Increasing speed can reduce coverage

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Boom sprayer

PHOTO: Brad Collis 

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.
Tractor spraying crop field.

Operating at high spraying speeds with a fine spray quality can lead to excessive amounts of spray remaining in the air.

PHOTO: Bill Campbell 

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.

water-sensitive paper cards showing difference in spray deposits

Figure 1: Placing water-sensitive paper cards in the inter-row and at the base of the standing stubble can highlight areas of poor deposition due to the wake effect. Figure 1 shows poor deposition in the centre of machine at base of stubble resulting from small droplets being transported upwards behind the machine.

SOURCE: Bill Gordon

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. 

Weed control in standing stubble

Grain stubble

The stubble load used in this trial was typical of the Narrabri, NSW, area. The increased stubble loads that can be found in higher-rainfall zones are likely to produce significantly different results.

PHOTO: Annabelle Guest 

Increased spraying speed can reduce weed control in standing stubble at different locations across the boom.

In a study conducted as a part of a GRDC-funded project delivered by Bill Gordon Consulting, the effect of spraying speed, spray quality and three nozzle designs on fallow weed control was conducted in standing wheat stubble at Narrabri, New South Wales, in March 2014.

In this study a marginal rate of one litre per hectare of a 470 gram/L glyphosate product was applied in a total application volume of 50L/ha, which was designed to highlight potential differences in efficacy as a result of the treatments used. All treatments were applied using a 36-metre-wide, self-propelled sprayer.

The study investigated:

Spraying speed: Two speeds were evaluated (20 and 27 kilometres per hour).

Nozzle type: Three 110° nozzle types at two orifice sizes (02 and 025) were used: TeeJet AIXR, TeeJet TTJ60 and the TeeJet TTI. All nozzles were operated at pressures between 3.5 and 4.0 bar to achieve 50L/ha. Spray quality was matched according to the manufacturer’s charts for the two orifice sizes of each of nozzle type used for the two spraying speeds. The AIXR and TTJ60 nozzles were selected based on their coarse (C) spray quality and the TTI was selected based on its extremely coarse (XC) spray quality.

Sampling position: Assessments of weed control were made at three positions across the sprayer: in the centre of the machine, adjacent to the wheel (0.5 metres downwind) and under the boom (3m downwind from the wheel). All spraying took place with a moderate crosswind averaging 12km/h.

The results highlighted the impact the wake effect can have on weed control as a result of increased spraying speed.

TTI nozzles producing an extremely coarse spray quality resulted in a 12 per cent reduction in awnless barnyard grass control in the standing stubble compared to the AIXR and TTJ60 nozzles, producing coarse spray qualities.

At the lower spraying speed of 20km/h, there was a 10 per cent reduction in weed control 3m downwind of the sprayer wheels compared with the centre of the machine or adjacent to the wheels.

At the higher spraying speed of 27km/h there was a 9.4 per cent reduction in weed control in the centre of the machine, compared with 3m downwind of the sprayer.

These results showed that, for the sprayer used in this trial, at the lower travel speeds the wake effect had an impact on the level of control at the 3m downwind sampling position of the machine; however, as the spraying speed was increased the level of weed control was reduced in the centre of the machine.

These results correspond with other studies comparing spray deposits onto water-sensitive paper from many different sprayers, which also show that the speed at which the wake effect becomes apparent can change between manufacturers and across models.

Generally, the wake effect occurs at lower speeds with trailing rigs rather than self-propelled sprayers. The higher clearance of self-propelled sprayers appears to help minimise some of the effects up to about 20 to 22km/h, but at higher speeds even the self-propelled sprayers have trouble landing droplets in the centre of the machine.

More information:

Bill Gordon,
0429 976 565,


Spray speak: droplet and drift terminology


GRDC Project Code BGC00002

Region National