Syngenta application specialist Tom Robinson examines the importance of a stable boom for good spray deposition, and offers tips for optimising forward speed

Suspension and tyres

Boom stability not only comes from correct set-up and maintenance of the boom suspension, but also operating with correct tyre pressures, since tyres act as both a spring and a shock absorber. Running with pressures too high not only reduces traction but also stiffens the ride.

Video: Guide to the use of booms for successful spraying

Combined together, the suspension and tyres therefore play a major role in smoothing out surface irregularities in tramlines while spraying. This is essential for getting the best result with any crop protection product, because it helps achieve an even spray deposition across the entire width of the spray swath.

With an unstable boom, major problems arise at the ends furthest away from the sprayer, since any movement created as the sprayer passes over uneven ground is magnified by the time it reaches the boom tips. This is especially so with a wider sprayer.

The result is that the outside of the boom travels either faster or slower as it bounces around compared with the inside, so will either deposit less or more chemical on the target than it should.

Spray coverage results

To illustrate just how dramatic this can be, look at the differences in the number of droplets in the photograph when water sensitive paper was placed horizontally on short posts in a wheat crop in April, and the sprayer then driven over a specially-prepared bumpy track.

The paper with the most droplets was from the outside end of the boom on a well set-up sprayer running with correct tyre pressures.

By contrast, the paper with fewest droplets was taken from the same position, but this time with poorly set sprayer suspension (in this case locked off to simulate badly maintained or seized suspension) and with the sprayer tyre pressures set too high (in this case 3 bars).

As an estimate, there was probably less than half the amount of spray deposited with the poorer set-up. If this coverage had been achieved using a crop protection product which produces a highly visible result, for example wild oat control, it is likely you'd be very disappointed with the level of control resulting from such under-dosing at the outside of the boom compared to the inside, particularly if already applying it at a reduced dose.

Putting it right

Maximum boom stability is therefore critical for product performance and on modern sprayers it can be relatively simple to check. To ensure correct set-up, check the tyre pressures against those in the manufacturer's handbook and ensure the various suspension components are working as they should be.

Very often, new tyres might come with the same pressures that were used to blow them on to the wheel rim. This might have been fine at the time, but not for spraying.

In the case of the sprayer used in the water sensitive paper experiment, the correct tyre pressure according to the handbook was 1.2 bars, which helped achieve the desired result.

Think for a moment, how often do you measure your tyre pressures? For optimum spray performance the tyre pressure should be the lowest recommended for the weight to be carried.

Yaw and roll

The two key components which upset boom stability are yaw, which is the boom swinging backwards and forwards at its outer edges, and roll, which is the boom ends moving up and down.

To check for roll, push the boom end down a foot and it should come back to the horizontal and not oscillate up and down. For yaw, when moving the boom backwards and forwards, check that all its suspension joints are fully lubricated and that springs and shock absorbers are working correctly. Listen out for the oil moving in the shock absorbers as a guide. All these checks take just a few moments to carry out on a regular basis, yet can have far-reaching results.

Forward speed

As well as boom stability, forward speed can also help spraying efficiency, by improving work rates by covering more hectares per day. Spraying faster isn't simply about increasing tractor speed and raising the spray pressure to maintain equivalent output.

This is because the amount of drift increases significantly with higher nozzle pressures and with the extra turbulence created as the sprayer moves faster. Drag and turbulence, for example, increase at the rate of the square of the increase in forward speed relative to the original speed. So if you increase the forward speed from 12 to 16kph, turbulence is increased by nearly 80%. For these reasons, it is always essential to ensure a stable boom for the speed you are spraying at, to use the correct nozzles in relation to forward speed, and to maintain the correct boom height.

Fortunately, with modern nozzles it is possible to swap from a standard spray to a less drifty alternative, such as the Amistar nozzle, very simply. This can give you double the work rate for about a quarter of the drift.

Wind direction also has an impact on spray drift. Clearly, it is essential to only spray when wind speeds permit. However, spraying in the same direction as a breeze tends to give less drift than when driving into one. This is because, with the breeze behind the sprayer, its direction tends to counter any turbulence caused by the movement of the sprayer, rather than add to it.

Currently, the average sprayer speed is around 12kph. There is a good case to increase this. Some fungicides, for example, have worked well at speeds up to 16kph.