Methods to minimise spray failures through tank mixing
- Check the pesticide label for tank-mix recommendations and note any restraints, for example, water quality, incompatible mixes.
- If the mix is not well known and no label recommendations exist, do a jar test prior to spraying.
- Mix all pesticides properly and according to labels.
- Make sure all components of your spray rig have the correct filters.
- If you have a spray blockage, try to retrieve the mix, before disposing of your tank mix.
Check the label
First, check the labels of each product to be mixed. If the recommendations state that those products should not be mixed, don't mix them. If no label recommendation exists, seek competent advice and do a jar test (see How to do a jar test below) if you wish to proceed.
Some mixes can not be sprayed at all, for example, 2, 4-D amine and copper sulphate form a very insoluble green precipitate which is difficult to clean from tanks, filters, nozzles and control valves.
Other mixes are synergistic and if sprayed onto crops may cause crop damage. When controlling weeds prior to sowing, such mixes may be useful as they can cause the weeds to die faster.
Also, be aware of the water requirements for chemicals. For instance some pesticides such as glyphosate are deactivated by calcium and other salts so adding fertilisers such as calcium nitrate or trace elements particularly soluble salts of zinc, copper, manganese or magnesium will reduce the effectiveness of the chemicals. Similarly, some insecticides including dimethoate are rapidly decomposed in alkaline water.
How to do a jar test
If you are considering using a complex mix for which there seems to be little available information and no label recommendations exist, carry out a jar test first to check if the mix is sprayable. The jar test will give the same concentration of products as a tank-mix that is to be applied at a spray volume of 50 litres per hectare (L/ha). If the mixture remains stable, that is, it is free from formulation and chemical incompatibilities, then it should not cause blockages and can be sprayed. However, biological incompatibilities are not revealed by this test.
- In a screw top jar add 500 millilitres (mL) of water.
- For emulsifiable concentrate (EC) or ultra low volume (ULV) formulations, for every 1L/ha of product to be applied in the field, add 10mL to the jar, for example, if you are spraying 1L/ha of glyphosate and 200mL/ha bifenthrin, place 10mL of glyphosate and 2mL of bifenthrin into the jar.
- Disposable syringes are very useful for measuring out small quantities of liquids. Some ECs will dissolve plastics very rapidly so the life expectancy of the syringe can be as short as 30 minutes.
- For granular formulations add ¼ teaspoon of granules to the jar for each 100 grams per hectare (g/ha) to be applied in the field.
- Cap the jar and shake it well.
- Let the jar stand undisturbed for at least two hours, but preferably overnight.
- Look for any obvious incompatibility such as flocculation or precipitation. Some settling of flowable or powder products is normal. But if you have difficulty in re-suspending sediment then extra agitation during spraying is needed or it may cause blockages and uneven application rates or it may be a reaction that seriously affects the pesticide.
- Dispose of the jar and its contents safely after this test.