Grass-clover response to cattle slurry N-rates: Yield, clover proportion, protein concentration and estimated N2-fixation

Abstract

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.