Alfalfa/grass mixtures are popular forages that are well adapted to the cool climates of the northern United States.  The addition of a grass to an alfalfa (Medicago sativa L.) crop in this region aids in weed management, winter survival, and forage yield, among other variables.  During establishment, few herbicides are labeled for control of weeds in alfalfa/grass mixtures.  Although some selective herbicides safely control broadleaf weeds in alfalfa/grass mixtures, choices are limited due to the lack of labeled products.  In addition, nothing is available to control grassy weeds in seedling alfalfa/grass forage mixtures.  Glyphosate-resistant Roundup Ready alfalfa became commercially available in late summer 2005.  This technology offers unprecedented weed control and crop safety in pure alfalfa stands, but may benefit alfalfa/grass forage stands as well.  The objective of this research was to evaluate the potential use of glyphosate for weed control in seedling Roundup Ready alfalfa/grass mixtures.  By varying the planting date of the grass in relation to alfalfa planting and a glyphosate application, the potential for good weed control exists.  In treatments that included herbicide application, glyphosate was applied 4 weeks after alfalfa planting.  Orchardgrass (Dactylis glomerata L.) was seeded into the alfalfa at 3 different times; seeded with the alfalfa, seeded 4 weeks after alfalfa, and seeded 5 weeks after alfalfa.  All of the treatments included an untreated check.  In alfalfa/grass mixtures not treated with glyphosate, alfalfa and orchardgrass dry matter was lower, and weed dry matter was higher than the respective treatments that included a post-emergence glyphosate application.  Preliminary results suggest that the species differences present during the establishment year may dissipate during the second year producing similar forages.  Initial observations show that Roundup Ready alfalfa could be a successful addition to weed management for alfalfa/grass mixtures.