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

iv) the availability of cheap and efficient mineral fertilizers, herbicides and pesticides
that substituted some of the beneficial effects of perennial crops on the following
annual crops (Entz et al., 2002; Katsvairo et al., 2006b).
A.IV EXPECTED IMPACTS OF PERENNIAL CROPS ON WEEDS
A.IV.1 Literature review
Empirical studies investigating the impacts of perennial crops on arable weeds are very
heterogeneous in terms of methodology and not so frequent, probably as the importance of
this crop type decreased during the 20th century. In this review, studies were found mainly by
using the following key words in popular search engines such as the ‘ISI web of knowledge’
(www.isiknowledge.com) and ‘Google Scholar’ (http://scholar.google.com): “crop rotation”,
“temporary grassland”, “forage”, “fodder”, “ley”, “mixed farming”, “alfalfa”, “lucerne”,
“clover”, “legume-grass mixture”, “weed management”, “weed community”, “crop
protection” and by searching both the references cited in these studies and younger articles
citing them. The review was limited to cropping systems in temperate climates and to papers
published after 1992, thus not included in the review of Liebman & Dyck (1993).
There are numerous studies comparing weed infestations in different crop rotations. However,
most studies include only annual crops. This may also be illustrated by the fact that 26 out of
29 comparisons between rotations and monocultures reviewed by Liebman & Dyck (1993),
crop rotations included only annual crops. In the literature search, 15 more recent studies
reporting impacts of PFCs on weeds (sometimes described by several successive publications)
were retained. These studies will be shortly reviewed in the following (see also the summary
in Table 1 of Article 1). 13 studies were based on field experiments, only one study on a weed
survey covering a large number of fields of a whole region and one study on interviews of
farmers.
A.IV.1.1 Farmers interview
Entz et al. (1995) interviewed 253 farmers known to include forages in their crop rotations in
two regions of Canada, Manitoba and Saskatchewan in 1992. 67% of them reported yield
benefits and 83 % weed control benefits from including forages in their rotations. Weed
control benefits lasted for one (11% of respondents), two (50% of respondents), or more (33%
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