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

significantly reduce the variability of the farmers’ income, which may be more important
than the absolute amount of income (Zentner cited in Entz et al., 2002).
• In a study of Olesen et al. (2002) in Denmark, the yield benefits caused by the inclusion of
a grass-clover ley could not fully compensate for the yield reduction as a result of leaving
25 % of the rotation out of cash crop (cereal) production.
• In a study of Liebman et al., (2008), the economic profitability of the farming system
could be improved by including alfalfa crops into corn–soybean rotations, but the
government subsidies reduced this advantage from 7 % to 1 %.
The need for (and therefore the prices of) herbal forages decreased during the last decades due
to the separation of crop and livestock production and the shift towards grain forage such as
maize. In the future, the demand and profitability of perennial crops might however increase
again as these kind of crops are increasingly used to produce energy or raw materials for
different industries. Tilman et al., (2006) suggested that ‘Low-Input High-Diversity’
grasslands may produce biomass (e.g. for bio-fuels) with a negative carbon balance and higher
yields compared to different monocultures.
A.VI THE RESEARCH PROJECT
A.VI.1 Objectives and questions
The diversified cropping system proposed in section A.V should alleviate the ‘weeds tradeoff’
and contribute towards a more sustainable agriculture. This system relies on the ‘weed
regulation function’ of PFCs. It is thus most essential to study the impacts of perennial crops
on weeds and to test the hypothesis of weed community shifts away from species that are most
noxious in annual crops. It is also necessary to improve the understanding of the underlying
mechanisms (Table 4). This is essential to predict the impacts on weed populations and
community dynamics and to successfully use this hypothetical ‘ecosystem service’ provided
by perennial crops. The following research questions will therefore be addressed in this thesis:
• How do weed populations and communities react to PFCs?
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