Archive Article: 2000/04/07

7 April 2000

A FEW years ago most spraying was done at 5mph. A lot is now done at 6-7.5mph, and some operators aspire to 10mph.

With relatively few ideal spray-days in a season, speed can help get the work done. But it must not be at the expense of delivering the product to the target.

Do not use faster speeds if your sprayers suspension and booms are not up to it – boom bounce and yaw are major causes of uneven application, warn experts.

High forward speed encourages spray droplets to move horizontally rather than downwards. If crop penetration is not critical that can be turned to advantage, for example, when applying a cereal flag-leaf fungicide where good coverage of the upper canopy is desired.

But where a product needs to penetrate, such as a graminicide applied to a GS31 cereal crop, trials show high speed application can give a poor result, says Novartis. The same effect is likely with broad-leaf weedkillers.

Some products need droplets at the fine end of the spectrum. But travelling at higher speed increases air turbulence and the risk of drift. That could be overcome by using an appropriate air induction bubble jet. But choosing the appropriate nozzle is critical.

That is especially so as the volume rate for a given nozzle at a given pressure goes down as speed rises and droplet size will decrease as the volume regulating control kicks in, says Paul Miller of Silsoe Research Institute.

Twin outlet nozzle tips or caps may be needed if the required volume and spray quality cannot be achieved at higher speed with a current nozzle, especially when using the high volumes needed for jobs like desiccation, he adds.

If houses, schools, crops, water or environmentally sensitive areas are nearby, the temptation is to reduce speed on the headland bout. But as speed is reduced pressure drops off rapidly and slowing from 10mph to less than 7.5mph is likely to need a nozzle change.

In water sensitive areas a change may be needed to meet LERAP requirements anyway. &#42

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