There is no excuse for off-target herbicide drift because the spray operator can reduce the potential impact using these methods:
- 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)
In 2014 we recently conducted a short demonstration to show the impact of these factors on spray drift using a backpack sprayer, a variable-rate fan and some water-sensitive paper positioned every 1.5 metres downwind.
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 kmh into a 10 kmh headwind is essentially spraying in a 20 kmh wind. The result of spraying with a medium spray quality in 10 kmh and 15 kmh 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. The result of spraying with a medium spray quality in a 10 kmh wind at 50 cm and 100 cm from the ground: water-sensitive papers and graph indicate that downwind drift increases as the boom gets higher.
The coarser the spray quality, the less likely the spray will drift off target. Remember that shifting to larger droplets means fewer droplets, so 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 and graph indicate that there is more downwind drift from smaller droplets.
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, don’t spray. Learn more about spray drift here.