The influence of salts in carrier water and adjuvants on glyphosate activity

Abstract

Glyphosate, a non-selective herbicide, is antagonized by salts in the spray carrier and responds to surfactant type and concentration. Glyphosate antagonism by dissolved salts such as calcium and magnesium was verified with natural water carriers and with carriers to which salts were added. Salt antagonism of glyphosate occurred from the formation of complexes that were less absorptive than the formulated isopropylamine glyphosate. Absorption of various salts of glyphosate varied as follows: isopropylamine > acid > ammonium > sodium > calcium. Ammonium sulphate increased the absorption of glyphosate both in distilled water carriers and in water carriers containing calcium chloride. Absorption and retention of glyphosate generally increased as surfactant (nonylphenol ethoxylate) hydrophilic/lipophilic balance (HLB) increased. The optimal HLB for glyphosate absorption was lower when ammonium sulphate was added to the spray carrier. An experimental adjuvant (trade name: Power-Up) that contained nonionic surfactant and ammonium sulphate, increased glyphosate efficacy more than the currently registered South African adjuvants. This could be as a result of increased foliar absorption and/or retention on foliage. The use of acid containing adjuvants was not essential for adequate glyphosate efficacy. Visual assessment of spray droplet residuals on leaves indicated that the appearance of spray droplet residuals was linked to glyphosate efficacy. Thick, amorphous and grainy spray droplet residuals on the leaf surface was an indicator of poor efficacy, whilst thinner, smoother residuals in close contact with the leaf surface was linked to increased efficacy.