Reduced lignin alfalfa (Medicago sativa L.) has the potential to improve alfalfa forage quality, yet forage morphology, biomass allocation, and stem and leaf forage nutritive value remains undetermined. The objectives of this study were to characterize changes in morphological development and forage nutritive value within stem and leaf fractions for reduced lignin alfalfa. Reduced lignin (‘54HVX41’) and reference (‘DKA43-22RR’) alfalfa cultivars were sampled weekly from the mid-vegetative stage through full flower during the seeding (2015) and first production (2016) year at two locations in Minnesota. Samples were staged to determine maturity, divided into stem and leaf fractions, weighed, and analyzed for forage nutritive value. Alfalfa cultivars were similar in stem and leaf yield, leaf to stem ratio (L/S), stem crude protein (CP), stem neutral detergent fiber (NDF), leaf CP, leaf NDF, leaf acid detergent lignin (ADL), and leaf NDF digestibility (NDFD). On average, reduced lignin alfalfa showed a 14% decrease in stem ADL and a 12% increase in stem NDFD compared to reference alfalfa. For both cultivars, increases in forage maturity increased stem and leaf yields and decreased L/S. Forage maturity also affected stem and leaf forage nutritive values; as maturity increased, NDF and ADL increased while CP and NDFD decreased. These results indicate that changes in forage nutritive value for reduced lignin alfalfa are largely a result of changes in ADL and NDFD within the stem fraction, and confirm that alfalfa forage nutritive value is negatively affected by forage maturity, particularly within the stem portion of the plant.