Keywords : Intensively managed grass, semi-natural grasslands, forage species, dairy cows, in vivo digestibility, feed degradation, energy metabolism, milk production, ruminant nutrition, rumen fermentation, rumen kinetics, voluntary intake, feed evaluation This thesis focuses on the nutritional value of grass from intensively managed as well as semi-natural grasslands in diets fed to dairy cows. Aims were to explain why performance of dairy cows, fed intensively managed grass, is lower than expected based on their calculated energy intake, and to obtain knowledge on the nutritional value of forages from semi-natural grasslands if fed to dairy cows. In order to understand the reason for the overestimation of the performance of dairy cows, several feeding and respiration trials were collected. From the feeding trials it was concluded that there was a discrepancy between energy input in grass and concentrates and energy output in milk and maintenance. This was due to the composition of grass and diet, but also due to higher maintenance requirements on grass-based diets than currently assumed. In the respiration trials it was observed that maintenance requirements for dairy cows on grass-based diets should be increased by 10%. This increase was attributed to nitrogen excretion and energy required for digestion. The digestibility of forages from semi-natural grasslands is often low, due to a delayed harvesting date, and thus an advanced stage of maturity. However, though the ( in situ and in vitro ) degradation rate of mature grasses was low, the degradation rate of some dicotyledonous species appeared to be high. Also the intake of silage from semi-natural grasslands, with high proportions of dicotyledonous species was observed to be relatively high, especially when compared to the intake of silages from semi-natural grasslands mainly consisting of grasses in an advanced stage of maturity. The milk production of diets containing large proportions of semi-natural silages was low compared to diets containing mostly ...