Performance and environmental effects of forage production on sandy soils. I. Impact of defoliation system and nitrogen input on performance and N balance of grassland

Grassland and its management is central to the productivity of and nitrogen (N) losses from dairy farms in north-west Europe. Botanical composition, production and N surplus of grassland were assessed during five consecutive years. The experiment consisted of all combinations of five defoliation systems: cutting-only (CO), rotational grazing (GO), grazing + one (MSI) or two silage cuts (MSII) and simulated grazing (SG). Four mineral N fertilization rates (0–300 kg N ha−1 year−1) and two slurry levels (0 and 20 m3 slurry ha−1 year−1) were applied. Fertilizer N was more efficient in producing net energy (NEL) in grazing-dominated, low white clover systems (GO and MSI systems: 70 and 88 MJ NEL kg−1 N) than in white clover-rich systems (MSII, CO and SG systems: ≤60 MJ NEL kg−1 N). While sward productivity in system MSI was similar to that in system GO, system MSII benefited from increased N2 fixation at low N rates. There were small differences in NEL concentrations of the herbage between defoliation systems. Crude protein concentration of the herbage increased with increasing N supply from fertilizer, excreta and N2 fixation. N surpluses (−63 to +369 kg N ha−1 year−1) increased with increasing grazing intensity and increasing N fertilization rate. The average response in N surplus applied was 0·81, 0·59, 0·40, 0·33 and 0·24 kg N ha−1 in systems GO, MSI, MSII, CO and SG respectively.