Pick the nozzle for spraying quality and then the adjuvant for optimising control. The new regulations for using 2,4-D means that growers must use very coarse (VC) to ultra-coarse (UC) spray qualities for their fallow spraying depending on the conditions and sensitive areas in the vicinity.
Operators using conventional sprayers will need to consider high-pressure air induction nozzles, which have a minimum operating pressure of about 3 bar, and ideally should be run at 5 to 6 bar pressure.
The GRDC nozzle selection guides ( https://grdc.com.au/resources-and-publications/all-publications/publications/2019/grdc-nozzle-selection-guide ) were updated in January 2019 to include a more extensive selection of nozzles that can produce very coarse (VC), extremely coarse (XC) and ultra-coarse (UC) spray qualities.
For growers operating pulse width modulation (PWM) systems, there has also been a new GRDC nozzle selection guide for PWM, produced to help owners of this technology with nozzle selection to meet new label requirements. There are several nozzle choices for PWM systems to achieve a VC spray quality or larger.
Application volumes for summer fallow spraying
The move to larger droplets means that application volumes must be increased to maintain spray coverage.
When using a VC spray quality, or larger, the starting application volume for fully translocated products such as 2,4-D should be not less than 70 litres per hectare in low stubble environments (less than 2.5 to 3.0 tonnes/ha) and at least 80L/ha in the heavier stubble and/or higher weed densities.
Trial work by the Northern Grower Alliance in northern NSW-southern Queensland has shown that control can be maintained on hard-to-wet weeds such as flaxleaf fleabane, even when using UC spray quality.
Regardless of the spray system used, spray coverage should be assessed using water-sensitive paper (WSP) to determine whether the application volume is appropriate for the stubble load present.
Useful instructions for using WSP can be found in the GRDC GrowNote on Spray Application for Grain Growers (https:// grdc.com.au/resources-and-publications/grownotes/technical-manuals/spray-application-manual ).
Growers and spray contractors must understand that measuring spray coverage will only tell you if the droplets are hitting the target – not whether the droplets stick to the target. Ultimately the best measure of spray coverage is the control achieved. If the results of a spray job are less than expected, check the sprayer set-up, tank mix and coverage before blaming a coarser spray quality.
Tank mix and adjuvant effects on drift
The nozzle type selected has the most significant impact on droplet size and drift potential, while tank mix and adjuvant have smaller effects.
Recent research by the Centre for Pesticide Application and Safety (CPAS) investigated the impact of nozzle type and tank mix on spray quality. Droplet size analysis to determine spray quality and the driftable fraction was conducted at the University of Queensland’s wind tunnel facility at Gatton. The driftable fraction is defined as “the per cent of the total spray volume with droplets less than 150 microns in diameter” .
The tests used nine typical summer fallow herbicides, either alone or in tank mixes, with and without the addition of common adjuvants (totalling 19 herbicide tank mixes). The mixtures were sprayed through three different 110-02 (yellow) nozzles operated at 4 bar (400 kPa) pressure. The nozzles were:

  • Turbo TeeJet ® (TT) – medium spray quality
  • Air Induction Extended Range TeeJet ® (AIXR) – the coarse end of medium spray quality
  • Turbo TeeJet ® Induction (TTI) – XC end of ultra-coarse spray quality

Graph 1: Effect of nozzle type on per cent volume less than 150 microns for the average of each herbicide + adjuvant mix.
Droplets Graph

NOTE: All treatments are the average of all herbicide mixtures + adjuvant except ‘Herbicides Only’ and ‘Water’.

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