Salt Antagonism of Glyphosate

Abstract

Glyphosate is often applied with diammonium sulfate to increase weed control. However, many other salts in the spray carrier have antagonized glyphosate phytotoxicity. Research was conducted with wheat as a bioassay species to further determine the influence of various salts on glyphosate phytotoxicity. Cation antagonism of glyphosate occurred with iron > zinc > calcium ≥ magnesium > sodium > potassium. Ammonium cation with hydroxide or most other anions was not antagonistic. Anions of ammonium compounds were of primary importance in overcoming glyphosate antagonistic salts, while the ammonium cation was neutral or slightly stimulatory with certain anions. Sulfate, phosphate, citrate, and acetate anions were not antagonistic, but nitrate and chloride anions were slightly antagonistic when applied as ammonium salts or acids. Antagonism of glyphosate action by sodium bicarbonate and calcium chloride was overcome by phosphoric, sulfuric, and citric acid and phosphate, sulfate, and citrate ammonium salts. Acid and ammonium salts of nitrate and chloride were more effective in overcoming sodium bicarbonate than calcium chloride antagonists of glyphosate. Ferric sulfate antagonism was overcome only by citric, partly by phosphoric and sulfuric but not by nitric and hydrochloric acids or their ammonium salts. Acetic acid, ammonium acetate, and ammonium hydroxide did not overcome any salt antagonism of glyphosate. Glyphosate response to salts was independent of spray carrier pH.

Carrier Volume Affects Wheat Response to Simulated Glyphosate Drift

“…Researchers have shown that the activity of glyphosate can be affected by carrier volume (Sanberg et al 1978;Stahlman and Phillips 1979). Most believe this is attributed to water hardness (Hatzios and Penner 1985;Nalewaja and Matysiak 1991). Water is determined to be hard if specific cations, such as Ca ++ and Mg ++ , are 100 ppmw or higher.…”

Why glyphosate is a global herbicide: strengths, weaknesses and prospects

“…Studies on the reasons for tolerance are rare. Calcium is a well‐known antagonistic ion when present in water used for spraying 21. However, calcium and magnesium, which is similarly antagonistic, are also present on the leaf surfaces of many plants 8.…”

Plants as sources of cations antagonistic to glyphosate activity

“…It is well established that polyvalent cations found in hard waters can antagonise the activity of the herbicide glyphosate [ N ‐(phosphonomethyl)glycine] 1–4. This antagonism is believed to result from the formation of glyphosate salts of low solubility which are less readily taken up into the plant with a consequent decrease in herbicidal activity 5–7.…”

“…It has been suggested that AMS alleviates cation antagonism through the precipitation of insoluble calcium sulphate 2, 4. Other work has shown that interactions between ammonium ions and glyphosate are also important 5.…”

“…AMS can also improve the activity of glyphosate when applied in deionised water. It is important to note that the degree of response was dependent on species 2, 9, 10. This suggests that additional sources of cations, other than in spray application water, are involved in glyphosate antagonism.…”