Glyphosate was applied alone and with either ammonium sulfate (AMS) or RUSA 703 (a premix of hydroxypropyl guar and AMS). Herbicide treatments were applied at 3-h intervals starting at 6:00 am and ending at 9:00 pm. The efficacy of glyphosate was dependent on the application time of day for all weed species tested, except for common waterhemp in the greenhouse. In general, applications at 6:00 am and 9:00 pm resulted in the least control of all weed species. The efficacy of glyphosate was often improved with the addition of AMS when glyphosate was applied at a suboptimal time of day. RUSA 703 did not result in any benefit in glyphosate efficacy beyond AMS. The addition of RUSA 703 to glyphosate improved the efficacy of glyphosate as often as AMS in greenhouse studies, but not in field studies.