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

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