Recent rainfall across much of the western winter cropping areas, has resulted in good soil moisture profiles. As a result effective summer weed control programs will be essential to maintain yield potential into 2015.

Timing of application is critical for effective control and as most summer weed herbicides provide little residual activity, controlling later germinations depends on stopping seed set.

Whether you decide to spray early or late, adjuvant selection is critical to the performance of most active ingredients.

Adjuvants improve herbicide efficacy by reducing evaporation and drift, increasing droplet survival on leaf surfaces and increasing penetration of herbicide active ingredients into the plant.

Which of these is best suited to your needs depends on the active ingredient being used, the weed spectrum and stage, water volume and desired droplet size, spraying conditions and timing of the year, which may require the use of a particular type of adjuvant.

SACOA’s ANTIEVAP® is excellent at improving the efficacy of most summer fallow herbicides such as Triclopyr and/or Glyphosate.

An acidifying surfactant such as COHORT 700® may be of use as a buffering agent if water quality is poor, otherwise provides little benefit in improving droplet survival and penetration.

The final efficacy of a herbicide results from the active ingredient getting from the nozzle into the plant for translocated herbicides or onto the leaf surface for contact herbicides. In most cases herbicides are sprayed out using water as a carrier and an adjuvant to improve this process.

Droplet survival on the leaf surface is an important indicator of herbicide efficacy, and is influenced by three key factors;

• Environmental loss through drift and evaporation influenced by temperature and humidity, travel and wind speed, droplet size and viscosity of spraying liquid. Generally measured using Delta T, volume median diameter of droplets (VMD) and leaf coverage (see Chart 1).
• Evaporation and run-off loss once droplets have landed on leaf surface
• Rate of adsorption through the leaf surface as a result of penetration and desired droplet size, spraying conditions and timing of the year, which may require the use of a particular type of adjuvant.

Adjuvants have an important role in improving herbicide efficacy by:

• Protecting the active ingredient from drift and evaporative losses
• Improving the coverage on the target surface and facilitating uptake through the leaf surface

Oils and adjuvants can be split into two broad types – penetrants and spreaders.

• Penetrants
  Penetrants have properties which break down waxy leaf surfaces, allowing the active ingredient to enter the plant faster. They are generally plant or mineral oil based and due to their higher viscosity have the ability to increase droplet size or Volume Median Diameter (VMD) (see Chart 1).Many grass selective herbicides such as the Group A’s are very poor on their own at entering leaf surfaces, particularly if they are waxy like ryegrass, but are translocated once inside the plant.Hence most are labelled to be used in conjunction with an oil based penetrant adjuvant, where leaf surface coverage is not as critical as penetration.

• Spreaders
  Most spreading type adjuvants are surfactant based, which work by reducing the surface tension of liquids and thereby improve coverage on the leaf surface.Improved coverage is important for non-translocated herbicides such as paraquat, which work on contact and don’t need to enter through the leaf surface.As a result of reduced surface tension, surfactants can decrease droplet size and increase drift. This may result in increased runoff of the active ingredient from the leaf surface, particularly fine leaf grasses, which may result in reduced efficacy (see Chart 1).

A 2012 trial conducted by AgSense at Goomalling examined the effect of time of day on herbicide efficacy and the efficacy of ANTIEVAP®, LI700  and ENHANCE® in some commonly used melon control mixtures.

The trial found;

•  ENHANCE® and ANTIEVAP® proved superior to LI700 in Triclopyr & LVE mixtures
•  ENHANCE® proved superior to LI700 in Triclopyr, Glyphosate and LVE mixtures
• Time of day did not affect the performance of ANTIEVAP® and LI700
• The Untreated Control had >300 surviving melons 3WAA

SACOA have been conducting research into the role of oils and adjuvants in improving droplet survival and final herbicide efficacy, under summer spraying conditions for a number of years.

SACOA’s research work will continue over the 2014/15 summer with work in Western Australia & the Victorian Mallee correlating the impact of droplet size on final herbicide efficacy. Data from a recently implemented trial in Western Australia comparing SACOA’s locally developed and manufactured products with imported alternatives will be published over the next few weeks.

• GRDC – Summer Fallow Spraying http://tinyurl.com/l5a2hy2
• GRDC – Summer Fallow Weed Management Reference Manual http://tinyurl.com/paf7j86
• PIR (SA) – Code of Practice, Summer Weed Control http://tinyurl.com/pdzbls6