On a standard single line sprayer, when spraying fallows with knockdown herbicides, many operators would normally fit a larger orifice size nozzle behind the wheel to compensate for the impact of dust and lower spray deposits adjacent to the wheels.

Simply increasing the orifice size of the nozzles on a multi-step system requires that the operator consider the impact on each of the transitions. The best solution may be for the operator to determine what the spraying speed will be for the fallow application and what nozzles will be engaged at this speed for the chosen application volume (L/ha). Once this has been established, only consider using a larger orifice size for one of the nozzles that will be engaged (e.g. replace the 025 air-induction with an 04 pre-orifice or low-drift nozzle producing a coarse spray quality adjacent to the wheels).

If the orifice size of one of the nozzles in the multi-step system is increased adjacent to the wheels, the operator should calculate the overall increase in flow rate that will occur when these nozzles are operating and make adjustments to the applied rate to compensate for this.

For example:  An operator is trying to achieve 70L/ha at a spraying speed of 24km/h, using a 36m-wide multi-step on a 0.5m nozzle spacing to spray a fallow with knockdown herbicides.

At this spraying speed and application volume this four-nozzle multi- step system is operating with 72 x 025 orifice nozzles engaged at 3 bar = 72.0L/min.

If four of the 025 orifice nozzles adjacent to the wheel tracks (two per wheel) were replaced with 04 orifice nozzles the flow rate for the nozzles would increase from 1.0L/min for the 025 orifice, to 1.58L/min for the 04 orifice. This would result in an extra 0.58L/min of flow x 4 nozzles, or an additional 2.32L/min of flow for the whole boom.

To maintain 3 bar at all of the nozzles, and deliver 70L/ha across the rest of the boom, the total flow rate for the whole boom should now be 74.32L/min. To achieve this, the applied rate needs to be increased by a factor of 74.32 ÷ 72.0 = 1.032.

1.0322 x 70L/ha = 72.25L/ha to maintain pressure in the spray lines at 3 bar and to achieve the applied rate of 70L/ha across the boom.

Mixing would still be based on the 70L/ha rate, and once the fallow spraying operations have been completed, the original 025 orifice size should be returned to its original position.

Find out more

Find more information on the positioning of wheel-track nozzles in the Weed Control in Wheel tracks factsheet

An alternative to changing nozzle orifice sizes would be to plumb additional wheel- track nozzles behind each wheel that can be switched on or off independently of the rest of the boom. By doing this the operator still needs to consider the impact of the increased flow rate and adjust the application volume, as indicated in the previous example.

Wheel-track two-nozzle set-up

Using wheel-track nozzles does not increase the boom width, so operators should not change the boom-width settings in the controller. Adjust application rate to match the required application rate by determining the change in total flow rate required to maintain the required pressure at the nozzles.