Effect of 20-years crop rotation and different strategies of fertilization on weed seedbank

Crop rotation is thought to reduce weed density and maintain species diversity, preventing the domination of few competitive weeds. In this work rotations of 1, 2, 4 and 6 years length have been compared in a long-term experiment since 1976. In order to detect the effect of rotation length and fertilization on weed community evolution, a specific study was performed on weed seedbank with soil sampling in 1993 and 2012. Results: show that weed density was not affected by rotation length or fertilization type or rate, and that about 98% of weed species were indifferent to the factors applied. The dominance of Portulaca oleracea and increment of a few grass weeds were consequences of an inadequate management of late emergence and post-harvest weeds. Furthermore, no significant changes in the distribution of seeds longevity groups occurred in the seedbank from 1993 to 2012. In cropping systems where herbicides are used according to best practices or to Integrated Weed Management principles, the effect of rotation on weeds is hard to detect even in long-term experiments. For this, the effect of herbicides must be included in a more general theory of rotational effects.