Drift mitigation, efficacy and 2,4-D

Take home messages

• All spray applications must comply with the directions on the product label or APVMA approved permit.  Applicators need to be aware of the changes to how 2,4-D must be applied.
• Applicators need to carefully select appropriate nozzles to meet the new spray quality requirements for products containing 2,4-D (on updated labels or according to APVMA approved permits).
• When using a very coarse spray quality or larger, an increase the application volume may be required to ensure the target weeds receive appropriate spray coverage.
• Be aware that nozzle selection has a greater impact on drift reduction than an adjuvant will
• APVMA approved permits exist that allow for the application of certain products containing 2,4-D through OSST (Optical Spot Spraying Technology, such as WeedIT® and WeedSeeker®) provided the weed cover does not exceed 10 percent and other permit requirements are met.

Label changes to products containing 2,4-D – a summary of recent changes

On October 3, 2018 the APVMA suspended all labels of products containing 2,4-D and replaced them with a permit (PER87174) outlining new instructions for how the products must be used. Many manufacturers have already updated their labels to reflect the new instructions.

In summary, the new 2,4-D labels, or existing permits (PER87174, PER87451 (aerial application), PER87338 (larger booms) and PER87570 (OSST)) provide updated instructions, such as;

• A mandatory requirement to apply products containing 2,4-D with a Very Coarse (VC) spray quality. During summer from October 1^st to April 15^th there is an advisory statement to use an Extremely-Coarse (XC) or Ultra-Coarse (UC) spray quality in cereals, fallow and pasture. Clearer definitions for recognising inversion conditions are included on the label or APVMA approved permit
• Mandatory no spray zones (downwind buffers) to aquatic areas and terrestrial vegetation (typically less than 50 metres for ground application) have been included
• Specific instructions and larger no spray zones included for aerial applications
• Additional record keeping requirements, including boom height.

Nozzles required to achieve very coarse, extremely coarse or ultra-coarse spray qualities

The GRDC nozzle selection guides have been updated to include a wider selection of nozzles that can produce very coarse, extremely coarse and ultra-coarse spray qualities.

For most operators, there will be very few low-pressure air induction nozzles that will be able to produce a very coarse spray quality or larger at reasonable pressures, unless they consider a 04 orifice size or larger, which may require a significant increase in application volume. To maintain reasonable pressures and application volumes, most operators will need to consider high pressure air induction nozzles, which have a minimum operating pressure around 3 bar, and ideally should be run at 5 to 6 bar pressure.

For growers operating Pulse Width Modulation (PWM) systems there has also been a new GRDC nozzle selection guide for PWM. This has been produced to assist owners of this technology with nozzle selection to meet new label requirements. There are several nozzle choices for PWM systems to achieve a very coarse spray quality or larger.

Determining suitable application volumes

The move to larger droplets may have implications on efficacy if spray coverage is not maintained. When using a very coarse spray quality or larger, the minimum application volume for fully translocated products, such as 2,4-D should be 70 L/ha in low stubble environments. In heavier stubble, this should be increased to at least 80 L/ha.

Where hard to wet weeds such as flax leaf fleabane exists, trial work conducted by Northern Grower Alliance (NGA) (funded by GRDC) have shown that when using 2,4-D, efficacy was maintained, even when using ultra coarse spray quality.

When moving to a coarser spray quality it may be useful to assess spray coverage using water sensitive paper (WSP) to determine if the application volume you are considering 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.

Tank mix and adjuvant effects on drift potential – GRDC funded research

Tank mix and adjuvant selection can impact on droplet size and drift potential, but their effect on the droplet spectrum  is not as great as that of the nozzle type itself.

Recent droplet size analysis funded by GRDC was conducted by the Centre for Pesticide Application and Safety (CPAS) at their wind tunnel facility at University of Queensland, Gatton. This work measured the impact of a range of common summer fallow tank mixes, with and without the addition of common adjuvants, on the spray quality and the driftable fraction produced through three different nozzle types.

Treatments used in this study consisted of 9 different herbicides, applied alone and in two or three way tank mix combinations, with or without the addition of 8 different adjuvants.

Each treatment was evaluated for the impact on the droplet sizes produced by three different nozzle types (TeeJet® TT, TeeJet AIXR and TeeJet TTI) operated at 4 bar pressure.  Tables 1 and 2 summarise a selection of the data generated from this study. A more comprehensive analysis will be included in a fact sheet to be released in coming months.

Table 1. Comparison of driftable fines (<150 microns) for all tank mixes by nozzle type

When comparing the nozzle tested there was a difference of between 2.31 times and 2.92 times as much of the volume existing as droplets less than 150 microns between the best tank mix and adjuvant combination and the worst tank mix and adjuvant combination.  However, the values obtained for each nozzle type did not overlap, meaning the nozzle type had a greater influence on the reduction of driftable droplets than the adjuvant or tank mix did.  As the initial spray quality increased, the impact of formulation and adjuvant became relatively smaller.

A TTI nozzle (extremely coarse spray quality) with any adjuvant added to the tank mixes had less than half the driftable fines than an AIXR (coarse spray quality) with the best drift reduction adjuvant added.

Where product labels or APVMA approved permits for products including 2,4-D require the use of a very coarse spray quality or larger, choose an adjuvant that provides the greatest increase in efficacy. Where an adjuvant can demonstrate both increased efficacy and drift reduction, it is a logical choice in areas where sensitive crops exist.

Table 2 illustrates the average impact of various adjuvants on the percentage of the volume existing as droplets less than 150 microns, average across all nozzle types and all tank mixes. A simple ranking system from 1 to 4 has been included, with a ranking of 1 indicating the best drift reduction properties.

Table 2. Adjuvant Impact on % volume < 150 microns averaged across nozzle types and tank mixes

In Table 2, the values around 100% in the third column are relatively neutral on the production of driftable fines (droplets less than 150 microns in diameter).  Adjuvants with values above 100% tend to increase driftable fines and adjuvants with values less than 100% tend to reduce driftable fines.

Permits to apply 2,4-D using Optical Spot Sprayer Technologies (OSST) sprayers

The APVMA has issued a permit (PER87570) that allow for the application of certain products containing 2,4-D through OSST, provided the permit conditions are fully met.

While the WeedIT® and WeedSeeker® setups currently produce a coarse spray quality, their use has been granted under permit for certain products containing 2,4-D provided the weed cover is not more than 10% and other label requirements, including record keeping are adhered to. Operators of these systems must only use registered products covered by the permit.

Conclusion

Like all things in agriculture, application technology is constantly changing, both in the equipment available and the rules and regulations governing how we can use the products that are available.

It is the grower and applicators responsibility to stay informed about their legal requirements and to seek out information and equipment that will help them maintain efficacy and work within the legal framework. Make use of the various GRDC publications related to nozzle selection, drift management and application technology.

Acknowledgements

The droplet size research undertaken by the Centre for Pesticide Application and Safety (CPAS) at the University of Queensland was made possible by the support of the GRDC, the author would like to thank them for their continued support.

Contact details

Bill Gordon
Bill Gordon Consulting Pty Ltd
Ph: 0429 976 565
Email: bill.gordon@ispray.com.au

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