MANAGEMENT OPTIONS TO LIMIT NITRATE LEACHING FROM GRASSLAND

Nitrate leaching can be reduced by the adoption of less intensive gras sland systems which, though requiring a greater land area to achieve t he same agricultural output, result in less nitrate leaching per unit of production than do intensively managed grasslands. The economic pen alties associated with reductions in output can be partly offset by gr eater reliance on symbiotic nitrogen fixation and the use of clover-ba sed swards in place of synthetic N fertilisers. Alternatively, specifi c measures can be adopted to improve the efficiency of nitrogen use in intensively managed systems in order to maintain high outputs but wit h reduced losses. Controls should take account of other forms of loss and flows of nitrogen between grassland and other components of the wh ole-farm system and, in most instances, should result in an overall re duction in nitrogen inputs. Removing stock from the fields earlier in the grazing season will reduce the accumulation of high concentrations of potentially leachable nitrate in the soil of grazed pastures but w ill increase the quantity of manure produced by housed animals and the need to recycle this effectively. Supplementing grass diets with low- nitrogen forages such as maize silage will reduce the quantity of nitr ogen excreted by livestock but may increase the potential for nitrate leaching elsewhere on the farm if changes to cropping patterns involve more frequent cultivation of grassland. Improved utilisation by the s ward of nitrogen in animal excreta and manures and released by mineral isation of soil organic matter will permit equivalent reductions to be made in fertiliser inputs, provided that adequate information is avai lable about the supply of nitrogen from these non-fertiliser sources.