Annual weed control may be improved when AMS is added to below-label glyphosate doses in glyphosate-tolerant maize (Zea mays L.)

In glyphosate-tolerant crops, weed control may be improved and glyphosate dose may be reduced when ammonium sulfate (AMS) is added to the spray mixture. Much research has investigated how AMS may reduce antagonism between salt ions present in the carrier water and glyphosate molecules, especially when hard water is used as the spray carrier. However, little information is available describing whether glyphosate dose may be reduced when soft water is used. Field trials were established at two Ontario and one Michigan location in 2003 and 2004 to evaluate control of several annual weeds when 0.25, 0.5, 0.75 and 1 x label doses of glyphosate +/-2% AMS were applied in glyphosate-tolerant maize. The concentration of salt and metal ions in the spray water at Harrow and Ridgetown were low, and the concentrations at East Lansing were moderate. Crop tolerance was excellent in all treatment plots. The addition of AMS only improved weed control at the Ridgetown location. Control of Abutilon theophrasti was improved at the 0.25 and 0.5 x glyphosate doses; however, improvements in control did not exceed 65%. Percent weed control below 80% is generally considered unacceptable. Control of Amaranthus retroflexus, Chenopodium album, and several annual grasses was not improved by AMS addition at any glyphosate dose in comparison to the application of glyphosate alone. Overall, the addition of AMS to a soft water carrier improved control of some weed species only at below-label glyphosate doses. Nonetheless, because weed control was not improved beyond 70% when AMS was added, we continue to recommend that the full label dose of glyphosate be applied to reduce the necessity for additional glyphosate applications to maintain acceptable weed control. (C) 2007 Elsevier Ltd. All rights reserved.