Michigan soybean producers have observed that glyphosate efficacy is sometimes reduced in tank mixtures with foliar manganese (Mn) fertilizers. The objectives of this study were to evaluate the effects of Mn formulation, Mn application timing, tank mixture adjuvants, and Mn rate on glyphosate efficacy. Three Mn formulations, manganese sulfate and ethylaminoacetate chelate (Mn-EAA), manganese sulfate and lignin sulfonate chelate (Mn-LS), and manganese sulfate monohydrate (MnSO4) reduced glyphosate efficacy in greenhouse and field bioassays, but Mn ethylenediaminetetraacetate (Mn-EDTA) did not. Mn-EAA applied less than 3 d before glyphosate reduced glyphosate efficacy on velvetleaf but not giant foxtail or common lambsquarters. The antagonism increased as the interval between treatment applications was shortened but did not appear when Mn was applied to velvetleaf 1 d or more after glyphosate. Including the adjuvants ammonium sulfate (AMS), EDTA, or citric acid in the glyphosate–Mn tank mixture increased control of giant foxtail and velvetleaf but only matched the efficacy of the glyphosate plus AMS control in three combinations: AMS with Mn-LS on velvetleaf, citric acid with MnSO4 on giant foxtail, and EDTA with Mn-EAA on giant foxtail. AMS increased the glyphosate efficacy in Mn tank mixtures as much as, or more than, citric acid and EDTA, with two exceptions: EDTA with Mn-EAA on giant foxtail and citric acid with MnSO4 on velvetleaf. Control of velvetleaf declined as the amount of Mn from Mn-EAA, Mn-LS, and MnSO4 in the tank mixture increased.
Nomenclature: Glyphosate; common lambsquarters, Chenopodium album L. #3 CHEAL; giant foxtail, Setaria faberi Herrm. # SETFA; velvetleaf, Abutilon theophrasti Medicus. # ABUTH; soybean, Glycine max L.
Additional index words: Fertilizer, hard-water antagonism, herbicide interaction, micronutrient, split application.
Abbreviations: AMS, ammonium sulfate; EDTA, ethylenediaminetetraacetate; glyphosate-IPA, isopropylamine salt of glyphosate; glyphosate-K, potassium salt of glyphosate; Mn-EAA, manganese sulfate and ethylaminoacetate chelate; Mn-EDTA, manganese ethylenediaminetetraacetate; Mn-LS, manganese sulfate and lignin sulfonate chelate; MnSO4, manganese sulfate monohydrate.