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

A.III APPROACHES TO ALLEVIATE THE ‘WEED TRADE-OFFS’
In this subchapter, several concepts and approaches will be exposed that might be useful to
alleviate the trade-offs. Some of these concepts are rather well known and the reader will be
referred to the corresponding literature, others will be explained in more detail, especially the
integration of ‘Perennial Forage Crops’ (PFCs) into crop rotations. For instance, possible
impacts of PFCs on weeds will be introduced in the following subchapter A.IV. Considering
these different approaches, a modified cropping system will then be proposed and its potential
for alleviating the weed trade-offs will be discussed in subchapter A.V. In view of the
literature review about what is already known on the impacts of such perennial crops on
weeds, subchapter A.VI will identify research needs and questions that will be addressed in
this thesis.
A.III.1 Overview of the approaches
Alternative, non-chemical weed control techniques have often limited efficiencies. The use of
a single alternative strategy might also cause the selection of ‘resistant’ weed biotypes in weed
populations and tolerant species within the communities. Herbicides must thus be replaced by
a combination of different techniques and changes in the cropping system to manage weed
populations, as proposed by Integrated Weed Management (see Ch. A.III.2).
Moreover, simply replacing herbicides by another (non-chemical) weed control technique
does not solve the trade-off with farmland biodiversity. Considerations and strategies how to
alleviate this third part of the ‘weed trade-off’ started only very recently (Storkey and
Westbury, 2007). This is a particularly difficult problem as there is certainly no ‘ideal’ weed
infestation level and no optimum balance between production and biodiversity (Firbank,
2005). Farmers usually prefer to keep weed densities as low as possible, while they are
required at certain densities to sustain populations at higher trophic levels. For example,
Moorecroft et al. (2002) showed that linnets (Carduelis cannabina) and red buntings
(Emberiza schoeniclus) only feed on fields where the densities of their dietary weed seeds
exceeded 250m -2
on the soil surface, densities possibly problematic for crop production.
However, weed species may differ both in their competitive ability and potential harm to crop
production as well as in their ‘biodiversity value’ (support of other organisms) which might
offer new possibilities for reducing the weed trade-off (see A.III.4 for more details). Another
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