The decision to leave nozzle positions on or off seems straight forward, but do you know how to check to see if you’ve make the right call? Once the air on your sprayer air is adjusted, your choice of on or off will rely on the following factors:
- the height of the crop,
- the angle of the spray (hollow cones generally produce a 60º to 80º cone as a function of design and operating pressure), and
- the orientation of the nozzle body.
Some nozzle bodies can be swivelled up or down a few degrees to adjust the spray angle. An alternative would be to permanently rotate the nozzle body fitting in the boom line.
When aiming nozzles using the roll-over, be careful not to swivel them too far or the valve will partially close and compromise the spray pattern.
Tying ribbons to the nozzle bodies and turning on the air shows where the centre of each nozzle is aimed. This informs your decision to turn off any nozzles that excessively over- or under-shoot the target canopy. Regarding the upper-most nozzle, be sure to leave a little over-shoot because wind is most likely to deflect spray at the top of the spray swath. In fact, studies have shown wind speed at the top of an orchard is >2 times that on the orchard floor.
Regarding the lower-most nozzle, remember that the ribbon only indicates the centre of the spray cone. The cone expands outward and overlaps adjacent cones. Therefore, even though the centre of the lower-most nozzle might intersect the bottom of the target canopy, you may still be able to turn it off because the nozzle above has that portion covered.
Many sprayer operators in orchards err on the side of caution and feel they should leave the lowest nozzle on. But, most products (e.g. foliar fungicides or ingestible insecticides) are designed to be on the leaves, not in the grass or on the trunk. Why waste it? In my experience, perhaps half the airblast sprayers I’ve worked with didn’t need the bottom-most nozzle on, and turning it off represented considerable savings in unneeded pesticide. Never turn the bottom nozzle off indiscriminately – always confirm coverage using water-sensitive paper, as described in another article.
Adjust spray distribution across the boom at the beginning and roughly mid-way through the spray season to ensure the sprayer will uniformly cover the target with the optimal volume. These adjustments should account for both canopy growth and fruit set. For example, as the season progresses in an orchard, fruit may cause limbs to hang lower and warrant a new spray distribution. Turning on the bottom nozzle position will help, but it doesn’t account for the possible increase in density throughout the canopy. You may need more volume distributed across the entire boom. Another example: as grape bunches begin to close, sprayer operators may direct fungicides exclusively at the fruit zone and not the entire canopy.
Remember to always check with water-sensitive paper to sure you’re covering the entire canopy.
It’s not worth saving a bit of spray if you’re missing a part of your target.