Is there a Weed Shift in Roundup Ready Maize?

The cultivation of transgenic glyphosate-resistant crop varieties can simplify weed control in crops such as maize and others. On the other hand, the repeated use of broadly effective herbicides like glyphosate, especially in the same crop, can have a negative effect on biodiversity. Additionally there is a risk for a spread of more tolerant or even resistant weed species. It was the aim of a 6-years field study (2003-2008) to investigate weed effects of use of glyphosate in a continuous transgenic herbicide-resistant maize rotation at three sites in Germany. These sites differed significantly in terms of soil, climate and initial weed spectrum. The studies focussed on the comparison between local herbicide standards and split applications of Roundup Ready (360 g L^-1^ glyphosate) applied at dosages of 1,5 + 1,5; 2 + 2 and 3 + 3 L ha^-1^ Roundup Ready. Concerning the potential changes in weed communities over longer periods of time, the study did not indicate any negative effect on weed infestation, communities or diversity, of the glyphosate treatments compared to the local herbicide standard. Concerning species richness, there was no clear trend of the Roundup Ready treatments. Possible changes of the abundances of individual species were affected by the initial weed spectrum and consequently by the site conditions. As far as the conventional herbicides are efficiently used, they have no other effect than the glyphosate applications. Similar results were seen for the Shannon’s diversity index and Shannon’s evenness index. In sum, and looking at annual effects, it could be concluded from this 6 years study that 1) there are no statistically significant differences between local standard herbicide treatments and the glyphosate treatments assessed in this study on the mean values of seed bank, species richness, species diversity and dominance; 2) the data collected on the different parameters showed an enormous variability within sites and years; 3) a dosage of 3 + 3 L ha^-1^ Roundup Ready avoids spread of certain weed species like _Chenopodium album_ or _Urtica urens_; 3) as far as the standard herbicides were applied correctly in terms of a high and wide efficiency, they will have the same effect as the Roundup Ready treatments