In 2014 one of our OMAFRA summer students designed a short-and-gritty demonstration using a backpack sprayer, a variable-speed fan and some water-sensitive paper positioned downwind at 1.5 metre intervals. The intent was to illustrate how sprayer operators could reduce the potential for off-target drift by recognizing and accounting for three factors:
- Apparent wind speed (i.e. the sum of wind speed and travel speed)
- Boom height (i.e. release height)
- Droplet size (i.e. nozzle spray quality)
Apparent Wind Speed
Spray operators know they should not spray when the air is calm or when the wind is too high, but they often forget that the nozzles experience “apparent wind speed” which means driving 10 km/h into a 10 km/h headwind is essentially spraying in a 20 km/h wind.
The result of spraying with a Medium spray quality in 10 km/h and 15 km/h wind: water-sensitive papers indicated that there is more downwind drift in higher winds.
Spray operators raise their booms to ensure their nozzles clear the crops, but this contributes to off target drift and greatly reduces coverage – particularly when using twin-fan style tips. Dr. Tom Wolf explains how to set your boom height here, or you could watch one of our Exploding Sprayer Myths videos on the subject.
The result of spraying with a Medium spray quality in a 10 km/h wind at 50 cm and 100 cm from the ground: water-sensitive papers indicated that downwind drift increases as the boom gets higher.
The coarser the spray quality, the less likely the spray will drift off target. Remember, for a given volume, shifting to larger droplets means fewer droplets. Application volumes may have to increase to compensate for potentially reduced coverage.
The result of spraying with a Medium spray quality versus spraying with an Extremely Coarse spray quality: water-sensitive papers indicated that there is more downwind drift from smaller droplets.
This demo used percent coverage as a metric, which is convenient but greatly underestimates drift. So even when the spray window is small and the spray has to go on, take a moment to drop the boom, use a coarser droplet size and if it’s too windy, just don’t spray.
WUR Drift Calculator
There are many drift calculators available for home use. Some require more expertise than others to get a reliable result. This free downloadable calculator from Wageningen University & Research was made available in 2021. It can quantify spray drift deposits onto surface waters and non-target terrestrial areas near a sprayed field or orchard
The calculator uses statistically obtained regression curves to calculate spray deposition next to the sprayed field. The spray drift curves are based on the latest experimental data for field crops, fruit orchards and avenue tree nurseries.
Download your copy here.