Increasing speed can reduce coverage
GroundCover™ Supplement Issue: 122 | 02 May 2016 | Author: Bill Gordon
Increasing spraying speed can influence more than pressure at the nozzle. Before you put the ‘pedal to the metal’ it is important to understand how increasing speed can affect sprayers aerodynamically and change how droplets are produced and where they land
Targeting the most susceptible stage in the life cycle of a pest or weed is a critical factor in achieving effective control.
Many growers prioritise the ability to get over as much country as possible with their sprayer while weeds are still fresh, grubs are small or before a fungal pathogen spreads.
However, the temptation to spray more hectares per hour by increasing spraying speed is not necessarily a valid strategy for improving the overall level of control.
When application speed is too fast for good spray deposits, it can start to erode efficacy and increase the potential for drift.
The impacts of higher spraying speeds include the following:
Increased pressure at the nozzle
The most obvious effect of changing spraying speed is on the pressure at the nozzle (the main exceptions are for spray application with manual pressure or pulse width modulation).
With conventional hydraulic nozzles, increased pressure produces smaller droplet sizes. Regardless of the system used, spraying speed can also influence how droplets behave close to the sprayer.
Increasing speed affects spray droplets at all stages from droplet formation to drop deposition on the target, including:
- at the nozzle, by increasing small droplet escape from the spray pattern;
- air movement around the sprayer (aerodynamically), which affects droplet movement close to the chassis (known as the wake effect) and adjacent to the wheels and tyres; and
- target coverage by influencing how the droplets may deposit and penetrate a canopy.
1 Increased escape of small droplets from the spray pattern
Air movement affects the nozzle and can change how the spray pattern is formed and the ability of droplets to remain within the spray pattern.
When the airspeed coming into contact with the spray pattern is fast enough, it can:
- change the shape of the spray pattern, causing it to narrow and wrap backwards – this can affect the overlap of the spray patterns and the evenness of the spray deposits onto the target;
- cause a loss in downward velocity of smaller droplets, which can reduce canopy penetration; and
- lead to the escape of small droplets from the pattern (known as detrainment), which can increase drift potential.
2 Increased wake effect
Increasing spraying speed increases the amount of air displaced by the sprayer as it moves across the ground. This can be significant directly behind the sprayer, where droplets may be transported in an upward direction several metres into the air.
Small droplets can be carried upwards by the wake leading to increased drift potential and lower deposition of droplets in the centre of the sprayer, particularly between the wheels when travelling into a headwind (Figure 1, right).
The spraying speed at which the wake effect becomes significant can change for different sprayer types. However, speeds of more than 15 to 16 kilometres per hour can cause this effect for most sprayers.
3 Increased displacement of spray adjacent to the wheels and tyres
The wheels and tyres on the spray rig tend to displace a lot of air. The faster the rotational speed of the tyres, and the more aggressive the lug pattern is, the more air will be displaced.
This tends to move smaller droplets away from the wheel tracks and causes lower spray deposit areas, particularly at the base of standing stubble. This air movement may also cause droplets to be drawn into the upward air movement behind the sprayer.
4 Reduced penetration into stubble and crop canopies
As droplet size increases droplets tend to hold their direction of travel. When using coarse droplets, increasing the spraying speed can increase deposition onto vertical surfaces.
Very coarse and larger spray qualities applied at higher spraying speeds can increase shadowing behind stubble, and below crop plants and larger weeds.
Increased spraying speed can reduce the penetration of spray droplets into dense canopies. This can occur at different spraying speeds for different nozzles.
Some nozzles that produce smaller droplets with low exit velocities will be affected at spraying speeds as low as 8 to 10km/h.
Spray operators can check the droplet deposition obtained around the sprayer and in crop canopies using water-sensitive paper and the SnapCard app to evaluate the impact of spraying speed and spray quality on where the droplets land.
Impacts of poor spray deposition on efficacy
Reduced spray deposition can limit the immediate level of control, but the longer-term effects of reduced efficiency are more concerning as they may lead to changes in the behaviour or biology of pests.
Insect pests such as diamondback moths have been shown to actively avoid some insecticides, so reduced penetration of sprays into crop canopies can change the behaviour of the pests, making them more difficult to control in the future.
How to spray more without increasing speed
There are several strategies that spray applicators can consider to increase the number of hectares sprayed per hour, without travelling at higher spraying speeds.
Wider booms allow the spray operator to cover more hectares per hour without increasing spraying speed.
Before increasing boom width, it is useful to consider a width that is a multiple of the header and seeder widths in order to reduce wheel tracks.
It is worth the extra expense of fitting an auto height control system to keep the boom stable and maintain boom height.
Faster filling and mixing systems can also reduce the time spent out of the paddock. One solution is to use a mixing trailer and water cart so the operator can mix close to the sprayed paddock. Increasing the number of fill points around the farm can also reduce the time spent travelling to and from water sources.
More information:Bill Gordon,
0429 976 565,
GRDC Project Code BGC00002
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