Optimal nitrogen (N) fertilization for grass-clover leys is important, since the N application affects yields and the clover and crude protein (CP) contents of the forage. Modeling of optimal N application to maximize economic output and minimize environmental consequences depends on grass-clover responses, e.g., on clover proportion, dry matter (DM) and CP yield. Thus, such parameters need to be quantified in field trials. Based on a three-year field trial (2017-2019; Foulumgaard Experimental Farm, Denmark) with four cuts of an organically managed grass-clover ley, the effect of increasing N fertilization from cattle slurry (0, 50, 100, 200 and 300 kg total N ha-1) on DM yield, CP yield and concentration as well as on clover proportion was examined. Results were analyzed separately for four sward ages (1-4 years). Dry matter yields ranged from 5.8 to 11.8 Mg ha-1 and the 200 and 300 kg N ha-1 treatments showed the highest DM yield for all sward ages; the 300 kg N ha-1 showed no significant additional yield benefit relative to the 200 kg N ha-1. The clover proportion generally decreased with increasing N application (36-65 % for 0 N and 8-35 % for 300 kg N ha-1). The herbage concentration of CP was either unaffected or decreased with increasing N application. The fourth-year swards showed DM yields comparable to first and third-year sward ages. In unfertilized swards, a linear relationship was found between the first cut in a given year and the fourth cut of the previous year (r2 = 0.82 within a range of autumn clover proportions of 33-74 %), indicating the possibility of using last cut for predicting next year's fertilizer requirements. The comprehensive data and results may form the basis for future modeling of optimal N application in organic grass -clover leys.
