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

including exotic species (Palmer and Maurer, 1997; Jiang et al., 2007; Lanta and Leps,
2008).
• In our experiment, the cutting dates and frequencies followed the experimental plan of 3
vs. 5 cuts per year with common cutting dates for all treatments (Table 6). This design was
used to disentangle the cutting frequency from the other experimental treatments (crop
species, sowing date) and to avoid introducing other variables such as the exact cutting
dates that may also be important for both crop and weeds growth. Cutting dates were thus
not optimized to specific crop growth dynamics and phenological stages that differed
between the crop species (Medicago and Dactylis), sowing dates and cutting frequencies
(Fig. 18). It may thus be recommend to use a flexible cutting frequency and to adapt the
cutting dates depending i) on the phenological stage of the forage crops to optimize the
crop growth and to maximize crop-weed competition but also ii) on the phenological stage
of the most noxious weed species to destroy high weed biomasses and to avoid seed
production. In the case of our experiment, the lower cutting frequency showed some
advantages:
o the Dactylis crops produced significantly more biomass than under the high
frequency (equal production for Medicago, Fig. 17),
o very frequent cuttings may reduce the regrowth ability of Medicago crops,
However, the cutting frequency should also not be too low, which may increase the risk of
high weed seed production and may lead to inferior forage quality.
While the weed seed bank analyses gives a good indication of the long-term effects of PFCs
on weed infestations, this should also be verified by long-term crop rotation experiments such
as the studies of Andersson & Milberg (1998) and Sosnoskie et al. (2006). Moreover, the
findings may depend on climate and soil conditions and must thus be repeated in other
locations. While such huge studies were not possible in our case, the effects of the perennial
and annual crops on the weed infestation of the following crop will be tested on our
experimental set up by a) analyzing the soil seed bank and by observing the weed vegetation
in a following test crop sown on all experimental plots.
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