Effect of surfactant concentration and spray volume on retention of organosilicone sprays on wheat

Abstract

The retention of organosilicone surfactant sprays (0-0.8% v/v) on wheat was determined, using a calibrated track sprayer, over a range of spray volumes representative of ground-based arable applications (37-280 litres/ha). Volume of spray retained on foliage generally increased with carrier volume and surfactant concentration. Spray run-off was not evident with any treatment. The importance of the interaction between concentration of organosilicone and spray volume was highlighted. Spray was captured most efficiently at low spray volumes and • 0.2% surfactant. It is expected that organosilicone surfactant addition to pesticide sprays on difficult-to-wet arable species would be beneficial over a wide range of application volumes. INTRODUCTION Many publications report on the effects of spray adjuvants on the retention of sprays on plants. Surfactants affect retention to a greater extent on water-repellent species than on more readily-wetted species (Holloway 1994; de Ruiter et al. 1990; Anderson and Hall 1987), where frequently, they do not enhance retention compared to water alone (Grayson et al. 1996; Holloway 1993). Organosilicone surfactants are known to increase adhesion (wetting) and spreading of pesticide sprays on foliage due to their high surface activity (Stevens et al. 1993; Zabkiewicz et al. 1988). They can either improve or reduce the retention of sprays, dependent on their concentration and the target plant/leaf characteristics (Holloway 1994; Stevens et al. 1994; Holloway 1993). The importance of the interaction between organosilicone concentration and application volume on spray efficacy has been highlighted with respect to high-volume, airblast applications onto tree fruit, where run-off may reduce efficacy (Stevens et al. 1994). Some efficacy data has also been generated on cereals (Stevens et al. 1995), but there are no reports on how organosilicone concentration and spray delivery volume may influence retention on such difficult-to-wet species. Accordingly, this systematic study examined retention by wheat of sprays containing an organosilicone, over a sixteen-fold concentration range and in four volumes representative of ground-based applications to cereals. METHODS Plants Wheat plants (Triticum aestivum, L. cv. Otane) were propagated from seed and grown to growth stage 15 (Zadoks; 3-4 weeks) in individual pots (12 cm 2) containing a proprietary potting mix (Bloom, Yates), under normal glasshouse conditions (temp 15-25ºC; 12 h photoperiod of daylight), with irrigation daily as required. Surfactant sprays Silwet 408 surfactant (Witco Corp. Organosilicones Group, NY) was diluted in water at concentrations 0, 0.05, 0.1, 0.2, 0.4 and 0.8% (v/v). All solutions contained CuSO 4 (4 g/litre), which had no effect on the spreading or dynamic surface tension of the surfactant (D. Murphy pers. comm.).