A COMPARISON OF ANIMAL OUTPUT AND NITROGEN LEACHING LOSSES RECORDED FROM DRAINED FERTILIZED GRASS AND GRASS CLOVER PASTURE/

Annual liveweight gain of beef cattle (steers) grazing grass pasture f ertilized with 200 kg N/ha was compared over a period of 7 years (1989 -95) with that of steers grazing grass/white clover pasture given no a rtificial N fertilizer at North Wyke, Devon, UK. Nitrogen lost by leac hing over the ensuing winter drainage periods was monitored from both pastures. Nitrogen leaching loss from the fertilized pasture over an e xtended period of 13 years (1983-95) is also reported. The average ann ual liveweight gain of the steers grazing the grass/clover pasture (0. 81 t/ha) was 19% lower than that of the steers grazing the N-fertilize d grass pasture (1.00 t/ha). The average annual loss of nitrate-N by l eaching in winter drainage from the grass/clover pasture (13 kg/ha) wa s only 26% of that recorded from the fertilized grass (50 kg/ha). A po ssible reason for this difference may arise from the previous history of the grass/clover pasture which had been ploughed in 1982, causing a flush of N mineralization and consequently greater immobilization of N in the soil in subsequent years. Losses of N each winter by leaching measured over a 13-year period from the fertilized grass were highly correlated (P < 0.001) with the preceding summer's soil moisture defic it, with the highest losses following dry summers. The nitrate-N conce ntration in the drainage water exceeded the European Union limit in dr inking water (11.3 mg/l) in the initial 25 mm of drainage during 11 of the 13 autumns. The average loss of N each winter (53 kg/ha) was equi valent to 26% of the fertilizer-N applied annually. Immediate losses o f N by leaching of fertilizer applied in early spring and throughout o ne very wet summer (1993) were minimal.