Keywords: Nitrogen legislation, Grassland fertilizer, Grazing, Nitrogen losses, Grassland management.

There is pressure to lower nitrogen (N) losses from intensively managed grassland. This paper examines the strategic use of fertilizer nitrogen for grassland with the objective of lowering costs and improving efficiency on farms while concomitantly lowering losses to the environment. A key issue is making use of N released by net mineralization of soil organic matter (SOM-N) under permanent grassland, particularly to meet the requirements for grass growth during the autumn, winter and early spring. During the main growing season there is a need to supplement the release of SOM-N with N inputs, usually fertilizer N. On farms where grass silage is produced it makes economic sense to maximise the area harvested for first-cut in May or June. To do this it is necessary to maximise stocking rate on the grazing area from April to June. Therefore greatest demand for grass on the farm coincides with the period of highest grass growth rates and when there is best production response to N fertilisation. The amount of second and third cut silage should be minimised to quantities sufficient to safely meet the winter feed requirements on the farm and hence, make the greatest area possible available for grazing during the second half of the grazing season. This lowers the requirement for fertilizer N on the farm during the summer and autumn resulting in lower N concentrations in pasture and less N excretion by grazing livestock. Lower N inputs will promote uptake of any residual N from the soil during the late summer and autumn, lowering the availability of mineral N in the soil when drainage recommences with increasing autumn rainfall. These factors combine to lower the potential for nitrate leaching losses from grassland during the late autumn and winter period.

James Humphreys, Teagasc Dairy Husbandry Research Center, Moorepark, Fermoy, Co. Cork, Ireland.
Kay O’Connell, Teagasc, Johnstown Castle Research Center, Wexford, Ireland.
School of Agriculture and Food Science, Queens University of Belfast, Newforge Lane, Belfast, BT9 5PX, UK.

36 Pages, 3 Figures, 3 Tables, 80 References.