Farmers who protect their crops against weeds and pests by using a tractor-driven boom sprayer should ensure that it is set to spray the exact volume of chemical needed per hectare.

Each season the availability and correct application of agricultural chemicals make the difference between crop failure and a profitable crop for many farmers. These chemicals are effective when applied at the correct concentrations and volumes.

When too much chemical is applied it may cause damage to the crop. If too little is applied, it will not be effective and the weeds or pests will continue to harm the crop.

The farmer should, therefore, ensure that the chemical is mixed to the correct concentration as specified on the label. Secondly he or she must ensure that the boom sprayer is set to spray the exact volume of the chemical per hectare according to the instructions on the label.

CHOOSING THE RIGHT NOZZLE

Three types of chemicals are sprayed on crops, namely herbicides, insecticides and fungicides.

Some are contact chemicals that need to be sprayed directly onto the pest and others are systemic chemicals that only need to come into contact with the plant in order to enter it and combat the pest from inside.

• In the case of contact chemicals, hollow-cone nozzles, producing small droplets, are needed.
• For systemic chemicals, flat fan nozzles producing coar¬ser droplets will do.
• It is recommended that the farmer consult the nozzle supplier when choosing a nozzle.
• The farmer also decides on the speed at which spraying takes place.
• The fastest possible speed will lead to the least cost per hectare.

On the other hand, especially if smaller tractors with mounted sprayers are to be used, too-high speeds may lead to instability and life-threatening danger to the operator.

The instructions on the chemical’s label will tell you how many litres of the chemical/water mixture are to be sprayed per hectare. If you know the quantity to be sprayed per hectare and the tractor’s land speed, you can choose the nozzle size from the manufacturer’s specification tables (see table below).

The chosen nozzles will be able to apply the volume of chemical needed, at the specified pressure, when operating the tractor at the specified speed.

If, for example, spraying is to be undertaken at 9 km/h and the label on the chemical container says that 250 litres/ha should be applied, choose the 9 km/h column and move your finger down down it until you find a delivery that’s close to 250 litres/ha. (See yellow arrow in the table).

You will see that the brown nozzle (XR8005VS) will provide the required delivery at a pressure of somewhere between 2 and 3 bars.

CALIBRATING THE SPRAYER

• It is assumed that the nozzles are spaced at 500 mm intervals.
• To accommodate this spacing, the boom will have to be set at 500 mm above the height of the crop to be sprayed.

The challenges that now face the farmer are to:

• Mark the correct land speed of the tractor on the tachometer, in a specific gear and at a specific engine speed.
• Fine-tune nozzle delivery at that engine speed by adjusting the spray pressure.
• Check the even delivery of all the nozzles on the sprayer.
• Check the correct boom height of the sprayer for even spraying.

CALIBRATING THE TRACTOR LAND SPEED

The tractor’s speedometer may be inaccurate and wheel slip might occur while working under field conditions.  The speedometer measures the wheel speed of the tractor, but if the wheels slip (spin), the land speed will be less than the wheel speed.

The farmer therefore has to determine where on the speedometer the chosen land speed will be under field conditions. This is done as follows:

• Mark off a 30 m or 60 m strip, using two droppers, in the field.
• Hitch the boom sprayer to the tractor, fill it with water and drive it to about 30 m before the first dropper.
• If, for example, the farmer wants to spray at a speed of 10 km/h, choose an appropriate gear and set the rev counter to a speed of 10.5 km/h for that gear.
• Start the tractor, ensuring that it reaches the desired speed at the first dropper.
• Note the time it takes to move from the first dropper to the second.

The actual land speed of the tractor can then be calculated using the following formula:

land speed (km/h) = distance (m) ÷ time (seconds)

• If the actual land speed is not the same as the land speed you need, increase or decrease the engine speed in the same gear and repeat the process outlined above, driving on a new strip in the field.
• Continue in this way until the exact desired land speed is reached.
• Mark this engine speed on the rev counter, using a permanent marker, and remember which gear you used.
• During spraying, the tractor will be in that specific gear and the engine speed will be set to the rpm indicated by the mark made on the rev counter using a permanent marker.

FINE-TUNE NOZZLE DELIVERY

The following formula can be used to determine the desired flow rate of the nozzle in litres/minute:
flow rate per nozzle (l/min) = spray volume/ha (l/ha) x speed (km/h) x nozzle spacing (cm) ÷ 60 000

In the example, the spray volume per hectare is 250 l/ha, the speed is 9 km/h and the nozzle spacing is 50 cm.

The desired flow rate per nozzle in l/min is therefore:
Flow rate per nozzle (l/min) = 250 (l/ha) x 9 (km/h) x 50 (cm) ÷ 60 000 = 1,875 l/min

Referring to the table above, you will see in the fourth column (blue arrow) that the delivered flow rate per nozzle will lie between 1,61 l/min and 1,97 l/min at pressures between 2 bars and 3 bars.

If the correct land speed has been determined, it is then known at which rpm setting the engine will run. Then you can do the following:

• Fill the boom sprayer tank with water after attaching it to the tractor.
• See to it that all the nozzles are inserted.
• Start the tractor and set the rpm to the setting as marked when calibrating the land speed.
• Engage the PTO-driven spray pump and set the pressure to somewhere between 2 and 3 bars.
• Measure the time it takes to fill a jug (1l) from a nozzle (in seconds).
• Use a new nozzle at the point where this measurement is taken.

The measured flow rate can then be calculated using the following formula:
flow rate per nozzle (l/min) = volume of water in measuring jug (L) x 60 ÷ time taken to fill (s)

• If the measured flow rate is lower than the required rate, adjust the system pressure upwards and repeat the flow rate measurement.
• If the measured flow rate is higher than the required rate, adjust the system pressure downwards and measure again until the required flow rate is achieved.
• Check that all nozzles deliver the same volume:
• If the sprayer is not fitted with new nozzles, one new nozzle should be installed and the pressure calibration, as described above, should be done using the new nozzle.
• After the correct setting has been achieved, the flow rates of all the nozzles should be checked to see if they deliver equally.
• If there is a difference of more than 5% between 2 or more nozzles and the new nozzle, all the nozzles should be replaced.
• Remember that nozzles are prone to wear and they need to be replaced when worn.
• The spraying pattern of worn nozzles changes and they will also deliver higher flow rates.
• This will lead to streaky spraying and unnecessary loss of chemical.

IN SUMMARY

• Know which pest is to be controlled.
• Choose a nozzle according to the spray pattern and droplet size needed to combat that pest.
• Check the label to find out what the chemical mixture and spray volume per hectare should be.
• Decide on the speed at which spraying will take place.
• Calibrate tractor speed according to the speed decided on and mark it on the rev counter.
• Use the nozzle catalogue to choose the right size of nozzle for the speed and spray volume needed per hectare.
• Install nozzles and fine-tune delivery at the rpm of the calibrated speed.
• Check that all nozzles deliver within 5% volume of each other.

Also read:
Mechanisation: A short guide for new farmers
How to effectively use knapsack sprayers
Multipurpose crop sprayer gives 90% coverage
Lightweight boom sprayer prevents compacting wet soil

• This article was written by Johan van Gass and first appeared in Farming SA.