Impact du système fourrager sur la qualité de l’eau. Enseignements issus du projet Green Dairy

Impact of the forage system on the quality of the water. Lessons from the Green Dairy project

The Green Dairy project, with a view of exploring the possible improvements regarding agricultural practices and the reduction of nitrogen and phosphorus excesses on dairy farms, has monitored 11 dairy farms on the Atlantic coast of Europe, in Ireland, Great Britain, France, Spain and Portugal. These systems cover a great diversity of production methods, of practices, and of pedo-climatic conditions. In the British Isles, the systems are based on grass and are intensive (per hectare Useable Farm Area : 8 500 l milk, 250 kg mineral nitrogen, and excesses of 225 kg N and 12 kg P). In Spain and in Portugal, the systems are still more intensive (4 L.U. per hectare, with much concentrate feed purchased and a heavy mineral fertilisation; excesses of 380 kg N and 56 kg P/ha). The systems in western France, with their arable crops and leys in rotation, are the most optimized, with excesses of 100-150 kg N and 15-22 kg P/ha. In the French systems however, the nitrate losses by leaching, approximately 50 kg N/ha, are larger than those observed in the British ones (about 20 kg N/ha), where the soil conditions and the humidity of the climate are more conducive to the organization of nitrogen in the soil and to denitrification. The proportions of maize and of grass seem to have little influence on the losses by leaching in the French systems, for equivalent stocking rates.