Mixtures and pure stands of perennial ryegrass, tall fescue, white clover and red clover were grown in a 3-cut and a 5-cut system in southern Norway, at a low fertilization rate (100 kg N ha-1 year-1). The nutritional quality (annual weighted averages) of the dried forage from the two first harvesting years was analysed. There was no significant effect of species diversity on crude protein (CP) concentration. In the 3-cut system we found a significant species diversity effect leading to 10% higher ADF (acid detergent fibre) concentrations, 20-22% lower WSC (water soluble carbohydrate) concentrations and 4% lower NEL (net energy for lactation) concentrations in mixtures as compared to pure stands (averaged across the two first years). In the 5-cut system similar effects were seen in the first year only. This diversity effect was associated with a reduction in WSC and NEL concentrations and an increase in ADF, NDF and CP concentrations in the grass species, and not in red clover, when grown in mixtures. This is thought to be a combined result of better N availability and more shading in mixtures. Species diversity reduced the intra-annual variability in nutritional quality in both cutting systems.