Nutritional value of grasses is determined by cell-wall digestibility (NDF). Accordingly, the optimal time of grass harvest is the beginning of heading. After this growing phase, the parameters of rumen degradability and cell-wall digestibility quickly deteriorate. In the methodology, recommended times harvesting in early- and late- maturing grass species are stated. These terms are to be adapted to grass variety, application of fertilizers, altitude, soil and climatic conditions. Combination of grass species with regard to their digestibility of NDF, which determines the grass quality, is recommended as well. In grass and grass-legume mixtures, the harvesting should be started at the time of heading of the first predominant grass species.