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Notes: The sustainability of white clover in grass/clover swards of an upland sheep system, which included silage making, was studied over 5 years for four nitrogen fertilizer rates [0 (N0), 50 (N50), 100 (N100) and 150 (N150) kg N ha−1]. A common stocking rate of 6 ewes ha−1 was used at all rates of N fertilizer with additional stocking rates at the N0 fertilizer rate of 4 ewes ha−1 and at the N150 fertilizer rate of 10 ewes ha−1. Grazed sward height was controlled, for ewes with their lambs, from spring until weaning in late summer by adjusting the proportions of the total area to be grazed in response to changes in herbage growth; surplus pasture areas were harvested for silage. Thereafter sward height was controlled on separate areas for ewes and weaned lambs. Areas of pasture continuously grazed in one year were used to make silage in the next year. For treatments N0 and N150, white clover stolon densities (s.e.m.) were 7670 (205·4) and 2296 (99·8) cm m−2, growing point densities were 4459 (148·9) and 1584 (76·0) m−2 and growing point densities per unit length of stolon were 0·71 (0·015) and 0·67 (0·026) cm−1 respectively, while grass tiller densities were 13 765 (209·1) and 18 825 (269·9) m−2 for treatments N0 and N150 respectively. White clover stolon density increased over the first year from 780 (91·7) cm m−2 and was maintained thereafter until year 5, reaching 8234 (814·3) and 2787 (570·8) cm m−2 for treatments N0 and N150 respectively. Growing point density of white clover increased on treatment N0 from 705 (123·1) m−2 to 2734 (260·7) m−2 in year 5 and it returned to the initial level on treatment N150 having peaked in the intermediate years. Stolon density of white clover was maintained when the management involved the annual interchange of continuously grazed and ensiled areas. The non-grazing period during ensiling reduced grass tiller density during the late spring and summer, when white clover has the most competitive advantage in relation to grass. The increase in stolon length of white clover in this period appears to compensate for the loss of stolon during periods when the sward is grazed and over winter when white clover is at a competitive disadvantage in relation to grass. The implications for the management of sheep systems and the sustainability of white clover are discussed.