Plant maturity substantially influences the yield and quality performance of grasses. Grass phenology is often not considered objectively to evaluate the new genotypes prior to registration. Measuring the mean stage by count (MSC) is time consuming, and simplified approaches are, therefore, required. Twenty diploid, intermediate heading Lolium perenne L. genotypes were evaluated in a 2-year field study in Northern Germany for yield and the content of Neutral Detergent Fiber (NDF), Acid Detergent Fiber (ADF), Acid Detergent Lignin (ADL), and digestible organic matter (DOM). Data from the first and second cut, each comprising three sampling dates, were included in this study. A simplified maturity index (SMI8), expressing the percentage of tillers at or beyond the boot stage, from MSC was derived. This index resulted in similar correlations with yield and quality parameters compared to MSC but is easier to use and less laborious. The SMI8 reduced the variations among genotypes, as for the first cut NDF and ADF content, where the genotype effect disappeared after considering SMI8 as the covariable. Moreover, the ranking of the genotypes was slightly modified for most studied traits, indicating that a large part of the variations in the studied parameters was caused by variations in maturity.