The Order Does Matter

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Tank-mixing more than one pesticide is happening more frequently, particularly as we have to deal with herbicide resistant weeds. So I thought it might be time to send out a reminder about the order that the pesticides go into the tank. Putting chemicals in the tank in the wrong order can result in the chemicals forming a “gunky mess” in the bottom of the tank. Sometimes the pesticides will settle out (form gunk) even when the proper order is followed, in which case a compatibility agent is needed. The order of putting pesticides in the tank is based on the formulation. The general order for mixing is as follows:

1. Compatibility agent (if needed)
2. Products in water soluble PVA bags (wait until they fully dissolve before continuing)
3. Wettable powders (first mixed with water in a bucket to form a slurry)
4. Dry flowables or water dispersible granules
5. Liquids (these are the true liquids, they do not turn the solution white when added to water)
6. Emusifiable concentrates (these do turn white when added to water) – this includes crop oil concentrate and methylated seed oil
7. Water-soluble additives (such as dry and liquid fertilizers, i.e. ammonium sulfate or UAN)

Some pesticides may suggest a different order for mixing, so refer to the label. Each type of formulation should be added and allowed to agitate for a few minutes before adding the next formulation. To further reduce chance of incompatibility, mix each pesticide with water prior to adding to the tank.

To check if a compatibility agent is needed prior to adding pesticides in the tank, use a jar test. Use a pint or quart jar and fill half full of the carrier (water or fertilizer). Calculate the proportion of each ingredient (i.e. water, liquid fertilizer, pesticides) to add to the jar. The proportions should be the same as will be added to the spray tank. Assuming spray volume is 25 gallons per acre, a dry formulated pesticide applied to a pint jar at 1½ teaspoons is the same as 1 lb/A. Likewise, a liquid formulation at ½ teaspoon in a pint jar is the same as 1 pint/A. Add the appropriate pesticides separately in the above order and shake the jar gently between pesticides. After all has been added, fill the jar with water, and give a final shaking. Let the jar set for about 10 minutes and look for the formation of large flakes, sludge, gels, or precipitates. This test could be run with two jars, one with a compatibility agent and one without. A compatibility agent added to a pint jar at ¼ teaspoon is the same as 2 pints per 100 gallons of carrier.

If your tank does end up with incompatible pesticides gunking up the bottom, try Dawn dish detergent. I have not done this myself but was told it was effective. Mixing Dawn (Dawn supposedly works better than all other dish detergents) with the water will re-suspend the incompatible pesticides. It is difficult to say how much Dawn to add, but start with a quart and add more if needed.

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