High concentration and low digestibility of stem fiber constituents in herbage of alfalfa (Medicago sativa L.) can limit dry matter intake and energy availability in ruminant (dairy and beef) production systems. Stem neutral detergent fiber (NDF), acid detergent lignin (ADL), and stem in vitro neutral detergent fiber digestibility (IVNDFD) are impacted by the environment. We identified 45 individual alfalfa plants that differed in stem NDF and ADL concentrations, ADL as a proportion of NDF (ADL/NDF), and stem IVNDFD after 16 and 96 h of digestion. All plants were vegetatively propagated and established in replicated trials and sampled 12 times (spring and early and late summer harvests for 2 yr at two field locations). A subset of clones were evaluated in two greenhouses and harvested for two regrowth cycles. Stability analyses demonstrated that stem ADL/NDF, NDF, and 96-h IVNDFD were more environmentally stable among the alfalfa clones in the field environments tested than stem 16-h IVNDFD or ADL. All stem fiber quality traits were considered consistent between the two field environments, but no relationship in trait responses were demonstrated between field and greenhouse environments. These results suggested that stem 96-h IVNDFD, NDF, and ADL/NDF would be the traits to use in a field selection program to develop alfalfa with improved nutrient value.