Influence of fodder crop rotations on the potential weed flora in the irrigated lowlands of Lombardy, Italy

Extensification has recently become an important option in Western European agriculture, driven both by economic considerations (product surpluses) and growing public concern on the possible adverse effects of intensive farming on the environment and human health. The adoption of rational fodder crop rotations, with the rediscovery of the beneficial effect of the meadow, is viewed as a possible means to reduce the impact of farming systems in the lowlands of northern Italy, characterised by highly intensive cropping and animal husbandry. Crop rotations are also expected to exert a positive effect in controlling the weed flora, thereby reducing the need for chemicals and increasing the sustainability of fanning. The current study examined, by the 'seedling emergence' technique, the quanti-qualitative composition of the potential weed flora (seed bank in the 0-20 and 20-40 cm soil layers) at the end of the second cycle (12 years since establishment) of a long-term crop rotation trial in northern Italy. The experiment compared five fodder crop systems, different in the degree of crop intensification and for the presence or absence of the meadow: a 1-year continuous cereal double cropping (R 1); a 3-year rotation (R3); a 6-year rotation (R6); a permanent meadow (PM); and a continuous grain maize cropping (CM). Each rotation was subjected to two input treatments, defined as optimal and sub-optimal (input reduction of approximate to25-30%) for the region, respectively, in terms of nutrient levels, herbicide doses, and soil tillage methods. The crop rotations exerted a marked influence on the potential weed flora, both qualitatively (mean number of weed species) and quantitatively (number of emerged weed seedlings m (-2)). Long rotations and maize monoculture had opposite trends with respect to the weed species diversity, the latter showing the narrower composition of the weed flora. The rotations including grain maize (viz., CM and R3) showed the highest seed stock, whereas the PM had the lowest weed infestation. Grain maize was particularly infested by warm-season, weed grasses, such as Digitaria sanguinalis and Panicum spp., against which neither the spring herbicide nor the post-emergence cultivation seemed effective. The meadow also showed an outstanding 'cleansing' effect when inserted into the R6 rotation after 3 years of annual double cropping, reducing to about one-third the initial seed stock. The input levels did not influence significantly the weed seed bank, suggesting the possibility of adopting less intensive agronomic practices without markedly worse weed control. (C) 2002 Elsevier Science B.V. All rights reserved.