The Impact of Travel Speed on Spraying -or- Slow Down to Spray the Roses

Whether boom or airblast spraying, travel speed impacts application rate, coverage uniformity, canopy penetration and drift.

As forward speed increases, spray can be diverted backwards into upward wind currents and vortices behind the sprayer. This increases variability in spray deposit, which is generally undesirable and adds to drift. This effect is amplified when driving into the wind because the shearing effect increases the number of driftable fines, even when using coarser droplets. One study on boom spraying showed that reducing speed from 8 to 6 kph has the potential to reduce drift by ~50%.

When performing airblast applications, canopy penetration and uniformity is greatly improved at slower speeds. Air and droplet velocity has a high rate of drop off, and this loss of momentum means it takes time for spray to get to the target.

Studies in grapes demonstrated that increasing air volume does not compensate for higher forward speeds; it reduced deposition on the spray side of a fully developed canopy, while it did not affect deposition on the far side. Moreover, the backward angles increased variability and ground deposition beneath vine rows. Adding more liquid will not permit a higher forward speed, either. This will only increase the material deposited on the area already sprayed.

So, why do growers spray so quickly?

Time Constraints

Obviously, there are time constraints associated with spraying; many pests and diseases develop rapidly under certain conditions, limiting the window of opportunity for control. Uncooperative weather often exasperates the issue by imposing further restrictions. Given that many growers have insufficient sprayer capacity for their plantings, they often resort to alternate row spraying and higher forward speeds in order to keep up. Employing additional operators or additional sprayers may not be economically feasible, so what are the alternatives?

Studies have shown the actual "spraying" part of a spray operation can be less than 50%. You can save some of that time by:

• installing a rapid-fill overhead water source and using large-bore hoses;
• using worksheets to allow operators to refill with all the facts on paper, rather than trying to calculate rates on the fly;
• moving water supply tanks closer to spray operations to reduce travel time when refilling;
• reducing spray volume to some extent, being aware that this could reduce efficacy;
• increasing boom width, being aware they are more difficult to keep level;
• employing over-the-row sprayers to apply to multiple rows in a single pass; and
• ensuring your speedometer is properly calibrated - see below.

Calibration

Speedometers can become inaccurate over time or because of wheel slippage during spraying. An important part of sprayer calibration is checking your driving speed by timing how long it takes to drive over a measured distance under true field conditions.

• Use a tape measure to place 2 stakes 50 metres (164 feet) apart in the field.
• Fill the sprayer half full of water.
• Select the throttle and gear settings you plan to use when spraying.
• Drive the distance between the stakes three times, timing each pass in seconds. Each time, make sure the tractor is at the desired spraying speed as you pass the first stake. Keep driving at this speed until you pass the second stake. Run the course in both directions and do not drive in the same tracks.
• Take the average time of three passes and use the formula below:

Driving Speed (km/hr) = 50 meters / average travel time (seconds) x 3.6 (a constant)

A modern alternative is to use a hand-held GPS receiver to determine accurate forward speed in specific gears at known RPMs. They're also handy for measuring row length and block areas.

So what is an effective speed?

Choosing an Effective Speed

Several factors must be considered in choosing the most effective operating speed:

• weight of sprayer being pulled;
• slope of terrain and ground conditions (leading to wheel slippage);
• the size and spacing of the crop;
• the density of the foliage;
• wind conditions; and
• the limitations of the equipment itself.

Generally, the recommended operational speed is between 6 and 10 kph for tractor-drawn boom sprayers and 4 to 6 kph for airblast sprayers. There will always be exceptions, but if the speed is too fast, the pesticide will drift and will not provide optimal results.

Don't compromise the effectiveness of an already expensive and time-consuming spray programme by speeding through applications, find some good music to listen to and slow down to spray the roses.