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

discussion on interactions between competition and (aboveground) disturbances below). The
second impact (stimulation of weed seed germination by soil tillage), may have positive or
negative effects on weed populations depending on the reproductive success of the
germinating seeds. Both impacts may occur simultaneously making the analysis rather
difficult. Moreover, different weed species might react differently according to their traits.
Previously published experimental results (Sjursen, 2001; Albrecht, 2005; Hiltbrunner et al.,
2008) as well as the large-scale weed surveys (chapter C.I) suggest that biennial and perennial
plant species are less well adapted to regular soil tillage than annual species, which is in line
with ecological succession theory. However, the present experiment was mainly concentrated
on annual weed species (that were sown on all plots at the beginning), while frequencies of
naturally occurring perennial weeds were mostly too low to see any significant differences.
Only Taraxacum officinale showed slightly increasing plant densities in some spring-sown
perennial crops (Fig. 14), which had, however, no detectable effects on weed biomass.
Perennial and annual crops differed by several tillage operations per year. The first differential
soil tillage took place only four weeks after the establishment of the experiment when the
sowing of winter wheat was prepared of the plots of the annual crops. This event was probably
the main cause of the lower weed densities and biomasses in winter wheat compared to most
plots of the autumn-sown perennial crops until spring 2007 (Fig. 13, Fig. 18) due to the two
mechanisms cited above. First, this supplementary tillage operation probably destroyed the big
number of weed seedlings that germinated after the initial soil tillage four weeks ago. This
corresponds to the well known ‘false-seed-bed technique’ that is used especially in IWM and
organic farming systems for depleting the superficial weed seed banks before crop sowing
(Rasmussen, 2004; Chikowo et al., 2009). Second, weed seed emergence after this second
tillage operation in October 2006 was much lower than four weeks earlier (due to modified
climatic conditions or weed seed dormancy). It is well known that the delaying of crop sowing
in autumn may considerably reduce the weed infestation in annual cereal crops (Rasmussen,
2004). Both the false seed bad technique and delayed sowing dates are often recommended for
IWM in annual crops and might also be beneficial for reducing weed pressures during the
establishment phase of perennial crops.
In the annual crops, established weed plants were also mechanically destroyed by soil tillage
during the intercrop periods after cereal harvest in late summer and autumn/winter of both
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