Nitrogen flows and balances in four grassland-based systems of dairy production on a clay-loam soil in a moist temperate climate

This study examined productivity, nitrogen (N) flows and N balances in grassland-based systems of dairy production in Ireland. There were four stocking densities of dairy cows on grass/white clover pastures and four inputs of N as fertilizers, concentrates and biological fixation over 2 years; 2001 and 2002. Annual stocking densities were 1.75, 2.10, 2.50 and 2.50 cows ha(-1). Associated N inputs were 205, 230, 300 and 400 kg ha(-1) respectively. There were eighteen cows per system. Cows calved within a 12-week interval in spring with a mean calving date of 28 February and lactation extended until mid-December in each year. There were no differences in annual milk yield (6337 kg cow(-1); s.e.m. 106.1), live weight or body condition score. Pre-grazing N concentrations in herbage increased (P < 0.001) with increasing N input, whereas there were no differences in N concentrations in silage reflecting optimum N inputs for silage production. Grazed herbage accounted for 0.64, silage 0.26 and concentrates 0.10 of annual dry matter consumed by the cows. Annual intakes of N ranged from 144 to 158 kg cow(-1) and were mostly influenced by N concentration in grazed herbage. Annual output of N in milk and liveweight change was 38 kg cow(-1) and was not different between systems. Annual N surpluses increased with increasing N inputs from 137 to 307 kg ha(-1), whereas the proportion of N inputs recovered in products declined from 0.34 to 0.24. More efficient N use was associated with lower N inputs and in particular lower N concentrations in grazed herbage.