Herbicide application

Page last updated: Thursday, 10 June 2021 - 8:05am

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Calibration of spray equipment

The importance of having your spray equipment accurately set up and calibrated cannot be overstressed. Huge losses can be incurred through incorrect herbicide application. These can range from total failure of the herbicide to kill the weeds, to the extreme where an overdose of herbicides kills both weeds and crop, and may even leave residues in the soil. Even if overdosing does not cause crop damage, the extra cost of the herbicide applied unnecessarily can be significant.

Calibration method

There are many methods for calibrating, usually involving calculations. A simplified method designed for boom sprayers with nozzles at 50cm spacings is summarised below.

  • Measure the output of each nozzle for one minute. This should already have been done when choosing an even set of nozzles.
  • Measure the combined output of all nozzles and divide by the number of nozzles. This gives average output per nozzle in millilitres per minute (mL/min).
  • Decide on a speed of travel for spraying.
  • Measure out a distance of 100m and record the time taken to cover the distance with the spray unit. It is important to calculate the speed on a surface similar to that being sprayed. You can calculate it by: speed (km/h) = 360 ÷ time (seconds) taken to travel 100m.
  • Calculate the output using the following formula:

Output in L/ha =

Average output of a nozzle (mL/min) x 60
Nozzle spacing (cm) x speed of spraying (km/h)

To calculate the amount of herbicide needed for each tank of spray, the tank size (volume) must be known. Dividing the tank size by the output (L/ha) gives the number of hectares that can be sprayed with each tank. Multiply the rate of herbicide required per hectare by the number of hectares that can be sprayed per tank to get the amount of herbicide added to each tank.


Sprayer with a 1000L tank; output 50L/ha

Number of hectares sprayed/tank: 1000/50 = 20ha

If the rate of herbicide application is 2L/ha, the amount of herbicide added to each tank is 2 x 20 = 40L

To better judge the appropriate rate of herbicide and water requires continuous monitoring of conditions and results to establish the causes of failure or success. To achieve this you must keep records of every day's spray operations, and assessment of herbicide performance.

Herbicide performance is influenced by weather, soil moisture, growth stage and density of the weeds, herbicide rate, water rate, droplet size, and growth stage of the crop.

Record all of these for each paddock each time you use the sprayer. Weather conditions should be recorded at intervals during the spraying operation and for the preceding and following days.

A good marking system is essential to prevent overlap or missed areas when spraying. Overlap of the spray swath results in areas being sprayed twice, which waste both herbicides and time.

Missed areas result in uncontrolled weeds, which can reduce yield, provide seeds for future generations or contaminate crops.

Misters are a useful but imprecise way of applying herbicides to large areas quickly. They rely on wind to drift the herbicide. If the wind is too light or the spraying speed too high, the swath width will decrease, possibly causing overdosing and wasted chemical. If the wind is too strong or gusty it increases the swath width, which will reduce the chemical application rate and increase the risk of damage from spray drift.