These tips are adapted from information provided by Thomas Wolf, Research Scientist at AAFC, Saskatoon.
1. Check conditions
Use an anemometer and compass to check wind speeds and direction. Spraying is best done when there is some wind and the operator can be sure that wind direction has stabilized. Low-drift nozzles allow for spraying in faster wind speeds. Avoid night spraying under conditions of temperature inversion when air near the ground is cooler than the air above it, causing poor spray dispersal.
Temperature and relative humidity (RH) affect how quickly spray droplets evaporate. Droplets evaporate more quickly at higher temperatures and lower RH, making them more prone to drift.
2. Use coarse spray
Coarse sprays are less prone to drift than fine sprays. Choose herbicides that are known to perform well when applied with coarser sprays, such as Group 2, 4 and 9, when drift is more likely to occur. Use finer sprays and be more careful near sensitive areas when using Groups 1, 10, 14 and 27.
3. Use low drift nozzles
Conventional fan nozzles or hollow cone nozzles are usually too drift prone and don’t provide an efficacy advantage. A larger nozzle opening will produce a coarser spray. Narrower fan angles generally produce larger droplets. Manufacturer tables show spray quality for nozzles at a range of pressures.
4. Use high carrier volumes
Most herbicides work well between 50 and 200 litres per hectare. Use of higher carrier volumes is a very effective way of reducing drift, for two reasons. First, if travel speed is maintained, a larger nozzle is used to apply the higher volumes. This results in a coarser, less drift-prone spray. Second, the spray solution is more dilute at the higher volume. Drift will contain less active ingredient, and have less potential to cause damage.
5. Adjust travel speed and boom height
The relationship between travel speed and boom height is very important. Generally a lower travel speed and lower boom height significantly reduces the potential for spray drift. Faster travel speeds cause increased air shear on the spray sheet which increases its breakup and produces a finer spray which is more prone to drift. Spray stays aloft longer at faster speeds because it is swept back due to wind resistance. Higher boom heights are usually required at higher travel speeds due to uneven terrain.
Orienting the spray forward or backward can allow boom height to be reduced as long as the nozzle to target distance is maintained at the minimum recommended for the direction it is pointing. For low drift sprays, boom height should ensure 100 per cent overlap.
6. Use Shrouds
Shrouds can reduce drift by up to 75 per cent in some cases, although not all booms can accommodate them. Cone shrouds are an alternative which have also been shown to reduce drift by up to 50 per cent, and allow more ground clearance for suspended booms. They won’t contaminate susceptible crops with spray residue on the shielding material. Shrouds become less effective at higher boom heights and faster travel speeds.
7. Air Assist
Air assist uses an air stream to increase the speed that the spray is carried down toward the target. This should reduce the time it hangs in the air, exposed to wind. The direction and velocity of the airblast must be matched with atmospheric and crop canopy conditions. Too strong an airblast into a small crop canopy, like seedling wheat, and the spray can bounce off the ground and return up into the airstream, increasing drift. Air assist must be adjustable to be useful and requires an experienced operator.
8. Know Your Product and What’s Downwind
Know the active ingredient and the likely impact of spray drift on anything downwind. Sulfonylurea herbicides, phenoxies and glyphosate are very potent and can damage susceptible plants at very small doses. Most other herbicides are less active and may have less effect on plants. Many insecticides and fungicides, however, are very harmful to aquatic or avian species and drift or run-off into water must be avoided.
Always make sure the wind is blowing away from sensitive areas. Buffer zones should be used to reduce the impact of spray. Buffer zones are the downwind distance from the edge of the spray swath to the upwind side of a sensitive area. Product labels list recommended buffer zones from sensitive ecological areas.
Keep people around you informed if you intend to spray, and take care when spraying near to yard sites and gardens. †