Abstract: The effects of ammonium sulfate and pelargonic acid on weed control with glufosinate and glyphosate and safety to glufosinate-resistant and glyphosate-resistant soybean were investigated in the greenhouse and field. Annual and perennial weeds varied in their sensitivity to the herbicides. Based on fresh weight reduction 10 d after treatment (DAT), common milkweed was more tolerant to glufosinate, and horsenettle was more tolerant to glyphosate. Giant foxtail was highly sensitive to both herbicides. The activity of glufosinate on common milkweed and of glyphosate on horsenettle was enhanced with the addition of 5% (wt/v) ammonium sulfate. The addition of pelargonic acid at 3% (v/v) did not enhance the activity of glufosinate or glyphosate on any weed, and it antagonized common lambsquarters and giant foxtail control with glufosinate and with glyphosate. Glyphosate was more effective than glufosinate in suppressing the regrowth of the perennial weeds horsenettle and common milkweed, but addition of ammonium sulfate and pelargonic acid was not beneficial with either herbicide. Under field conditions, the addition of ammonium sulfate or pelargonic acid to glufosinate or glyphosate did not improve efficacy on annual weeds. The addition of pelargonic acid improved yellow nutsedge control with glufosinate, but only at 6 DAT. Glufosinate and glyphosate applied alone or in combination with ammonium sulfate were safe to transgenic soybeans resistant to the respective herbicide. The addition of pelargonic acid to glufosinate or glyphosate in the greenhouse caused a rate-dependent reduction in soybean fresh weight. In the field, slight soybean injury with the addition of pelargonic acid was evident 6 DAT, but not 23 DAT. Addition of ammonium sulfate can increase the efficacy of glufosinate and glyphosate on perennial weeds without negatively affecting soybean yield.
Nomenclature: Glufosinate; glyphosate; pelargonic acid; common lambsquarters, Chenopodium album L. #3 CHEAL; common milkweed, Asclepias syriaca L. # ASCSY; giant foxtail, Setaria faberi Herrm. # SETFA; horsenettle, Solanum carolinense L. # SOLCA; yellow nutsedge, Cyperus esculentus L. # CYPES; soybean, Glycine max (L.) Merr. ‘Asgrow 5547LL’ and ‘Asgrow 4501RR’.
Additional index words: Herbicide additives, Amaranthus retroflexus, Ambrosia artemisifolia, Ipomoea hederacea, Ipomoea lacunosa, Senna obtusifolia, glufosinate-resistant soybean, glyphosate-resistant soybean, AMARE, AMBEL, ASCSY, CASOB, CHEAL, CYPES, IPOHE, IPOLA, SETFA, SOLCA.
Abbreviations: DAT, days after treatment; GR50, 50% growth reduction (the herbicide rate needed to achieve a 50% reduction in fresh weight).