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

D GENERAL DISCUSSION
Data from weed surveys on commercial fields and from the small-scale field experiments
suggested that PFCs have strong impacts on the arable weed vegetation which is in line with
the initial hypothesis. The large-scale weed surveys (Article 1) showed that current annual and
perennial crops differed strongly in species composition (discussed in D.I.1). Comparisons
between wheat fields following either perennial or annual crops as well as the analysis of
several stages of a crop rotation before, during and after perennial crops using the space-for-
time substitution design (Article 2) suggested that the inclusion of perennial crops in rotations
based on annual grain crops also impacts the weed communities in the subsequent crop
following PFC (seed details in D.I.2). While the large-scale studies were based on a high
number of fields of a whole region during three years with a high variety of natural conditions
and crop management techniques (information that was not available for analysis), different
crop management treatment could be compared in the small-scale field experiment. This
experiment suggested contrasting population dynamics and thus increasing differences in the
communities between annual and perennial crops (discussed in D.I.3). A comparison of the
behaviour of individual weed species in the large- and small-scale studies is provided in
chapter D.I.4; reactions of functional groups, weed abundances and diversities are discussed in
D.I.5 and D.I.6.
Results suggest that several mechanisms are involved in the effects of the impacts of PFCs on
weeds (D.II). Mechanisms linked to (the absence of) soil tillage (D.II.1) and competition
(D.II.2) are much better known than others linked to the regular hay cuttings (D.II.3),
interactions between competition and disturbances (D.II.4) and weed seed predation (D.II.5),
which were thus studied in more detail. Strengths and weaknesses concerning the different
approaches are discussed after each section.
Given that the identified processes are intended to be integrated into mathematical models
simulating the weed population dynamics as influenced by cropping systems and natural
conditions, some suggestions for modelling formalisms are discussed in section D.III.
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