Effect of a spring application of nitrogen on the performance of perennial ryegrass white clover swards at two sites in the Netherlands

In the Netherlands, the introduction of milk quota and concern about N losses has led to a lower N application on grassland. Moreover, the present government policy will lead to a further reduction of N application. These developments have renewed the interest in white clover. Two cutting trails were set up to (i) evaluate the potential performance of mixed swards in the Netherlands and (ii) to quantify the effect of spring N on mixed swards. In a five year trial on clay soil five rates of N application in spring (0, 25, 50, 75 and 100 kg ha(-1) year(-1)) were combined with two cutting frequencies (4-5 cuts year(-1) and 6-7 cuts year(-1)). In a three year trial on sandy soil only the effects of spring N application (0, 50, and 100 kg ha(-1) year(-1)) were studied. Average annual DM yields were 14.66 and 13.76 t ha(-1) year(-1) for the clay and sandy soil, respectively. Spring application of 100 kg ha(-1) increased the yield in the first cut with 11.1 kg DM per kg applied N. White clover contents decreased with increasing N rate, reducing the DM yield in the remaining cuts on the fertilised treatments. Nevertheless, on an annual basis, 100 kg N ha(-1) increased the yield with 7.4 kg per kg applied N. Annual N yields were not affected by spring N application. Compared to a cutting frequency of 4 to 5 cuts year(-1) a frequency of 6 to 7 cuts year(-1) increased the white clover content from 36 to 47% and the N yield from 458 to 524 kg ha(-1), but did not affect the DM yield. It was concluded that spring application of N can be a practical tool to secure herbage production in spring with only a short-lived negative effect on white clover content.