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

2004). These aims are often considered contradictory, forming several trade-offs (Firbank,
2005; Green et al., 2005): increasing the agricultural production would either require to
expand the farmed area, implying further destruction of natural habitats, or to increase the
farming intensity, implying to increase the use of external inputs (but see Badgley et al.,
2007). Inversely, some of the actions (‘agri-environment schemes’ Berger et al., 2006)
implemented to reduce or compensate the negative impacts of farming on the environment or
on biodiversity, (also called ‘wildlife friendly farming’), may reduce the per hectare crop
production which may increase the pressure to expand the farmed land on natural areas (Green
et al., 2005).
A.II THE ‘WEEDS TRADE-OFF’
Arable weeds and weed control play a central role in this conflict between (i) crop production,
(ii) environmental protection and (iii) biodiversity conservation, which will be described in the
following sections. For clarity, a definition and short discussion of the term ‘weed’ is given in
Annexe 2.
A.II.1 Weeds & crop production
Weeds are frequently considered as one of the most serious factors threatening crop
production. Crop yield loss caused by weeds is estimated at 10% on average worldwide
(Oerke, 2006). Competition with the crop for growth resources (light, water, nutrients) is the
most important factor (Caussanel, 1989; Zimdahl, 2004), but weeds may also have other
negative effects such as the contamination of harvested grains by weed seeds, especially when
crop and weed seeds have similar sizes, and mechanical crop harvest difficulties, especially
for weed species with climbing morphologies such as Galium aparine. The presence of weed
plants is also an important factor for the presence of animals and micro-organisms. This may
include crop ‘pests’ such as aphids or slugs and crop diseases such as fungi developing on
weed species that are taxonomically close to the crops (see e.g., Dulout et al., 1997), but also
‘auxiliary’ organisms feeding on crop pests such as predatory carabids, rove beetles and
spiders (e.g., Schellhorn and Sork, 1997). For all these reasons, farmers try to keep weed
densities low using different weed control techniques (see Ch. A.II.2 below). Weed scientist
have intended to define theoretical ‘economic thresholds’ of weed densities above which the
expected loss of crop yield or quality exceeds the costs of weed control (Coble and Mortensen,
1991), but this threshold concept was shown to be unsuitable for weed management for long-
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