Permanent grassland and 3-year leys alternating with 3 years of arable land: 31 years of comparison

During 31 years (1969–1999), we measured the yields of permanent grassland and of 3-year temporary leys alternating with 3year periods of arable forage crops. The average feed energy yields of both types of grasslands, respectively 75.1 and 73.3 GJ Net Energy for Lactation ha-1, were not significantly different. Possible preconditions for the lasting high production level of the 31-year old, never reseeded permanent pasture were the high fertilization level (200–350 kgN ha-1 year-1) and the preservation of a fairly good botanical composition. The temporary grasslands produced as much as the permanent grassland without the necessity to apply higher amounts of fertilizer N during their 3-year lifetime.