There is a temptation amongst growers and agronomists to mix multiple products in the spray tank to reduce the number of spray passes required in a season. It is not uncommon for spray mixes to contain two or three herbicides, an insecticide, a fungicide and liquid fertiliser, plus one or two adjuvants.

While this might seem an efficient use of time, the applicator may be getting into unknown territory once more than two or three products are mixed.

There can be both chemical reactions that cause sediment and blockages and/or mode-of-action antagonism (products mix ok, but biological performance is reduced) that lead to suboptimal control or total spray failure.

Where a chemical reaction occurs that causes sediment or blockages it is often common to blame the product. However there are many factors that influence compatibility:

• temperature of the water (usually warmer water increases solubility which assists compatibility)

• water quality (higher concentration of cations, bicarbonates or organic matter usually causes more problems)

• water pH (for some products the chemical reactions will change at different pH levels)

• tank mixed adjuvants

• adjuvants in the formulation (which can be different between different brands of the same herbicide)

• concentration (generally problems are more likely when lower spray volumes are used, or products pre-mixed in batch tanks using small volumes of water)

• products added to the spray tank too early (fill the tank to above 70% capacity and then add water conditioning agents such as ammonium sulphate before adding pesticides)

• correct mixing order (usually products are added from least soluble to most soluble)

• time (low soluble products need time to fully disperse in the tank before adding the next product. This will take longer with colder water)

• formulation quality / stability.

These interactions become even more complex when more than two products are mixed. Due to the number of different factors involved, it may be difficult to replicate the exact conditions that caused the reaction.

Often haste to refill the sprayer is one of the primary cause of problems. Sometimes these ‘spray cauldrons’ can lead, at best, to blocked filters all the way through to tanks full of solid mix.

Biological antagonism is often more difficult to observe, as products may mix well in the spray tank but performance is reduced in the field due to the way the products interact in the spray tank or inside the plant. 

Where biological antagonism is weak to moderate, it may be possible to still use the mix by increasing the rate of the affected partner. A common example of this is mixing glyphosate with many broadleaf partners such as Group 4 or Group 14 herbicides. These mixtures often reduce the performance of glyphosate on grass weeds, however the mix may still be able to be applied where the rate of the partner herbicide is kept low and the rate of glyphosate is increased to compensate for the antagonism. As glyphosate resistance develops, the magnitude of these interactions becomes more evident.

Always read the label for directions of use and compatibilities. It is also very useful to refer to state crop management notes or GRDC publications related to specific crops and products.

What is in the herbicide container?

Herbicides come in a range of formulations, determined by the herbicide’s solubility in water and the use for which it is intended. A container of herbicide contains the active ingredient, typically the surfactant required for that product and other 'inert' ingredients required for formulation stability and performance. Herbicides are formulated to be:

• as biologically effective as possible over a range of conditions;

• physically and chemically stable;

• easy to mix;

• compatible with a limited number of other products; and

• a stable spray solution in a range of water qualities.

To do this the active ingredient is mixed with a range of other chemicals such as solvents, emulsifiers, wetters, dispersants, safeners, buffers, dyes. For products such as paraquat, a stenching agent is included to add smell as a warning agent and an emetic is added to induce vomiting if accidentally ingested.

Typically the manufacturer will add the right type and concentration of surfactant that is required for the particular herbicide and its recommended use pattern. Some surfactant types are bulky, for example crop-oil-concentrates or spraying oils and will not be added to the herbicide formulation. Where this occurs, the label will provide directions of what tank-mixed adjuvant is required.

If there is no label recommendation requiring a tank-mixed adjuvant then the starting position should be to assume that the correct adjuvant is already included in the formulation. Adding extra tank-mixed surfactant may be wasting money if not needed, or in several cases may reduce performance if the wrong type of surfactant is added for the particular herbicide type.

Sometimes the label may only require additional surfactant for a particular weed species. This does not imply that the tank mixed surfactant will also improve control of other weeds, and in some cases control may actually be decreased if the wrong surfactant type is added.

Surfactant recommendations become problematic when tank mixing multiple products as commonly the surfactant package required to optimise one product may be completely different to what is required for the other product.

Herbicides are already complex products before the operator even considers tank- mixing them with other products.