Diversifying crop rotations with temporary grasslands - Université de ...

governing the presence and abundance of seeds (i) and predators (iii) are strongly dependent
on the environment such as the pedo-climatic conditions and landscape characteristics and, in
the case of agro-ecosystems, on the cropping system and the different farming practices (Fig.
25). One of these environmental factors determining the habitat quality for seed predators,
vegetation cover, has been shown in Article 8 to have a positive impact on seed predation rate.
Future developments of models simulating weed seed predation might use this information.
However, the lack of mechanistic knowledge on the impact of vegetation cover, the
differences between weed seeds and all the other factors still hampers the proposition of a
mechanistic modelling framework at this stage.
D.IV GENERAL CONCLUSION
The results of this research project provide direct evidence that perennial forage crops might
play a significant role in arable crop rotations, both by contributing to the management of
weeds and by maintaining or increasing the floristic diversity. The typical trajectories of weed
communities identified from the preceding crop to the crop following alfalfa in the space-fortime
substitution design in the large-scale surveys were consistent with the more precise,
longitudinal observations of community and population dynamics in the field experiment. In
both studies, strong changes in the weed species composition away from the most noxious
species in annual crops were observed. Two examples of weed species that were clearly
suppressed by PFCs were Galium aparine and Cirsium arvense, two very competitive species
often requiring specific supplementary herbicide applications in current cereal-based cropping
systems. Most important, the effects of PFCs on weed communities were still marked in the
subsequent annual crop, hence indicating that the impacts not only concerned the currently
emerged flora, but also the belowground seed bank. The observations thus validated one
hypothesis of the project, i.e. the value of PFCs as a significant component of Integrated Weed
Management. Introducing PFCs in arable crop rotations is thus a powerful means for weed
management, probably allowing reductions in herbicides or excessive mechanical weed
control. Moreover, diversified crop rotations including annual and perennial crops creates
highly variable selection pressures for weeds, and this may also reduce the risk for selecting
herbicide resistant weed biotypes.
According to the large-scale surveys, the species richness and the richness/abundance ratios
were somewhat improved in alfalfa compared to annual grain crops, increasing floristic
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