Nitrogen fertilization of grass/clover swards under cutting or grazing by dairy cows

Intensively managed perennial ryegrass/white clover (Lolium perenne L. and Trifolium repens L.) swards receive relatively high levels of fertilizer N, and high N surpluses can subsequently be found. The N-fertilization effects on growth, yield, and herbage quality were therefore examined on three farms over a period of three years. Nitrogen was applied at four rates (0, 75, 150, and 225 kg N year-1) with cutting or grazing regime in Year 1 and Year 2, after establishment. A spring-only application of 150 kg N was compared with four applications during the season, which was the fertilization management in the rest of the experiment. N-response was greatly affected by management. Under cutting it was higher in Year 1 [16 kg dry matter (DM) kg-1 N] than in Year 2 (9 and 13 kg DM kg-1 N with and without residual effects of fertilization in Year 1). Under grazing with one or two rest periods for cuts the N-response was only approximately half that under cutting. Under grazing there was only a response at the beginning of the season (May-June), while under cutting there was a response throughout the season. N-response decreased with increasing clover content in herbage by 4 kg DM kg-1 N per 10%-units of clover content. A spring-only application of 150 N increased the growth rate in spring-early summer, and decreased the rate later in the season. The annual yield was unaffected by the application method in fields with good clover establishment, but the seasonal clover content was highly affected. The results indicate different possibilities for strategic fertilization both at farm and field level, and in swards with a high clover content it demonstrates how the clover content can be used as a buffer both for maximizing the N-response and for manipulating the production profile.