Coarser spray qualities will deliver effective summer spray coverage

Good coverage with very coarse spray quality can be easily achievable


Innovation
A spray rig going through its paces at the Yuna spray field day, February 2019. PHOTO Evan Collis

A spray rig going through its paces at the Yuna spray field day, February 2019. PHOTO Evan Collis

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Field day demonstrates very coarse spray coverage does the job.

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Key Points

  • Using a VC spray quality in summer with an application volume of 70L/ha will give effective spray coverage for most herbicide applications, except for contact herbicides such as paraquat. Contact products can be applied as a coarse spray quality and/or a higher application volume
  • Spray coverage should be measured using water-sensitive paper and the SnapCard® app
  • All spray applications must comply with the directions on the product label or APVMA approved permit

Applicators should 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)

Earlier this year, growers from the Yuna Farm Improvement Group and the Northern Agri Group near Geraldton in Western Australia teamed up to look at ways they could improve their summer fallow spraying.

The best way to assess spray coverage is to look at the level of weed control obtained.

Many growers and advisers were initially sceptical that the requirement for a very coarse (VC) spray quality or larger when using any herbicide containing 2,4-D would provide equivalent coverage to the boom set-ups they were currently using.

The actual results may have been a little surprising for the more than 100 growers who attended the field day in February.

After measuring the spray coverage from five different sprayers on water-sensitive paper placed in standing stubble, the VC spray qualities performed much better than most people were expecting.

Placement of water-sensitive paper for assessing fallow sprays in crop stubble. PHOTO Bill Gordon

Placement of water-sensitive paper for assessing fallow sprays in crop stubble. PHOTO Bill Gordon

Four self-propelled sprayers (one conventional hydraulic pressure and three with pulse width modulation) and a conventional trailing rig were put through their paces.

All sprayers applied 70 litres per hectare of water at travel speeds between 22 kilometres per hour and 28km/h, using a range of nozzle spacings and nozzle types.

Each sprayer was initially set up and operated as it would have been before the requirement for VC spray quality or larger was introduced in October 2018 for applying products containing 2,4-D.

Spray coverage on water-sensitive paper was measured using the SnapCard® app. The water-sensitive paper was placed at the base of standing stubble and in the inter-row.

The meteorological conditions on the day of the field day were typical for February in the northern cropping region of Western Australia, with high temperatures and low humidity leading to a high Delta T (Figure 1) during most of the day. These conditions cause finer droplets to evaporate before hitting the weeds.

What do the coverage results mean?

When assessing spray coverage using water-sensitive paper and the SnapCard® app, being able to obtain a similar coverage in the stubble and the inter-row means that your set-up (speed, volume and spray quality) is reasonably good. Where coverage into the stubble is less than the inter-row, adjustments to application volume, travel speed or spray quality should be made.

Achieving more than 10 to 12 per cent coverage at 70L/ha will be adequate for most herbicide applications, provided a robust product rate is used and the droplets are retained on the target weeds. However, large droplets on small hard-to-wet weeds such as grass seedlings and those with waxy leaves could reduce the percentage of droplets retained for absorption, and require an increase in application volume.

FIGURE 1 Hourly Delta T at Yuna on the day of the field day. SOURCE DPIRD

FIGURE 1 Hourly Delta T at Yuna on the day of the field day. SOURCE DPIRD

How did very coarse compare?

After comparing the existing set-ups, a John Deere SP machine with 50-centimetre nozzle spacing was set up with larger nozzles to produce a VC spray quality. This set-up was able to achieve coverage in stubble and inter-row above 12 per cent at 70L/ha with the same application volume and ground speed as used by the grower.

This was a better result than many of the existing set-ups were able to achieve, or that many of the participants expected.

While an application volume of 70L/ha was sufficient in this situation, moving to extremely coarse or ultra coarse spray qualities or applications in heavier stubble will require a higher application volume, often more than 80L/ha.

The best way to assess spray coverage is to look at the level of weed control obtained. However, measuring coverage and making adjustments before spraying takes place will improve results and reduce the chances of a spray failure.

More information:

Maintaining efficacy with larger droplets - New 2,4-D application requirements

GRDC Spray Application Manual Module 21: Assessing spray deposits

Department of Primary Industries and Regional Development SnapCard® app, https://www.agric.wa.gov.au/grains/snapcard-spray-app

Pulse width modulation sprayers (correct operation)

Nozzle selection guide

Pulse width modulation - nozzle selection guide

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