White clover nitrogen N content is generally high and constant throughout the growing season. It is also difficult to manipulate, as N2 fixation tends to maintain a high N supply to the plant. White clover is a highly heterozygous, natural out-breeder, but self-fertile lines have recently been developed, including one which fails to develop nodules when grown under appropriate physiological conditions. An advantage of this line is that leaf protein content can be manipulated by altering the N supply to the plant and, in addition, it offers the opportunity to examine the effect of protein content on silage quality. This line was grown under varying levels of nitrate–N, giving rise to plants with a range of protein contents which were subsequently conserved as silage in two separate experiments. In experiment 1, both herbage protein and water-soluble carbohydrate (WSC) content varied, with values ranging from 17 to 31 g/kg dry matter (DM) and 52 to 72 g/kg DM, respectively. Protein content also varied in herbage grown for experiment 2, ranging from 20 to 33 g/kg DM, but variation in WSC content (91–101 g/kg DM) was not statistically significant. Silage quality was dependent on the protein content of the fresh herbage in both experiments. A comparison of herbages with high and low initial protein content, from experiments 1 and 2, respectively, revealed that a high protein content resulted in silages with a higher pH (4.81 versus 3.74 and 4.46 versus 4.23), a lower protein N content when expressed as a proportion of total silage N (536 g/kg versus 752 g/kg and 523 g/kg versus 659 g/kg TN), and an increase in free amino acid content when expressed as a proportion of total N (267 g/kg versus 90 g/kg and 323 g/kg versus 200 g/kg TN). A higher initial protein content results in a higher proportion of protein degradation during ensiling.