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

herbicides, soil erosion due to intensive soil tillage, reduced carbon fixation and increased
CO2-emissions).
Intensive chemical and mechanical weed control may also have negative
effects on different elements of farmland biodiversity, either by direct destruction of
organisms or by alternating the food availability and habitat quality (Semere and Slater, 2007).
Both mechanical and chemical weed control may have very high economic costs and need a
lot of non-renewable resources. Finally, the long-term efficiency of herbicides is also
questioned due to the appearance of herbicide resistant weed biotypes. All these reasons
increased the need for developing more sustainable weed management systems.
The diversification of crop rotations may be an important element to achieve this goal. In
alternating the selection pressures, it may prohibit strong population increases of particular
weed species, which may be the case in monocultures. It is frequently cited as an important
element of IWM (Liebman and Dyck, 1993; Buhler, 2002; Cardina et al., 2002a; Nazarko et
al., 2005). Crop rotations may be diversified either by introducing additional annual cash
crops, by growing ‘cover’ or ‘catch’ crops in the period between successive cash crops, but
also by introducing perennial crops lasting several years on the fields. In central Europe,
perennial crops consisting of grasses, legumes or mixtures were widely used for livestock
forage production in mixed farming systems (perennial forage crops, PFCs) but the
importance of these systems declined during the last 60 years (Freyer, 2003). However,
perennial crops recently gained new interest for sustainable farming design as they may
increase soil organic matter and carbon storage important for the global climate, improve soil
fertility, which may have positive effects on crop yields and may reduce fertilizer inputs,
especially after perennial legume crops, reduce soil erosion and nitrogen leaching and increase
the landscape heterogeneity, biodiversity and associated ecosystem services (Katsvairo et al.,
2006b). Perennial crops have recently gained new interest for the production of bio-energy
and other renewable resources (Tilman et al., 2006; Ceotto, 2008).
The (re-)introduction of perennial crops into arable crop rotations may also cause benefits for
weed management. Perennial and annual crops differ in numerous aspects that may be
important for weeds. PFCs are e.g. characterized by (a) the complete absence of soil tillage
and sowing operations during several years, (b) permanent vegetation that is present nearly all
year round, but (c) several hay cuttings per year regularly destroying large parts of the
aboveground biomass of crops and weeds. In contrast, most annual crops are characterized by
annual soil tillage and sowing operations, the crop vegetation is only present during shorter
periods of the year and they are harvested only once per year. These modified conditions in
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