A comparison of methods of applying pesticides to cereal grains before storage
About this project
The project examined methods of improving the application of pesticide to grain streams and so minimise the dose required for effective pest control. An initial hypothesis was that coarser sprays with a small percentage of the spray volume in small droplets (< 100 µm), would give better impaction and retention on a moving grain stream but a less uniform coverage of the treated grain. Experiments were therefore conducted to assess the retention when using coarse sprays compared with conventional nozzles, and the biological performance of liquid pesticides applied as such coarse sprays or as liquid streams.
A shrouded spinning disc and dribble bar arrangement were used to apply both tracer dyes and a liquid pesticide formulation to a moving grain stream at flow rates of nominally 5 tonne h-l. Comparative experiments were conducted with a range of nozzle types, including a twin-fluid design and a conventional cone nozzle commonly used in grain spraying equipment. Measurements showed that the spray from the spinning disc system was coarser than that from the conventional cone nozzle, having a volume median diameter of 252 µm with 0.3% of the spray volume in droplets < 100 µm in diameter compared with 195µm and 6.1% for the conventional cone nozzle respectively. An effective technique was established based on the use of tracer dyes to determine the comparative retention of liquids applied to moving grain streams with different application systems.
The main results from the work were:
Spray recoveries from treated grain were between 60 and 85% with tracer dyes, but were less than 60% with the active pesticide formulation. The lower recoveries with the active pesticide may be related to the method of analysis or the condition of the grain. There was some evidence that a proportion of the 15-40% of spray lost from the grain was absorbed by dust and debris.
Differences in spray recoveries when using the different application systems were small but there was a trend towards higher recoveries with the coarser sprays from the disc and dribble bar systems. This trend was evident in both tracer dye and pesticide experiments.
Insect response related directly to pesticide capture by the grain and was consistent with the results from laboratory pests. This means that systems such as the dribble bar or spinning disc that could increase retention would give improved biological control at a given application rate or the same level of control at a lower rate in comparison with conventional nozzle systems.
It was concluded that both the spinning disc and dribble bar arrangement are application systems suitable for further development. They have the potential advantages of increased pesticide retention on the grain for a given nozzle output and improved uniformity in the case of the spinning disc. It would be possible with both systems to match pesticide application rate to grain flow rate without affecting the physical parameters of the pesticide liquid. If realised, these advantages could represent significant savings in pesticide use.
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