Factors affecting herbicide performance

Tank mixing and compatibility of herbicides

Care needs to be taken when tank mixing as some herbicides are incompatible. Tank-mixing can minimise cost; each application has a cost in labour, fuel and wear on the spray unit and secondly, make best use of suitable spray weather, provide synergism - some herbicides enhance the performance of others, providing better weed control than when either herbicide is used alone, or when used in separate applications.

Warning: Some herbicides can be either physically or chemically incompatible and cannot be tank-mixed. Incompatibility may lead to application problems, have poorer weed control and/or cause crop damage. Always seek professional advice before trying a new mix and always check for product specific mixing directions on the label.

Formulation incompatibility

Formulation incompatibility is usually a problem of emulsifiable and flowable mixes. The solvent and/or emulsifier in the emulsifiable product can strip the dispersant from some flowable formulations. As a result, the active ingredient from the flowable formulation will flocculate and settle out. Certain combinations of simazine and trifluralin have done this in the past.

Manufacturers may change the emulsifiers and dispersants in their formulations from time to time in response to supply or price changes. A mix that was stable one year cannot be assumed to be compatible later. It should always be tested first, using the jar test described below.

Chemical incompatibility

Some products cannot be mixed because the two components react. This is usually a problem where the positive ion of one product reacts with the negative ion from another. The reaction between the ions reduces efficiency at best, but at worst will produce a tank full of gel. A striking example is 2,4-D amine and copper sulphate. Tank-mixing these produces a thick gelatinous precipitate which is extremely insoluble.

Compatibility jar test

Before making a sprayer-full of tank-mix for the first time, it is advisable to check the compatibility of the components, unless it is a recognised mix shown on the label. This can be done conveniently on a small scale using a medium size screw-top jar.

Instructions

1. Use water at the same temperature as the spray supply (preferably use the same water).
2. Take a 2 litre (L) jug or sealable jar
3. Add 500 millilitres (mL) water
4. Add flowable, granular or powdered products and mix.
5. Add emulsifiable concentrate (ester or oil soluble) products and mix.
6. Add water soluble products and mix.
7. Add another 25mL and mix.
8. Add other adjuvants and trace elements etcetera and mix.
9. Add wetting agents and mix.
10. Add spray oils and mix.
11. Add water to make up to 1L and mix.
12. Leave the mixture to stand for several hours and note any separation or layering. Any settling of flowable or powder products is normal, but any difficulty in resuspending any sediment should be noted.

Warning – physical compatibility does not imply biological compatibility.