Biomass yield and composition from semi-extensively cultivated perennial fodder grasses

Which perennial fodder grass species is most suitable for energy production from extensive permanent grassland implying low nitrogen (N) fertilizer application and low cutting frequency?
As part of an experiment comparing annual, perennial and woody candidate biomass crops, the perennial fodder grasses: perennial ryegrass (Lolium perenne L.), timothy (Phleum pretense L.), tall fescue (Festuca arundinacea Schreb.), cocksfoot (Dactylis glomerata L.) and a mixture of timothy and red (Trifolium pratense L.) and white clover (Trifolium repens L.) were sown in 2007 on a sandy soil in Melle (Flanders) in a split plot trial with two levels of N application (250 and 150 kg N/ha.year). The plots were cut 3 times a year (in May, July and September) in 2008, 2009 and 2010. We determined the dry matter yield, the contents of crude protein (CP), water soluble carbohydrates (WSC), cell wall (NDF), crude fiber (ADF) and lignin ADL and the organic matter digestibility (OMD).
The average dry matter yield amounted to 17.2 Mg/ha. year at 250 kg N and to 14.4 Mg/ ha.year at 150 kg N. Almost half of the yield was harvested in the first cut. Timothy was the highest yielding species at 250 kg N (19.4 Mg/ha.year) and the mixture of timothy with clover was the highest at 150 kg N (18.2 Mg/ha.year). Perennial ryegrass had the lowest dry matter yield. Although the higher sugar content and digestibility of the ryegrass, also its area-specific digestible organic matter yield was significantly lower than that of the more fiber rich species timothy and tall fescue and the timothy/clover mixture at the low N application rate.