Apparent nitrogen fertilizer replacement value of grass-clover leys and of farmyard manure in an arable rotation. Part I: grass-clover leys

Apparent nitrogen fertilizer replacement values of grass-clover leys (NFRVGCL) and farmyard manure (NFRVFYM) were studied in a long-term (24 years) experiment. This paper reports the results for grass-clover leys (GCL). Five rates of farmyard manure (FYM) and four rates of fertilizer nitrogen (N) were applied to two arable rotations (RotA -arable+ silage maize; RotB - arable+GCL). Polynomial response surfaces fitted to dry matter (DM) yield and N offtake in the three arable crops were used to assess NFRVGCL by interpolation, at high (Method H) and low (Method L) N fertilizer rates, always comparing RotA and RotB at equal FYM rates. In Expt1 (started in 1980), arable crops in RotB required 188, 246, 270, 295 and 312 kg N/ha less fertilizer per cycle than in RotA, to match the DM yields found at highest fertilizer rate in RotA (Method H). These values refer to FYM rates of 0, 20, 40, 60 and 80 t/ha per cycle, respectively. Corresponding fertilizer savings in Expt2 (started in 1981) were 287, 323, 341, 346 and 337 kg N/ha per cycle. These NFRVGCL values represent savings of 50-83% (Expt1) or 77-92% (Expt2) in N fertilizer applied to the arable phase. Slightly lower NFRVGCL was found by Method L. Extra N offtake in RotB over RotA was a poor estimator of NFRVGCL, showing much lower values and different responses to FYM and fertilizer than NFRVGCL. Mixed leys enable large fertilizer savings in arable rotations.