Abstract:Due to great human awareness of environmental conservation and public health, pesticides must be applied in economically viable and environmentally sensitive ways, and it requires deep understanding on the distributions of pesticide application. The distribution is the process occurring immediately after application. When applied, the pesticide can distribute in the following way: loss by wind drift, deposition on leaves or other parts of the target plant and deposition on ground (soil). The sum of these three components (plant, soil and drift) should equal to the amount of spray emitted. However, there are concerns over the intended effect (deposition on plant) and unintended effect (deposition on ground and wind drift) of pesticide application on the effectiveness and risks associated with the use of pesticides. Oriented to distribution of pesticide application, droplet deposition on different parts was measured in the research. Spray deposition was tested by recovery of a fluorescent tracer (pyranine) in the 1.75m wide, 1.75m high and 10m long working section of the wind tunnel. A single nozzle was positioned in the centre of the wind tunnel at a height of 0.6m above the wind tunnel floor and then sprayed moving along the length of the tunnel at a speed of 1m/s. The nozzle moved over the top of the plant so that application amount was the same to a typical spray in agriculture and the pesticide distribution was measured. Mylar cards (plastic) were used to collect the deposition on ground, diameter polythene lines were horizontally mounted to provide an estimate of the wind drift of spray, and the sow thistle plant was put under the nozzle to test the deposition on the target. To evaluate the influence of different factors on spray distribution, the trial was carried out for three plant types (sow thistle, cotton and barnyard grass) at three growth stages (leaf area were 15cm2, 135cm2 and 300cm2, respectively). After spraying, fluorescent dyes were easily washed off the three different samples so good recoveries can be got. The amount of spray on the plant, ground and wind drift was calculated and expressed as a percent (or fraction) of the amount of spray that came out of the nozzle. Based on the frame straddling technique, velocity of droplet was investigated by particle image velocimetry (PIV), and the test system included laser imaging system, pulse generator and analysis software. The influences of droplet size (VMD), droplet velocity, spray angle, flow rate, height of nozzle, wind speed and growth stage of plant on deposition at various parts were investigated. Correlation between different factors and spray distribution (fraction of spray deposited on ground, plant and drift) was calculated to judge the impact. The results indicated that the distribution of the spray was influenced by droplet size, release height wind speed and growth stage. Meanwhile, spray angle, sheet velocity flow rate and plant type had no significant effect on the spray distribution. The ground deposition was the lowest with the fine sprays. It was found that ground deposition from application of pesticides was 57.7% for finer droplet（VMD was 181μm) which was increased to 82.7% for coarser droplet (VMD was 445μm). Deposition on plant surfaces was also found to be more than 13.4% with release height at 40cm to 60cm and wind speed less than 4m/s. For soil-active herbicide, a proportion of deposit on the ground was increased by bigger VMD, and this is a desirable result. For controlling flying pest, airborne deposit can increase chemical’s spread on its body, so smaller VMD was more effective. It was also shown that the proportion of spray depositing on plant surfaces was increased as the plants got larger and the amount depositing on the ground was decreased. The result can be used to effectively aid spray decisions to maximize the effectiveness of pesticides and minimize risks to the environment from chemical spraying activities. Chemical application must be as precise as possible, so populations of unwanted organisms (insects or diseases) can be reduced and less environmental impacts created in the agricultural production. An understanding of the pesticide application process can be utilized to improve the estimate of the distribution on pesticide from a spray operation.
