Efficiency is key to maintaining margin. Combining herbicides, fungicides, insecticides, and/or even liquid fertilizer in a single pass fits this thinking (I'm discounting the convenience argument as I would argue that 'convenient' and 'farming' are mutually exclusive).
Individual pesticide labels clearly state whether that product can be tank mixed. Many formulations of the pesticides we use are emulsified concentrates or ECs. These are liquid formulations in which the active ingredient is dissolved in an organic solvent. They often also contain surfactants that help with the uptake of the active ingredient by the plant or insect. Combining several ECs in a mix means that you keep adding to your surfactant load. This, in turn, can, under the wrong environmental conditions, cause some unexpected problems including injury and reduced control (antagonism) even if the individual labels allowed for tank mixing of said products.
What are these wrong environmental conditions? Anything that stresses the plant and slows its metabolism down. Extremely cold or hot weather or saturated soils are all instances that slow the crops' metabolism and can create problems. An example of antagonism in small grains is the Group 6 herbicide bromoxynil in combination with a group Group 1 or Group 2 grass herbicide, a fungicide, and an insecticide mixed in the same tank. The surfactant load in this combination is high enough that with the wrong environmental conditions, the bromoxynil caused enough injury on wild oats that uptake of the grass herbicide was reduced, eventually resulting in reduced control.
Many wheat fields need to be sprayed for weeds. The conditions have been favorable for tan spot and there are aphids in the region. The weather is forecasted to remain relatively cool and wet for the coming week. Adding a fungicide and insecticide seems like a foregone conclusion as cheap insurance and cost-effective. I urge you, however, to scout first and determine the presence of both tan spot and aphids before adding the fungicide and insecticide to the tank. Do not add the insecticide or the fungicide to the tank when the problem you target isn't present. When either or both are present, follow label directions and understand that some crop injury is possible even if the label allows for tank mixing.
Source : umn.edu