Mahoney, K. J., Nurse, R. E. and Sikkema, P. H. 2014. The effect of hard water, spray solution storage time, and ammonium sulfate on glyphosate efficacy and yield of glyphosate-resistant corn. Can. J. Plant Sci. 94: 1401-1405. Effects of ammonium sulfate (AMS) on reduced glyphosate rates are well documented; however, these rates are not used by farmers. Studies in 2011 and 2012 determined the effects of AMS and hard water on glyphosate applied at a field rate. AMS (0 or 2.5 L ha^-1) and glyphosate (900 g a.e. ha^-1) were added to distilled or hard water carriers. Glyphosate plus AMS, in either distilled or hard water, did not usually affect velvetleaf, pigweed species, common ragweed, common lambsquarters and large crabgrass above-ground biomass and density 8 wk after treatment (WAT); however, velvetleaf biomass was reduced with the addition of AMS to glyphosate. Compared with the weedy control 8 WAT, glyphosate plus AMS reduced velvetleaf biomass by 93%, while treatments with no AMS reduced biomass by 77%. In distilled water, 4.8 common lambsquarters plants m^-2 remained compared with 7.8 plants m^-2 with glyphosate in hard water. For corn yield, an AMS×water hardness interaction was detected; however, no differences were observed among the treatment combinations and the addition of AMS to glyphosate did not affect yield. Therefore, advocating adding AMS to spray solutions containing glyphosate, especially when a labeled rate is used, should be called into question.