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Mixed swards of white clover–grass mixtures in highly productive environments often fail to reach the minimum recommended annual clover proportion of about 0·30. This study assessed the effect on clover content and total dry matter (DM) yield of two spring N applications (0 and 45 kg N ha−1) and two distances between drilled grass-rows (0·18 and 0·36 m) over 3 years for mown swards of white clover–Italian ryegrass ( Trifolium repens–Lolium multiflorum) in binary mixtures in northern Italy. An additional aim was to determine the advantage of association of grass–clover compared with grass and clover monocultures. On average, N fertilization of mixtures resulted in almost 9% higher total yield ( P < 0·01; mean response = 18·1 kg of total DM per kg of N) but decreased the clover proportion (0·250 vs. 0·312). Wider grass-row spacing increased clover proportion (0·327 vs. 0·234; P < 0·01) with no reduction of total DM yield. N fertilization × grass-row spacing interaction occurred only for clover content ( P < 0·01). Without N fertilization, mixtures out-yielded clover and grass pure stands. With N fertilization, at double rate to pure grass, yields from mixtures were greater than from clover and comparable to Italian ryegrass. [ABSTRACT FROM AUTHOR]