The effects of nitrogen fertilizer applied in spring on swards of ryegrass sown with four cultivars of white clover

Abstract Nitrogen fertilizer was applied at 0, 30,60 and 90 kg N ha‐1 in March 1976, 1977 and 1978 to plots containing perennial ryegrass cv. Barlenna sown either alone or with white clover cv. Blanca, Sabeda, Olwen and S100 in four replicated blocks. Dry matter (DM) yield of all swards responded positively to N with the response being highest in the no‐clover swards in two of the three years. Annual DM yields were lower in the no‐clover than in the mixed swards at any given N level. The higher levels of N reduced DM yield in mixed swards at some harvests in midsummer. Swards of S100 were consistently among the lower yielding mixed swards, whereas the other three clover cultivars varied in their relative yields. There were no N x cultivar interactions for white clover yields; irrespective of cultivar, N application reduced annual white clover yield by similar amounts. It is concluded that it is beneficial to apply moderate amounts of N fertilizer to mixed swards in spring and that under these conditions the yield differences between swards including medium‐large and smaller leaved white clover cultivars are similar to differences between the inherent yield potential of these cultivars in mixed swards receiving no N.