Efficacy of Water-Conditioning Adjuvants for Dicamba-Tolerant Soybean

摘要

Glyphosate and dicamba are weak-acid herbicides and can bind with antagonistic salts in the spray carrier. Diammonium sulfate (AMS) is commonly used as an adjuvant with several weak-acid herbicides to enhance activity and overcome antagonistic salts. Dicamba use in resistant soybean will prohibit the addition of AMS due to the formation of volatile dicamba acid. Glyphosate and dicamba were each applied with AMS and ten commercial non-AMS water-conditioning (WC) adjuvants. Data were combined across 5 geographic locations and 17 grass and broadleaf species. The efficacy of glyphosate and dicamba was reduced when applied in 1,000 ppm hard water. AMS at 8.5 lb/100 gal water completely overcame hard-water antagonism. Control from glyphosate and dicamba applied in hard water with ten WC adjuvants was generally less than the standard of AMS but relatively similar in control. A WC adjuvant containing methylated seed oil provided control equal to the control obtained with AMS. Adjuvants did not contain sulfate salts but did contain acetic and carboxylic acids. At least one adjuvant contained a dipotassium salt. At least three adjuvants contained monocarbamide dihydrogen sulfate (AMADS). AMADS in WC adjuvants apparently was neutralized to some level to prevent a spray solution pH below 5.5. Hard water conditioned by sulfate from AMADS and the slow conversion of urea in AMADS to ammonia may enhance and optimize herbicide phytotoxicity similar to AMS.