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

forage crops sometimes cited the increased competition or the mowing or grazing activities as
possible causes (e.g., Norris and Ayres, 1991; Schoofs and Entz, 2000), while increased
abundances were sometimes linked to reduced herbicide use or reduced soil tillage (e.g.,
Cardina et al., 2002a; Bellinder et al., 2004). Few details were given for identifying which
phases of the weed life cycle were mostly affected by PFCs. Only Heggenstaller and Liebman
(2006) showed that alfalfa reduced the seedling survival and fecundity of Abutilon
theophrasti, the most important weed species in their system.
A.IV.2 Hypothetical mechanisms causing the impacts
Annual and perennial crops differ in several important aspects concerning both the
characteristics of the crop plants and the crop management, including weed control actions
(see also Table 4). The impacts of PFCs on arable weeds might therefore be caused by various
mechanisms.
1) PFCs are characterized by the absence of soil tillage during the whole duration of the crop
(about 2-6 years), thus often much longer than with annual crops, where soil tillage and
sowing operations are mostly effectuated once or even several times per year. This may
have various impacts on weeds such as a reduced germination of weed species needing
light or oxygen stimulus for germination (Huarte and Arnold, 2003) and an increased
survivorship of established weed plants.
2) In contrast to these reduced soil disturbances, mowing or grazing may lead to frequent
mechanical disturbances of the aboveground vegetation. While most annual crops are
harvested only once per year, forage crop cutting is effectuated about 2-5 times per year,
thus often both at earlier and later times of the year compared to the single harvesting date
of annual crops. This may reduce the survivorship, biomass and seed production of weeds
(Gill and Holmes, 1997), although species may strongly differ in their sensitivity to
cutting.
3) PFCs are often characterized by strong canopy closures and deep and dense rooting
systems. This may cause intense competition against weeds. Compared to annual crops,
competition may not only be stronger, but also more extended in time. While annual crops
are often characterized by periods of weak competition (i) during crop establishment, (ii)
crop ripening/senescence and (iii) after harvest, perennial crops have only one
establishment phase in the first year and regrowth after cutting may be faster than initial
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