But many premixed water-conditioning adjuvants in the United States are delivering half or less of the minimum recommended AMS, warns Mark Bernards, University of Nebraska Extension weed specialist.

"Many of these adjuvants don't have enough AMS to consistently improve weed control," Mr Bernards said.

"In many cases, performance is no better than glyphosate alone."

In a recent Nebraska trial, some actually reduced control – lending to the rise in glyphosate resistance.

The top-performing adjuvants in this study were the ones that delivered the minimum recommended volume of AMS.

The relationship between control and high AMS content has shown up in numerous land-grant field trials.

"Ammonium sulfate reduces the antagonism that can occur when glyphosate ions bind to mineral ions in hard water," Mr Bernards said.

"If you add enough ammonium sulfate to the spray tank, then the glyphosate ions will bind to the ammonium ions instead of the hard water minerals."