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

the first year of the temporary grasslands. In contrast, species composition varied much less
among 2-6 year old grasslands, which is consistent with observations by Critchley et al.
(2006) in sown field margin strips in England.
While the large-scale study demonstrates the effect of perennial crops on weed communities
in the following annual crop, future studies must also determine how long this effect may last.
Canadian farmers estimated that weed control benefits of PFCs lasted for one, two, and three
or more years (11%, 50%, and 33% of respondents, respectively, Entz et al., 1995). While the
large-scale studies allowed a good estimation of (instantaneous) species composition and
species richness, abundance estimates were only based on one evaluation of the species
frequencies on the 32 quadrats per field and per year, which is not very exact and strongly
dependent on recent weed control actions. Therefore, high numbers of fields were required to
detect the effects despite this variability. Future studies may thus also take into account the
variability in crop management practices. Some of these shortcomings could be addressed by
the field experiment discussed below.
D.I.3 Weed population dynamics under various crop management
practices in the small-scale field experiment
Regular monitoring of the weed vegetation during the 2.5-years field experiment in Dijon-
Epoisses also showed contrasted weed population dynamics in annual and perennial crops
(chapter C.II, Manuscript 3). Differences in weed plant densities, diversities, biomasses and
species composition were strongest between the annual and perennial crops, before all other
contrasts concerning the crop management practices (see C.II.4.1 and summary in Table 11).
Many typical arable weed species showed decreasing plant densities in perennial crops and
increases in annual crops (Fig. 13 and Fig. 14). Therefore, impacts of PFCs on weed
communities observed in the field experiment are quite close to the large-scale weed surveys,
despite the differences between the two studies (see the next section D.I.4 for details). Results
of the field experiment are broadly also in line with several of the previous experiments in
other cropping systems reporting weed suppression by PFCs (e.g., Schoofs and Entz, 2000;
Bellinder et al., 2004; Teasdale et al., 2004; Albrecht, 2005; Heggenstaller and Liebman,
2006, and other studies reviewed in Article 1).
Among the three perennial crop management factors tested in the small-scale field
experiment, the sowing date (autumn vs. spring) had strongest impacts on weed species
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