Dry and cold conditions put weeds under stress which makes them more difficult to kill, particularly when using herbicides that are poorly translocated, like grass selectives. A number of aspects are important for setting up a boomspray to target stressed weeds – these are aimed at achieving two things;

1. Hitting the target with adequate coverage – by starting out with the boom set up correctly
2. Getting the herbicide into weeds – by matching the adjuvant to the mode of action being used, and ensuring weeds are actively growing prior to spraying

There are a number of tools available to optimise target coverage and herbicide penetration into leaf surfaces. Proper boomspray setup has the biggest impact on coverage, whilst selecting  the correct adjuvant and matching it to the mode of action being used has the biggest influence on leaf surface spreading and penetration.

There are four basic variables that can be managed to get the most out of a boomspray. Water rate, nozzle type, spraying pressure and weather conditions. These will vary for every application but should always be aimed at producing the required spray quality – fine, medium or coarse, to produce the optimum target coverage whilst minimising drift.

A huge variety of boomspray and nozzle types are available.Each manufacturer publishes specific guidelines of pressure, travel speeds etc. to target the required spray quality and water volume. Spending some time calibrating, particularly when changing from summer fallow or knockdown jobs to post emergent spraying is invaluable, particularly to meet APVMA labelling requirements, around spray quality.

Targeting weeds which are not stressed from lack of moisture or immediately following frosts is also important, along with spraying during ideal Delta T conditions.

Adjuvants can influence how well a herbicide works mainly by modifying two things;

1. Coverage – through modifying the viscosity of the spraying solution and;
2. Herbicide leaf surface spreading and penetration – by reducing the surface tension of the spray solution and breaking down waxy leaf coatings (see Figure 2).

The four major types of adjuvant – emulsified mineral oils, seed oils soyal phospholipids and non-ionic surfactants and compatibilisers – vary greatly in their properties as droplet modifiers, leaf surface spreading/penetrating agents, improvers of mixing multiple tank component, and improvers of poor water quality (see Table 1).

Calibrating the boomspray with commonly used mixes versus water can be useful to understand how the mixture modifies sprayer output through viscosity changes. As an example, in aerial applications, using a carrier such as CROPSHIELD® or BIOPEST® can be useful to improve coverage.

In a demonstration trial conducted by Agvivo in 2014, the addition of a high viscosity product such as BIOPEST® resulted in an equivalent improvement in coverage to an additional 10L/Ha of water. BIOPEST® also has insecticidal properties, particularly on developing aphid populations, and as such can make an ideal tank mix partner for herbicide applications.

The effect of adjuvant type on spray quality was also evaluated in a range of summer fallow trials. As clearly seen on water sensitive paper – non-ionic surfactant products produce finer, more readily driftable droplets than PLANTOCROP® or emulsified mineral oils ANTIEVAP® and ENHANCE® (see Figure 3).

• SACOA Fallow Trial, December 2014.
• Agvivo Demonstration Trial Avon Valley, June 2014.