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

The first two limits could be addressed by the small-scale field experiment, where the weed
vegetation was monitored on a much finer temporal scale during the whole vegetation period
(see discussion on weed population dynamics, page 155 below). First evidence for long-term
effects could be obtained by comparing fields with annual or perennial preceding crops and by
following the development of the weed species composition during crop rotation using the
space-for-time substitution design (discussed in the following paragraph).
D.I.2 Weed community trajectories during crop rotations
As long-term studies analyzing one or several cycles of long rotations with annual and
perennial crops are very costly and impossible to conduct during ‘standard’ 2-4 years research
projects, an alternative space-for-time substitution design was used in this work. It
simultaneously compared a large number of fields being in different phases of such long crop
rotations (before, at the beginning, at the end and after perennial crops). First, this analysis
confirmed results of Ominski et al. (1999) who compared the weed species composition after
either annual or perennial crops in Canada. Second, it suggested that the weed community
composition varies in a cyclic way during the crop rotation. It thus allowed to retrace typical
weed community trajectories during crop rotations including perennial crops and thus to better
comprehend the differences in the weed species and functional groups composition in the
following annual crop (see Article 2 for details).
Many species causing problematic infestation in wheat in cereal-based cropping systems were
both less frequent and less abundant in wheat immediately following alfalfa as compared to
wheat following a sequence of annual crops (main examples : Galium aparine, Cirsium
arvense, Sinapis arvensis). On the other hand, perennial alfalfa favoured some species in the
following wheat (main examples: Taraxacum officinale, Silene latifolia, Veronica persica),
but most of those species would be likely to disappear quickly after some consecutive years of
annual crops, because they were not frequent or abundant in wheat following annual crops,
and are not considered as very harmful species in cereal-based cropping systems. As a
consequence, the introduction of perennial alfalfa in annual crop sequences can be considered
as a powerful mean to manage weeds, in spite of the low use of herbicide during the years of
alfalfa.
Both in the large-scale comparisons and the field experiment, highest modifications in species
composition away from the ‘typical’ weed composition in annual crops were observed during
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