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

PFCs might inhibit the successful germination, growth and reproduction of weed species that
are adapted to annual crops but may favour the growth of other species at the same time
resulting in plant community shifts. Such community shifts were observed in a large-scale
weed surveys in Canada (Ominski et al., 1999) and recently in France (Meiss et al., 2010a;
Meiss et al., 2010b) and are also suggested by several field experiments (reviewed in Meiss et
al., 2010a).
Understanding the mechanisms involved in the observed effects is necessary for generalizing
the results, for understanding the potential antagonistic observations reported by some authors,
and for developing predictive models. Previous experiments frequently confounded the
differences between annual and perennial crops (cited above) with differences in herbicide
treatments, as PFCs are frequently characterized by reduced herbicide treatments or the use of
herbicides with other active ingredients (Summers, 1998; Meiss et al., 2010a). However,
besides the strong and rather well-known effects of herbicides, many other differences
between annual and perennial crops may impact the weed species composition, that are not
well understood.
In this manuscript, it will first be analyzed which weed species are favoured and suppressed in
annual and perennial crops. Then, the impacts of contrasted crop management options will be
analyzed (including crop species, sowing date and cutting frequency in the perennial crops and
management options of the intercrops between successive annual crops). Finally, these results
will be used to investigate and discuss the underlying mechanisms of the impacts on weeds.
A 2.5-year field experiment was set up to compare the population dynamics of different
common annual weed species between PFCs and a succession of annual crops. For the
perennial crops, six crop management treatments were compared that differed by three factors:
1) the crop species, opposing a legume and a grass crop, which differ e.g. in establishment
and growth dynamics, symbiotic nitrogen fixation and fertilisation, hence affecting crop-weed
competition and other weed growth conditions; 2) the sowing date, opposing autumn and
spring sowing, which is known to affect weed recruitment in annual crops (Hald, 1999); and
3) the crop cutting frequency, which may have strong impacts on weed plant survival and
seed production. These three factors were partially combined in order to create a variety of
weed growth conditions. These perennial crop treatments were compared to a succession of
annual cereal crops which differed in various aspects to the perennial crops including soil
tillage, crop growth dynamics, and harvesting dates (only one cereal harvest per year). For the
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