Our objective was to evaluate a method for comparing forage quality of grass varieties harvested at different maturity stages. Forty eight tall fescue (Festuca arundinacea Schreb.) varieties were sown in a randomized complete block design with five replicates at two sites in 2004. Three replicates were harvested for yield and two were used for heading date evaluations and periodic spring quality sampling in 2005. Heading date was determined as the date when at least five heads in a plot were visible emerging from the boot. Heading dates ranged from May 21 to June 6, with an average range of 10 days within a site. Yield and quality samples were taken on six dates from heading date plots from mid May to early June. Changes in parameters were generally linear between mid May and early June, selection of four sampling dates during this interval allowed the calculation of rates of change for forage yield and quality parameters. An exception was yield at one site, which was linear when transformed to growing degree days, base 30 (GDD). Linear rates of change were 172 kg DM/ha/day and 7.7 kg DM/ha/GDD for forage yield, 244 kg milk/ha/day and 13.7 kg milk/ha/GDD for Milk2000 yield, 8.2 and 9.4 g/kg/day for NDF, -6.8 and -5.9 g/kg/day for IVTD, -10.9 and -8.3 g/kg/day for NDFD, -6.4 and -5.9 g/kg/day for CP and -6.9 and -7.3 units/day for relative forage quality (RFQ). Rates of change were used to generate heading date corrected quality and yield of varieties harvested on the same date at each site. Heading date corrections of 3 days or more generally resulted in parameter changes larger than the LSD value.