Getting the right mix

Author: | Date: 24 Feb 2015

With growers trying to reduce the number of spray passes on a paddock there is a great temptation to start mixing multiple products in the one tank, often with five or six products, which often include two or three herbicides, an insecticide and some form of adjuvant. This really starts to get complicated with the wide variation in the quality of water being used with these mixes.

Add to this the fact that there are over 300 branded adjuvant products on the Australian market with 30 different active ingredients, it is no wonder agronomists and farmers can get into trouble.

Reading the herbicide label usually gives a good indication of what the company considers as compatible with their product although this information might not be as detailed as the user would desire.

An excellent resource is Table 33 in NSW Department of Primary Industries 'Weed Control in Winter Crops 2014' which looks at a wide range of pesticide compatibilities.

The pesticide user needs to remember a pesticide active ingredient has to be formulated to be:

  • As biologically effective as possible in controlling the pest/weed/disease
  • Physically and chemically stable in storage (minimum of 2 years)
  • Easy to mix
  • Compatible with other pesticides
  • Stable as a spray solution

To achieve these qualities a range of other chemicals are mixed with the active ingredient and these are known as 'inerts''. These 'inerts' include:

  • Buffers
  • Dispersants
  • Defoamers
  • Solvents
  • Emulsifiers
  • Dyes

Another complication is that formulations for the same active ingredient usually vary from manufacturer to manufacturer. The 'inert' ingredients will also vary in quantity and quality.

Tips about chemical compatibility and how to assess what mixes work

It might sound trite, but read the label:; there is a lot of good information on spray volumes and compatibility to be found.

The major factors that cause spray tank incompatibility include:

  • Spray volumes - higher spray volumes tend to have fewer problems as there is 'more room' in the solution for the various components.
  • The greater the variety of formulations (formulation type and active type) added to the spray tank, the greater the chance of problems, such as the active coming out of solution and becoming ineffectual.
  • pH – This is only a problem in extreme cases.. Typically glyphosate adjusts the pH to the correct level so test your spray mix pH, not your water pH. Acidifiers (such as Li-700) are only required in extreme cases.
  • Hardness – Metal ions such as calcium and magnesium can create tank mix problems with soluble liquids (SLs) such as glyphosate, and emulsifiable concentrates (ECs) such as triclopyr. High levels of bicarbonate can also affect Group A herbicides and 2,4-D amine. Avoid brackish water and ensure you test bore water for hardness before using. Rain water is always the best bet to avoid any hardness issues.

With soluble liquids (SL) the amine salt used in the formulation (such as dimethylamine, isopropylamine, etc) will dictate the compatibility of the formulation with other formulations and water quality. Some companies will add conditioners to their formulations to improve the compatibility. Suspension concentrates (SC) generally mix well with all water and other formulations.

In general, follow the correct mixing order, give lots of spray volume and try a jar test.

How to conduct a jar test:

  • In a 500 mL jar mix the same ratio of chemicals with the same water at the same temperature and let it sit for 5 hours.
  • Look carefully for crystals that would cause spray filter blockages
  • Look for oil or general glug - a slight cream is not a problem.
  • Remember, a small amount of problem in the jar can mean big problems in the tank!

Follow the proper mixing order (general guidelines below, but read the label to verify):

  • Fill tank two thirds with clean water, turn on agitation. Make sure there is lots of room for the fomulations to dilute.
  • Add water conditioners such as ammonium sulphate or EDTA. This covers up hard water ions that may cause incompatibility issues.
  • Add granules / flowables / powders and mix well. These take time to mix in so don’t rush it.
  • Add ECs and crop oils before SLs, this allows the emulsion to set up first. Add soluble liquids (SLs), such as glyphosate - the ions added at this stage are less likely to interfere with theemulsification
  • Add wetters (e.g. BS-1000). These are surfactants and have to be added after ECs have formed an emulsion. Adding wetters at the end also results in less foaming.
  • Top up with water to final spray volume.

Further information:

Contact details

Rob Buttimor
Dow AgroSciences
Ph: 1800 700 096
Email: rbuttimor@dow.com