"It is a balancing act," says Kuehl. "In the past, if glyphosate was in a tank mix, we would recommend an adjuvant that optimized glyphosate activity, as it was doing 90% of the work. With resistant weeds, you may want to optimize the activity of a tank-mix partner and give up some glyphosate performance."

Increased specialization of adjuvants also adds complexity. Water quality importance has created demand for adjuvants to counter problems like salts, which can antagonize glyphosate, and pH ranges, that can speed breakdown of insecticidal active ingredients. Even the weed being sprayed can influence adjuvant selection.

"You can clean up the water at its source, but a number of weeds carry calcium on the surface of the leaf, and that calcium can contaminate a herbicide when it hits and cause antagonism without the right adjuvant being added," says Kuehl.