Nutritive value improvements in both alfalfa and perennial grasses have the potential to significantly increase economic returns when fed to dairy cattle. Our goal was to assess yield and nutritive value of genetically modified reduced-lignin alfalfa in monoculture and in binary mixtures with perennial grass. A HarvXtra-type alfalfa (Hx14376) and a conventional alfalfa (WL355RR) were sown in small plots as pure stands and in mixture with festulolium (Fojtan), meadow fescue (Driftless), or orchardgrass (Dividend VL). Studies were sown in the spring of 2016 in Ithaca, NY, Lexington, KY, and Rosemount, MN, and harvested in 2016 (NY and KY only), 2017, and the spring of 2018. Two harvests were taken in the seeding year, and 3 harvests (MN), 4 harvests (NY), and 4 (flower stage) or 5 (bud stage) harvests (KY) were taken in 2017. Plots were harvested at alfalfa first bud and first flower stages in NY and KY, and harvested at first flower stage in MN. Alfalfa-grass mixtures outyielded pure alfalfa (P < 0.05) for 3 of 8 site-year combinations, but never yielded less than pure alfalfa stands (P < 0.05). Delayed harvest to flower stage for alfalfa resulted in increased yield if an equal number of cuttings were taken for both bud and flower stages, but reduced yield if more harvests could be taken from the bud stage than flower stage over the season (P < 0.5) (e.g., KY in 2017). Orchardgrass proportion of mixtures greatly increased between 2016 and 2018, from 10% to over 80% grass in NY, and from 19% to 62% grass in KY. Minnesota had no 2016 data, but orchardgrass mixtures had only 15% grass in 2018. Reduced-lignin alfalfa averaged (annually) between 11.0 and 18.5% less lignin over the 3 years of the study, and averaged between 5.8 and 10.6% greater fiber digestibility, compared with WL355RR. Alfalfa nutritive value response was relatively consistent across regions, but grass response was variable across regions for both grass proportion in mixtures and nutritive value. Perennial grasses grown with alfalfa must be evaluated for both yield and nutritive value on a regional basis to produce meaningful results.