Tall fescue (Lolium arundinaceum (Schreb.) S.J. Darbyshire) (formerly Festuca arundinacea Schreb.) often ranks highest for yield among cool-season grass species in the Northeast. Many new varieties are now available with modifications of leaf texture or infected with a novel endophyte. Our objective was to develop a system to adjust yield and quality of varieties to a common NDF value in spring harvested forage. Our management goal is to harvest grasses to optimize the fiber content for the class of livestock being fed; therefore our focus is on NDF. Optimum NDF is the most useful harvest date target, and standardizing varieties to a common NDF value offers the most potential for meaningful comparisons of varieties. Standardization can be accomplished by determining the rate of change of yield and quality over time, which was found to be essentially linear from mid May to early June. While rates of change do vary among varieties, an average rate of change over time for yield or quality can adequately represent all varieties in a trial. Sampling across years, locations and varieties, replicated rates of change averaged 0.85 NDF units per day, -1.0 fiber digestibility (NDFD) units per day, and 205 lbs DM per day. The coefficient of determination for these parameters in individual varieties typically exceeded 0.90. Based on the rate of change of NDF per day, NDFD and yield of varieties were adjusted to a common NDF date. The mean NDF for a given trial was used as the target NDF for adjustment. Comparing 45 tall fescue varieties, adjusted NDFD and yield resulted in changes to some variety means that exceeded the trial LSD value. Adjustment of values to a common NDF date changed the order of some entries from near the highest ranked value to near the lowest ranked value and vice versa.