wetting agents in the Authorised List. 3.2. Coverage As found with #uorescein retention, large di!erences were observed between the various adjuvants tested. However, similar trends in their coverage e!ects (r2"0.98) were recorded on pea (Table 4) and barley (Table 5). There was essentially complete leaf coverage from sprays containing the organosilicone (e.g. Fig. 1E for pea), as would be predicted from its highly surfaceactive nature; coverage achieved using the nonylphenol and tallow amine (e.g. Fig. 1D for pea) surfactants was much lower in comparison (ca 30}50%). Coverage obtained from the three oil-based adjuvants was much less than that with the surfactants but could be ranked in the same order as that observed for their retention enhancement, viz., methylated vegetable'vegetable'mineral; on pea leaves, the corresponding coverage values were 34.0 (Fig. 1C), 13.4 and 6.7% (Fig. 1B), respectively. While the phospholipid adjuvant gave ca 20% spray coverage (e.g. Fig. 1G for pea) on both species, addition of the latex and pinolene (e.g. Fig. 1F for pea) products did little to improve coverage in comparison with water.

31

The behaviour of the acetone}water benchmark (e.g. Fig. 1H for pea) was similar to the nonylphenol surfactant; there was (3% coverage after application of water alone (e.g. Fig. 1A for pea) or the ammonium sulphate solution to leaves of either pea or barley. Deposit counts and mean deposit areas (Tables 4 and 5) also provided an estimate of adjuvant coverage performance in terms of the number and sizes of spray droplets retained, together with a measure of their subsequent spreading and/or coalescence. For example, addition of the tallow amine to the spray liquid would appear to generate large numbers of small droplets that do not spread much after deposition, eventually giving rise to many discrete spray deposits (mean deposit area 0.17 mm2 on peas, 0.07 mm2 on barley); the phospholipid adjuvant also shows this pattern of spray redistribution. On the other hand, although deposit numbers obtained with the nonylphenol are lower than those recorded for the tallow amine, mean deposit areas are greater (0.33 mm2 on pea, 0.17 mm2 on barley), indicating, in turn, that some spreading has occurred before droplet dry-down; the behaviour of aqueous acetone is similar. The organosilicone is the extreme example of spreading, giving a "lm-like deposit which becomes perforated in some places along its surface. Deposit numbers for the oil-based adjuvants were probably a re#ection of their relative retention e$ciencies (Tables 2 and 3) but their spreadabilities were markedly di!erent; droplets containing the methylated vegetable oil spread much more than those with mineral oil added, e.g. mean deposit areas on pea 0.25 and 0.08 mm2, respectively. There was little spreading of retained spray droplets containing the latex, phospholipid or ammonium sulphate adjuvants. Because detached leaves mounted horizontally were used for assessments, coverage values for all of the adjuvants tested

Table 4 Adjuvant e!ects on spray coverage of pea leaves after application with an even spray nozzle Additive!

Mean deposit count (for 500 mm2)

Mean deposit area (mm2)

Coverage (% Leaf area)"

None Mineral oil Vegetable oil Methylated vegetable oil Tallow amine Nonylphenol Organosilicone Latex Pinolene Phospholipid Ammonium sulphate Acetone}water

79 500 592 939 1821 895 117 173 446 1086 326 801

0.02 0.08 0.13 0.25 0.17 0.33 58.67 0.02 0.04 0.11 0.02 0.36

!1.12 1.91 2.60 3.53 3.70 3.96 4.54 !0.77 1.14 3.03 !0.22 3.98

LSD (p"0.05)

*

*

!Application rates and composition given in Table 1. "ln values for statistical analysis; back-transformed values in parenthesis.

0.49

(0.3) (6.7) (13.4) (34.0) (40.4) (52.4) (93.2) (0.5) (3.1) (20.8) (0.8) (53.6)

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P.J. Holloway et al. / Crop Protection 19 (2000) 27}37

Fig. 1. Representative images of spray deposits on pea leaves visualised with Uvitex 2B. A-water, B-mineral oil, C-methylated vegetable oil, D-tallow amine surfactant, E-organosilicone surfactant, F-pinolene, G-phospholipid and H-acetone}water. Magni"cation ]2.5.

P.J. Holloway et al. / Crop Protection 19 (2000) 27}37

33

Table 5 Adjuvant e!ects on spray coverage of barley leaves after application with an even spray nozzle Additive!

Mean deposit count (for 200 mm2)

Mean deposit area (mm2)

Coverage (% Leaf area)"

None Mineral oil Vegetable oil Methylated vegetable oil Tallow amine Nonylphenol Organosilicone Latex Pinolene Phospholipid Ammonium sulphate Acetone}water

21 156 217 303 872 513 89 123 132 385 142 553

0.02 0.05 0.12 0.25 0.07 0.17 19.82 0.01 0.04 0.11 0.02 0.16

!1.44 1.25 2.56 3.58 3.42 3.69 4.51 !0.29 0.76 3.01 0.09 3.76

LSD (p"0.05)

*

*

(0.2) (3.5) (13.0) (35.7) (30.4) (40.2) (90.8) (0.8) (2.1) (20.4) (1.1) (43.1)

0.36

!Application rates and composition given in Table 1. "ln values for statistical analysis; back-transformed values in parenthesis.

should be regarded only as comparative indicators of potential performance on intact plants. 3.3. Spray quality VMDs and SDCs obtained with the even spray nozzle for liquids containing the additives tested are summarised in Table 6. Although the PDPA may not provide precise data about droplets with internal structure, such as those produced by spraying oil-in-water emulsions or water-dispersible surfactants (Tuck et al., 1997), consistent trends in the changes to spray quality were observed irrespective of the method of spray cloud measurement (PDPA vs PMS, r2"0.97 for VMD, y"1.105x!7.995; r2"0.93 for SDC, y"1.189x!4.860, where y"PDPA value and x"PMS value (Table 6)). Addition of the three oil-based adjuvants increased VMDs considerably in comparison with water; this phenomenon is already well documented for this type of additive (Merritt and Morrison, 1988; Miller et al., 1995; Hall et al., 1997b; Butler Ellis et al., 1997; Hall et al., 1998; Butler Ellis and Tuck, 1999; Western et al., 1999). Also in good agreement with published work, the water-soluble tallow amine and nonylphenol surfactants caused a signi"cant reduction in droplet VMDs, unlike the waterdispersible organosilicone surfactant which increased VMDs substantially (Holloway, 1994; Miller et al., 1995; Butler Ellis et al., 1997; Butler Ellis and Tuck, 1999). For example, using the PMS, VMDs were 207, 210 and 248 lm for the tallow amine, nonylphenol and organosilicone, respectively (cf. water 218 lm). The pinolene and phospholipid products exhibited similar e!ects on spray formation, giving VMDs close to those observed for oil-based adjuvants and the organosilicone. Increased VMDs in sprays containing the

Table 6 Adjuvant e!ects on spray droplet spectra from an even spray nozzle Additive!

VMD (lm)

SDC (%)"

PMS

PDPA

PMS

PDPA

None Mineral oil Vegetable oil Methylated vegetable oil Tallow amine Nonylphenol Organosilicone Latex Pinolene Phospholipid Ammonium sulphate Acetone}water

218 241 251 252 207 210 248 221 248 247 ND ND

220 251 278 293 200 209 261 235 275 278 221 197

39.5 30.1 26.8 28.4 45.9 43.4 29.5 37.8 27.7 28.2 ND ND

44.8 33.7 26.7 21.2 53.1 49.5 29.1 40.3 26.5 22.8 44.6 54.2

LSD (p"0.05)

4.0

7.6

2.0

2.3

!Application rates and composition given in Table 1. "Droplets (200 lm diameter.

phospholipid have also been noted previously using three di!erent types of particle analyser (Quinn et al., 1986; Miller et al., 1995; Butler Ellis et al., 1997; Butler Ellis and Tuck, 1999). Addition of the latex-based adjuvant and ammonium sulphate had little e!ect on spray quality. The acetone}water benchmark used for optimum spray retention enhancement showed the largest decrease in droplet VMD (PDPA 197 lm). Droplet VMDs were inversely related to SDCs (Table 6), which ranged from 26.8 to 45.9% for the PMS and from 21.2 to 54.2% for the PDPA. VMDs also correlated well with droplet velocities (Butler Ellis and Tuck, 1999), giving a r2 value of 0.99 with the PMS for

34

P.J. Holloway et al. / Crop Protection 19 (2000) 27}37

200 lm diameter droplets (data not shown); mean velocities ranged from 1.5 to 3.7 m s~1. The corresponding velocity range for the PDPA was 3.4 to 5.3 m s~1. Because spray drift is known to decline with increasing droplet size and velocity, and since additives which make droplets larger also tend to increase their velocity, the likelihood is that drift potential will be reduced with increasing VMD. This has been con"rmed recently for a number of oil-based adjuvants using standard #at fan nozzles in conjunction with PDPA and wind tunnel drift measurements (Western et al., 1999). The adjuvant-induced changes described in the present work have all been determined by scanning the central 160 mm of the spray swath from an even spray nozzle. However, similar e!ects have been con"rmed for six out of the 10 additives listed in Table 1 using the full spray from this nozzle operating over a range of di!erent pressures (Butler Ellis and Tuck, 1999). Nevertheless, adjuvant e!ects have been shown to vary according to nozzle type (Butler Ellis and Tuck, 1999). Whereas standard and low-pressure #at fan nozzles generally showed changes in spray characteristics similar to the even spray, not all of the adjuvant oil emulsions increased VMDs when sprayed through a pre-ori"ce nozzle and reductions in VMDs with water-soluble surfactants were not recorded using a hollow cone nozzle. It should be noted that the #uid dynamics of the latter are fundamentally di!erent from those of the #at fan types. 3.4. Inyuence of liquid properties The various spray liquids tested have disparate physical properties and, consequently, substantial di!erences between them in performance would be anticipated. However, they can be divided into three main physicochemical groups. Firstly, solutions of acetone and ammonium sulphate in water and water itself, are Newtonian liquids and, thus, their hydrodynamic behaviour is straightforward because they have constant surface tension and viscosity. The second group comprises the three surfactants which behave as non-Newtonian liquids possessing dynamic components of surface tension and probably some elastic or viscoelastic properties (Holloway, 1994); these properties can be determined from independent measurements on the bulk liquid. The behaviour of the organosilicone would be expected to be di!erent from the tallow amine and nonylphenol, because it has low water solubility, forming a dispersion of ca 40 nm diameter aggregates at the concentration used (Svitova et al., 1996). The last category is also composed of non-Newtonian liquids and includes the three oilbased adjuvants and the latex-, pinolene- and phospholipid-containing products. These are all classi"ed as ECs and contain di!erent amounts of lipophilic surfactants as emulsi"cation agents for the water-insoluble active principles. Thus, their hydrodynamic behaviour

is exceedingly complex and probably impossible to measure precisely. Another complicating factor in making comparisons between the spray liquids examined was that the additives were used at widely di!erent concentrations. It is well known that the e$ciency of spray retention by foliage depends mainly on the size and velocity of impacting droplets, their intrinsic physicochemical properties and those of the target surface, especially if di$cult to wet. Although the VMDs of adjuvant sprays correlated with their SDC and droplet velocity, there were no clear relationships between these parameters for the even spray nozzle and their retention e$ciency on waterrepellent targets. Nevertheless, the enhanced retention of spray liquids containing the two water-soluble surfactants was associated with a signi"cant reduction in VMD and an increase in SDC, which can probably be ascribed to a lowering of surface tension in both the spray liquid and in the droplets formed therefrom (Holloway, 1994). An additional factor contributing to the e!ectiveness of the tallow amine as a spray deposition agent is likely to be its low dilational modulus (Reekmans, 1998); we have recently con"rmed this result using quasi-elastic light scattering (Richards and Holloway, unpublished). Changes in droplet spectra produced by these types of adjuvant are associated with increases in the length of the liquid sheet emitted from the nozzle (Butler Ellis et al., 1997). Large reductions in surface tension are also produced by the organosilicone surfactant but these are partially o!set by considerable increases in droplet size, resulting in less e$cient spray deposition compared with more water-soluble types of surfactants. This behaviour is similar to that observed for lower linear alcohol and nonylphenol ethoxylates that have high surface activity but are only water dispersible (Holloway, 1994). Most of the emulsi"able adjuvants also increased VMDs, probably by decreasing the length of the liquid sheet during atomisation (Butler Ellis et al., 1997), but were still able to enhance spray retention. Their relative enhancement e$ciencies appeared to be related to emulsi"er content rather than composition, residual surface activity again probably compensating for the potential loss in performance caused by the increase in droplet size. Retention e$ciency of the emulsions generally declined with decreasing emulsi"er content in the approximate order, methylated vegetable oil (2.5 g l~1), vegetable oil (0.5 g l~1), phospholipid (0.5 g l~1), mineral oil (0.32 g l~1), latex (0.14 g l~1) and pinolene (0.09 g l~1). Dynamic surface tension (DST) values at a surface age of ca 50 ms were recorded for bulk samples of all the spray liquids tested. The values obtained showed some relationships with the corresponding VMDs (Table 6) and DUE values for #uorescein (Tables 2 and 3). Thus, with the exception of the organosilicone (DST 48.0 mN m~1), retention e$ciency appeared to increase with decreasing DST (Fig. 2). Maximum retention

P.J. Holloway et al. / Crop Protection 19 (2000) 27}37

35

Fig. 2. Dynamic surface tensions of the spray liquids (Table 1) at 50 ms surface age and their retention enhancing performance on pea (triangles) and barley (squares) foliage. For pea, y"!3.533x#20.438 and !2.819x #17.547 for Experiments 1 and 2, respectively, and y"!2.569x#15.878 and !2.486x#17.262 for barley in Experiments 1 and 2, respectively; y"ln DUE and x"ln dynamic surface tension.

enhancement was always observed for acetone}water, which had the smallest VMD (197 lm) and the lowest DST (35 mN m~1). At the other extreme, sprays containing additives that gave DSTs similar to water (ca 71 mN m~1) but produced larger VMDs were poorly retained in comparison, e.g. mineral oil, latex and pinolene ECs. The DSTs of the vegetable and methylated vegetable oils were much lower than that of the mineral oil (59.0 and 56.0 mN m~1, respectively) and DUE values and VMDs were correspondingly higher. Despite its higher DST of 63.0 mN m~1, the retention behaviour of the phospholipid was similar to that of the two vegetable oils. The tallow amine usually gave better spray deposition than the nonylphenol, in accordance with its lower DST (51.1 cf. 59.2 mN m~1), although their VMDs were similar. It should be noted that DST data for heterogeneous spray liquids, such as oil-in-water emulsions, will probably only provide an indication of the amount of `excessa emulsi"er present at the air}liquid interface. Target coverage will depend on both the number and sizes of spray droplets retained and the amount of spreading that occurs before droplet evaporation is complete; therefore, some relationship between these factors would be expected. For pea, the correlations between coverage and retention enhancement for the adjuvants tested had r2 values of 0.84 and 0.86 for Experiments 1 and 2, respectively (y"2.011x!9.922 for Experiment 1; y"2.542x!13.171 for Experiment 2, where y"ln

coverage and x"ln DUE). However, the corresponding values for barley were much lower, viz., 0.60 and 0.67, respectively, (y"2.480x!11.305 for Experiment 1; y"2.271x!10.748 for Experiment 2, where y"ln coverage and x"ln DUE), probably re#ecting the di!erences between the gross morphology of the two target species in terms of leaf orientation to the incident spray. In both cases, there was a better data "t if the results for the organosilicone were omitted from the regression analysis. Droplet spreading is in#uenced greatly by the surface tension of the liquid applied and equilibrium values of bulk solutions of the three surfactants correlated well with their covering abilities on water-repellent leaves, viz., organosilicone 22'nonylphenol 30'tallow amine 41 mN m~1 (Holloway, 1994). However, the corresponding surface tension values for droplets in contact with the leaf surface will be higher due to surfactant adsorption at the cuticle}liquid interface; these e!ects will be concentration dependent (Holloway, 1994). Leaf surface adsorption does not occur with acetone}water and, therefore, any spreading observed is a true surface tension e!ect for 35 mN m~1. Overall coverage from sprays containing the emulsi"able adjuvants was usually much less than that obtained using the surfactants or aqueous acetone. But, as found with retention enhancing properties, the covering ability of the emulsions also increased with their emulsi"er content, the best coverage being achieved by the methylated vegetable oil (emulsi"er 2.5 g l~1), the worst with the

36

P.J. Holloway et al. / Crop Protection 19 (2000) 27}37

latex (emulsi"er 0.14 g l~1). Although the precise nature of the emulsi"ers used in the various commercial products is not disclosed, they are likely to be lipophilic and highly surface-active.

4. Conclusions The present work has con"rmed that tank-mix adjuvants can have a considerable in#uence on the e$ciency of delivery of water sprays. However, the magnitude of this e!ect is dependent on the composition of the additive, the way in which it is formulated and the amount present in the spray liquid. This, in turn, a!ects the physicochemical properties of spray droplets in terms of their size and velocity, and governs their ultimate impaction and spreading behaviour. Surfactant adjuvants represent the simplest situation because coformulants are not normally used; retention enhancing performance of water-soluble products is superior to water-dispersible ones and vice versa for spreading (Holloway, 1994). DST values are good predictors for retention e$ciency for closely related series of surfactants but become unreliable if comparing di!erent classes of surface-active compounds (Holloway, 1994; Reekmans, 1998). Predicting the behaviour of other classes of adjuvant is more di$cult, because most of them also contain surfactants as emulsi"ers or dispersants, e.g. latex-, pinolene-, phospholipid- and oil-based products. However, it would appear that their spray performance is in#uenced greatly by the amounts of these coformulants present in the spray liquid after dilution of the corresponding EC; in some cases, applications using the emulsi"er alone have been shown to provide retention enhancement similar to the adjuvant oil-in-water emulsion itself (Hall et al., 1997a). It should be noted that our investigation has been carried out with glasshouse grown plant material, using sprays which do not contain a pesticide formulation. Nevertheless, for waxy, water-repellent species this represents a `worst case scenarioa for retention e$ciency and, thus, is a valid model system for evaluating the relative e!ectiveness of di!erent adjuvants. Performance with "eld grown crops and weeds of this type would be expected to be di!erent due to weathering and abrasion e!ects within the plant canopy; this aspect will be examined in future work. However, this criticism probably does not apply to plant species that are easily wetted. There are few quantitative data available in the open literature describing the retention e$ciency of di!erent types of pesticide formulation; such information is essential in order to establish baselines for assessing formulation performance using adjuvants. However, for a given application system, it is likely that e$ciency of delivery will be in#uenced mainly by the chemical nature of any surface-active or polymeric coformulants present and especially by their concentrations in the diluted pesticide

spray. Initial work with some blank pesticide ECs has already shown that atomisation and spray quality were a!ected by six of the adjuvants listed in Table 1 (Butler Ellis et al., 1997,1999), whereas retention enhancement by surfactant and oil-based adjuvants was signi"cant only at high EC dilution rates (Hall et al., 1997a). Work is in progress using complete EC, suspension concentrate and wettable powder formulations containing di!erent proportions of active ingredient and coformulants.

Acknowledgements The authors are grateful to the various adjuvant companies who kindly supplied the products used in this work. Our research is funded by commissions from the Ministry of Agriculture, Fisheries and Food. IACR-LARS and SRI receive grant-aided support from the Biotechnology and Biological Research Council of the UK.

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