EFFECT OF DROPLET SIZE AND CARRIER VOLUME ON PERFORMANCE OF FOLIAGE-APPLIED HERBICIDES

Frequently, spray application of agrochemicals is effective but inefficient. A better understanding of the effects of spray application factors on spray performance may contribute to increased efficacy. In this paper the effects of droplet size and carrier volume on performance of foliage-applied herbicides are reviewed. Generally, performance increased as droplet size decreased at constant carrier volume, irrespective of the droplet size range investigated. Performance of systemic herbicides increased more consistently with decreasing droplet size, compared with herbicides with a contact mode of action. For monocotyledons with a predominantly vertical structure, decreasing droplet size in the size class <150 mum diameter enhanced performance more consistently than in the droplet size class >150 mum, or compared with dicotyledons with a predominantly horizontal structure. Further, decreasing droplet size more frequently enhanced herbicide performance on difficult-to-wet (contact angle of water droplets >110 degrees) than on easy-to-wet plants (contact angle <110 degrees). Carrier volume effects on herbicide performance were less consistent. At low volumes (<100 l ha-1), performance more frequently decreased as carrier volume decreased, whereas the reverse trend was observed at high carrier volumes (>400 l ha-1). A significant interaction between the effect of carrier volume and herbicide type was observed. For glyphosate, plant response consistently increased as carrier volume decreased, but for other herbicides performance generally decreased as carrier volume decreased. There was no difference in carrier volume effects between herbicides with a systemic or a contact mode of action, or between response of monocotyledonous or dicotyledonous species. However, on difficult-to-wet plants, decreasing carrier volume more frequently increased herbicide performance compared with easy-to-wet plants. The effects of the spray application factors droplet size and carrier volume on individual stages of the composite spray application process (i.e. canopy penetration and droplet impaction, retention, foliar uptake and the induction of a biological response) arc discussed.