Occurrence and distribution of important weed species in German winter oilseed rape fields

Data on weed species currently found in winter oilseed rape, the extent of their occurrence and regional distinctions were collected in autumn 2005, 2006 and 2007 from a total of 1463 oilseed rape (OSR) fields throughout Germany. Investigations took place in untreated parts of OSR fields at the four to six leaf stage of rape. Weed frequencies and densities were determined by randomly placing a 0.1 m(2) square frame ten times in each plot, identifying and counting all plants within the frame. Results presented here mainly focus the distribution of important weed species and differences in their frequencies and densities between German Federal states. The majority of weed species occurring in winter oilseed rape is not evenly spread over Germany. There are weed species with a limited distribution which can be frequently found in some regions but are absent in others. Sisymbrium officinale, Descurainia sophia, Centaurea cyanus, Anchusa spp., Sonchus spp., Rumex spp. or Euphorbia spp. are some weed species with mainly regional importance. Several ubiquitous weed species show regional differences in their densities. Capsella bursa-pastoris, Chenopodium album, Galium aparine, Lamium spp., Matricaria spp., Stellaria media, Thlaspi arvense, Veronica spp. and Viola arvensis were among the most frequent weed species in OSR in all German federal states though sometimes differing significantly in regional densities. Recent field observations reporting an increased spreading of Geranium spp., Sisymbrium spp. or Anchusa arvensis in intense OSR cropping areas are verified by these results.