Spray quality data for nozzles – Implications for use and advice

Call to action/take home messages

Advisors and growers need to critically evaluate the claims made on adjuvant labels or in technical literature about the products they plan on using, as well as the spray quality data for nozzles supplied by their manufacturers for legal compliance, efficacy and drift control.

Ensure growers select nozzles based on current spray quality information, such as the GRDC nozzle selection guide, 2017.

Why do we use adjuvants?

The primary purpose of adding an adjuvant to the tank mix should be to improve efficacy.

This may be achieved through different mechanisms, such as;

• increasing spread of the droplet on the leaf surface,
• modifying the leaf cuticle to improve penetration,
• adjusting the pH of the solution to reduce interactions with cations in the water or on the leaf surface,
• reducing evaporation to allow more time for the product to enter the target,
• reducing undesirable interactions between products in the tank mix, or
• improving droplet retention by reducing droplet bounce or shatter.

Figure 1. Behaviour of droplets on a leaf surface (with and without an adjuvant).
Source: Adjuvants – Oils, surfactants and other additives for farm chemicals, GRDC 2012.

To change the behaviour of the spray solution, or of a droplet on the leaf surface, the physical and chemical properties of the spray solution usually need to be modified in some way. The most obvious effect of adding an adjuvant to the spray solution is a change in the dynamic surface tension.

Lowering the surface tension causes droplets to spread on the leaf surface, which can increase contact with the leaf surface, improving uptake. However, reducing surface tension of the spray solution can also modify how the droplets themselves are formed as they leave the nozzle, typically reducing their size (compared to water alone).

Table 1. Typical dynamic surface tension values (dynes/cm) for some common adjuvant types

One of the main factors influencing the droplet sizes produced by a nozzle is the nozzle design itself, that is some nozzles are coarser or finer than others. The spray solution also has an influence, where products with a lower dynamic surface tension tend to produce finer droplets than product with a higher dynamic surface tension. Other factors including viscosity and solution temperature can also impact on how droplets are made through various nozzles. Typically, the more uniform the pattern is as is begins to break up, the more uniform the range of droplet sizes produced will be (compare the uniformity of the emulsion in figure 2, to the other solutions).

Figure 2. Effect of various adjuvant types on a TeeJet® AIXR11002 at the same pressure.
Source: University of Queensland, C-START

Spray quality according to various standards

Spray quality is not a direct measurement of drift, but a measurement of the range of droplet sizes produced by a nozzle.  Spray quality data may be reported by nozzle manufacturers against a couple of different standards including the British Crop Protection Council (BCPC) or the older American Society for Agricultural Engineers (ASAE) Standard S572, which are both mentioned on some Australian labels.

Both the BCPC and older ASAE standards report spray quality based on water alone being sprayed through the nozzle.

More recently the ASAE has changed its name to the American Society for Agricultural and Biological Engineers (ASABE) and has adopted a new standard for spray quality known as the ASABE S 572.1. The new standard requires that testing of pre-orifice and air induction nozzles include the addition of a 40 dynes/cm adjuvant to water as the test solution. This has been designed to provide data that better reflects the spray quality that a typical tank mix may produce, rather than water alone. As a result, recent nozzle charts (see figure 3) may show spray qualities that may appear to be finer than older charts that may still be in circulation. It is important that nozzles are selected based on the best available data.

Figure 3. Comparing old and new spray quality data for the same nozzle.
Source: GRDC Grownote – Spray Application for Grain Growers, 2017.

Contact details

Bill Gordon
Nufarm Australia
Ph: 0418 794 514
Email: bill.gordon@nufarm.com

® Registered trademark

^TM trademark