Compatibility of Pesticides
Part 2, Section 1: Pest Management
Pesticides and Their Application
Compatibility of pesticides
Pesticides are not always compatible with each other or with the water or liquid fertilizer carrier. Lack of compatibility may result only in the formation of a gel, precipitate, or sludge that plugs up screens and nozzles. But extreme incompatibility may produce a settling out of material that can harden like concrete in the bottom of the tank and in hoses, pumps, and other internal parts of the sprayer. The result may be total loss of the pesticide and use of the sprayer. Incompatibility also can reduce effectiveness or cause crop injury.
Herbicides may be combined with liquid fertilizers to minimize trips over the field; however, information is scarce concerning the compatibility of herbicides with specific fertilizer solutions. Herbicide-fertilizer solution combinations may form a gel or precipitate that settles to the bottom of the sprayer tank or that will not flow through the sprayer equipment.
Tank-mixing several pesticides, although convenient, may create other problems. Foliar activity may be enhanced and result in crop leaf burn, or the activity of one or more of the pesticides may be reduced (“antagonism”).
To prevent the main water tank or liquid-fertilizer measuring tank from becoming contaminated, commercial applicators may want to mix the herbicides and other ingredients in a separate holding tank. The herbicide mixture then is sucked into the main line as the truck tank is being filled and is thoroughly mixed by the truck's agitation system. Compatibility problems are more likely to result when concentrated herbicides are mixed together, so a compatibility test should be done before new mixtures are tried.
Use only labeled tank mixtures or mixtures recommended by experienced scientists whose recommendations are backed by research. Tables 2.2-16 and 2.2-17 (corn) and 2.4-14 and 2.4-15 (soybeans) list approved herbicide tank-mixes. Compatibility of selected herbicides and insecticides is given in Table 2.2-23. For all unlabeled tank mixtures, a jar test is strongly recommended to test for compatibility. The compatibility of herbicide-fertilizer combinations should be tested before large batches are mixed. In some cases, adding a compatibility agent (Compex, Unite, or comparable surfactants) may help maintain component dispersion.
The following “two-jar test” may be used to test the compatibility of herbicides with each other or of herbicides and other pesticides with liquid fertilizers.
- Add 1 pint of carrier (water, liquid fertilizer) to each of two quart jars.
- Add ¼ teaspoon of compatibility agent to one jar (equivalent to 2 pints per 100 gallons of spray solution).
- To each jar, add the required amount of pesticide in the order suggested above for tank-mixing herbicides. Shake well after each addition to simulate continuous agitation.
- When all ingredients have been added, shake both jars for 15 seconds and let stand for 30 minutes or more. Then, inspect the mixture for flakes, sludge, gels, or nondispersible oils, all of which may indicate incompatibility.
- If, after standing for 30 minutes, the components in the jar with no compatibility agent are dispersed, the herbicides are compatible and no compatibility agent is needed.
- If the components are dispersed only in the jar containing the compatibility agent, the herbicide is compatible only if a compatibility agent is added.
- If the components are not dispersed in either jar, the herbicide-carrier mixture is not compatible and should not be used.