Diversifying crop rotations with temporary grasslands - Université de ...
Diversifying crop rotations with temporary grasslands - Université de ...
Diversifying crop rotations with temporary grasslands - Université de ...
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4)<br />
Herbici<strong>de</strong> use<br />
5)<br />
Fertilisation<br />
Normal herbici<strong>de</strong> use Lower than in annual <strong>crop</strong>s or<br />
completely absent<br />
Use of mineral N-fertilizer Symbiotic fixation of<br />
atmospheric N<br />
29<br />
Increased weed plant<br />
survivorship and seed<br />
production<br />
Delayed N-availability may<br />
favour <strong>crop</strong>s over small-see<strong>de</strong>d<br />
weeds (Liebman and Ohno,<br />
1998; Liebman and Davis,<br />
2000).<br />
Besi<strong>de</strong>s these different direct effects on weeds, the differences between annual and perennial<br />
<strong>crop</strong>s may also have several indirect effects, involving e.g. modified microclimate and soil<br />
characteristics resulting from the permanent vegetation cover and the absence of soil tillage in<br />
perennial <strong>crop</strong>s (Entz et al., 2002). Weeds might also be affected by allelopathic compounds<br />
released by some perennial <strong>crop</strong> species including alfalfa (Xuan et al., 2004; Khanh et al.,<br />
2005) and clover (Liebman and Ohno, 1998; Ohno et al., 2000). Such phytotoxic effects may<br />
be stronger for weeds than for <strong>crop</strong>s, which may be caused by the small seed size of most<br />
arable weeds compared to <strong>crop</strong>s (Liebman and Davis, 2000). Due to increases of soil fertility<br />
(organic matter and nitrogen fixation) during the perennial <strong>crop</strong>s, mineral fertilisation may<br />
also be reduced in the following annual <strong>crop</strong>s. This may lead to a <strong>de</strong>layed N-availability<br />
compared to mineral fertilizer application, which may also give an advantage to big see<strong>de</strong>d<br />
<strong>crop</strong>s over small see<strong>de</strong>d weeds (Liebman and Ohno, 1998; Liebman and Davis, 2000). Indirect<br />
impacts on weeds may also arise if the <strong>crop</strong> plants grown after PFCs show a more vigorous<br />
and competitive growth due to a better soil structure and fertility or lower pest and pathogen<br />
pressures than after annual <strong>crop</strong>s. Finally, indirect impacts may also be caused through<br />
interactions <strong>with</strong> animals and micro-organisms that find a modified habitat in perennial <strong>crop</strong>s.<br />
One hypothesis would be that organisms feeding on weed seeds (seed predators) find better<br />
habitat conditions in perennial <strong>crop</strong>s which may cause higher weed seed mortality compared<br />
to annual <strong>crop</strong>s (Westerman et al., 2005; Heggenstaller et al., 2006).<br />
All these different hypothetical impacts of PFCs may strongly differ between weed species<br />
according to their morphological, physiological and phonological traits (MPP-traits, Violle et<br />
al., 2007). PFCs may thus provoke weed community shifts suppressing some species and<br />
favouring others. Weed species that are best adapted to annual arable <strong>crop</strong>s (including the<br />
most noxious ones) would be rather disfavoured while other species less or not occurring in<br />
annual <strong>crop</strong>s would profit. In this case, the integration of perennial <strong>crop</strong>s in arable <strong>crop</strong><br />
<strong>rotations</strong> would reduce the weed pressure in the following annual <strong>crop</strong>s and thus allow a high<br />
yielding <strong>crop</strong> production <strong>with</strong> fewer inputs.