THE BASIS FOR THE HARD-WATER ANTAGONISM OF GLYPHOSATE ACTIVITY

Hard-water cations, such as Ca+2 and Mg+2, present in the spray soluti on can greatly reduce the efficacy of glyphosate, These cations potent ially compete with the isopropylamine in the formulation for associati on with the glyphosate anion, C-14-Glyphosate absorption by sunflower was reduced in the presence of Ca+2, The addition of ammonium sulfate overcame the observed decrease in C-14-glyphosate absorption, Nuclear Magnetic Resonance (NMR) was used to study the chemical effects of cal cium and calcium plus ammonium sulfate (AMS) on the glyphosate molecul e, Data indicate an association of calcium with both the carboxyl and phosphonate groups on the glyphosate molecule, Initially, a random ass ociation of the compounds occurred; however, the reaction progressed t o yield a more structured, chelate type complex over time, NH4+ from A MS effectively competed with calcium for complexation sites on the gly phosate molecule, Data suggest that the observed calcium antagonism of glyphosate and AMS reversal of the antagonism are chemically based.