Evaluation of changes in weed flora in arable fields of Nordic countries - based on Danish long-term surveys

P>During the last 50 years, agricultural productivity has increased tremendously, as have changes in the weed flora. In several European countries, weed surveys have been conducted regularly, of which the most frequent and consistent ones are Danish surveys conducted in 1911-1915, 1945, 1960-1970, 1987-1989 and 2001-2004. The surveys were carried out on fields not sprayed with herbicide in the sampling year. On the basis of this frame of reference, we discuss the changes in the weed flora during this period and relate them to findings in other Nordic countries and review the role of different farming practices on the weed flora of the past. Numerous agronomic factors and political initiatives to protect the environment have operated in concert. During the last 30 years, winter-sown crops have increased by almost 70%, at the expense of spring annual crops and grass leys, and this has favoured winter annual weed species. Maize is now widely grown and new weed species are invading arable land. Also, increased fertilisation levels over time have affected the weed flora. It is, however, obvious that herbicides play a major role in determining the composition, diversity and abundance of weed flora, although other factors such as mechanisation and the Common Agricultural Policy of the EU are also important. For the last 20 years, we have experienced an increase in some of the most frequent weed species without reductions in yield. This is probably due to governmental policies mandating lower herbicide efficacy goals for farmers, but it has been partly counteracted by genetic improvement of crops and better management.