Weed community evolution in conservative and conventional agricultural systems

The potential shift in weed communities is a common concern in conservation tillage systems. The aim of this study was to determine, in a field study, the weed community evolution over time in conventional and conservative cultural systems. The study was carried out in northern Italy over the period 1997-2012. Two cultural systems were compared: conventional (CONV), based on ploughing, and conservative (CONS), based on minimum tillage. A four-year crop rotation, including wheat, maize, pea, sunflower, rape and soybean was adopted. In both systems weeds were controlled with herbicides. Weed density was assessed for each crop at least twice during the growing season in non treated areas, while weed seed bank (at 0-15 cm depth) was monitored at the end of each crop rotation cycle. For the majority of the crops grown in CONS total weed density was significantly higher than in CONV. In summer crops the most abundant weeds were Echinochloa crus-galli, Chenopodium album and Galinsoga quadriradiata, with the highest density in system CONS. In wheat, weed infestation was mostly represented by Stellaria media, Veronica persica and Lamium purpureum. At the beginning of the study weed seed bank varied from 11838 seed/m2 in CONV, to 13616 seed/m2 in CONS. In the following assessments, weed seed bank decreased remarkably in both systems, particularly in CONV. Over the period weed species richness was influenced by the rotation cycle and by the changes in agronomic practices occurred. The highest number of weed species was generally observed in summer crops and in the system CONV. The CONS system was characterized by a higher Simpson index and a lower Shannon value, compared to CONV system. Weed management resulted more simplified in CONV system, due to a more diversified and generally less abundant infestation.