Using sprayers efficiently

Spraying plant protection products (fungicides, insecticides and herbicides) is a critical operation on most crop producing farms.

The annual spend of a standard programme of plant protection products can vary from €171/ha for spring barley to €292/ha for winter wheat.
The products must be applied to the crop evenly at the correct rate, with the correct spray quality (droplet size) in the correct volume of water and all at the right time.
Loss of spray product to the environment through drift, spillages or incorrect sprayer cleaning has to be avoided.

The sprayer, whether it’s a large new trailed sprayer costing €75,000 or a second-hand unit costing €3,000 has the task of applying the product evenly. While a sprayer consists of a tank, pump, boom and other components to load and clean the spray solution, the nozzles and a pressure gauge are key components.

Nozzles

Nozzles determine the spray quality (droplet size); the volume to be applied, the evenness of the spray pattern; and the susceptibility to drift.
Agricultural nozzles are available in standard ISO sizes usually with standard colours (e.g. blue 03) which will have a standard output (e.g. 1.2 litres per min at 3 bar). This allows nozzle sets to be easily changed or low-drift versions to be selected, without changing outputs, forward speeds or pressures.
The product container label should be consulted to determine the required water volume per hectare (usually between 100 and 200 litres/ha) and if stated the required spray quality (droplet size) which could be defined as fine, medium, coarse etc. This helps select a suitable nozzle size from manufacturer’s charts.
While spray charts can be consulted for forward speed, pressure and nozzle size to give the required application map, increasingly, simple-to-use phone apps allow the settings to be checked in the field.
Air induction nozzles are the most common, effective and simplest type of drift reduction technology on the market. By drawing in air to mix with the fluid in the nozzle, droplet size is increased and drift is greatly reduced, allowing spraying to be safely carried out in conditions that would cause extreme drift with standard nozzles. With much larger droplets air induction nozzles often produce a spray quality outside of the parameters suggested on product labels; however independent trials indicate in many cases that this has only a small effect on product performance and may be compensated for by the lack of drift.
The best approach is to have the most common size nozzle fitted to the sprayer in standard and air-induction versions to allow switching between each type when conditions allow. Be aware too that different brands of air-induction nozzle have different droplet spectrum sizes.
Replace nozzles as a complete set when there is any indication of wear such as increased flow rate (>10% than rated capacity) or uneven spray patterns.

Pressure gauge /transducer and forward speed sensor

An accurate and easy to read pressure gauge is essential for accurate spraying. While full testing is best, if a new nozzle gives an incorrect output, it often indicates an inaccurate gauge or transducer.
Accurate forward speed indication is also essential and it should be checked; many simple GPS guidance monitors give an accurate speed output.

Sprayer control systems

Many modern sprayers will have a flow transducer fitted which allows the application rate per hectare to be calculated and controlled, but they depend on the accuracy of the forward speed and flow transducers which should be checked.
These control systems use system pressure to keep the output constant if forward speed changes, so this can impact on spray drift and spray quality. The correct forward speed must still be chosen for the nozzle size and application rate.

Filling, Filtration and cleaning

Sprayers have suction, pressure, (possibly in-line) and nozzle filters fitted to prevent suspended material blocking nozzles. Some air-induction nozzles are prone to blockage and need correctly sized filters particularly at the nozzle to avoid blockages. Some filter types (often in nozzle bodies) have a much greater filter area than others and are less easily blocked.
Cleaning the sprayer is essential to avoid contaminating the environment or damaging the next crop.
Accurate calibration will avoid the need for excessive disposal as the sprayer should be emptied in the field. Any left-over can be sprayed on the target field at a very low rate provided the maximum allowable product rate/ha is not exceeded.
Internal sprayer washing systems should then be used to clean the residues from the sprayer which in very dilute form can again be sprayed out in the field. The level of cleaning required beyond this rinsing will depend on the products used and the next crop being treated. As always particular care must be taken when moving from using herbicides in cereals to using the sprayer on a broad-leaved crop such as beans, beet or rape. Invariably a number of washings are required often including a complete fill and use of a specific cleaning product.

Newer Technologies

Recent sprayer purchases with TAMS support has brought extra control to the spraying operation with many sprayers having position-based automated control of the boom sections on short ground or headlands. These need to be carefully set-up to ensure they operate correctly. They can save on costs, but the savings will depend on the size of boom section being controlled (single 0.5m nozzle up to 6m sections) and the type of sprayer – a trailed sprayer will likely gain more in accuracy. But the savings may be a lot less than some of the sales literature claims.
Multiple nozzle holders which allow automatic switching of nozzle sizes or the number of nozzles working at one point, on the move, offer a very flexible level of control of application rate. These are available currently and an alternative single nozzle system (pulse width modulation) is also being evaluated. These allow application rate to remain constant when varying forward speed and can even ensure application rate is the same across all areas sprayed by a boom when the sprayer is turning to follow a headland contour. More importantly they will allow variable rate technology to be easily applied in the field, once we have a driver for this technology.

Dermot Forristal April 2020