Achieving good pre-emergent spray results

Author: | Date: 10 Feb 2011

Introduction 

While there are new pre-emergent herbicides becoming available for the control of problem weeds such as Rye grass, there are no magic bullets when it comes to pre-emergent spraying. Getting a good result from pre-emergent herbicides is about getting every part of the application process right, starting with your product selection to match the weed spectrum present, through to monitoring the spray results carefully.

Good application technique will not prevent the development of resistance, what it will do is allow you to identify it much sooner. Hence application should be one component of an integrated weed management program.

The aim of any pre-emergent application should be to maximize the deposit onto the soil surface, and to achieve a uniform distribution of the product across the soil surface. The evenness required will be dependent on the solubility of the product and its ability to move within the soil. The time between application and incorporation may also be very important, generally the more volatile the product is, the more important it is to ensure incorporation occurs soon after application.

The Basics of Spraying Pre-emergent Herbicides

There are three key things to consider as a part of the spraying process; the target, the timing and the technique.

The Target:

Know your target. It is critical to ensure that the product selected is the most appropriate
for the weeds present and the situation. It is important to consider many factors, such as the previous weed burden, potential resistance, the products residual activity, solubility and soil moisture requirements, ability to tank mix, and the target area including stubble load and ground cover.

Always consult product labels and refer to manufacturers technical data to make an informed decision.

The Timing:

It is important to consider the most appropriate conditions for spraying a pre-emergent and when the incorporation will need to be completed by?

Conditions for Spraying

Suitable conditions for spraying pre-emergent products are the same as for any spraying operation. Extremes of temperature, relative humidity and wind speed all effect the amount of product available to deposit onto the target area. Often these factors will interact with droplet size, speed and boom height.

Always refer to the product label for appropriate conditions for spraying, but as a rule of thumb, the following guidelines may be useful:

  • Always measure and record conditions at the site of application, before, during and at the end of the days spraying operations.
  • Never spray when the wind speed is less than 3 km/h and avoid spraying when wind speeds exceed an average of 15 km/h.
  • Do not spray when the wind is blowing towards a sensitive area (including fallows to be planted to different crops) unless you have a sufficient buffer in place (typically 300m or more when using a coarse spray quality).
  • Never spray during highly stable conditions, such as surface temperature inversion conditions (especially in the hours leading up to sunrise and immediately after)
  • Carefully monitor the Delta T value (temperature and relative humidity).
  • When the Delta T exceeds 10 it is probably time to stop spraying. The only situation where it may be reasonable to continue would be when you are using a spray quality that is larger than coarse (very coarse or extremely coarse), and you have adequate soil moisture and ground cover, such that the delta T value at the target (soil level) is below 8.
  • Continue to monitor and record factors such as rainfall after the application.

The Technique:

Many small adjustments to the sprayer setup can improve the deposit onto the soil and minimise losses, such as tie up on stubble and loss to atmosphere.

The heavier the stubble load, the greater the need for a coarse spray quality or larger droplets. Larger droplets usually require greater application volumes (more than 80 L/ha) to ensure uniformity of the deposit, particularly for pre-emergent herbicides with a low solubility in the soil.

There are many ways to deliver the desired spray quality and to achieve the desired application volume, which can require thinking about more than just the nozzle type.

Nozzle Style: Solid Droplets or Air Inducted Droplets?

Solid Droplets (non air induction)

When using solid droplets in light stubbles (much less than 50 % ground cover) a medium to coarse spray quality may be appropriate, provided the conditions are suitable.

If using a medium spray quality expect that a high proportion of the droplets will be retained on stubble, and the risk of spray drift and loss to the atmosphere will be increased. Some retention on the stubble may be acceptable for highly soluble products, if rainfall is likely to wash the product into the soil. For other pre-emergent herbicides that bind strongly with organic matter, retention on the stubble means a tie up of the product, unless the stubble was completely buried during incorporation.

In heavy stubbles, penetration is critical. With non air induction nozzles (solid droplets) use coarse to very coarse spray qualities (such as those produced by larger orifice pre-orifice nozzles like the Hardi Low Drift (LD), Turbo Teejet (TT) and Turbo Twinjet (TTJ). Larger solid droplets tend to bounce off the standing stubble onto the soil surface.

Air Inducted Droplets

If using air inducted nozzles in situations with low stubble or minimal ground cover situations
a coarse to very coarse spray quality will be sufficient.

When using air induction nozzles in heavy stubbles, consider using larger than a very coarse spray quality (e.g. extremely coarse) to maximize the penetration and minimize retention of the product on the stubble.

Larger Droplets Usually Require Higher Application Volumes

The trade off with increasing droplet size is usually a large reduction in droplet numbers. 

Moving from one spray quality classification to another (e.g. coarse to very coarse) can result in a reduction in the number of droplets produced between 3 to 5 times, which can lead to problems such as shadowing (areas of lower deposition behind stubble and large clods). To improve the evenness of deposition, an increase in application volume is usually required, particularly with pre-emergent herbicides with low solubility.

Increases in application volume can be achieved several ways, such as using larger orifice size, using more nozzles on narrower nozzle spacings, increasing pressure or simply reducing speed.

Nozzle Style: Single Fan, Angled Fan or Twin Fan?

If travelling at lower speeds (around 16 km/h or less) a twin fan nozzle, such as the TTJ may be a good choice to improve penetration and increase uniformity.

At speeds above 16-20km/h a twin fan becomes less suitable, particularly for medium to coarse spray qualities, where nozzles producing a standard fan pattern would be more appropriate.

At higher speeds (above 16km/h) it may be of benefit to use nozzles which are slightly offset (usually angled backwards), such as the TT style nozzles, or some of the newer nozzles released by Syngenta.

Boom Height:

Release height above the target affects overlap, deposition and loss to the atmosphere.

As droplets fall, the larger droplets tend to accelerate, while the small droplets rapidly slow down – both can affect deposition.

Increasing the release height tends to reduce penetration into stubble with smaller droplets, and may increase large droplet shatter and bounce (not useful when tank mixing with a knock down, if you need deposition on emerged weeds).

If the nozzle spacing is different to the row width, the ideal height above the target is at a point where double overlap is achieved at the top of the stubble. Hence narrower nozzle spacings allow for reduced boom heights above the target.

It is important to remember that taller standing stubble usually requires a higher release height to ensure uniformity, and will tend to intercept more of your droplets before they reach the ground. However, standing stubble is often easier to manage and penetrate than fallen stubble or thick chaff that is thatched on the ground surface (these require higher application volumes, over 100 L/ha).

If the nozzle spacing matches the row width, and the nozzle is set midway between the stubble, then it may be possible to further reduce boom heights, such that the overlap from adjacent nozzles meet on the stubble line. This strategy may also reduce the potential for shadowing.

Boom height is often set in relation to the travel speed and boom width. Travel speed for any sprayer should be restricted to a point where boom stability and the correct height to achieve the appropriate overlap can be maintained.

Conclusion

There is no single setup that will maximize deposition for all situations. Good pre-emergent spraying is about doing all of the little things well.

The best way to maximize deposition with your machine is to conduct your own tests on deposition using water sensitive paper. Check the impact of variations in nozzle type, application volume, boom height and travel speed to compare their effects on deposition in and around the stubble to determine what works best in your situation.

Contact Details

Bill Gordon, Bill Gordon Consulting Pty. Ltd.
PO BOX 4197 Lawrence, NSW 2460.
email: bill.gordon@bigpond.com
mobile: 0429 976565