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