Sprayer technique

Good sources of information on how agrochemicals should be applied are often found on the agrochemical label. 

You may still need to adjust your sprayer to the specific spraying conditions you choose for volume rate, driving speed, nozzles and spray pressure. 

Calibration for field crop sprayers

Check driving speed:

1) Half fill the spray tank with water.

2) Mark out 100 m and note time to drive this distance.

3) Driving speed formula: Distance driven (m) x 3.6 / Time (sec) = km/h

Example:If driving 100 m takes 50 seconds, then the spraying speed (100 x 3.6 / 50) is 7.2 km/hour.

Select nozzle:

Use the calibration disc or formula (nozzle spacing 50 cm):

Example: By aligning 7.2 km/h over 150 l/ha on the calibration disc the line in the 'window' shows we need a flow of 0.9 l/min.

Then a suitable combination of nozzle size and pressure can be found on the lower half of the calibration disc. e.g. ISO 025 at 2,3 bar (or ISO 02 at 3,6 bar).

(The rate formula is: speed x coverage / 1200.
e.g. 7.2 km/h x 150 l/ha / 1200 = 0.9 l/min )

Protecting the environment when spraying

For many years, 7 to 8 km/h (or 4 to 6 km/h in dense crops needing canopy penetration) have been considered good spraying speeds.

Increasingly, it is recognised that the slower the boom moves the less air turbulence it generates, and therefore less drift and less disturbance in the quality of spray distribution.

Some important considerations needs to be made before choosing a higher tractor gear:

Side effects from higher speed:
How to deal with the side effects:
1) More turbulence / more wind drift
Bigger droplets or TWIN air assistance
2) More boom movementReadjust the boom for optimum performance at a higher driving speed
Droplet sizes

The droplet spectrum
All agricultural nozzles produce a range of droplet sizes. This is a useful feature as the crops to be sprayed always present a 3-dimensional target with contrasting leaf surfaces and angles. In a dense crop, for example, finer droplets are likely to be deposited on the top leaves and larger droplets lower down.

Droplet sizes are measured in microns.
1 micron is 1/1,000,000 m or 1/1,000 mm

To describe the median droplet sizes produced from a specific nozzle, the term VMD is used.

VMD = Volume Median Diameter
VMD is the drop dimension where the accumulated volume of smaller droplets accounts for 50% of the liquid leaving the nozzle; and all larger droplets account for the remaining 50% of total volume.

Wind drift is generally regarded to be mostly caused by droplets below 150 micron in diameter.

Calibration for handheld spayers
Spray pressure

Spray pressure influences spraying effectiveness in crops by 3 ways:

  • Pressure influences the spray angle: the higher the pressure the wider the spray angle. If the pressure is too low (below 1,5 bar for flat fans and 3 bar for INJET) the spray angle is not wide enough to ensure full overlap and liquid distribution along the boom.
  • The higher the pressure, the smaller the droplets and the more likely they are to be deposited within the upper canopy of the foliage. Smaller drops are also more sensitive to wind movement.
  • The higher the pressure, the more surrounding air will be entrained with the spray, which helps larger droplets to penetrate the leaf canopy. The resulting air turbulence within the crop can also increase the deposition of smaller droplets on the underside of leaves, especially when using low output conventional nozzles

Pressures recommended for field spraying with standard and lowdrift nozzles

  • Pressures of 2 to 3 bar for normal flat fan and lowdrift nozzles are recommended for most applications.
  • Only for well developed dense canopies where penetration is needed - such as weeds that are concealed at the base of a vigorous crop - the pressure could be increased to 5 bar with larger nozzles (03, 04 and above).

Pressures recommended for field spraying with INJET nozzles

  • INJET nozzles require a minimum of 3 bar to have a full spray angle but can be used up to 8 bars
Optimize spray technique

Be prepared to optimize spraying technique “on the go” - droplet size can be more flexible than timing.

In many cases, delaying spraying can result in the need for a higher chemical dose or an increased number of applications.

The possible reduction in efficacy from using a larger droplet size, which offers less risk of drift, will usually be far less consequential - so long as a good liquid distribution is achieved.

So it is a good idea to have a set of low drift or INJET nozzles on stand-by in case the wind speed increases. It is a lot more convenient and safer to change nozzles and finish spraying than return with a half full spray tank.

Because there can be up to 10% difference in flow for new and worn nozzles both sets of nozzles should be calibrated - even if both sets are ISO nozzles.