Influence of diammonium sulfate and other salts on glyphosate phytotoxicity

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Abstract
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.