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

diversities were even higher in crop rotations containing hay crops (alfalfa-ryegrass mixtures)
compared to rotations containing only soybean or maize and monocultures of these annual
crops.
As the nature of species associated with young and old alfalfa is very different from weed
communities in wheats in annual crop sequences, the introduction of alfalfa also contributed to
the overall diversity of the flora at the landscape level (in addition to the increased floristic
richness in young alfalfa as compared to wheat crops).
Stable or increased plant diversities and improved richness/abundance ratios might be due to
three mechanisms. 1) The reduced soil tillage (and reduced herbicide use on commercial
fields) might lead to more stable habitats which would favour species diversity as predicted by
the intermediate disturbance hypothesis (Connell, 1978). 2) Perennial crops might also show
more small-scale habitat heterogeneities within the field than regularly tilled annual crops
(Ominski et al., 1999), which may favour plant species diversity, as shown e.g. by the review
of Benton et al. (2003). 3) Finally, crop rotations including PFCs are more diverse than
rotations including only annual crops, which may favour different species types in different
crops and thus also increase the instantaneous plant diversity (cf. General Introduction).
D.I.7 Conclusion: PFCs, useful tools for Integrated Weed Management
The different large-scale analyses and the field experiment generally agreed with the main
hypothesis of cyclic variation in weed abundance and species composition during the rotation
cycle (Fig. 7 in the General Introduction). Perennial alfalfa suppressed several of the most
widespread and most noxious weeds of annual arable crops. This may decrease weed pressure
and thus the need for curative weed control in the following annual crops. At the same time,
alfalfa favoured other plant species leading to stable or even slightly increased species
diversities during the crop rotations. These new species are not known as mayor weeds in
annual crops, and several of them (including Rumex crispus, Crepis sp., Picris sp., Cerastium
sp.) showed already strongly decreased frequencies in the following annual crops (Table 3 of
Article 2). It is thus not likely that the favoured species will cause strong weed problems in the
following annual crops. The other way round, the results might also indicate that an
‘interruption of grassland periods with annual crops’ may be favourable for managing
(perennial) grassland weeds such as Rumex sp. These results thus suggest that rotations of
annual and perennial crops may contribute to weed control without reducing plant diversity.
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