Dairy systems based on grazing reach their highest economic performance when utilized as much as possible under grazing. Standing crops make it possible to control the costs by reducing the proportion of harvested forage. This technique is presently well mastered with Perennial Ryegrass. Measurements of the quality of this type of forage are welcome. From 1999 onwards, cultivars of Perennial Ryegrass were grown as standing crops. Three samplings, made after 21, 42, and 63 days of regrowth, showed how the quality of the forage changed with time. This quality was assessed by measuring the disappearance of the dry matter in the rumen after 9, 24, or 72 hours of incubation. The results showed that with this management of delayed grazings there existed a genetic variation for digestibility among the Perennial Ryegrass cultivars ; that there could also exist differences in voluntary intake ; and there was a considerable effect of the type of grazing management on the quality of the forage.