SIZE AND COMPOSITION OF THE WEED SEEDBANK UNDER DIFFERENT MANAGEMENT-SYSTEMS FOR CONTINUOUS MAIZE CROPPING

The size and composition of the weed seedbank after 5 years of maize ( Zea mays L.) continuous cropping under four crop management systems wa s evaluated. The management systems included a conventional system (pl oughing, full-rate fertilization, broadcast pre-emergence herbicides), an organic system (ploughing, organic fertilization, physical weed co ntrol), a reduced-input system (rotary harrowing, low-rate fertilizati on, physical post-emergence herbicides) and a strip-cultivation system (non-tillage, banded fertilization and herbicides at sowing, broadcas t toy-dress fertilization and post-emergence herbicides). Seedbank sam pling was performed after maize harvest by taking 270 soil cores per m anagement system at depths of 0-10, 10 20 and 30 30 cm. Weed seedbank size was largest under the organic system (100 761 seeds m(-2)) and sm allest under the conventional system (27 601 seeds m(-2)). Differences among maize manage ment systems depended mainly upon weed control eff icacy rather than upon tillage effects. Four of the eight major weed s pecies showed differential responses to tillage and weed control metho ds carried out within maize management systems. Canonical discriminant analysis showed that maize management systems based upon reduced till age had more characteristic weeds than systems based upon ploughing. A maranthus retroflexus L., Chenopodium album L., Digitaria sanguinalis L. and Solanum nigrum L. were the most troublesome weeds for the cropp ing system under study; their relative importance in the seedbank usua lly varied according to maize management system.