Fertilisation azotée du maïs ensilage sur la bordure est du Massif Central. I- Choix des potentiels de production et de fourniture du sol en azote

Nitrogen fertilization of silage maize on the eastern slopes of the Massif central. I- Choice of production potentials and of nitrogen supply by the soil

On the eastern slopes of the Massif central (départements "Rhône" and "Loire"), a large proportion of the farm effluents (mainly farmyard manure) is used (50 t/ha on average) on land prepared for silage maize. The nitrogen fertilization of this crop, often excessive, can be better controlled by using local references regarding the values of production potentials and the nitrogen-supplying power of the soils. A network of experimental plots was set up on farms (20 each year during 4 years) to this purpose. The first paper deals here with the typology of the plots, based on various environmental constraints (altitude, exposure, soil type and structural accidents...) and on the crop succession. The results vary much with the years and with the plot type (from 7 to 17 t DM/ha). The production potential is defined on the basis of experimental plot without nitrogen constraints (from 12 to 15.5 t DM/ha on average, depending on the zone) and the nitrogen-supplying power of the soil is calculated on the basis of the unfertilized controls (from 150 to 218 kg N/ha). Depending on plot type, referential values are given, which make it possible to have some guidance for a nitrogen fertilization policy, with two possible strategies in mind : one for the maximization of production and risk limitation, the other best for vulnerable zones (regarding water resource).