Boom spray technology - improving coverage and managing drift

Author: | Date: 26 Jul 2017

Take home messages

  • Classification of droplet sizes has changed.
  • Future spray drift challenges will be presented by new crop technologies.
  • The tank mix has the potential to affect droplet size and drift risk.
  • Summer spray drift can be managed by using extra coarse to ultra-coarse droplets, reducing ground speed, adjusting application volume, ensuring nozzle pressure is in the optimum range and adopting surfactants that positively manipulate droplet behaviour.


Spray application target - hitting or missing?

Today, the person applying the pesticide (spray applicator) has available to them a range of drift reducing technologies that can assist them in reducing the risk of drift. If the risk of drift is to be reduced it is vital that the applicator is aware of what is available to them to reduce the risk of pesticide moving outside the target area.

Australia’s regulatory environment has changed with the adoption of a Spray Drift Risk Assessment to reduce the impacts of off target movement of pesticides. The APVMA has adopted, and is currently still working on, a drift risk assessment approach to the review processes that are in place for existing and new agricultural and veterinary chemical products. This drift risk assessment approach has seen a strengthening of directions on the product labels to ensure that the applicator makes informed decisions on how they can best manage the application of pesticide without adverse impacts on the neighbouring land users and the environment.

Definition of drift and drift drivers

The APVMA currently defines ‘spray drift’ as:

The physical movement of spray droplets (and their dried remnants) through the air from the nozzles to any non-or off target sites at the time of application or soon thereafter. Spray drift shall not include secondary movement of agricultural chemicals to non-or off-target sites caused by volatility, erosion, surface or ground water transport or windblown soil particles that occurs after application.

The above definition is intended to encompass the uninterrupted flight of a droplet from the nozzle to the impact point and point of capture by a physical object such as soil or plant surfaces. The words ‘soon thereafter’ typically refer to a time period of less than an hour. However, if an application was conducted during a surface temperature inversion condition, it is possible for very small droplets to remain suspended in the air for a number of hours until atmospheric conditions change and air movement carries the droplets to some impact point. The essential concept is that spray drift occurs during the flight of the droplet from nozzle to eventual impact and capture off target and is not properly defined to include secondary movement of a chemical due to transport by other mechanisms such as subsequent soil erosion or direct volatility (APVMA Operating Principles in Relation to Spray Drift Risk 15 July 2008)

The key drivers of spray drift are:

  • Droplet size (spray quality).
  • Spray boom height.
  • Speed of operation.
  • Wind speed.
  • Temperature/humidity/inversions.
  • Formulation type in the spray mix

These drivers of spray drift can be addressed by examining the following extract from the Nufarm Amicide Advance 700 Selective Herbicide Label - Version: 21Mar2014:

DO NOT apply when wind speed is less than 3 or more than 20 kilometres per hour (ground application) as measured at the application site.
DO NOT apply when wind speed is less than 3 or more than 15 kilometres per hour (aerial application) as measured at the application site.
USE ONLY COARSE or larger spray quality according to the ASAE S572 definition for standard nozzles.


  • Have you cleaned/decontaminated your boom sprayer?
  • Have you contacted your neighbour prior to spraying?
  • Is your sprayer set-up correctly for the particular application?
  • Check:
    • Boom calibration.
    • At nozzle - nozzle choice.
    • Low drift/what spray quality.
    • Coarse or larger spray quality?
    • Boom height - speed of intended application - water volume
  • You must check, determine and record the weather conditions immediately prior to, and immediately after the spray application is made.
  • Record:
    • Temperatures.
    • Relative Humidity.
    • Delta T.
    • Wind speed.
    • Is there a temperature inversion?
  • Night spraying - extra care is required to ensure that inversion conditions are not present. Use a smoke generator to determine the wind direction and presence of inversion conditions.

For further information refer to nufarm website.

Droplet size

The required spray quality is specified on the registered labels of crop protection products. To reduce the possibility of spray drift it is important that you are using a coarse spray quality for application.

Note, the ASAE S572 standardhas recently been updated. Therefore, applicators need to check the ASABE Standard S-572.1, the new spray quality standard, to ensure that their spray quality still meets the legal requirement. For example, the new classification for the AIXR 11002 nozzle is only coarse to 3.5 bar whereas, previously this nozzle was classified as coarse to 5 bar (Table 1).

Table 1. Changes to the ASABE Standard.

Air Induction Exchange Range1.

(Source: TeeJet Technologies ASABE Standard S-572.1 July 2016)

The spray qualities depicted in Table 1 reflect the performance of the nozzles with water and do not make any allowance for the effect tank mix has on spray quality.

Figure 1. The ASABE Standard S-572.1. (Source: ASABE S572.1 Droplet Size Classification)

Driftable fractions

The driftable fraction is the percentage of spray volume smaller than 141 microns. At this size, a droplet will not fall to the ground under its own weight.

Table 2. Driftable fractions within a spray quality class.

Droplet size spray quality class

Very Fine




Very Coarse

Extremely coarse

% of spray volume smaller than 141 microns





Less than



An example of how droplet size and driftable fraction can affect level of spray drift

Operator A is summer spraying with an AIXR 11002 nozzle at 4 bar, operating at 18km/hr with an application rate of 61L per ha using Amicide Advance 700® (700g/L 2,4-D) at 0.5L per ha, and Weedmaster® DST at 1.3L per ha. These products have label requirements for coarse to very coarse spray qualities.

Estimates are that it takes 0.73grams of 2,4-D to damage one hectare of grapes (Ag Vic). In this example, the applicator is applying 350grams/ha of 2,4-D through a nozzle that is potentially losing 10 to 12% of its volume in driftable droplets (Table 2). At a 10% loss that equates to 35grams of 2,4-D in the atmosphere per ha. A spraying rate of 50ha/hr has the potential to contribute 1.75kg of 2,4-D into the atmosphere in an hour which can be exacerbated if all of Operator A’s neighbours are spraying as well. Nozzle choice is paramount (Figure 2).

Figure 2. Cross section of TeeJet® nozzles with high, medium and low propensity for spray drift (Source: Catalog 51A-M, Teejet Technologies, p.151).

Boom height

Keep the boom height as low as possible. At 50cm nozzle spacings, ideally the boom should be maintained at 50cm above the target for double overlap of nozzle output with 110o fan angle nozzles. Drift potential will increase as boom height increases. Moving from 50cm to 70cm above a target will see a four-fold increase in drift potential. Boom height variations can be caused by instability issues as a result of high ground speed.

Operating speed

Operating speed should be adjusted to suit various operations. Higher ground speeds of greater than 25km/hr will deposit spray higher in the crop canopy. Slowing down allows for better penetration into stubble or crop canopy. To avoid problems, spray applicators should not exceed 25km/hr.

Wind speed

New crop protection label directions give allowable wind speed limits that must be observed during application. For example, the Amicide Advance 700® label specifies a wind speed of 3 to 20km/hr. However, a wind speed of 3km/hr is far too low for night spraying if there is a risk of a surface temperature inversion (see discussion within Temperature/humidity/inversions section).


Depending on the formulation of the crop protection product spray tank mixtures, temperature and humidity can affect the potential for drift.

Water based tank mixes if sprayed within the fine droplet spectra, are rapidly evaporated when temperature is high and humidity is low. High temperature and low humidity causes the droplets to lose mass or size, and therefore, they remain suspended in the atmosphere for a longer period of time and become more prone to drift. Once a droplet achieves a droplet size of less than 150 microns, it is at the will of the wind direction as to where it lands.

The applicator must observe the temperature and humidity at the time of spraying and select droplet sizes that are coarser in order to reduce evaporation and give the droplets greater mass to manage the drift risk in this situation. Spraying with the wind blowing away from the sensitive areas will further reduce the risk of spray drifting onto the sensitive areas.

Weather conditions of low temperature and high humidity can also increase the drift potential of fine droplets. These conditions reduce the rate of evaporation and enable small droplets to be transported for longer distances before they completely evaporate. These conditions occur commonly in association with a stable, still environment that may have layers of air with temperature variations or surface temperature inversions. A surface temperature inversion consists of layers of atmosphere near the earth’s surface in which temperature increases with height (Tepper 2014). This is the inverse of the normal as temperature usually decreases with height. When the surface temperature inversion first forms it is only a few metres deep, however as the night progressively cools, the temperature inversion increases in depth reaching a maximum depth at about sunrise at which it may be tens of metres or a few hundred metres deep.

Calm or very light wind conditions often accompany an inversion. Therefore, night spraying poses a particular risk, especially when wind speeds fall below 11km/hr. This minimum wind speed of 11km/hr is a much higher rate than the daytime spraying level of a minimum of 3km/hr. This is because a higher wind speed is needed to mix the cooler denser air and prevent it from separating into layers. Daylight spraying is far safer when there is the potential for surface temperature inversions to form.

It is unsafe to spray when surface temperature inversion conditions exist as the potential for spray drift is high. In 2004, there were several major drift incidents along the Murray River affecting horticultural crops. These drift incidents were the direct result of 2, 4-D material being sprayed under surface temperature inversion conditions. The significance of 2004 is that this was the period when the adoption of Globally Positioned Systems (GPS) escalated rapidly and as a consequence widespread night spraying occurred. At this time, the spray application industry was focussing solely on the evaporation rate of the atmosphere, or Delta T. Spray recommendations were to avoid Delta T levels of less than two and greater than eight. Ironically, this attempt to minimise drift risk by avoiding these Delta T levels facilitated major drift incidents. Applicators measured Delta T and found measurements of Delta T greater than eight close to or not long after sunrise. Consequently the preference was to spray at night and GPS gave the applicators the ability to do this as the overlap or underlap of spray was avoided by the guidance system. The industry now realises the implications of surface temperature inversions and has shifted its opinion on Delta T. The preference is for an applicator to spray at Delta T measurements between 3 to 10 rather than attempt night spraying during a surface temperature inversion, this is with the proviso that the plants are still actively growing and will take in pesticide. If spray applicators chose to spray at these higher Delta T levels it is advisable that very coarse spray droplets be used and if near sensitive crops, use extra coarse or ultra-coarse spray droplets.

Why then in 2016 and 2017 were there still an unacceptable level of drift onto sensitive crops?

This phenomenon is not just isolated to Australia and it is vital that the industry is prepared to manage issues associated with emerging crop technologies, such as, Dicamba tolerant cotton and soy.

Formulation type in the spray mix

Some products, such as glyphosate combined with non-ionic surfactants (herewithin referred to as ‘fully loaded glyphosate’) will increase the potential for drift (Hewitt et al., 2016). A decrease in droplet size was measured in the wind tunnel across a range of nozzles when fully loaded glyphosate products were added to the tank mix.

The addition of oily products, for example HastenTM, UptakeTM, Liberate® or LI-700 will reduce the volume of air that is sucked into a droplet produced by an air inducted jet. This causes a decrease in the fan angle. To overcome this, the applicator should avoid operating the nozzle at the bottom end of its pressure range.

An app is currently being developed (Hewitt et al. 2017) to determine the implications of tank mix on droplet size. The use of this app will assist with the development of a drift risk plan.

Currently, the question perplexing United States agriculture concerns the effect of ammonium sulphate on the volatility of dicamba. It has been found that the use of ammonium sulphate increases the volatility of dicamba which leads to an increase in drift incidents. These incidents have emerged since the adoption of Roundup Ready 2 Xtend® Soybeans and Bollgard II® XtendFlex® Cotton (Monsanto) and Engenia™ Herbicideɸ(BASF) for the use in dicamba tolerant crops.

ɸCurrently this product is not registered in Australia.

The 2015 Weed Control Guide on the North Dakota State University (NDSU) Weed Science home page (NDSU Weed Science website) clearly states:

  • DO NOT add ammonium sulphate (AMS); AMS will increase dicamba volatility.

2016 and 2017 has seen an unreasonable number of drift issues using these crop technologies with the current call for the suspension of dicamba registration in Arkansas. The Engenia label not only states DO NOT use ammonium sulphate, but it also prevents the use of an acidifying buffer in order to avoid volatilisation of the dicamba product.

These crops have been developed to manage herbicide resistance, particularly to glyphosate. However before adopting these technologies in Australia, research is needed to test the potential drift risk effect of ammonium sulphate and acidifiers.

The drift occurring in the United States is having a disastrous impact on non-dicamba tolerant crops. Great effort has been extended on a stewardship program that only permits the use of ultra-coarse and extra coarse spray qualities when using these products (Figure 3).

Figure 3. Extract from label (ɸNote: XtendiMaxTM is currently not registered in Australia).

Wind speed limits have also been set at 4.8km/hr to 16km/hr and a maximum ground speed of 24km/hr.

Considerations for Australia

Do our water quality issues and our reliance on ammonium sulphate as an adjuvant contribute to the off-target movement of pesticide?

Does the use of acidifiers enhance the potential for spray drift? If so, can we manage this in the mixing procedure?

If there is sensitive crop within a particular postcode or shire, should extra coarse to ultra-coarse nozzles be adopted?

Should a drought proofing scheme be undertaken that enables growers to establish large rain water tank reserves, and therefore, reduce growers’ reliance on ammonium sulphate and acidifiers?

Consider the adoption of a drift reducing surfactant, such as, the product Dead Sure® from Caltex.


Before you spray, or advise others on whether to spray or not, check the spray quality. Take into consideration registered product label requirements, new ASABE standards and the tank mix.

The decision to release a pesticide into the environment places a great deal of responsibility on the person applying the pesticide (spray applicator). The applicator is bound by a ‘duty of care’ and must adopt a risk assessment approach to ensure that the activity does not affect the environment, people’s health and property.

Operating speeds should not exceed 25km/hr.

Do not spray under inversion conditions. Ensure all spray applicators can recognise the presence of an inversion.

If you do spray near sensitive crops, for example within a postcode or shire that produces sensitive crops, you need to utilise better spray technology than would be the current industry standard.

Use adjuvants that will enable extra coarse to ultra-coarse droplets to be more effective.

Lastly, and most importantly, develop a spray drift management plan for your enterprise.

Useful resources

For information on spray planning, calibration and more:

GRDC Grow Notes Spray Application


New herbicide tech demands new nozzle thinking 10 quick points

Xtendimax soy label 2ipuo8e

A Hewitt, C O’Donnell and G Dorr, 2016 Update on GRDC Projects at The University of Queensland as presented at the National Woking Party for Pesticide Application.

Tepper, Graeme (2014) Weather Essentials for Pesticide Application.

2015 Weed Control Guide NDSU Weed Science home page

Hofman, V and Wilson, J (Oct 2005) Choosing Drift-reducing Nozzles, International Conference on Pesticide Application for Drift Management.

TeeJet Technologies Catolog 51A-M

TeeJet Technologies Nozzle Selection Guide ASABE Standard S-572.1

Ferrell, J and Leon, R (February 2017) Using Dicamba in Dicamba-Tolerant Crops

Contact details

Craig Day
‘Shadeland’ 100 Greenethorpe-Bumbaldry Rd, Cowra NSW 2794
02 6345 5818; 0437 432529