Trends in Australian broadacre spraying

Author: | Date: 26 Feb 2013

“Bigger Sprayers and Bigger Droplets”

What does this mean for advisors and applicators?

Bill Gordon, Bill Gordon Consulting Pty Ltd

Keywords

Application Technology

GRDC code

BGC00002

Take Home Messages

  • Increased spraying capacity can improve spray results, if the machine is operated within appropriate limits
  • Higher travel speeds have created the majority of issues associated with poor application
  • Applicators and advisors need to be able to identify when application has contributed to reduced efficacy and be able to modify the application technique when required.

Introduction

There has been a continuing trend towards more spraying capacity through larger and wider spray rigs, often self-propelled sprayers, to increase spraying capacity (more hectares per hour) with the desire to improve the timeliness of sprays.

The other major trend has been towards air induction nozzles, typically producing larger droplets.

Both of these trends have resulted in both positive and negative effects on the spray results, depending on what practices are used to increase that capacity.

Increasing spraying capacity by completing more sprayed hectares per hour also comes with an increase in the risk of off target damage if spraying takes place with the wrong set up or during unsuitable weather conditions. Many operators (and advisors) still do not fully understand the risks associated with spraying during the presence of surface temperature inversions, which occur on most nights. This should be considered before an application takes place.

Applicators and advisors need to consider what impact the increased capacity of a larger machine may have on the results in the paddock. We should know all what to look for in relation to application technique when assessing the outcomes of a spray job, so that we can make informed decisions about what parameters may need to be adjusted.

Ways of increasing capacity and potential impacts on efficacy and the risk of spray drift

Impact of increased speed

The greatest temptation for operators of self-propelled sprayers is to travel at higher speeds than they may have previously considered, simply because their machines allow them to. 

Increased speed can produce effects at the nozzle, at the target, and around the machine itself.

  • Increased air movement past the nozzle can narrow the effective sprayed width of each nozzle and lead to detrainment (escape) of smaller droplets from the pattern, which increases the risk of spray drift. Narrower patterns may also impact on overlap of spray patterns, unless height is adequate.
  • When using a coarse spray quality, the larger droplets tend to hold their trajectory (direction as a function of downward velocity from the nozzle and forward travel speed). Faster travel speeds tend to increase the horizontal movement of larger droplets. This can lead to shadowing or misses of smaller weeds behind stubble, more spray depositing on one side of the target than the other, and poorer penetration into larger canopies.
  • Increased travel speed can also increase the wake affect around the machine and around the wheels, which can lead to poorer deposition of spray in and around the wheel tracks on many machines. However the increased clearance of many self propelled sprayers can reduce the impact of the wake effect when compared to trailing rigs and lower clearance machines. Hence wheel track problems with high clearance machines can often be improved with additional wheel track nozzles for knock down (non-residual) products, provided that dust is not contributing to the problem.

Impact of increased boom width

If a suitable travel speed is used, increasing the boom width can greatly improve capacity. If the boom height and stability can be maintained the result should be improved application efficiency.  When increasing boom width it is important that the boom width is a multiple of the header and planter/seeder width to minimize the number of wheel tracks in the paddock.

Very wide booms often require automatic height control to maintain a suitable height above the target (stubble or crop). Auto height control may be limited in its effectiveness when poorly setup or when travelling a higher speeds. As a result of higher travel speeds, wider booms are often set at heights well above that required to maintain a double overlap. Greater release heights increase the potential for detrainment of small droplets as well as the lateral movement of larger droplets.

Impact of increasing tank size

Increased tank size can mean more time in the paddock due to less filling operations, increasing productivity. Consideration should be given to the impact of the weight of larger machines on the correct tyre selection and on compaction and wheel track maintenance.

Productivity can also be increased by reducing fill times through larger pumps, providing more filling points, or the use portable mixing sites and water carts.

However, when mixing larger tank volumes careful consideration should be given to the agitation requirements of the mix, the actual mixing order and the mixing procedure. These should be considered by the advisor, with mixing order included on the recommendation.

Implications for advisors and operators assessing spray results

Often the advisor and applicator only consider the application parameters when they believe the efficacy is less than they had anticipated. However some knowledge of the application method can be critical in identifying issues during an inspection of spray results.

When inspecting the results of a spray job I believe the advisor or grower should be equipped with the following information to determine if application has impacted on the anticipated efficacy:

  • Actual products used, mixing amounts and mixing order
  • Dates and times of application (each load)
  • Details of the water source and water quality used
  • Application Volumes, Speed (minimum, average and maximum), Nozzles (brand, type orifice size and spray angle) and the Pressures (at the nozzle)
  • Weather parameters for each load, especially wind direction, temperature and humidity (including at the target).
  • Direction of travel of the sprayer
  • GPS map of areas sprayed
  • Details of rate controller settings, such as minimum pressure/speed/flow hold
  • Nozzle charts to indicate spray quality at various pressures and speeds
  • Decontamination procedure following previous spray job

Essentially this means ensuring that good spray records are maintained, and that you have a copy of them with you before you assess a spray job.

What should we be looking for when assessing the results of a spray job?

If there is an issue with the application, often the pattern is repeated within the paddock. This requires the inspection must be done at an appropriate number of locations within each paddock that adequately represents the variability in paddock conditions and the sprayers operation.

  • When inspecting a spray job, we should look at multiple sets of wheel tracks, which include both directions of travel.
  • Compare efficacy in the midpoint of boom, adjacent to wheels, between the wheel tracks and at the overlaps of the boom (particularly where auto section control is used to ensure is it engaging correctly).
  • Compare ends of runs and areas where speed may be lower (hence pressure or duty cycle) with points in the spray runs where desired speed is obtained.
  • Inspect and compare control in the stubble line (both sides of the stubble relative to the direction of travel and to the wind direction) with areas between the stubble line.
  • Inspect control on both sides of target weeds, relative to direction of travel and wind direction.
  • Inspect fence line control and potential green bridges.
  • Map areas of poor control relative to other parts of the paddock in relation to soil type, moisture, cropping history to the evaluate the potential impact of  factors such as plant stress, and to monitor the potential for resistance development over time.
  • Assess control of susceptible species outside of the application area to identify potential off target movement.

Implications for recommendations from the advisor

The advisor needs to understand how the products are being applied to determine if that is having an impact on efficacy and on the risk of off target movement.

Recommendations should provide enough information to ensure the job is done correctly, but should not be too restrictive for the operator or have legal implications for the advisor.

Recommendations should include at least the following information

  • target species and situation
  • product trade names, active ingredients and rate of active to be applied
  • mixing order and water quality requirements
  • minimum legal spray quality
  • minimum and maximum application volumes
  • advice regarding no spray zones (such as a general statement to read the label and adhere to all restraints)
  • a general statement regarding risk of spray drift, sensitive areas, plant back, re-entry and withholding periods
  • an expiry date for the recommendation

Any other information about application technique, such as nozzle selection, pressure, speed and rate controller settings should not be included in the recommendation. These decisions should be discussed pre-season, where the operator and the advisor develop a series of spray plans, so both on the same page during the season.

I believe all advisors should provide a short report on spray results to follow-up all recommendations, which indicates if an application or control issue has been identified, and how that may be remedied and prevented in the future. Growers should request this kind of feedback in a written form.

Good spray jobs require a partnership between the advisor and the applicator, with both having a good understanding of the application process

This means understanding how the products are being applied, what impact this may be having on efficacy and identifying where improvements can be made.

Contact details

Bill Gordon
Bill Gordon Consulting
Ph: 0429 976565
Email: bill.gordon@bigpond.com

GRDC Project Code: BGC00002,