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.
Glyphosate toxicity to wheat was antagonized more by calcium chloride than sodium bicarbonate. Mixtures of the salts at greater than 100 mg L−1sodium bicarbonate and 200 mg L−1calcium chloride were additive in antagonism of glyphosate in the greenhouse experiments. Surfactant and oil adjuvants did not overcome sodium bicarbonate or calcium chloride antagonism of glyphosate. Oil adjuvants were generally antagonistic to glyphosate. An equation is presented that determines the amount of diammonium sulfate required to overcome glyphosate antagonism based upon the sodium, potassium, calcium, and magnesium cations in the spray carrier.
Diammonium sulfate is commonly used as an adjuvant with glyphosate, but reports vary regarding its effect on weed control, and its possible function in enhancing glyphosate phytotoxicity is not fully understood. Several experiments were conducted in the glasshouse to determine glyphosate phytotoxicity to various species as influenced by diammonium sulfate in distilled water and in the presence of antagonistic salts. Diammonium sulfate overcame sodium hydrogen carbonate, calcium chloride, and 2,4‐D antagonism of glyphosate phytotoxicity to wheat (Triticum aestivum L.). Sulfate anions were important for overcoming calcium antagonism, possibly by forming calcium sulfate. Scanning electron micrographs of spray droplets of glyphosate with calcium chloride and diammonium sulfate indicated the presence of crystals, presumed to be calcium sulfate, that were independent of the glyphosate deposit. Diammonium sulfate may also provide ammonium ions to form effective glyphosate‐ammonia complexes rather than less effective calcium, sodium, diethylamine, or other cation complexes. Diammonium sulfate also may influence susceptibility to glyphosate by affecting herbicide absorption into foliage of certain species. Glyphosate phytotoxicity to sunflower (Helianthus annuus L.) was increased whereas phytotoxicity to kochia [Kochia scoparia (L.) Schrad.] and soybean [Glycine max (L.) Merr.] was reduced by diammonium sulfate applied in the absence of antagonistic salts; diammonium sulfate apparently has several functions as an adjuvant with glyphosate.
Calcium chloride in the spray carrier antagonized the toxicity of diethanolamine 2,4-D and sodium 2,4-D, dimethylamine MCPA, sodium bentazon, dimethylamine dicamba and sodium dicamba, sodium acifluorfen, imazamethabenz, ammonium imazethapyr, and isopropylamine glyphosate to kochia in greenhouse experiments. Diammonium sulfate overcame calcium chloride antagonism of the above herbicides, except for glyphosate and imazethapyr. Diammonium sulfate or ammonium nitrate adjuvants overcame calcium chloride and sodium bicarbonate antagonism of dicamba toxicity to kochia and enhanced toxicity of sodium dicamba to nearly equal that of dimethylamine dicamba.
Experiments were conducted to determine the influence of spray carrier salts, UV light, and temperature on sethoxydim phytotoxicity to oat or yellow foxtail. Spray solution pH and ions present were both important to sethoxydim phytotoxicity to oat. Sodium and calcium salts were antagonistic to sethoxydim phytotoxicity only when the spray carrier pH exceeded 7. Ammonium salts and ammonium hydroxide were synergistic with sethoxydim, and the synergism was independent of spray solution pH. Ammonium sulfate, but not ammonium hydroxide, overcame sodium bicarbonate antagonism of sethoxydim. The antagonism of sethoxydim phytotoxicity by sodium bicarbonate was greatest in the presence of UV light and most pronounced when treated plants were exposed to mid-day sunlight. Sodium bicarbonate or low temperature may reduce the speed of sethoxydim absorption allowing for greater UV degradation of unabsorbed sethoxydim on the leaf surface.
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