[Towards a Better Management of Nitrogen in Maize - Undersowing, Sowing in Residues of a Cover Crop, Nitrogen Placement]

We tested two methods aimed at providing soil coverage before and after a maize crop, to limit erosion and nitrate leaching. The first method consisted in undersowing a companion crop between maize rows, and leaving this companion crop after maize harvest- The companion crop is sown between seedling emergence and canopy closure of the maize crop. Thirteen species were tested including grasses (rye grass, red and tall fescue, smooth-stalked meadowgrass, common bent, cocksfoot), cereals (rye), legumes (field bean, pea, red clover, lucerne), cruciferae (rape) and phacelia. The technique allowed a fixation of 31 to 42 kg/ha of nitrate nitrogen, according to the species and provided soil cover after harvest of maize. Undersowing a perennial ryegrass (25 and 50 kg/ha seeding rate) reduced soil erosion by 13 and 54 % respectively. Species which can be sown early (to achieve sufficient development before maize canopy closure) and do not compete excessively with maize (to avoid yield losses) appeared preferable. Such a species was red fescue. The use of species such as rape, field bean, pea and smoth-stalked meadowgrass should be avoided. The second method consisted in maize sowing in the residues of a cover crop sown in autumn and destroyed by winter frosts or chemically, according to the species. The cover crop provided soil coverage during the interval between the preceding crop and maize sowing. Direct drilling of maize (no-till or minimum tillage system) in the residues makes this technique especially attractive in erosion sensitive soils. Soil warming up was however slower in early season. Wheat, radish, rye, ryegrass, rye and ryegrass mixture, rape, lucerne, phacelia, triticale and vetch were tested as cover crops. The cover crop must be destroyed early : destruction was easier with a younger crop, risk of competition by survivors was less, and sowing of maize was easier (amount of residues is smaller). Under those conditions only, maize yields were not decreased. Mixture of cover crops, ryegrass and radish appeared to be more difficult to destroy. The effect on the environment was favourable, since 40 to 70 kg/ha of nitrate nitrogen were fixed by the cover crop during the interseason preceding maize sowing. Both techniques had therefore short term favourable effects on the environment since nitrates were incorporated in the companion crop or in the cover crop organic matter. The fate of this organic matter remains however to be taken into account, and the mineral dressings of the following crops must be adjusted. A last technique tested concerned nitrogen fertilizer application : placement and splitting of different forms and different amounts. Side dressing allowed a 35 to 45 kg N/ha reduction of the amount broadcast applied, without yield decrease. This preliminary result remains to be confirmed, however.