The GRDC nozzle selection guides were updated in January 2019 to include a wider selection of nozzles that can produce very coarse (VC), extremely coarse (XC) and ultra coarse (UC) spray qualities.
As more manufacturers release new nozzle models, the range of nozzles available to meet the coarser spray qualities will continue to increase, so spray operators need to continually check whether the data they are relying on is the most current.
To maintain reasonable pressures and application volumes, most operators using conventional sprayers will need to consider high-pressure air induction nozzles, which have a minimum operating pressure of about three bar, and ideally should be run at five to six bar pressure.
Ultimately the best measure of spray coverage will be the control achieved. If the control from a spray job is less than anticipated, it is important to check the sprayer set-up, tank mix and coverage before blaming a coarser spray quality.
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 may have implications for control if spray coverage is not adequate.
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 heavier stubble and/or higher weed densities.
GRDC-invested fallow trial work by the Northern Grower Alliance has shown that control can be maintained on hard-to-wet weeds such as flaxleaf fleabane, even when using UC spray quality.
When moving to a coarser spray quality, 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.
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 will be the control achieved. If the results of a spray job are less than anticipated, it is important to check the sprayer set-up, tank mix and coverage before blaming a coarser spray quality.
Tank mix and adjuvant effects on drift
Tank mix and adjuvant selection can impact on droplet size and drift potential, but their effect on spray quality and drift potential does not have as much impact as the nozzle type itself.
A recent GRDC investment with the Centre for Pesticide Application and Safety (CPAS) investigated the impact of 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 percentage of the total spray volume with droplets less than 150 microns in diameter.
The tests used nine common 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 four 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; and
- Turbo TeeJet® Induction (TTI) - XC end of ultra coarse spray quality.
The most effective way to minimise the risk of drift is by selecting the correct nozzle. The data shows that across the range of tank mixes, there is a 19-fold decrease in the percentage of droplets less than 150 micron (1.4 per cent compared to 26.7 per cent) by selecting a TTI nozzle over a TT.
Adding herbicides to the spray tank 'fines up' the spray quality for every nozzle tested, resulting in a greater percentage of driftable fines compared to spraying water alone. However, when an adjuvant is then added to this chemical mix, the impact can be quite varied and is not necessarily consistent between nozzles.
For all nozzles, there is over a two-fold difference in the volume of driftable fines between the worst and best tank mix.
This study shows that for some common summer fallow tank mixes, more than one-third of the spray volume in the tank is at risk of not hitting the target if the wrong nozzle is selected. This can have significant impacts on the economics and efficacy of the spray job, and the potential drift risk to off-site targets.
The most effective way to minimise the risk of drift is by selecting the correct nozzle.
The TTI nozzle produced a UC spray quality irrespective of the adjuvant added and had less than half the driftable fines of an AIXR with the best-performing drift reduction adjuvant added.
Given that nozzle choice has a far greater influence on the percentage of driftable fines than the adjuvant, it is important to choose the nozzle first and then an adjuvant based on its role in improving weed control, either for modifying unsuitable water or improving retention and uptake. Further information regarding adjuvants can be found in the GRDC publication Adjuvants.
Spray quality for products containing 2,4-D
On 3 October 2018 the APVMA suspended all labels of products containing 2,4-D and replaced them with a permit outlining new instructions for how the products must be used (https://apvma.gov.au/node/15581). Many manufacturers have already updated their labels to reflect the new instructions.
A mandatory requirement on the labels and permit is to apply products containing 2,4-D with a very coarse (VC) spray quality or larger. From 1 October to 15 April there is an advisory statement to use an extremely coarse (XC) or ultra coarse (UC) spray quality in areas where sensitive crops may be present.
Application technology is constantly evolving, both in the equipment available and the rules and regulations governing how we can use the available products.
It is the grower and the applicator's responsibility to stay informed about their legal requirements and to seek out information and equipment that will help maintain efficacy while working within the legal framework.
Where product labels or APVMA-approved permits for products require the use of a VC spray quality or larger, operators should consider the nozzle first and then choose an adjuvant that enhances weed control.
The droplet size research undertaken by the Centre for Pesticide Application and Safety at the University of Queensland was a GRDC investment.